CHAPTER-1 INTRODUCTION

Heat transfer in pool boiling is mainly used in flooded evaporators as in heat pumps,

refrigerating technique, air conditioning and within the process industry. The trend towards a

better understanding of the fundamentals of bubble formation is supported by new developments

in computer calculations and in measurement technique. However, little progress has been made

in the development of theoretically based predictive methods. A literature survey shows that

numerous influencing parameters for the heat transfer have been identified. Anyhow, it is not

clear how the certain influence parameters can be taken into account.

In applications where it is desirable to keep the temperature of a boiling surface low,

reducing the saturation pressure may be a useful solution. A reduction in the saturation pressure

causes a corresponding decrease in the saturation or boiling temperature, allowing a given

superheat level to be achieved with a lower surface temperature. This approach is particularly

useful when water is used as the boiling liquid.

Water is a desirable liquid since it has such a high heat of vaporization, high thermal

conductivity, and is non-toxic and non-flammable. Boiling in sealed vessels is a typical

application of low pressure boiling. Heat pipes, thermo siphons, and some heat pump cycles may

rely on low pressures to provide low surface temperatures while moving significant quantities of

heat. For example, it is often desirable to maintain a low temperature on the heated end of a heat

pipe or thermo siphon in spot cooling electronic components. Heat fluxes from current electronic

components are approaching 50 W/cm2. These fluxes are not easily handled by solid heat sinks.

Phase-change heat sinks, which operate with a nearly isothermal interior, are becoming

increasingly attractive.

Low temperature operation of these heat sinks may be prescribed by creating a saturated

liquid and vapor state in the vessel at very low pressures. Therefore the boiling occurs in the

heated end of the vessel at a low temperature. The system studied in this investigation consists of

boiling of water in a finite pool at sub atmospheric and atmospheric pressures from a small

horizontal thermo siphon surface. Knowledge of the boiling characteristics of the small heated

surface is necessary to insure that Steady and safe operating conditions are maintained. In

particular, the boiling heats transfer Characteristics of low-frequency intermittent bubble

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departures, the onset to continuous boiling, and the CHF condition. Because intermittent bubble

departure may cause undesirable surface temperature oscillations, continuous boiling is desired.

The CHF condition prescribes the upper limit on the system operation so as to avoid high surface

temperatures characteristic of film boiling. The characteristics of pool boiling of water at low

pressure are known to be much different from the corresponding process at atmospheric pressure.

Dissipation of large heat fluxes at relatively small temperature differences is possible in

systems utilizing pool boiling phenomenon as long as the heated wall remains wetted with the

liquid. With the wetted wall condition at the heated surface, heat is transferred by a combination

of two mechanisms: (i) bubbles are formed at the active nucleation cavities on the heated surface,

and heat is transferred by the nucleate boiling mechanism, and (ii) heat is transferred from the

wall to the liquid film by convection and goes into the bulk liquid or causes evaporation at the

liquid-vapor interface. The large amount of energy associated with the latent heat transfer

(compared to the sensible energy change in the liquid corresponding to the available temperature

potential in the system) in the case of nucleate boiling, or the efficient heat transfer due to liquid

convection at the wall, both lead to very high heat transfer coefficients in flow boiling systems.

Removal or depletion of liquid from the heated wall therefore leads to a sudden degradation in

the heat transfer.

MODES OF BOILING

1.1 NATURAL CONVECTION:

Water that is not in contact with its own vapour does not boil at the so called normal boiling, T sat

instead it continues to rise in temperature until bubble finally to begin to form. On convectional

machined metal surfaces, this occurs when the surface is few degree above Tsat.

1.2 POOL BOILING:

Pool boiling is the process in which the heating surface is submerged in a large body of stagnant

liquid. The relative motion of the vapor produced and the surrounding liquid near the heating

surface is due primarily to the buoyancy effect of the vapor. Nevertheless, the body of the liquid

as a whole is essentially at rest. Heating of the water at various pressures with a heated

cylindrical copper section submerged horizontally under the water level. The boiling curve is

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divided into 6 regions based on the observable patterns of vapor production. Region I, is so

small that the vapor is produced by the evaporation of the liquid into gas nuclei on the exposed

surface of the liquid. Region II, is large enough that additional small bubbles are produced

along the heating surface but later condense in the region above the superheated liquid. Region

III, is enough to sustain "nucleate boiling", with the creation of the bubbles such that they

depart and rise through the liquid regardless of the condensation rate. Region IV, an unstable

film of vapor was formed over the heating surface, and oscillates due to the variable presence of

the film. In this region, the heat transfer rate decreases due to the increased presence of the vapor

film. Region V, the film becomes stable and the heat transfer rate reaches a minimum point. In

Region VI, the is very large, and "film boiling" is stable such that the radiation through the

film becomes significant and thus increases the heat transfer rate with the increasing .

