A Water-Based Heat Pipe for Molten

Steel Applications in a Tundish

by

Nogol Madani

Department of Mining and Materials Engineering

McGill University

Montreal, Canada

July 2010

A Thesis Submitted to the Faculty of Graduate Studies and Research

In Partial Fulfillment of the Requirements

For the Degree of Master of Engineering

Nogol Madani

2010

II

Abstract

In the continuous casting of steel shapes, a tundish feeds the molten steel alloy into

oscillating moulds. Therefore, a better knowledge of some of the parameters of the steel

in this phase of the process will lead to better quality with more productivity and in a

safer environment. There are many control parameters to be measured in the tundish,

especially on a continuous basis, yet in this project we are focused on the following 3

main ones that are the basics of tundish metallurgy: the temperature of the steel, the

thickness of the slag, and the composition of the steel.

A viewing tube capable of withstanding the intense environment of the steel tundish

enables one to obtain information about temperature, slag thickness and composition of

the melt on a continuous basis. This probe based on heat pipe technology has been

designed and fabricated and tested in two different environments. One was in gaseous

environment in a natural gas filled furnace. The other environment in which the heat pipe

has been tested was in a molten aluminum bath. The results of the first experiments

showed that the heat pipe is capable of extracting 23 kW at a heat flux of 0.5 MW/m2

while in the next set of tests the heat pipe could not overcome the film boiling limitation.

This was a result of the internal design of the pipe, thus, some design modifications are

suggested to solve the problem.

III

Résumé

Dans la coulée en continu pour produire des formes en acier, un panier de coulée alimente

des moules basculants en acier en fusion. Par conséquence, une meilleure connaissance de

certains des paramètres de l'acier dans cette phase du processus permettra d’améliorer la

qualité ainsi que la productivité dans un environnement plus sûr. Il y a beaucoup de

paramètres de contrôle qui peuvent être mesurés dans le panier de coulée, surtout sur une

base continue, cependant cette étude concerne les trois principaux qui représentent la

base fondamentale métallurgique du panier de coulée : la température de l'acier,

l'épaisseur des scories, et la composition de l'acier.

Un tube capable de résister à l'environnement intense du panier de coulée et permettant

l’observation in-situ a été utilisé pour obtenir l'un rend capable d'obtenir des informations

sur la température, l'épaisseur de scories et la composition du fond sur une base continue.

Cette sonde fondée sur la technologie des caloducs a été conçue, fabriquée et essayée

dans deux environnements différents. L'un était dans un environnement gazeux dans une

fournaise alimentée en gaz naturel. L'autre environnement dans lequel le caloduc a été

essayé était dans un bain d'aluminium en fusion. Les résultats des premières expériences

ont montré que le caloduc est capable d'extraire 23 kW à un flux de chaleur de 0,5

MW/m pendant que dans la prochaine série de tests le caloduc ne pouvait pas surmonter

la limite imposée par le film qui bout. Ceci était un résultat de la conception interne du

caloduc, ainsi, quelques modifications de conception sont suggérées pour résoudre le

problème.

IV

Acknowledgments

I would like to express my sincere gratitude to my thesis supervisor, Professor Frank

Mucciardi and co-supervisor, Dr. André Moreau for their guidance, encouragement and

support through the whole project. Their scientific approach and hard work are examples

to emulate in the future.

Special thanks are due to Salar Tavakoli, Csaba Szalacsi, Roberto Tariello, Jean-Guy

Allard and Antoine Pelletier for their enormous help in the design and the fabrication of

the heat pipe. I would also like to thank CMQ, Centre de métallurgie du Québec for

sharing their space and facilities with us for the aluminum tests.

The pleasant and friendly international environment created by my fellow graduate

students along with their knowledge and skills are deeply appreciated. I gratefully

acknowledge McGill University especially the Mining and Materials Engineering

department for giving me this opportunity and IMI-NRC for the financial support.

Last but not least, I would like to thank my parent, sister, brother and my best friend

Saadi Daftari for their understanding, patience and continuous support.

V

Table of Contents

Abstract -------------------------------------------------------------------------------------- II

Acknowledgments--------------------------------------------------------------------------- IV

Table of Contents--------------------------------------------------------------------------- V

List of Figures------------------------------------------------------------------------------- VIII

List of Tables ------------------------------------------------------------------------------- X

Chapter 1 Introduction-------------------------------------------------------------------- 1

1.1 CONTINUOUS STEEL TUNDISH MEASUREMENTS----------------------- 1

1.2 OBJECTIVES-------------------------------------------------------------------------- 1

1.3 THESIS ORGANIZATION---------------------------------------------------------- 3

Chapter 2 Literature Survey------------------------------------------------------------- 4

2.1 TUNDISH MEASUREMENTS IN CONTINUOUS STEEL CASTING----- 4

2.1.1 Various Methods for Measuring Temperature in the Steel Tundish--------- 5

2.1.2 Slag Thickness Measurements in Steel Tundish------------------------------- 7

VI

2.1.3 Analysis of the Composition of the Melt in the Steel Tundish--------------- 9

2.2 STRATEGY---------------------------------------------------------------------------- 10

2.3 HEAT PIPE TECHNOLOGY-------------------------------------------------------- 14

2.3.1 Heat Pipe History------------------------------------------------------------------ 14

2.3.2 Classical Heat Pipes--------------------------------------------------------------- 15

2.3.2.1 Classical Heat Pipe Applications--------------------------------------------- 17

2.3.2.2 Classical Heat Pipe Limitations---------------------------------------------- 18

2.3.3 McGill Heat Pipe------------------------------------------------------------------- 22

2.3.3.1 Development-------------------------------------------------------------------- 22

2.3.3.2 Design and Operating Principles of the McGill Heat Pipe---------------- 23

2.3.3.3 Choking Limitation in the McGill Heat Pipe------------------------------- 26

2.3.3.4 McGill Heat Pipe Applications----------------------------------------------- 27

2.3.4 Heat Pipe Probe for a Tundish--------------------------------------------------- 31

Chapter 3 Experimental Procedure---------------------------------------------------- 34

3.1 THE DESIGN AND FABRICATION OF THE HEAT PIPE------------------- 34

3.1.1 Evaporator--------------------------------------------------------------------------- 34

3.1.2 Condenser--------------------------------------------------------------------------- 39

3.2 EXPERIMENTAL SETUP----------------------------------------------------------- 42

3.2.1 Sensors------------------------------------------------------------------------------- 45

Chapter 4 Results and Discussions------------------------------------------------------ 47

PART 1: GAS FURNACE TESTS-------------------------------------------------------- 47

VII

4.1 REACHING STEADY STATE------------------------------------------------------ 48

4.2 MEASURING THE EXTRACTED HEAT----------------------------------------- 52

4.3 COMPENSATING FOR HEAT LOSSES------------------------------------------ 53

4.4 CHOKING LIMITATION (DRY-OUT)-------------------------------------------- 61

PART 2: MOLTEN ALUMINUM TESTS----------------------------------------------- 63

4.5 MODIFYING THE INTERNAL DESIGN OF THE EVAPORATOR-------- 66

Chapter 5 Conclusions--------------------------------------------------------------------- 67

References------------------------------------------------------------------------------------- 69

VIII

List of Figures

Figure 1.1- A schematic of the probe in a steel tundish-------------------------------- 2

Figure 2.1- Continuous steel casting------------------------------------------------------ 4

Figure 2.2- Schematic of the LIBS-------------------------------------------------------- 10

Figure 2.3- The emissivity of liquid steel and molten slag---------------------------- 12

Figure 2.4- Pouring sequences of (a) steel and (b) slag-------------------------------- 13

Figure 2.5- Classical heat pipe------------------------------------------------------------ 16

Figure 2.6- Schematic of loop heat pipe------------------------------------------------- 18

Figure 2.7- Natural and forced convection boiling curves----------------------------- 20

Figure 2.8- An example of a flow modifier---------------------------------------------- 24

Figure 2.9- McGill heat pipe after stable and unstable operations------------------- 27

Figure 2.10- Conventional and heat pipe cooled copper blocks---------------------- 28

Figure 2.11- Copper taphole block under heat loading from a gas burner---------- 29

Figure 2.12- Slag launder (copper)------------------------------------------------------- 30

Figure 2.13- Copper launder under intense heat load---------------------------------- 31

Figure 2.14- Temperature mapping of launder------------------------------------------ 31

Figure 2.15- The views of water based heat pipe immersed in aluminum---------- 32

Figure 3.1- Flow modifier used in McGill heat pipe----------------------------------- 35

Figure 3.2- Solid Works drawing of the evaporator------------------------------------ 36

IX

Figure 3.3- The top and side view of the evaporator----------------------------------- 37

Figure 3.4- Nozzle in the evaporator from different angles--------------------------- 38

Figure 3.5- Different views of fabricated nozzle---------------------------------------- 38

Figure 3.6- Assembled evaporator ------------------------------------------------------- 39

Figure 3.7- (a) Schematic of the condenser, (b) Fabricated condenser-------------- 41

Figure 3.8- Heat pipe during an experiment in gas furnace--------------------------- 42

Figure 3.9- Equipments for heat measurement------------------------------------------ 45

Figure 4.1- Heat pipe experiment in gas furnace level 1------------------------------- 49

Figure 4.2- Heat pipe experiment in gas furnace level 2------------------------------- 51

Figure 4.3- Heat pipe experiment in gas furnace level 3------------------------------- 52

Figure 4.4- The curve of extracted heat vs. temperature in setup level 1----------- 55

Figure 4.5- The curve of extracted heat vs. temperature in setup level 2----------- 57

Figure 4.6- The curve of extracted heat vs. temperature in setup level 3----------- 59

Figure 4.7- The dry-out phenomenon in an experiment in gas furnace level 3----- 62

Figure 4.8- The evaporator after gas furnace experiments---------------------------- 62

Figure 4.9- The behaviour of the heat pipe and the condenser in aluminum bath-- 64

Figure 4.10- The thin frozen aluminum layer on the evaporator’s surface---------- 65

Figure 4.11- The view of corroded area on the evaporator---------------------------- 65

X

List of Tables

Table 4.1- A set of data captured in furnace setup level 1---------------------------- 54

Table 4.2- A set of data captured in furnace setup level 2---------------------------- 58

Table 4.3- A set of data captured in furnace setup level 3---------------------------- 59

[1]

Chapter 1

Introduction

1.1 CONTINUOUS STEEL TUNDISH MEASURMENTS

Several technologies have been coupled in the development of an innovative probe for

use in the tundish. This probe can be used as a viewing tube to measure the three most

important parameters of temperature, slag thickness and composition on a continuous

basis. The corrosive environment of the steel tundish along with its high temperature

makes this target difficult to achieve. Even though, there are some continuous methods

for temperature and slag thickness measurements in continuous steel casting, this industry

still lacks a method with which one can obtain a composition analysis in the steel tundish

on a continuous basis. Among the methods that have been developed recently for

temperature and slag thickness measurements, there is no single method which meets all

criteria in this industry such as cost, safety, accuracy and sufficient life time ensemble.

Therefore, researchers and experts in this field are still struggling to find a solution for a

more appropriate continuous measurement technique.

1.2 OBJECTIVES

In this project we intend to develop a probe that is based on heat pipe technology, and

which will allow electronic devices of various types to view the liquid steel from above

the tundish in order to make strategic measurements of the melt (Figure 1.1). We have

[2]

formulated four test programs to pursue the ultimate target of a steel environment. They

are:

a) Gas furnace

b) Molten aluminum

c) Molten cast iron

d) Molten steel

The purpose of the research presented in this thesis is the following:

1) To design a heat pipe capable of withstanding the steel tundish environment

2) To evaluate the functionality of the design in addressing the heat extraction,

durability, safety and cost in the first two test programs listed above.

Figure 1.1- A schematic of the probe in a steel tundish

[3]

1.3 THESIS ORGANIZATION

Chapter 2 reviews the literature pertaining to this thesis. A summary of the current state

of measurements technologies in the field of steel continuous casting is given. In addition,

the strategy of this project along with the history of heat pipe technology is described in

detail. Finally, the McGill heat pipe with its new features and applications are presented.

Chapter 3 describes the design of the new heat pipe and the process of its fabrication.

Moreover, this chapter includes the experimental setups for the first two phases of the

project meaning gas furnace and molten aluminum tests.

In Chapter 4 we describe the results which have been obtained from the experiments. In

each setup the capability of the system for maximum heat extraction along with the

behaviour of the heat pipe towards the limitations of film boiling and choking is verified.

In addition, some ideas for further experiments in the future are presented. Chapter 5

summarizes the conclusions of this work.

[4]

Chapter 2

Literature Survey

2.1 TUNDISH MEASUREMENTS IN CONTINUOUS STEEL

CASTING

Several technologies have been developed in the search of an innovative probe to be used

in the steel tundish. Steel represents 93% of all metal production out of which 96% is

continuously cast. In continuous casting, the molten steel is poured from the ladle into the

tundish where it is fed to oscillating moulds for casting (Figure 2.1). The phenomena that

occur in the steel ladle affect those taking place in the tundish directly and later, what

goes on in the continuous casting mould and downstream will be reflected in the

intermediate and semi-finished quality of the cast products.

Figure 2.1- Continuous steel casting [1]

[5]

There are several parameters that can be of use if they can be measured in the tundish

environment, especially, on a continuous basis. Among them, there are three main ones

which we are focused on in this project. They are the temperature of the steel, the

thickness of the slag and the composition of the melt. More knowledge of these

parameters will lead to greater quality and higher productivity in a safe environment in

the production of continuously cast steel products.

2.1.1 Various Methods for Measuring Temperature in the Steel Tundish

In steel continuous casting, both high and low superheats can have negative effects. High

superheat above the liquidus temperature in the tundish can increase central segregation

and affect grain size which would finally interrupt the continuous casting sequence. In

addition, high superheats also lead to reduced productivity. On the other hand, low

superheat in the tundish can clog tundish nozzles, entrap macro inclusions which affect

flux powder melting and consequently increase the probability of mould sticker formation

[2]. Therefore, the benefits of continuous temperature measurement are better steel

quality, and an increase in machine productivity in a safer environment.

Steel industry uses the instruments such as radiation pyrometers, optical pyrometers, and

thermocouples, etc. to measure the temperature of molten steel. For a batch measurement,

thermocouples are most preferable. Most commonly used thermocouples are consumable

ones having a heat sensing portion which comprises platinum-rhodium-based

thermocouple and a quartz glass tube to protect the tip of the thermocouple. When this

thermocouple is immersed in molten steel, the heat sensing leading end is destroyed in a

[6]

very short time (10 to 20 seconds) so that the temperature measurement operation is

carried out in a short time frame. In addition, the heat sensing portion must be replaced

for each measurement. Therefore, we can conclude that this kind of thermocouple cannot

continuously measure a temperature for a long interval, and it is expensive to measure

temperatures frequently. Moreover, the characteristics of each thermocouple differ from

one another, and it is difficult to maintain the accuracy of temperature measurement [3].

Nowadays, the adopted method is the optical infra-red pyrometer, whose operating

principal is based on the light emitted by a certain material. The light is captured by a

sensor and transmitted by optical fibre up to a signal converter. The readings are

processed by the electronic signal converter by using a mathematical equation that

calculates and displays the temperature. This system consists of a measuring device,

articulated manipulator, signal conversion device and a control system. The measuring

tube is a ceramic tube that is immersed into the molten steel and works as a viewing port

for the optical sensor [4].

From the economic point of view, the continuous method has proved to be more

advantageous than the batch measurements, yet it still encounters certain technical and

operating problems such as high temperature in the sensor body, the cooling of the sensor

and optical fibre without interfering with the measurement, the loss in accuracy arising

from the distance between the sensor and the bottom of the ceramic tube and the effect of

the steel bath level changing during the measurements [4].

Therefore, in 2008, a measuring device featuring a cooled jacket has been developed to

protect the optical infra-red sensor. The infra-red sensor which is located inside a ceramic

tube made of pressed graphite alumina enables the system to measure the temperature

accurately while the steel is in the tundish.

[7]

Using this measuring device has some advantages such as exposing the operator to high

temperature for less time which leads to enhanced operating safety, lower operating cost,

reduced maintenance cost and higher measurement reliability. Yet this method has a

major problem and that is the replacement of the ceramic tube every 15 hours of

operation [4].

2.1.2 Slag Thickness Measurements in Steel Tundish

In steelmaking, after the basic oxygen process, some of the slag floating on top of the

molten steel follows the steel when it is poured into ladles and a fraction of that flows

from the ladle into the tundish. While some slag is required to absorb impurities and

inclusions, excessive quantities of slag can lead to serious processing scenarios which can

impact the quality of the cast product and create dangerous working conditions for the

personnel. Therefore, it is necessary to know the amount of the slag that resides on the

steel in the tundish.

There are several methods to measure the thickness of the slag in the steel tundish.

Richard E. Kracich and Kenneth Goodson, in 1996, described a slag depth measuring

system including measuring the lower level of slag by a "slag/steel interface electronics"

coil, a probe which must penetrate the slag layer and beyond, where the induction effects

of the coil are changed by the presence of the molten steel [5].

The other kind of probe used in the past is a simple steel bar that is inserted until its

extremity touches the bottom of the molten steel. The thickness of the slag is the length of

the bar that has been less heated. Because of the much higher thermal conductivity of the

steel, the portion that resides in the steel appears to be hotter than that which was in the

[8]

slag. Sometimes, the upper crust of the slag does not heat the bar enough and hence the

measurement may be compromised. Nevertheless, such devices can often be relied upon

at least for determination of the upper level of the molten steel that can be measured

reliably at a single point.

Microwave radar which has been used to measure the levels of various substances is

another option for the slag thickness measurements. These devices that have recently

become more popular for their cost, size and accuracy work with a basic principle and

that is the distance from the radar sender/receiver which is directly related to the elapsed

time between sending and receiving the radar signal [6]. However, despite the advantages

that these systems have, Kracich and Goodson disapproved using of a microwave unit for

measuring slag thickness due to time constraint, cost and durability in the harsh

environment [5].

But in 2000, a new radar unit has been developed which can measure the slag thickness

with high accuracy. This device is based on the enormous difference between the

electrical conductivities of slag and steel. The electrically conductive materials such as

steel with a high electrical conductivity of λ=7140 Ω-1

at steelmaking temperatures have

good reflection characteristics for reliable measurements. On the other hand, slag with

electrical conductivities λ of about λ=0.4 to 0.7 Ω-1

is desirable for its low reflection. The

difference in conductivity, a factor of about 7000, between slag and molten steel can be

used in the radar to determine the position of the interface of the slag and the molten steel.

Despite other microwave radars which report solely the upper level of the slag, this

invention recognizes the distance from both the upper and lower surfaces of the slag and

consequently the difference which is the thickness [6].

[9]

The device appears to work well but it has been described as expensive and delicate.

2.1.3 Analysis of the Composition of the Melt in the Steel Tundish

At this time, in the steelmaking industry, a technology that enables operators to have an

on-line and in-situ analysis of the composition of the melt does not exist. Some methods

of in-situ analysis have been proposed with more or less success. Attempts have been

made by some researchers to generate an aerosol powder from a melt either using a probe

containing an atomization die or using laser ablation to evaporate a sample from the

molten metal. Thereafter, the removed material is transported by an inert gas line to an

inductively-coupled plasma torch remote from the probe, where the metal powder is

finally analyzed. However, the transport of the aerosol may induce subtle errors in the

results due to selective evaporation of the volatile elements in the particles [7, 8].

Moreover, some in-situ analysis has been done using LIBS (laser-induced breakdown

spectroscopy) in lab scales [9]. In the LIBS technique, a laser pulse is sent to a melt

surface and this creates a plasma on the surface of the melt. As the plasma cools down, it

emits light which is focused by collector lenses that direct the light to a fibre optic as

shown in Figure 2.2 [10]. The light is carried by the fibre to a spectrometer which

performs the chemical analysis. In lab scale, the plasma is created on the molten steel

surface and hence one can analyse its composition by installing the device near to the

steel crucible while in the industrial scale, the molten steel is covered by slag and

therefore we need a viewing tube to be immersed into the tundish deep enough that the

technique would be able to analyse the composition of steel and not the slag.

[10]

Figure 2.2- Schematic of the LIBS [10]

Many have tried to make a variety of such tubes for prolonged immersion in steel. Given

the temperatures that the tube is exposed to, and given the corrosive slag it is in contact

with, and given that it may become frozen in the top layer of the slag, finding a viable

tube (and configuration) to use has been a formidable task that has yet to be resolved.

Therefore, laboratory analysis on a batch basis is the only feasible method to determine

the composition of the melt for now.

2.2 STRATEGY

Our research work is tied into providing the other groups of researchers a clear view of

the molten steel from above the tundish. With the use of a viewing tube based on heat

pipe technology, one can measure the temperature of the molten steel on a continuous

basis with a radiation pyrometer. This technique that has been used for a number of

decades uses the emissivity of clean steel surface to measure its temperature. However, in

many cases, the temperature of the steel is measured when the steel is tapped.