This behavior as described above occurred when the temperature of the wire was the controlled

parameter, Twall-Tsat = ∆T. If the power is the controlled variable then the increase in the power

(or heat flux, q") in Region III results in a jump in the wire surface temperature to a point in

Region VI. This point of transition is known as the critical heat flux and occurs due to

hydrodynamic fluid instabilities as discussed later. This results in the stable vapor film being

formed, and the wire surface temperature increases as the heat transfer resistance increases for a

fixed input power. If the power is now decreased, the vapor film remains stable in Region VI and

the decreases to the minimum point for film boiling within Region V. At this point the vapor

film becomes unstable and it collapses, with "nucleate boiling" becoming the mode of energy

transfer. Thus, one passes quickly through Region IV and III to a lower wire surface temperature.

This "hysteresis" behavior is always seen when the power (or heat flux) is the controlled

parameter.

On the effect of the pressure, Farber and Scorah suggested that increasing the pressure, for the

same temperature difference, would result in the decreasing of the size of the bubbles. At the

same time, the film becomes thinner and less circulation would be observed. This effect is

counter balanced by the increased density of the vapor and the attendant increase in its enthalpy.

Thus, the increase in pressure initially increased the heat transfer rate in pool boiling.

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The objective of this section is to present an overall picture of the pool boiling process with an

emphasis on the practical models used (1) to identify the transition between natural convection

and nucleate boiling, as well as nucleate and film boiling, and (2) to estimate the heat flux during

nucleate and film boiling given the difference between the heater surface temperature and the

bulk liquid. First, we consider the process of bubble nucleation and then begin to "construct" the

conceptual picture of the pool boiling curve with suggested.

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1.3 BUBBLE GROWTH AND NUCLEATE BOILING

The first phase of growth is controlled by the large ΔP that initially exists balanced by the

inertia of the surrounding liquid; i.e., inertial growth. As the bubble expands the is

maintained by vaporization of the surrounding liquid, caused by energy transfer from the

superheated liquid. The second phase of growth is controlled by the rate of energy transfer from

the liquid to the vapor-liquid interface to produce vapor and maintain the pressure; i.e., thermal

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growth. There is a doubt that bubbles act in some way as small pumps that keeps replacing

liquid heated at the wall with cold liquid. The equation is that of specifying the correct

mechanism. the bubble probably acts to remove hot liquid from the wall and introduce cold

liquid to be heated. It is appeared that the numbers of active nucleation sites generating bubbles

with a strong influence q. on the basis of this experiment Yamagata shows that

Q α ΔTanb

Where ΔT=Tw-Tsat and n is the site density or no. of active sites per square meter. A great deal of

subsequent work has been done to fix the constant of proportionality and the constant exponent

a&b . The exponent term are approximately a=1.2, b=0.33.

1.4 TRANSITION BOILING

"Transition boiling is a combination of unstable film boiling and unstable nucleate

boiling alternately existing at any given location on a heating surface. The variation in heat

transfer rate with temperature is primarily a result of a change in the fraction of time each boiling

regime exists at a given location. The transition boiling section of the boiling curve is bounded

by the critical heat flux and the minimum heat flux. The critical heat flux has been extensively

studied and can be predicted by a variety of correlations. The minimum heat flux has undergone

less study. it is known to be affected by flow, pressure, fluid properties and heated surface

properties. At surface temperatures in excess of the CHF temperature, the heated surface will be

partially covered with unstable vapour patches, varying with space and time. Ellion studied

forced convective transition boiling in sub cooled water and observed frequent replacement of

vapour patches by liquid. Although this may seem similar to transition pool boiling as described

above, the introduction of the convective component will improve the film boiling component by

reducing the vapour film thickness and changing the heat transfer mode, whether dry or wet,

from free convection to forced convection. This will result in an increase in qmin and also can

increase Tmfb (if Tmfb is hydrodynamically controlled). For low qualities and sub cooled

conditions the slope of the transition boiling is always negative, just as in pool boiling. The

amount of heat transfer in the transition boiling region is primarily governed by liquid-solid

contact. At the critical heat flux point the contact-area (or time) fraction F is close to unity and,

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therefore, the liquid contact heat flux is close to the CHF. The value of F strongly decreases

with increasing wall temperature. Initially, at surface temperatures just in excess of the boiling

crisis temperature, a significant fraction of the droplets will deposit on the heated surface but at

higher wall superheats the vapour repulsion forces become significant in repelling most of the

droplets before they can contact the heated surface. The repelled droplets will contribute to the

heat transfer by disturbing the boundary layer sufficiently to enhance the heat transfer to the

vapour. The periodic contacts between liquid and heated surface in the transition boiling region

of the boiling curve result in the formation of both large amounts of vapour, which forces liquid

away from the surface, and creates an unstable vapour film or blanket. Because of this, the

surface heat flux and the surface temperature can experience variations both with time, and

position on a heater. However, the average heat transfer coefficient decreases as the temperature

increases, because the time of contact between the liquid and the heater surface is decreased. To

gain a better understanding of the transition boiling mechanism, the phenomena occurring at the

interface between fluid and a heated surface (i.e. the mechanism of fluid- solid contact including

the frequency of this contact; heat transfer in the contact areas; time history of such contact) need

to be considered.