[11]

It is not possible to measure the steel temperature accurately if it is covered by slag. Thus,

in a tundish where the steel pool is covered by a slag layer, the use of a radiation

pyrometer to directly measure the steel temperature is not possible by simply pointing the

pyrometer at the slag surface. Now, if one inserts a tube of suitable diameter in the melt

such that it penetrates through the slag and into the steel, one then has a clear view of the

steel melt through the tube. To keep the slag or steel from entering the tube, an inert gas

such as argon is continuously bubbled through the tube. With a clear view of the steel

melt, one can then use a pyrometer to measure the temperature of the steel [11].

For a given wavelength and temperature, the amount of thermal radiation emitted by an

object is directly proportional to the spectral emissivity of the object's surface. This is

summarized in Planck's radiation equation (equation 2-1):

2

5

1( , )

1C

T

CI T

e

(2-1)

Where ε is the spectral emissivity, the wavelength, T the temperature, I (, T) is the

spectral emissive power and c1 and c2 are constants. Thus, it can be seen that the

temperature of the surface can be determined by measuring the spectral emissive power

of the surface. This is the basic principle most pyrometers are based on [11].

To measure the location of the liquid steel/liquid slag interface in the tundish, one can

design a technique that is underpinned by the fact that the emissivity of steel and slag are

substantially different for most wavelengths. This variation between the emissivity of

steel and slag is the basic principle behind a device that was developed by Tata Steel to

detect the flow of slag during the tapping of a furnace. Normally, as one taps a furnace,

[12]

the stream comprises steel. Near the end of the tapping, the stream will also carry slag.

The device from Tata Steel detects the appearance of slag in the stream and alerts the

operator [11].

Liquid slag has an emissivity that is about 0.8 while liquid steel has a relatively low

emissivity of about 0.3 (except at the very small wavelengths). This is shown in Figure

2.3 [12]. Photos of the steel tapping stream and the slag tapping stream are shown in

Figure 2.4 [12]. One notes that the steel stream shown in Fig. 5a appears dull as viewed

by the camera while the slag stream in Fig. 5b appears substantially brighter. The

difference between the 2 streams is the emissivity. Because steel has a lower emissivity

(about 0.4 at a wavelength of 2 μm), its heat flux is ½ that of slag at the same

temperature.

Figure2.3- The emissivity of liquid steel and molten slag [12]

[13]

(a) (b)

Figure 2.4- Pouring sequences of: (a) steel and (b) slag [12]

Our strategy for measuring the steel/slag interface requires that the tube through which

the steel is sighted for temperature measurements be moved in increments through the

steel into the slag phase. As the tube is moved in the vertical direction, temperature

measurements are taken with the pyrometer. When the front opening of the tube crosses

from the steel phase into the slag phase, the pyrometer will show a relatively large jump

in temperature (in the order of several hundred oC). This occurs not because the

temperature is higher but rather because the emissivity of the slag is higher. The

pyrometer picks up more infrared radiation and translates this to a sharp increase in

temperature. In this way, this device can be used to measure how much slag there is in the

tundish at any given time [11].

For measuring the third parameter, composition of the molten steel, with a clear pathway

to the molten steel, one can use the LIBS technique that has been described earlier to

generate an in-situ and online analysis of the composition of the molten steel.

[14]

The next section describes the basics of heat pipe technology, its applications and

limitations following some of the significant technological advances that the McGill heat

pipe group have made in recent years and with particular emphasis on those factors that

will help us achieve the objectives.

2.3 HEAT PIPE TECHNOLOGY

2.3.1 Heat Pipe History

Peterson in his book “An Introduction to Heat Pipe Modeling, Testing and Applications”

explains the initial concept of the heat pipe which goes back to mid 1800‟s in the patents

of A. M. Perkins and J. Perkins. Perkins tube which has been described in these patents

used either single or two-phase processor to transfer heat form a furnace to a boiler. Later,

Gay used the idea of the Perkins tube and added number of vertical tubes that were

arranged with the evaporator placed below the condenser. All these devices which are

categorized as thermosiphons generated the idea for the later development of what we

today call the heat pipe [13]. The modern concept for a capillary driven heat pipe was

first suggested by R.S. Gaugler of General Motors in 1944 who patented the idea [14].

Early 60s‟interest in heat pipes followed its independent invention by Grover in 1962 [15]

as a solution to heat transfer problems encountered in nuclear-powered thermionic

generators for use as space power supplies. During the late 1960s NASA played a large

role in heat pipe development by funding a significant amount of research on their

applications and reliability in space flight following from Grover's suggestion.

[15]

The first application of heat pipes in the space program was in thermal equilibration of

satellite transponders. As satellites orbit one side is exposed to the direct radiation of the

sun while the opposite side is completely dark and exposed to the deep cold of outer

space. This causes severe discrepancies in the temperature (and thus reliability and

accuracy) of the transponders. The heat pipe cooling system designed for this purpose

managed the high heat fluxes and demonstrated flawless operation with and without the

influence of gravity. The developed cooling system was the first description and usage of

variable conductance heat pipes to actively regulate heat flow or evaporator temperature

[16].

2.3.2 Classical Heat Pipes

A heat pipe is a sealed container of any shape that is evacuated of any substance other

than an appropriate working fluid. Therefore, the working fluid exists both as a liquid and

as a gas within a container, and is constantly at the saturation pressure and temperature, in

the other word the working fluid is constantly at its boiling point (Figure 2.5). The basic

construction of a classical heat pipe consists of the following:

a) An evaporator section, where an external heat source is applied

b) A condenser section, where external cooling is applied

c) And a capillary structure (wick)

Working fluid inside the device is boiled in the evaporator, is then transported to the

condenser as a vapour via pressure-driven flow, and is then subsequently condensed. The

condensed working fluid is returned to the evaporator primarily by capillary forces

supplied by the wick [17].

[16]

Figure 2.5- Classical heat pipe

Heat energy is stored in the working fluid as a phase change. In the evaporator, the

following process occurs:

Liquid + heat from heat source vapour

And in the condenser, the following occurs:

Vapour liquid + heat to external cooling circuit

Therefore, by adding the above equations, the overall process at steady-state is:

Heat from heat source heat to external cooling circuit

Heat pipe is, in effect, a „superconductor‟ of heat energy. Tests have shown that a heat

pipe can be as effective in transporting energy as 1,000 times the equivalent quantity of

copper under similar heat transfer conditions.

[17]

2.3.2.1 Classical Heat Pipe Applications

Grover and his colleagues were working on cooling systems for nuclear power cells for

space craft, where extreme thermal conditions are found. Heat pipes have since been used

extensively in spacecraft as a means for managing internal temperature conditions [15].

Present applications of classical heat pipes include electronics, plastic mould injection,

cryogenic medicine, cutting tools and etc. [18]. Nowadays, they are being used in many

modern computer systems, where increased power requirements and subsequent increases

in heat emission have resulted in greater demands on cooling systems. Heat pipes are

typically used to move heat away from components such as CPUs (central processing

unit) and GPUs (graphics processing unit) to heat sinks where thermal energy may be

dissipated into the environment.

One of the other applications of the heat pipe is in the Trans-Alaska pipeline where heat

pipes function as thermal diodes and are used to stabilize the permafrost foundation of the

pipeline [19].

In 1972, Loop Heat Pipes (LHPs) were developed when the first such device with a

length of 1.2 m and a capacity of about 1 kW, with water as a working fluid was created

and tested successfully by the Russian scientists Grasimov and Maydanik from the Ural

Polytechnical Institute. The appearance of LHPs was a response to the challenge

connected with the acute demand of aerospace technology for highly efficient heat-

transfer devices with all the main advantages of conventional heat pipes, but at the same

time much less sensitive to the change of orientation in the gravity field.

Loop heat pipes (LHPs) are two-phase heat-transfer devices with capillary pumping of a

working fluid (Figure 2.6). They possess all the main advantages of conventional heat

[18]

pipes, but owing to the original design and special properties of the capillary structure are

capable of transferring heat efficiently for distances up to several meters at any

orientation in the gravity field, or for several tens of meters in a horizontal position.

Besides, the LHP conception allows a wide variety of different design embodiments,

which essentially extends the sphere of functional possibilities and practical application

of these devices [20].

Figure 2.6- Schematic of loop heat pipe [21]

2.3.2.2 Classical Heat Pipe Limitations

There exist several limitations which can diminish heat transfer in classical heat pipes.

Some of the less significant limitations include the vapour pressure limit, the sonic limit,

the flooding limit and the dry-out limit. But there are two main limitations: film boiling

and liquid entrainment.

[19]

Film Boiling Limitation

To better understand the film boiling phenomenon, a brief introduction about boiling heat

transfer is necessary. Boiling or convection with a phase change is defined as evaporation

occurring at a solid-liquid interface. Utilizing the latent heat of vaporization as a means to

store and transfer heat can greatly enhance heat transfer. For instance, one can store 405

Joules in a single gram of water as sensible heat by increasing its temperature from 0°C to

100°C while by boiling the same gram of water at 100°C, one can store 2260 Joules.

Nukiyama identified the existence of several boiling regimes, namely natural convection

boiling, nucleate boiling and film boiling [22]. His experiment consisted of passing an

electrical current through a platinum wire immersed in water at atmospheric conditions.

By measuring the power through the wire, the temperature of the wire (T w ) and the

temperature of the water which was maintained at the saturation temperature (T sat ), the

classical boiling curve shown in Figure 2.7 has been constructed. This excess temperature

differential (T w -T sat ) represents the driving force for the boiling. If the wall temperature

is less than the saturation temperature, boiling will not occur. However, if the wall

temperature exceeds the saturation of the liquid, boiling heat transfer will be dominant.

This curve delineated by points A to E describes pool boiling. The curve between A and

B represents natural convention boiling, where bubbles are being formed on isolated spots

and fluid mixing occurs principally by natural convection. The curve between points B

and C shows nucleate boiling, where the surface becomes densely populated with

bubbles, and bubble separation induces considerable mixing. Point C is the critical heat

flux (CHF), beyond which film boiling is initiated. From point C to point D, partial film

boiling occurs, whereas the section between point D and point E represents fully

[20]

developed film boiling. The reason why heat transfer increases in this region is related to

the fact that radiation across the vapour film dominates. Given that radiative heat transfer

increases by the fourth power of temperature, heat flux from the wall increases in the film

boiling regime.

Figure 2.7- Natural and forced convection boiling curves [23]

Film boiling occurs when the vapour bubble density at the solid/fluid interface is large

enough to cause coalescence. This forms a stable vapour film, which encompasses a large

thermal resistance and temporarily reduces heat transfer. During this time, the applied

heat load is not being completely removed from the solid. Equation 2-2 can be used to

predict the critical heat flux for nucleate pool boiling [24, 25, 26].

[21]

q″ CHF = 0.149Hfg

ρ v [σg(ρ l -ρ v )/ρ 2

v ] 4/1 (2-2)

where

q″ CHF is the critical heat flux (W/m2

)

Hfg

is the latent heat of vaporization of the fluid (J/kg)

ρ v and ρ l are the densities of the vapour and liquid, respectively (kg/m3)

σ is the surface tension (N/m)

g is the gravitational acceleration normal to the surface (m/s2

)

Assuming that ρ v is much smaller than ρ l , equation 2-2 implies that the critical heat flux

increases with:

a) fluids with larger values of Hfg

b) larger vapour densities (at larger pressures)

c) larger body forces normal to the solid surface

Heat exchangers which utilize boiling to transfer heat are operated in the nucleate boiling

regime. In the case where the heat flux is beyond the CHF of the heat exchanger, film

boiling results in surface temperature, T w , which leads to failure. Therefore, using boiling

as a heat transfer mechanism is a double-edged sword: although very large heat transfer

coefficients are attainable, we have to be careful not to attain the critical heat flux and

induce film boiling.

In a classical heat pipe, the wick is the primary means by which liquid working fluid

refluxes the evaporator. However, at elevated heat fluxes, vapour formed at the liquid/gas

[22]

interface can expel liquid working fluid from the wick. The porous nature of the wick can

effectively trap the vapour, stabilizing a vapour film, and enhancing film boiling.

The Entrainment Limitation

The flow of fluid within conventional heat pipes is counter-current meaning the vapour

travels from the evaporator to the condenser, and liquid travels from the condenser to the

evaporator. Therefore, a shear force exists at the free liquid/vapour interface. The

entrainment limitation occurs at large vapour velocities where a significant portion of the

returning liquid can be entrained into the vapour stream, causing evaporator dry-out.

The limitations of classical heat pipes have impeded the realization of boiling‟s full

potential as a heat transfer mechanism, and thus heat pipes have only been successful in

applications with relatively small heat fluxes and heat loads. These limitations have

effectively prevented the metallurgical industry from using heat pipes as heat exchangers.

2.3.3 McGill Heat Pipe

2.3.3.1 Development

Recent research at McGill University has led to the development of a newly designed heat

pipe, referred to in the literature as a “McGill heat pipe” [27, 28, 29, 30, 31]. This type of

heat pipe is capable of overcoming the traditional limitations encountered at large heat

loads and heat fluxes by incorporating two features in the design [32]. In this report, the

term “heat pipe” refers to the novel design developed at McGill University.

[23]

2.3.3.2 Design and Operating Principles of the McGill Heat Pipe

Similar to classical heat pipes, McGill heat pipes consist of three parts: evaporator,

condenser and a working fluid contained within. Yet by incorporating two features they

are distinguished from the conventional heat pipes. These two features are as follows:

1. Flow modifier

2. Return line

Flow Modifier

The flow modifier imposes a resistance on the rising fluid in the evaporator, causing it to

rotate (Figure 2.8). It is believed that the resulting helical flow propels the denser liquid

against the inner evaporator wall due to the centrifugal forces generated, which ensures

homogeneous wetting and stabilizes the annular two-phase flow regime. In classical heat

pipes, capillary wicks serve as a way to distribute liquid homogenously, hence by adding

this feature, the wick which would be considered as a resistance, can be eliminated from

the heat pipe design [33, 34]. Moreover, the flow modifier is able to break any vapour

film which may form and thus can increase heat transfer dramatically. Measurements

with a water based heat pipe have shown that the critical heat flux can be greatly

increased. If film boiling is established in a water based heat pipe, a representative value

of the rate of heat extraction is about 80 kW/m2 when the heat pipe is operated at about

100oC while a McGill heat pipe operated at 100

oC can extract more than 2,500 kW/m

2

and still not be subjected to film boiling. Thus, it is clear that a swirled flow is of major

importance [11].

[24]

Figure 2.8- An example of a flow modifier [11]

Return Line

By eliminating the capillary wick, the McGill heat pipe relies solely on gravity to return

the condensed working fluid to the evaporator. Therefore, the condenser section of this

heat pipe must be located above the evaporator section. The return of liquid working fluid

is done through a pipe referred to as the return line.

The role of the return line is to provide a separate path through which the liquid can travel

down to the bottom of the evaporator. This liquid can be returned to the evaporator

because of the pressure head between the reservoir and the return line discharge without

any entrainment from the rising vapor. It is important to mention that the reservoir is the

section at the bottom of the condenser where condensed working fluid collects [33].

Working Fluid

The working fluid used in conventional heat pipes is usually either water or sodium,

based on (1) their favourable properties suited for high heat fluxes, and (2) operating

temperatures which fall within those desired for metallurgical applications. In addition to

the mentioned changes of the design of a conventional heat pipe, the viable working

[25]

substances were expanded to include sulphur. Ever since the early days of heat pipe

development, researchers have sought to use sulphur as a working substance. The reason

for this was that there are a number of working substances that can operate at low

temperatures such as water with a normal boiling point of 100°C while for high

temperature applications we have the alkali metals, and, in particular, sodium with a

normal boiling point of 880°C. Therefore, there is quite a large gap between the two.

The element, sulphur, is a material that is ideally suited to fill this gap. It has a normal

boiling point of about 445oC. For 40 years researchers have tried to incorporate sulphur in

conventional heat pipes. However, success has been limited because of the unique

viscosity curve of liquid sulphur. Sulphur at its melting point of about 115oC is relatively

fluid, however, as the liquid is heated to about 200oC, the viscosity increases dramatically

to the point that it hardly flows. As the temperature is increased further, the viscosity

drops rapidly. When the liquid is at its normal boiling point of about 445oC, the viscosity

of the liquid is similar to the viscosity it had when it melted. Additional details about

sulphur in a McGill heat pipe have been detailed by Zhao [35].

The three working substances (water, sodium and sulphur) have been used extensively in

McGill heat pipes. In comparison, water is the preferred one, sulphur is the second and

sodium follows as the third choice. Its high boiling point (880oC) implies that a sodium

pipe must operate at temperatures in excess of 800oC and possibly 900

oC. The plant trials

done with McGill heat pipe [36] have clearly demonstrated that sodium is an excellent

heat pipe working substance; however those same trials showed that a heat pipe (316L

stainless steel) which operates at such elevated temperatures will eventually deteriorate

and crack. Sulphur has a very low thermal conductivity and a relatively high viscosity.

[26]

These properties are opposite to what is required. Thus, sulphur was ranked second. The

clear winner as the most viable working substance which can be used at high heat fluxes

is water. Numerous tests for a number of applications with the patented design have been

conducted and considering the results we have concluded that with this design, water is

the preferred working substance, especially at high heat fluxes.

2.3.3.3 Choking Limitation in the McGill Heat Pipe

Because it is very difficult to get visual observations or precise flow measurements within

a heat pipe, concepts such as choking (dry-out) are still not completely understood. It is

generally accepted that the condenser has the lowest pressure as it is cooled and vapour

condenses there. On the other hand, the evaporator has the highest pressure as vapour if

formed there. Therefore, there exists a pressure difference between the evaporator (high)

and the condenser (low). Flow of liquid through the return line is primarily due to the

head of liquid working fluid that fills the return line, form the point of discharge in the

bottom of the evaporator to the reservoir in the condenser. The driving force must

overcome the pressure difference between the evaporator and the condenser such that

sufficient condensed working fluid flows down the return line and is discharged into the

evaporator, cooling it by vaporizing. Under high heat loads and low operating

temperatures (causing low vapour densities), the vapour velocities within a heat pipe are

rather large. With large velocities, it is thought that the flow resistance of vapour as it

rises to the condenser can be quite high, as flow resistance is generally found to be

proportional to the fluid velocity squared. The high flow resistance causes a large

[27]

pressure gradient in the evaporator, resulting in a relatively high pressure at the bottom of

the evaporator. Therefore, at the onset of choking, the large flow resistance caused by the

high vapour velocities accounts for a pressure drop that exceeds the hydrostatic pressure

of the liquid column in the return line. This slows the flow of liquid down the return line

such that an insufficient flow rate of liquid working fluid is discharged into the evaporator

and as a result the evaporator dries out. This phenomenon was first observed by Yuan in a

laboratory test reported in 2002 (Figure 2.9) [37].

Figure 2.9- (A) McGill heat pipe after 2 hour test, stable operation, (B) McGill heat pipe after 1 minute

test, conditions for choking limitation [37]

2.3.3.4 McGill Heat Pipe Applications

There are three groups of applications using water as the working substance that will be

briefly reviewed here. The fourth application is the objective of our project which will be

highlighted after reviewing the other 3 applications.

The 3 applications are as follows:

1) Taphole cooling blocks

[28]

2) Slag launder

3) Cooling for blast furnace refractory

Taphole Cooling Block

The taphole of a lead blast furnace is simply a hole that runs through the copper block and

which is cooled extensively by water. In a conventional copper block, passages for water

are embedded within the block. In this case, a crack in the copper can lead to a water leak

and a subsequent explosion. For this reason, an alternative copper block that is cooled by

two symmetrical heat pipes was designed and tested to determine if it could be a viable

substitute for the conventional block [38]. Schematics of the conventional water cooled

block and the modified heat pipe cooled block are shown in Figures 2.10a and 2.10 b.

(a) (b)

Figure 2.10- (a) Conventional copper block with water cooled passages and (b) Heat pipe cooled copper

block

[29]

Figure 2.11- Copper taphole block under intense heat loading from an oxygen/natural gas burner

The specifications were such that the conventional block extracts about 300 kW and this

heat is carried away by the cooling water. McGill heat pipe using water as the working

substance was to accomplish this rate of heat extraction. Moreover, the condenser of each

heat pipe was also water cooled. The trials were successful and clearly demonstrated the

capabilities of the McGill heat pipe under extreme heat loading conditions that exceeded

2 MW/m2. Shown in Figure 2.11 is a photo (and corresponding schematic) of the copper

block under the intense heat loading generated by an oxygen/natural gas burner.

[30]

Figure 2.12-Slag launder (copper)

Slag Lauder

When the slag is tapped from a lead blast furnace it flows over a runner and into another

vessel for processing. The focus of the application to be described is the runner (Figure

2.12). Prior to our study, the runner was made by creating a steel trough which was lined

with refractory. While this application was not deemed critical as was the previous one

with the taphole, it was nonetheless carried out to study the viability of using three

horizontal pipes and of connecting multiple evaporators to one condenser. More details

about this application are reported in [38].

The runner was subjected to intense heating from an oxygen/natural gas burner for

extended periods of time (days) (Figure 2.13).The runner was modeled extensively with

Fluent software. The temperature distribution within the copper at steady state was

computed and is shown for a typical run in Figure 2.14. One sees that the intense heat

extraction creates sizeable temperature gradients within the copper.