Multiphase flow is characterized by the existence of interfaces between the phases and

discontinuities of associated properties. In contrast multiphase flow is classified according to the

internal phase distributions or "flow patterns" or "regimes". For a two-phase mixture of a gas or

vapor and a liquid flowing together in a channel, different internal flow geometries or structures

can occur depending on the size or orientation of the flow channel, the magnitudes of the gas and

liquid flow parameters, the relative magnitudes of these flow parameters, and on the fluid

properties of the two phases.

The rate of exchange of mass, momentum and energy between gas and liquid phases

as well as between any multiphase mixture and the external boundaries depend on these internal

flow geometries and interfacial area; hence is dependent on flow-pattern. For instance, the

relationships for pressure drop and heat transfer are likely to be different for a dispersed flow

consisting of bubbles in a liquid (bubbly flow) than for a separated flow consisting of a liquid

film on a channel wall with a central gas core (annular flow). This leads to the use of flow-

pattern dependent models for mass, momentum and energy transfer, together with appropriate

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flow-pattern transition criteria. Given the existence of any one pattern, it is possible to model the

two-phase flow field and to select a proper set of flow-pattern dependent equations so as to

predict the important process design parameters. However, the central task is to predict which

flow-pattern will exist under any set of operating conditions as well as to predict the value of

characteristic fluid and flow parameters (e.g., bubble or droplet size) at which the transition from

one flow-pattern to another will take place. Therefore, in order to accomplish a reliable design of

gas-liquid systems such as pipe lines, evaporation and condensers, a prior knowledge of the

flow-pattern is required.

1.5 CRITICAL HEAT FLUX:

When liquid coolant undergoes a change in phase due to the absorption of heat from a heated

solid surface, a higher transfer rate occurs. the more efficient heat transfer from the heated

surface (in the form of heat of vaporization plus sensible heat) and the motions of the bubbles

(bubble-driven turbulence and convection) leads to rapid mixing of the fluid. therefore, boiling

heat transfer has played an important role in industrial heat transfer processes such as

macroscopic heat transfer exchangers in nuclear and fossil power plants, and in microscopic heat

transfer devices such as heat pipes and micro channels for cooling electronic chips. The use of

boiling is limited by a condition called critical heat flux which is also called a boiling crisis or

departure from nucleate boiling. The most serious problem is that the boiling limitation can be

directly related to the physical burnout of the materials of a heated surface due to the suddenly

inefficient heat transfer through a vapor film formed across the surface resulting from the

replacement of liquid by vapor adjacent to the heated surface. Consequently, the occurrence of

CHF is accompanied by an inordinate increase in the surface temperature for a surface-heat-flux-

controlled system. Otherwise, an inordinate decrease of the heat transfer rate occurs for a

surface-temperature-controlled system. This can be explained with Newton’s law of cooling:

q=h(tw-tf)

Where q represents the heat flux, h represents the heat transfer coefficient, tw represents the wall

temperature and tf represents the fluid temperature. if h decreases significantly due to the

occurrence of the CHF condition, tw will increase for fixed q and tf while q will decrease for fixed

δt. the understanding of CHF phenomenon and an accurate prediction of the CHF condition are

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important for safe and economic design of many heat transfer units including nuclear reactors,

fossil fuel boilers, fusion reactors, electronic chips, etc. therefore, the phenomenon has been

investigated extensively over the world since Nukiyama first characterized it in 1950.

Kutateladze suggested the hydrodynamical theory of the burnout crisis. Much of significant work

has been done during the last decades with the development of water cooled nuclear reactors.

Now many aspects of the phenomenon are well understood and several reliable prediction

models are available for conditions of common interests.

1.6 FILM BOILING:

Post-CHF heat transfer is encountered when the surface temperature becomes too high to

maintain a continuous liquid contact, and the surface becomes covered by a continuous or

intermittent vapour blanket. Post-CHF heat transfer includes transition boiling, where

intermittent wetting of the heated surface takes place, and film boiling, where the heated surface

is too hot to permit liquid contact. The boundary between these post-CHF heat transfer modes is

the minimum film boiling temperature, or TMFB. Due to the poor heat transport properties of the

vapour, high heated surface temperatures are often encountered during film boiling.