[31]

Figure 2.13- Copper launder under intense heat load Figure 2.14- Temperature mapping of

launder

Cooling for Blast Furnace Refractory

The iron blast furnace requires cooling of the tuyeres and the refractory to maintain a long

and stable operation. One of the current methods of cooling the refractory is to use what

are referred to as „cigar‟ coolers. These are copper pipes several inches in diameter and

about 2 feet in length which are embedded in the refractory at numerous locations around

the hearth of the furnace. Each of these is water cooled. In the event of failure, the

situation can be dangerous. In addition, it can happen that water leaks into the furnace and

freezes some of the melt. This can cause problems related to furnace productivity. As an

alternative to water cooled units, a new heat pipe based cooler has been designed, built

and tested successfully [11].

2.3.4 Heat Pipe Probe for a Tundish

Before discussing the use of heat pipes in the steel tundish, it is instructive to briefly

review the testing of a water based heat pipe in molten aluminum that has been carried

out at McGill University [37]. In Figure 2.15a the evaporator of the heat pipe which is

immersed into the molten aluminum is shown. Typically, the heat flux on the heat pipe

[32]

surface was about 2 MW/m2. Moreover, the uniformity of the heat flux is demonstrated

by examining the frozen Al shell on the pipe. Since the outer surface of the shell

represents an isotherm, a smooth uniform contour of the outer shell surface indicates

uniform heat extraction from the inner surface of the pipe (Figure 2.15b).

Figure 2.15- (a) water based heat pipe immersed in Aluminum, (b) the view of heat pipe after it was

removed from the Al melt.

The research with molten aluminum showed that the water based heat pipe can extract 2

MW/m2. To put this flux in perspective it should be noted that a typical DC casting mold

for aluminum extracts heat at a peak heat flux of about 1 MW/m2 at the top of the mold.

As stated, the application we are now focused on is the use of a water heat pipe in molten

steel. The most visible difference between the 2 configurations is the melt temperature. In

the aluminum tests the melt temperature was about 800oC, however, in the steel tests the

melt will be in excess of 1500oC. While we tend to focus on temperature, it is the heat

flux that the heat pipe is most concerned with. Our estimate based on classical heat

transfer shows that the anticipated heat flux in the steel system will be around 1 MW/m2.

[33]

This is about ½ of the rate the heat pipe has been operated at in the lab. In addition, one

should recall that the heat pipe in the taphole copper block extracted heat at peak fluxes of

about 2.5 MW/m2. Thus, we are confident that the water based heat pipe which is

immersed in a molten steel environment is not only viable but very close to being a

reality. Once the stability of McGill heat pipe in molten steel tundish is approved, it can

be used as the viewing port with which one can measure the tundish temperature, slag

thickness and composition of the melt using the methods explained earlier in the strategy

section (2.2).

[34]

Chapter 3

Experimental Procedure

This section consists of the following:

1) The design and fabrication of the heat pipe

2) Experimental setup

3.1 THE DESIGN AND FABRICATION OF THE HEAT PIPE

As explained in Chapter 2, the McGill heat pipe is equipped with two new features: flow

modifier and liquid return line. Therefore, this heat pipe which has been granted US

patent 7,115,227 does not have wicks that are used in conventional heat pipes and it only

relies on the flow modifier to distribute the liquid homogenously [40]. Consequently, this

design consists of two parts:

1) Evaporator

2) Condenser

3.1.1 Evaporator

The evaporator is an annular tube which has been fabricated at IMI-NRC in Boucherville,

Québec. It is made of stainless steel 316L due to its good strength and corrosion

resistance. The bigger pipe has a diameter of 7.6 cm with 6 mm thick wall for safety

reasons while the smaller pipe has a diameter of 2.5 cm with a 3 mm thick wall. The pipe

[35]

is designed to be annular to serve as a viewing port for various measuring devices.

Moreover, it is used to transfer an inert gas such as argon into the bath in order to prevent

the molten metal or slag from blocking the viewing tube. It is important to mention that

all the designs have been drawn using Solid Works software.

Flow Modifier

One of the additional features in the McGill heat pipe is the flow modifier which has been

sized based on past experiences (Figure 3.1). The outside diameter of this flow modifier

is 6mm and it has a 3.8 cm pitch.

Figure 3.1- Flow modifier used in McGill heat pipe

Return Line

The second new feature of the McGill heat pipe are the two return lines in the evaporator

with an outside diameter of 1.2 cm. They are approximately 77 cm long and at 10 cm

from the top of the evaporator they are connected to the related hoses to bring the liquid

from the condenser to the pipe. On each return line, a valve is installed in order to be able

Viewing tube Flow modifier

Turns: 22

Outside diameter:

6 mm

Pitch: 3.8 cm

[36]

to effectively turn off the heat pipe. By shutting off the valve, the flow of liquid to the

evaporator can be stopped and the evaporator eventually dries out. In the absence of

liquid working fluid in the evaporator, no cooling would take place.

Vapour Line

There are two vapour paths to bring the generated vapour to the condenser. These holes

which have the inside diameter of 1.2 cm are connected to the related hoses at 3 cm from

the top of the evaporator. The schematic of the evaporator is shown in Figure 3.2 and 3.3.

Figure 3.2- Solid Works drawing of the evaporator

Return Line

Vapour Path

Flow Modifier

Thermocouples

[37]

Figure 3.3- The top and side view of the evaporator

Bottom Part (Nozzle)

The most critical part of the evaporator is its 6cm bottom part which is inserted into the

molten metal. Precision in the design and fabrication of this nozzle which is made of

stainless steel 316L has been considered. If the heat pipe works properly, a layer of metal

will freeze on the nozzle. However, if the thickness of the frozen material exceeds a

certain amount, it may block the viewing port and interfere with the continuous

measurement process. Therefore, the nozzle is designed to be tapered as shown in Figures

3.4 and 3.5. In Figure 3.6 the assembled evaporator is shown.

[38]

Figure 3.4- Nozzle in the evaporator from different angles

(a) (b)

Figure 3.5- Different views of fabricated nozzle: (a) inside view, (b) outside view

[39]

Figure 3.6- Assembled Evaporator

3.1.2 Condenser

The condenser of the heat pipe which has been described in Chapter 2 has a lower heat

flux than the evaporator due to its distance from the heat source. Therefore, it is made of

stainless steel 304. It is a cylinder with 60 cm outside diameter and height which has been

fabricated along with the evaporator at IMI-NRC in Boucherville, Québec. The system is

water cooled, thus, the condenser is equipped with 4 copper coils to bring the cooling

water to the device. The coils are 50 cm long with 10 turns and approximately 1cm

outside diameter (3/8 inch). Moreover, there are 2 vapour pipes (30cm long and 1.25 cm

inside diameter) which bring the vapour from the evaporator to the hoses and into the

condenser (Figure 3.7). The vapour is condensed by the water coils and the liquid is

collected at the bottom of the condenser. There are two holes to bring the liquid to the

evaporator. Last but not least feature in the condenser is the vent which is used to

minimize the possibility of blocking the liquid return pipe by vapour.

[40]

Working Fluid

Water was chosen as the working fluid in this project. Previous work has shown that

water can dissipate a heat flux in excess of 1.5 MW/m2

without experiencing film boiling

[37]. As explained in Chapter 2, sodium could not be used due to its high operating

temperature (>800ºC). In addition, sodium is flammable and requires special startup

procedures as it is solid at room temperature.

The bottom of the condenser which is called the reservoir was charged with 5 litres of

water. This quantity of water is sufficient to completely fill the evaporator, return line,

and hoses linking the evaporator to the condenser. This ensures that the evaporator walls

are completely wetted when starting a test.

All the hoses used in the system are Swagelok products and they are made of stainless

steel inside and outside. On top of the condenser, a pressure relief valve was installed for

safety issues. Moreover, there is a pressure gauge transmitter which enables us to read

pressure at any time during the process.

The fabricated heat pipe was first tested in a gas furnace as demonstrated in Figure 3.8.

[41]

(a)

(b)

Figure 3.7- (a) Schematic of the condenser, (b) Fabricated condenser

Vent

Cooling Coil

Vapour Pipe

Reservoir

Liquid Path

Pressure Gauge Transmitter

Pressure Relief Valve

Vent

[42]

Figure 3.8- Heat pipe during an experiment in gas furnace

3.2 EXPERIMENTAL SETUP

The ultimate target of this project is to insert the newly designed heat pipe into molten

steel to serve as a viewing tube for continuous measurements of temperature, slag

thickness and composition of the melt. However, before inserting the stainless steel

device into the molten steel, we have to have a thorough knowledge of its functionality

and behaviour in different environments. Therefore, considering the three main criteria of

safety, temperature and heat flux, we have formulated a 4 step experimental program to

pursue this target. The test program comprises:

Condenser

Hoses

Evaporator

[43]

a) Gas furnace test

b) Aluminum test

c) Cast iron test

d) Steel test

From the safety aspect, inserting the heat pipe in the gas furnace is the safest process of

the 4. The maximum temperature of the gas furnace is around 1400°C, yet considering the

relatively low heat flux of the gas on the outer wall of the heat pipe, this step is the safest

one. The second safest procedure is the test in molten aluminum wherein the temperature

of the furnace is around 700°C while the third is the test in cast iron with a melting

temperature of approximately 1100°C. The least safe procedure is the steel test, the metal

of which the device is made and that has a high melting temperature of about 1500°C.

The last but not least criterion to be discussed is „heat flux‟. Heat flux is the flow of

energy per unit of area per unit of time. As described in Chapter 2, film boiling which is

considered as the main limitation in heat pipe technology plays an important role.

Referring to Figure 2.8, film boiling happens due to large vapour density at solid/liquid

interface which leads to heat flux decrease despite the increase in temperature differential

(T w -T sat ). The critical heat flux, the peak where film boiling occurs, depends on the heat

pipe configuration and design. In this project we are experimenting with a newly designed

heat pipe in which the critical heat flux cannot be determined except with experiments. If

film boiling takes place, heat extraction due to poor heat flux would decrease

substantially and the temperature of the heat pipe would increase. If the environment

where heat pipe is exposed to has high temperature as the steel furnace for instance, the

heat pipe temperature may exceed 1500°C and the device will melt. Therefore, finding

[44]

the critical heat flux for the current design before the molten steel test is a must. Our

estimate based on classical heat transfer shows that the anticipated heat flux in the steel

system will be around 1 MW/m2

. Among the mentioned steps, molten aluminum which

has the highest heat flux of 2 MW/m2 can confirm the durability of heat pipe in the

molten steel furnace. In all 3 steps (gas, molten aluminum and cast iron), we are in search

of critical heat flux in a safe and controlled environment. On the other hand in the steel

furnace we are expecting to extract 30 kW in the molten steel furnace and thus the

capability of the design to extract this amount of heat along with critical heat flux in each

step is being verified.

In molten metal furnaces the heat pipe due to high heat extraction works at a temperature

lower than metal‟s freezing point which then leads to the formation of a frozen layer of

molten metal on the evaporator. This layer which has different thicknesses depending on

the heat conductivity of the metals would form up to the point where its outer surface

experiences the temperature more than metal‟s melting point. There, the liquid would no

longer solidify. This phenomenon is shown in Figure 2.15.

[45]

3.2.1 Sensors

In order to measure the amount of heat being extracted and to calculate the corresponding

heat flux in the gas furnace, two thermocouple wells are incorporated in the system: 1 in

the evaporator and 1 in the condenser. In the evaporator two thermocouples are inserted

at different depths, yielding the temperature of the bottom and 0.6 m from the bottom of

the evaporator (almost where the evaporator is inserted in the furnace). However, in the

condenser the two thermocouples report the temperatures of the bottom and the mid-point

of the condenser. In phase 2, in the aluminum furnace, another thermocouple is added on

the wall of the evaporator to report the temperature of the frozen aluminum formed on the

outside of the evaporator.

Using enthalpy balance formula, we can easily measure the amount of heat picked up by

the heat pipe (Figure 3.9, Equation 3-1)

(a) (b)

Figure 3.9- Equipments for heat measurement, (a) container for water flow rate measurement, (b) digital

thermometer

[46]

p (3-1)

where

is the extracted heat (W)

is the water flow rate (g/s)

p

is the specific heat capacity (J/(g·K) (4.2 for water)

is the temperature rise of the cooling water (°C)

However, when the heat pipe is running at high temperatures, heat is being lost from the

condenser‟s wall which is not insulated, from the vapour formed in plastic hoses along

with hot water which is neglected in measurements and etc. Therefore, the amount we are

measuring based on the cooling water is not the exact amount of heat that the heat pipe

has picked up from the furnace. There must be a solution to compensate for the heat

losses. All the solutions and results of the heat pipe experiments in gas furnace and

molten aluminum test will be presented in next chapter.

[47]

Chapter 4

Results and Discussions

This chapter consists of two parts: part 1 which describes the results of gas furnace tests

and part 2 where the results of molten aluminum tests are presented.

Part 1: GAS FURNACE TESTS

Newly designed heat pipe has been tested in a gas furnace at McGill University over a

number of days from January 10th

to April 10th

, 2010. The first experiments were

preliminary tests which explored the operability of the heat pipe while the rest aimed to

determine the maximum heat extraction capacity and to verify the film boiling

phenomenon. It is important to mention that LABVIEW software has been used to record

the behaviour of the heat pipe through the experiments.

The tests in the gas furnace were divided into the following 3 segments:

a) Gas furnace temperature at approximately 730 °C (Level 1)

b) Gas furnace temperature at approximately 1370 °C (Level 2 )

c) Gas furnace temperature at approximately 1480 °C (Level 3)

In each setup, there are 3 steps in order to calculate the total extracted heat. To have

reliable information, the measurements have to be taken when the experiment is

stabilized, and thus reaching steady state is the first step in each measurement. Second, as

[48]

described earlier, the amount of extracted heat should be measured. The third step is to

account for the heat losses from the condenser, etc...

4.1 REACHING STEADY STATE

When the furnace is turned on, the evaporator is exposed to the heat source generated in

the furnace. Consequently, its temperature increases and the water inside the pipe

evaporates. The generated vapour will travel through the hoses and enter the condenser

environment where at the beginning it is at the room temperature. As a result, the

temperature of the condenser starts to increase but with slower speed than the evaporator

due to its distance from the heat source. Therefore, there will be a larger temperature

differential between the condenser and the evaporator at the beginning of the test (Figure

4.1). In this phase which is called start-up, the change of liquid to vapour in the

evaporator and vapour to liquid in the condenser does not happen uniformly because of

the instability of the system. Figure 4.1 presents the temperature distribution of the heat

pipe system versus time through a 3 hour experiment of heat pipe in gas furnace level 1

meaning a furnace temperature around 700°C. As explained earlier there are 4

thermocouples inserted in the evaporator and the condenser. In the graphs evap. B

presents the thermocouple at the bottom of the evaporator while evap. M shows the

temperature at 0.6 m from the bottom. In a lower temperature range, cond. B stands for

the thermocouple at the bottom of the condenser and cond. M demonstrates the

temperature at 0.3 m from the bottom of the condenser. In the start-up phase, as it is seen

[49]

in Figure 4.1, the system is experiencing an unstable situation where the temperatures in

both evaporator and condenser change substantially in short periods of time.

Figure 4.1- Heat pipe experiment in gas furnace level 1, temperature vs. time

. The thermocouples at the bottom of the evaporator and condenser where the liquid water

is collected show more instability than the other two thermocouples. In the start-up phase,

in the evaporator, water evaporates and is replaced by the liquid coming from the

condenser irregularly. On the other hand, in the condenser, there is some liquid left in the

reservoir when the condensed vapour is added to this volume of liquid. This volume of

liquid has different temperature and that justifies the oscillation in green thermocouple.

Time wise, all these phenomena occur so fast that the system cannot have the expected

uniform cycle.

0

10

20

30

40

50

60

70

80

90

1001

50

1

10

01

15

01

20

01

25

01

30

01

35

01

40

01

45

01

50

01

55

01

60

01

65

01

70

01

75

01

80

01

85

01

90

01

95

01

10

00

1

10

50

1

evap B

evap M

Cond B

Cond M

Time

(s)

Tem

per

ature

(°C

)

Cooling Start-up Increase Cooling

[50]

Another important point to mention is the temperature in red and velvet thermocouples

which mostly report higher temperatures than the other two. In the evaporator, the liquid

at the bottom evaporates and the flow modifier rotates the formed vapour all the way up

the vapour path. At the bottom we have the combination of liquid coming from the

condenser with a lower temperature and vapour while at the position of evap. M, there is

only a large density of generated vapour. Because of some undercooling in the reservoir

pool in the condenser, the returning liquid can be at a temperature below the saturation

point. Thus, the temperature at evap B can be slightly below that at evap M because the

fluid will have reheated to saturation.

Similarly, in the condenser cond. M is hotter than the cond. B. This thermocouple has the

same elevation as the vapour pipes which bring the hot vapour into the condenser.

Comparing to the bottom of the heat exchanger where the liquid stays with lower

temperature, higher temperature in the mentioned location is predictable.

Figure 4.2 shows the heat pipe behaviour in the gas furnace operating at level 2 meaning

an environment temperature of 1350°C. In this setup, the temperature of both evaporator

and condenser increases faster due to the hotter environment which the evaporator is

exposed to. Consequently the cycle of phase change (liquid to vapour and vice versa)

occurs earlier in comparison to level 1. As a result, despite a larger instability that the heat

pipe experiences, this phase takes less time than level 1 and the system reaches steady

state earlier.

In steady state where all four thermocouples follow a very smooth and stable trend, we

modified the situation by adding some cooling water. As seen in Figure 4.2, by increasing

[51]

cooling water flow the temperature in both evaporator and condenser decreases in a

uniform manner. Surprisingly enough, the temperature in the system does not exceed

90°C while the evaporator is in a 1350°C environment.

Figure 4.2- Heat pipe experiment in gas furnace level 2, temperature vs. time

In level 3, the heat pipe is experiencing an environment temperature of 1500°C. Figure

4.3 shows the behaviour of heat pipe towards this environment. It is important to mention

that in these graphs, level 3 comparing to level 2 has a larger start-up phase. This is due to

the fact that the data in level 2 has been captured when the furnace was on for an hour.

Therefore, the heat pipe has taken less time to reach steady state in level 2 than in level 3.

However, in an equal condition level 3 needs less time than level 2 to reach steady state.

0

10

20

30

40

50

60

70

80

90

100

1

50

1

10

01

15

01

20

01

25

01

30

01

35

01

40

01

45

01

50

01

55

01

60

01

65

01

70

01

75

01

80

01

85

01

90

01

95

01

10

00

1

10

50

1

11

00

1

evap B

evap M

cond B

cond M

Time

(s)

Tem

per

ature

(°C

)

Cooling

Start-up

Increase more cooling Increase cooling

[52]

Figure 4.3- Heat pipe experiment in gas furnace level 3, temperature vs. time

4.2 MEASURING THE EXTRACTED HEAT

Each furnace setup allows the heat pipe to extract a certain amount of heat and

consequently heat flux. Our target in this step is to measure the heat that the heat pipe has

captured at each furnace level. To pursue this target, we have measured the extracted heat

with the process described in Chapter 3 for at least 5 points in each setup.

In each furnace level, despite the fact that all the measurements have been done in the

same furnace temperature, the results were different due to heat losses primarily from the

condenser. There are several sources of heat loss in this system. For instance, the wall of

the condenser is not insulated and is exposed to an ambient temperature of approximately

0

10

20

30

40

50

60

70

80

90

100

110

1

50

1

10

01

15

01

20

01

25

01

30

01

35

01

40

01

45

01

50

01

55

01

60

01

65

01

70

01

75

01

80

01

85

01

90

01

95

01

10

00

1

10

50

1

11

00

1

11

50

1

evap B

evap M

cond B

cond M

Time

(s)

Start-up

Tem

per

ature

(°C

)

Cooling

[53]

25°C and this can be considered as a source of heat loss. In addition, in high operating

temperatures, the air trapped in the hot water coming from the condenser would evaporate

and release some vapour which its energy is neglected in the measurements. Moreover,

there is some heat being lost in the hoses and plastic tubes. Therefore, we can conclude

that the measured heat does not present the heat which the system has picked up in each

furnace setup. Consequently, we should find a solution to compensate for the heat losses.

4.3 COMPENSATING FOR HEAT LOSSES

Figure 4.4 shows the curve of extracted heat versus the condenser temperature in level 1.

The dashed red line extrapolates the curve to 25°C which is defined as room temperature.

If the temperature difference between the heat pipe system and the environment where the

experiment is being done decreases, the heat losses would be minimized. Thus, we

assume that the condenser has the room temperature as the environment to eliminate the

heat losses from condenser, hoses and vapour formed in plastic tubes. However, in reality

this condition does not occur. With this assumption we can declare that the amount of

heat captured from this method can represent the exact amount of heat extracted from the

furnace by the evaporator.

Solving equation 4-1by inserting 25 for the x axis will lead to calculate the exact amount

of extracted heat, q , in level 1.

(4-1)

W

[54]

In furnace setup level 1with approximately 700°C environment, the heat pipe system is

capable of extracting 3940 W from the furnace. By calculating the active area of the

evaporator in the furnace, we can find out the correspondent heat flux, " (equation 4-2,

4-3).

active (4-2)

active=0.4 cm

"avg

=

kW/m

2 (4-3)

Knowledge of total extracted heat aids to obtain more information regarding volumetric

flow rate and vapour velocity which is presented in Table 4.1.