Post-CHF heat transfer is initiated as soon as the critical heat flux condition is

exceeded. It persists until quenching or rewetting of the surface occurs. The occurrence of film

boiling depends on surface temperature and flow conditions. The post-CHF heat transfer modes

in flow boiling can be classified as:

(i) Transition boiling (also referred to as “sputtering”)

(ii) inverted-annular film boiling (IAFB) associated with sub cooled or low quality flow

(iii) dispersed-flow film boiling (DFFB) associated with intermediate and high quality flow.

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CHAPTER-2 OBJECTIVE OF THE PROJECT

The main aim of this project is to design and fabricate a pool boiling apparatus and study

the various parameters involving heat transfer flux for varying pressures. In this paper, the most

common calculation methods for nucleate boiling are presented and compared to experimental

results. New investigations are carried out for water and organic fluids in order to improve this

method especially for low pressures. Therefore, an existing standard apparatus for pool boiling at

ambient pressure is prepared to fit the new requirements for variable pressures.

The saturated pool boiling heat transfer characteristics, waiting time analysis, departure

time analysis and the critical heat flux (CHF) condition are to be determined in the

experiments and compare the experimental results for sub- atmospheric, ambient and

overpressure.

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CHAPTER-2

EXPERIMENTAL SETUP

Figure 1 shows the experimental test section and system used in this investigation. The

copper test sections were machined to accommodate two heaters in the bottom end. The top half

of the copper piece was milled to provide 3 cm diameter circular section heater which is 4cm

long. Within these sections, 0.8 mm holes were drilled to the center to hold thermocouple wires.

The copper and thermocouples were then cast in a low viscosity epoxy. Round one of 7.04cm2

were used since they could accommodate a range of typical electronic component sizes in a

thermosiphon application.

The main body of the pool boiling container was made with 3.5 cm I.D. tubing. In order

to examine boiling under different below and above atmospheric pressures. Nucleate boiling is

very dependent on cavity size, distribution, and wetting properties. The most difficult extraneous

nucleation sites to control were at the interface between the copper test section and the epoxy.

This interface had to both maintain a vacuum seal and not become a cavity for nucleation. The

epoxy that was eventually selected, adhered and sealed well enough to the copper so that boiling

did not occur at the edge where the copper and epoxy met, even after repeated thermal cycling.

Once the epoxy cured, the top surface of the copper could be treated. Excess epoxy was milled

down to be flush with the copper surface. The copper/epoxy surface was then finished with

emery paper and cleaned with alcohol.

The epoxy surface was then bonded to the end of a clear tube to allow observations. The clear

tube fit inside a collar fixed at the bottom of the condenser so that repeated assembly was

impossible. The condenser was made of a 15cm long section of copper tubing which had radial

copper fins wound and welded onto its O.D. Heat was removed from the fins with an fan. A

thermocouple probe extended down through the inside of the tube and could be positioned

vertically to monitor fluid or vapor temperatures. A vacuum pressure gauge measured the

internal pressure of the thermosiphon. During initial start up of the system, a valve allowed a

two-stage vane-type vacuum pump to pull the internal pressure down to very low values. The

liquid was boiled during this step to remove gasses from the internal volume. pressure, indicated

the presence of non-condensible gasses.

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PARTS OF APPARATUS

1. T-section of Copper metal

2. Glass tube

3. Condenser Section

4. Collar

5. Thermocouple

6. Temperature Reader

7. Air Blower

8. Heater

9. Ammeter

10. Epoxy

11. Insulation

12. Vacuum pump

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REFRENCES

1. R. Cole and H. L. Shulman., Bubble Departure Diameters at Subatmospheric Pressures.

Chemical Engineering Progresses Symposium Series

2. S. J. D. Van Stralen, R. Cole, W.M. Sluyter, and M. S. Sohal. Bubble Growth Rates in

Nucleate Boiling of Water at Subatmospheric Pressures. International Journal of Heat

and Mass Transfer

3. A. Niro and G. P. Beretta. Boiling Regimes in a Closed Two-Phase Thermosyphon.

International Journal of Heat and Mass Transfer 33(10)

4. W. R. McGillis, V. P. Carey, J. S. Fitch, and W. R. Hamburgen. Pool Boiling on a Small

Heat Dissipating Element in Water at Low Pressure. To be presented at ASME/AIChE

National Heat Transfer Conference, Minneapolis Minnesota.

5. W. Nakayama, T. Daikoku, H. Kuwahara, and T. Nakajima. Dynamic Model of

Enhanced Boiling Heat Transfer on Porous Surfaces, Part I: Experimental Investigation.

Journal of Heat Transfer 102:445-450,

6. A. Niro and G. P. Beretta Boiling Regimes in a Closed Two-Phase Thermosyphon.

International Journal of Heat and Mass Transfer 33(10)

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