Evap. B

Temp.

(°C)

Evap.M

Temp.

(°C)

Cond.B

Temp.

(°C)

Cond.M

Temp.

(°C)

Water

Temp.

(°C)

Pressure

(atm)

Q water

(W)

Q lost

(W)

Volumetric

flow rate (1)

(m3

/s)

(2)

(m/s)

81

81.3

71.5

72.5

60.6

0.50

1422

2518

0.005

11.3

7 3

73.3

59.6

60.3

55

0.37

1864

2076

0.007

15.9

70

71

56

56

53.5

0.33

2205

1735

0.008

18.8

68

69

53.5

52.5

51.5

0.30

2802

1138

0.009

20.4

62

63.5

44

41.5

43.5

0.24

2845

1095

0.011

25.0

59.5

60.5

38.5

33.5

37

0.22

3348

592

0.012

27.2

Table 4.1- A set of data captured in furnace setup level 1

(1) Volumetric flow rate of water vapour

(2) Velocity of water vapour at the discharge

0.042 m2

[55]

Figure 4.4- The curve of extracted heat vs. condenser temperature in gas furnace setup level 1

The first four columns in Table 4.1 reflect the behaviour of 4 thermocouples used in the

system while the fifth column shows the temperature of the cooling water measured for

each data point using a digital thermometer. The fifth column demonstrates the pressure

in the condenser revealed on the pressure gauge screen. The pressure inside the

evaporator cannot be easily obtained due to the fact that the swirler inside the evaporator

causes vortex inside the environment which leads to a dynamic behaviour and therefore

we cannot detect its pressure directly from temperature as in equilibrium status.

Unfortunately, at this moment we do not have enough information to explain the

phenomenon in more detail.

By knowing the total extracted heat and the heat captured from the cooling water, one can

calculate the heat loss for each data point. As mentioned earlier, the heat loss decreases

when the temperature of the system decreases and this statement is justified in column 8

of Table 4.1.

The last column in table 4.1 shows the vapour velocity in the heat pipe discharge. The

heat pipe is a sealed system and this property challenges the researcher to discover all the

y = -53.908x + 5289

0

1000

2000

3000

4000

0 20 40 60 80

q w

ater

(W

)

Temperature

(°C)25

[56]

phenomena that occur in this technology. One of these phenomena is choking or dry-out

which has been explained earlier in Chapter 2 and is directly related to vapour velocity.

Although it is difficult to measure this term, yet using ideal gas law one can calculate the

vapour velocity in each furnace setup (equation 4-4)

(4-4)

Where

is the pressure of the system (Pa)

is the volumetric flow rate (m3/3)

is the rate of vapour formation per time (g/s)

is the gas constant (J/molK) (8.314 J/molK)

is the average temperature in the evaporator (K)

If we accept that the heat captured from the heat pipe vaporises the liquid inside the

evaporator and also consider that for 1 g of liquid water which is 1/18 mole, 2260 kJ/kg

as a latent heat of vaporization is needed, we can easily calculate the quantity of vapour

( being formed in each furnace setup.

Once we obtain the volumetric flow rate from equation 4-4, by solving equation 4-5 we

can calculate the vapour velocity.

* (4-5)

[57]

* is the area of the hose discharge and is 4

m2

in the design.

The results of the experiment in furnace setup level 2 where the environment has the

temperature of 1370° are demonstrated in Figure 4.5 and Table 4.2.

Figure 4.5- The curve of extracted heat vs. condenser temperature in gas furnace setup level 2

Equations 4-6 and 4-7 show that the heat pipe has extracted a substantially greater amount

of heat when compared to level 1.

(4-6)

W

And

"avg

= kW/m2

(4-7)

The vapour velocity in this setup exceeds 100 m/s meaning 360 km/hr which reflects the

intense environment that the heat pipe experiences. It is an accomplishment to extract

18.5 kW with the maximum operating temperature of 80°C. More important, in the first

two setups none of the limitations of film boiling and choking have been detected.

y = -172.15x + 22901

0

5000

10000

15000

20000

0 20 40 60 80

Temperature

(°C) 25

q w

ater

(W

)

[58]

Evap. B

Temp.

(°C)

Evap.M

Temp.

(°C)

Cond.B

Temp.

(°C)

Cond.M

Temp.

(°C)

Water

Temp.

(°C)

Pressure

(atm)

Q water

(W)

Q lost

(W)

Volumetric

flow rate (1)

(m3

/s)

(2)

(m/s)

79

80

70

69.5

69.3

0.48

10230

8370

0.027

61.3

79

79.7

68

67.5

66.5

0.47

11480

7120

0.029

65.9

75.7

76.4

62

61.5

61

0.42

13400

5200

0.031

70.4

74.9

75.7

62

61.5

61.4

0.41

11860

6740

0.032

72.7

71.1

71.8

54.6

54

54.2

0.35

12900

5700

0.037

84.0

70.7

71.4

52

51.5

50

0.34

15000

3600

0.038

86.3

69

69.7

48

47.5

44.5

0.31

15870

2730

0.041

93.1

65.9

66.7

43.5

42.5

37

0.28

14770

3830

0.045

102.2

65.7

66.8

41.5

40.5

27.3

0.28

16660

1940

0.045

102.2

Table 4.2- A set of data captured in furnace setup level 2

(1) Volumetric flow rate of water vapour

(2) Velocity of water vapour at the discharge

In furnace setup level 3, the heat pipe reached its maximum capacity for heat extraction in

the gas furnace experiments and that is 23500 kW which leads to a heat flux of 560

kW/m2 (equations 4-8 and 4-9).The results of this setup experiment is shown in Figure 4.6

and Table 4.3. It is important to mention that in molten steel furnace, we are expecting to

have a heat flux of 1000 kW/m2

. Comparing to level 3 in gas furnace, this heat flux

seems to be easily achievable unless we encounter the design limitation of film boiling. In

gas furnace experiments no film boiling has been detected otherwise, the heat extraction

due to the formation of vapour layer on the wall of the evaporator would have

substantially decreased. However, we pushed the system to the limit of dry-out in order to

[59]

verify the maximum vapour velocity which the heat pipe can handle in a safe and

controlled environment.

Evap. B

Temp.

(°C)

Evap.M

Temp.

(°C)

Cond.B

Temp.

(°C)

Cond.M

Temp.

(°C)

Water

Temp.

(°C)

Pressure

(atm)

Q water

(W)

Q lost

(W)

Volumetric

flow rate (1)

(m3

/s)

(2)

(m/s)

94.2

94.4

88.4

88.2

85.9

0.82

10850

12700

0.021

48

87.5

88

79

78.8

76.8

0.64

13500

10050

0.027

61

82.9

83.4

71.9

71.5

70.2

0.54

15330

8220

0.031

71

79.1

79.5

65.5

64.9

63.8

0.47

16615

6935

0.035

81

77.5

78.1

63.2

62.5

61.8

0.44

15550

8000

0.038

86

76.8

77.4

61.5

61

60.4

0.43

17400

6150

0.039

88

72.2

73

53.8

53.2

51.4

0.36

17950

5600

0.046

104

71.1

72.1

51.6

51

48

0.34

18310

5240

0.048

110

69.3

70.5

48.2

47.4

42.8

0.33

18960

4590

0.049

113

Table 4.3- A set of data captured in furnace setup level 3

(1) Volumetric flow rate of water vapour

(2) Velocity of water vapour at the discharge

Figure 4.6- The curve of extracted heat vs. Condenser temperature in gas furnace setup level 3

y = -189.71x + 28294

0

5000

10000

15000

20000

25000

0 10 20 30 40 50 60 70 80 90 1002

Temperature

(°C)

q w

ater

(W

)

[60]

(4-6)

W

And

"avg

= kW/m2

(4-7)

[61]

4.4 CHOKING LIMITATION (DRY-OUT)

By decreasing the operating temperature of the evaporator in the gas furnace setup level 3

which has a large heat load, the system choked as shown in Figure 4.7. As explained in

Chapter 2, the increased vapour velocities cause large pressure gradients to develop

across the evaporator and hoses. This pressure gradient opposes the pressure head

between the reservoir and return line ( ). It is the pressure head that causes the return

of liquid working fluid to the evaporator. In Figure 4.7 the thermocouple with the higher

elevation in the evaporator reports that the temperature increases substantially while

choking occurs in the system. This is because of the pressure drop which prevents liquid

from reaching the evaporator and therefore, the dry-out phenomenon starts from the top

of the evaporator while at the bottom due to the presence of some liquid water the

temperature is not changed dramatically. Figure 4.8 shows the evaporator being darker in

the middle where it dried out and silver at the bottom and this justifies the fact that the

bottom has not been hot when compared to the middle section. The pressure drop within

a pipe is generally found to be a function of the vapour velocity squared yet because of

the complexity (both geometric and mechanical) of the evaporator, very little theory is

available to predict the velocity field within it. However, using equations 4-4 and 4-5 one

can calculate the velocity that the system tolerates without drying out.

The maximum vapour velocity in this set of experiments is 103 m/s, which is equal to

about 1/3 Mach (the speed of sound). The fact that gases start to compress at this velocity

also justifies the statement that at this velocity the system chokes due to the pressure drop.

However, we have to keep in mind that this limitation is user controllable and by

[62]

decreasing cooling water flow and thus decreasing the vapour velocity we can easily

avoid the choking phenomenon.

Figure 4.7- The dry-out phenomenon in the heat pipe system in an experiment in gas furnace level 3

Figure 4.8- The evaporator after gas furnace experiments

0

50

100

150

200

250

1 101 201 301 401 501 601 701 801

evap B

evap M

cond B

cond M

Tem

per

atu

re

(°C

)

Time

(s)

[63]

Part 2: MOLTEN ALUMINUM TESTS

After studying the results of the first phase which were satisfying, we decided to insert the

McGill heat pipe into molten aluminum. The experiments took place in CMQ (Centre de

métallurgie du Québec), Trois-Rivières, in a furnace with 0.4 m diameter crucible filled

with molten aluminum 356 alloy. As described in Chapter 3, in this phase we were

expecting to experience a heat flux of 2 MW/m2

. We were investigating the capability of

the pipe to avoid the film boiling limitation. Compared to the choking limitation, film

boiling is a consequence of a design problem which cannot be controlled during an

experiment. By encountering film boiling, due to the vapour layer formed on the wall of

the evaporator, the system would not be able to extract the expected amount of heat and

thus the molten aluminum would not freeze on the wall as predicted.

In the aluminum phase of the test program, the heat pipe was preheated by a burner

before its insertion into molten aluminum. The maximum heat extracted with the burner

was 10 kW. By immersing the evaporator into the molten aluminum at 800°C, the

thermocouple in the evaporator was expected to report a relatively higher temperature

than with the burner but, Figure 4.9 does not show the predicted trend because of what is

suspected to be film boiling in the system.

The evaporator was pulled out several times to investigate the formation of frozen

material. However, due to insufficient heat extraction the system was not able to freeze

enough aluminum (Figure 4.10). The frozen material can play a role in protecting the

evaporator and the lack of the frozen coating can conclude in partial corrosion of the

evaporator which is demonstrated in Figure 4.11. The corrosion can be a result of several

[64]

parameters such as Ar flow that enhances convective mass transfer. It is important to

mention that the maximum heat extraction in the aluminum test was 11 kW meaning half

of the heat extracted in gas furnace experiment and this can be only justified with the

existence of film boiling during the process. The next section proposes a solution to

overcome this limitation.

Figure 4.9- The behaviour of the heat pipe and the condenser in aluminum bath

0

10

20

30

40

50

60

70

80

90

1 501 1001 1501 2001 2501 3001 3501 4001 4501 5001 5501 6001

evap in

cond M

cond B

Time (s)

Tem

per

ature

(°C

)

[65]

Figure 4.10- The thin frozen aluminum layer on the evaporator‟s surface

Figure 4.11- The view of corroded area on the evaporator after 2 hour experiment in aluminum bath

Corroded area

[66]

4.5 MODIFYING THE INTERNAL DESIGN OF THE EVAPORATOR

We have reviewed the design and construction of the heat pipe and concluded that the

design can be improved. We do believe it is possible to attain 2MW/m2

heat flux because

it has been done with other pipes at McGill University [37]. Thus, by improving the

design, we can hit the target heat extraction rate.

Our suggestion for the next step is to build a new evaporator. This evaporator would have

3 hoses instead of 6 meaning the elimination of one liquid return and vapour path. This

configuration along with larger flow modifier diameter (7.6 cm instead of 3.14 cm) can

give the fluid in the evaporator more space and better swirl and decrease the chance of

vapour layer formation on the wall and therefore more heat extraction.

[67]

Chapter 5

Conclusions

The newly designed McGill heat pipe has been tested repeatedly in a gas furnace for a

time frame of 4 months. This set of experiments was conducted at McGill University and

they were the first phase among the four steps formulated for the project (gas furnace,

molten aluminum test, cast iron test and molten steel test).

In order to leave the heat pipe in a steel tundish for long period of time and capture

information on temperature, slag thickness and composition on a continuous basis, the

system has to be able to extract 30 kW heat and 1 MW/m2

heat flux. In gas furnace test

the maximum heat extraction of the heat pipe is 23.5 kW with 0.56 MW/m2

heat flux. No

film boiling was detected and the results are satisfying. The operating temperature of the

heat pipe is 70°C which proves the capability of the system to extract 30kW in steel

furnace with an operating temperature less than 100°C meaning a pressure less than 1

atm. This statement takes into account some of the safety issues.

Moreover, the choking limitation is identified and the vapour velocity of maximum 103

m/s is determined to be the vapour velocity limit in the system to prevent dry-out

phenomenon. However, this limitation can be controlled by changing the water cooling

rate in the heat pipe.

The second phase is immersing the heat pipe into molten aluminum bath. This experiment

was done in CMQ (Centre de métallurgie du Québec), Trois-Rivières. The heat pipe

[68]

which was preheated by a burner before its insertion into aluminum bath could only

extract 11 kW heat from molten aluminum. This is half of the heat extraction in the gas

furnace test. Therefore, the system is not able to extract 2 Mw/m2

as predicted. This is

due to film boiling that occurs during the process and the formation of the vapour layer

which leads to lower heat transfer and thus less heat extraction. To overcome this

limitation, some design modification is suggested. The flow modifier can be made larger

in diameter and a vapour path and liquid return line can be eliminated. The improvements

in the design would give the fluid inside the evaporator more space to rotate and break the

vapour films on the wall and as a result increase the chance that the heat pipe extracts

sufficient heat to survive in molten metal baths.

[69]

References

1) Met Soc website http://www.metsoc.org/virtualtour/processes/steel

2) Gastόn, A., Sánchez Sarmiento G., and Sylvestre Begnis, J.S., “Thermal Analysis

of a Continuous Casting Tundish by an Integrated FEM Code”, Latin American

Applied Research, Argentina, 2008, pp. 259-266.

3) Shia et al., “Thermocouple Temperature Sensor and a Method of Measuring

Temperature of Molten Iron”, United State Patent, No. 5181779, 1993.

4) Barbosa et. al, “Device for Continuous Measurements of Molten Steel in the

Tundish Using Optical Fibre and Infra-red Pyrometer”, United State Patent, No.

0205480-A1, 2008.

5) Kracich, R.E., and Goodson, K., “Ladle Slag Depth Measurement”, Steelmaking

Conference Proceedings, pp. 53-60, 1996.

6) Meszaros et al., “Measuring the Thickness of Hot Slag in Steelmaking”, United

States Patents, No. 6130637, 2000.

7) Kenney, G. B., U.S. Patent, 4,578,022.

8) Jowitt, R., and Abell, I.D., U.S. Patent, 4,598,577.

9) Gruber, J., Heitz, J., Strasser, H., Bauerle, D., and Ramaseder, N., “Rapid in-situ

analysis of liquid steel by laser-induced breakdown spectroscopy”,

Spectrochimica, 2001.

10) Ocean Optics web site http://www.libsresources.com/technology/index.asp

[70]

11) Mucciardi, F., Yuan, Z., and Madani, N., “Measuring the Steel Temperature and

Slag Thickness in a Tundish on a Continuous Basis”, 12 p, 2008.

12) Tata Steel website http://www.automationtatasteel.com/products.htm

13) Peterson, G., “An Introduction to Heat Pipe Modeling, Testing and Applications.”

chapter 1, pp 1-16.

14) Gaugler, Richard (1944), Heat Transfer Devices, Dayton, Ohio: U.S. Patent Office,

pp. 4, 2350348.

15) Grover G M, Cotter T P and Erikson G F 1964 J. appl. Phys. 35 1990-1.

16) http://www.lanl.gov/orgs/esa/epe/Heat_Pipe_Site/ancient.shtml (Los Alamos

National Laboratory website)

17) Dunn, P.D., Reay, D.A., “Heat Pipes”, 4th

Edition, Elsevier Science Ltd, Oxford,

England, 1994.

18) Koepfer, C., “Drilling Dry with a Heat Pipe”, Modern Machine Shop, pp. 56-57,

May 2003.

19) http://www.alyeskapipe.com/InTheNews/MonthlyNews/2004/December/dec2004_

featurestory.asp

20) Gerasimov, Yu.F., Maydanik, G.T. Shchogolev, et al., “Low-temperature heat

pipes with separate channels for vapor and liquid”, Eng.-Phys. J. 28 (6) (1975)

957–960 (in Russian).

21) China Heat Pipe net http://www.china-heatpipe.net

[71]

22) Nukiyama, S., “The Maximum and Minimum Values of the Heat Q Transmitted

from Metal to Boiling Water under Atmospheric Pressure”, Journal of the Japanese

Society of Mechanical Engineers, 37, pp.v367-374, 1934.

23) Mucciardi, F., Zheng, G., “Heat Transfer in the Boiling Regime”, CIM Conf. of

Metallurgists Ottawa, Canada, 2000.

24) Kutateladze, S.S, “On the Transition to Film Boiling under Natural Convection”,

Kotloturbostroenie, No. 3, pp. 10-12, 1948.

25) Zuber, N., “On the Stability of Boiling Heat Transfer”, Trans. ASME, Vol. 80,

pp. 711-720, 1958.

26) Lienhard, J. H., Dhir, V.K., and Riherd, D.M., “Peak Pool Boiling Heat Flux

Measurements on Finite Horizontal Flat Plates”, Journal of Heat Transfer, 95,477-

482, 1973.

27) Navarra, P., “Heat Pipe Cooling of Metallurgical Furnace Equipment”, Ph.D.

Thesis, McGill University, Montreal, Canada, 2006.

28) Mucciardi, and Jin, N., “Extending Lance Life in Top Blowing”, Copper 99,

TMS, Oct.1999, pp. 207-222.

29) Mucciardi, F., “Improving Injection Lances with Heat Pipe Technology”, The

Brimacombe Memorial Sym.-Poster Proceedings, MetSoc-CIM, Oct. 2000, pp.

205-218.

30) Mucciardi, F., and Jin, N., “Mathematical Model of a Heat Pipe Lance”, (Paper

presented at the Computer Applications in Metallurgy and Materials Processing

Sym., CIM, 1998.

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31) Mucciardi, F., and Yuan, Z., “Waterless Oxygen Lance for Steelmaking and

Refining”, Sym. on Innovation Technologies for Steel and Other Materials,

MetSoc-CIM, 2001, pp. 159-170.

32) Mucciardi, F., et al., “Heat Pipe”, U.S. Patent Applications, Field on Feb. 25,

2002.

33) Zhang, C., Mucciardi, F., and Gruzleski, J.E., “Controlled Cooling of Permanent

Moulds in the Casting of Aluminum”, Light Metals Sym. Of MetSoc, CIM, Aug.

2001, pp. 431-441.

34) Mucciardi, F., and Zheng, G., “Heat Transfer in the Boiling Regime”,

Fundamentals of Metallurgical Processing- J.M. Toguri Sym., MetSoc-CIM, Aug.

2000, pp. 335-350.

35) Zhao, H., “Development of a Sulphur-Based McGill Heat Pipe.” , Ph.D. Thesis,

McGill University, Montreal, Canada, 2007.

36) Yaun, Z., and Mucciardi, F., “Waterless, Non-Consumable Thermopump

Lances”, Ecomaterials and Ecoprocesses, MetSoc of CIM, Vancouver, 2003, pp.

229-242.

37) Yuan, Z., and Mucciardi, F., “Heat Fluxes from Aluminum Melts to Isothermal

Surfaces”, MetSoc-CIM, 2002, pp. 335-349.

38) Navarra, P., Mucciardi, F., and Van Rompaey, T., “Heat Pipe Cooling of a Slag

Tapblock”, Converter and Fire Refining Processes, EPD Congress (TMS), 2005,

pp. 701-712.

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39) Navarra, P., Zhao, H., and Mucciardi, F., “Improvement of Flow Modifiers in

McGill Hear Pipes”, Multiphase Phenomena in Materials Processing (TMS), 2005,

pp. 31-41.

40) Mucciardi, F., Gruzleski, J. E., Zheng, G., Zhang, C., and Yuan, Z. US Patent

7,115,227, “Heat Pipe”, Oct., 2006.

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