fluid measurement

44

CHAPTER 3 EXPERIMENTAL INVESTIGATION 3.0 Introduction

From the literature review it is observed that there are a

number of variables that control the heat transfer rate in impingement

cooling and play an important role in the fluid flow. Most important

parameters are the fluid velocity, geometry of nozzle, spacing between

the nozzle and target plate, temperature of the fluid and the target

plate. To conduct an experimental investigation in which there are

several variables like this, it is necessary to develop a test facility

keeping all the operational requirements in mind.

3.1 Experimental test facility

As a part of the present investigation an experimental test

facility is designed, developed, tested and commissioned. The facility

consists of four sub systems namely:

(1) Fluid flow measurement and monitoring system,

(2) Heat flow regulating system,

(3) Instrumentation system, and

(4) Data acquisition and storage system.

These subsystems are integrated to form the final experimental

facility, shown diagrammatically in Fig.3.1.

45

Fig. 3.1 Schematic diagram of experimental test facility

3.1.1 Fluid flow measurement and monitoring system

A two stage reciprocating air compressor driven by a prime

mover DC motor through a belt is shown in Fig.3.2. The test rig

consists of base on which the tank (air reservoir) is mounted. The out

let of the air compressor is connected to the reservoir at 20-bar

pressure and 160 liters capacity. The temperature and pressure of the

compressed air is indicated by a thermometer and a pressure gauge

respectively. The suction is connected to the air tank through a

calibrated orifice plate with a water manometer for facilitating the flow

measurement. During the experiment, air is drawn from the reservoir

through the rotameter and led subsequently to the manifold, to which

the nozzles are attached. Two rotameters are employed parallely, one

for the larger mass flow rates, (0-150 LPM) and the other one for low

46

(0-30 LPM) mass flow rate. The flow rate is measured with rotameters,

calibrated as per ASME standards with 1% accuracy. The system

also includes of regulating valves to change the flow rate as per the

operational requirements.

Fig.3.2 Schematic diagram of two stage air compressor

47

3.1.2 Heat flow regulating system

Fig.3.3 Schematic diagram of heat flow regulating system

Heat flow regulating system consists of a stabilized power

supply from UPS, a dimmerstat to vary the voltage and a voltmeter (0

to 250 V) and an ammeter (0 to 200 mA) to indicate the supply voltage

and current to the 500W heater plate. The heater plate is a 240 mm

diameter and 20 mm thick is shown in Fig.3.3. The temperature of the

hot plate (Target plate) can be regulated by changing the supply

voltage.

3.1.3 Instrumentation system

Present experimental setup consists of thermocouple sensors to

measure temperature at various locations and an eight channel

temperature scanner (M83 BP 407, Masibus Digital Scanner 85XX) to

48

record the corresponding temperatures. The programming, calibration

and the operation of the scanner are accomplished by nine simple

keys with two independent displays for channel no and data value for

the channel. Channel display is of two digits to differentiate it from

data display of four digits. Each of 0.56 seven segments LED is

shown schematically in Fig.3.4. J- Type thermocouple (Iron-

constantan) would normally have an error of approximately 0.1% of

the target temperatures when used in the temperatures ranging from

0 to 400 0C. These types of thermocouples are used for temperature

measurement due to their excellent sensitivity.

Fig.3.4 Schematic diagram of instrumentation facility

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3.1.4 Calibration of temperature sensors (Thermocouples)

Fig.3.5 Schematic diagram of calibration setup

A schematic diagram for the arrangement for temperature

calibration setup, used in the present investigation is shown in

Fig.3.5. The temperature sensors used in the present experimental

investigation (J-type thermocouple) are properly calibrated as per the

standard procedure, described in detail in the following section. J-type

thermocouples, are drawn from a single spool are cut to the required

sizes based on the distance between the measuring point and the

temperature scanner. Thermocouple beads are formed by gas welding

technique in a nitrogen environment. This ensures formation of beads

with out metal oxide coating on the surface of the bead. Thermocouple

beads thus formed are given a varnish coating to prevent electric short

circuiting between the sensor and the measured. A representative

thermocouple is arbitrarily selected and calibrated in the range of 0 to

50

2000C. The bead is dipped in a thermo fluid and the cold junction of

the thermocouple is connected to a precision volt meter through a ice

melting bath that served as a zero temperature reference. The

temperature of the thermic fluid is gradually increased with an electric

heater. The temperature of thermic fluid is measured with a standard

reference thermometer with an accuracy of 0.10C. The e.m.f.

developed at the cold junction is recorded with the precision

multimeter. The data thus generated, is used to draw the calibration

graph between the input temperature and output voltage. The output

voltage is in turn used to measure the temperature of the measurand.

The calibration data for J-type thermocouples used in experiment is

given in Table 3.1.and Table 3.2.

Table 3.1

Details of instrument calibrated:

MAKE/TYPE TAG NO. SR.NO RANGE ACCURACY

J TYPE

THERMOCOUPLE MJIT-01 TM-TC-01

0-200

DEG.C 0.5%

Environmental conditions: Room Temperature: (23 20C) Humidity : (55 5% RH)

Details of standard used:

MAKE & TYPE SR.NO RANGE ACCURACY

SPECIFIED

CAL.REPORT

NO NEXT CAL

Temperature Bath with

Honey-Well controller

DC1010

0-4000C

0.1% Cal/0601/1167

30/7/08

51

Table 3.2

1. Calibration values for thermo-couple (TC-01)

S.No

Set

Temperature(0C)

Actual

Temperature(0C)

Observed

Temperature(0C) Error(0C)

1 50 50.0 50.2 +0.2

2 100 100.0 100.1 +0.1

3 150 150.0 150.1 +0.1

4 200 200.0 199.8 -0.2


S.No Set

Temperature(0C)

Actual

Temperature(0C)

Observed


1 50 50.0 49.6 -0.4

2 100 100.0 99.8 -0.2

3 150 150.0 149.7 -0.3

4 200 200.0 199.6 -0.4


S.No Set Temperature(0C)

Actual Temperature(0C)

Observed Temperature(0C)

Error(0C)

1 50 50.0 50.2 -0.2

2 100 100.0 100.1 +0.1

3 150 150.0 150.1 +0.1

4 200 200.0 200.2 +0.2


S.No Set

Temperature(0C)

Actual

Temperature(0C)

Observed


1 50 50.0 50.3 +0.3

2 100 100.0 100.3 +0.3

3 150 150.0 150.1 +0.1

4 200 200.0 200.1 +0.1





Error(0C)

1 50 50.0 49.7 -0.3

2 100 100.0 99.7 -0.3

3 150 150.0 149.6 -0.4

4 200 200.0 199.5 -0.5

52





Error(0C)

1 50 50.0 49.8 -0.2

2 100 100.0 99.9 -0.1

3 150 150.0 150.1 +0.1

4 200 200.0 200.2 +0.2


S.No Set

Temperature(0C)

Actual

Temperature(0C)

Observed


1 50 50.0 50.1 +0.1

2 100 100.0 100.2 +0.2

3 150 150.0 150.3 +0.3

4 200 200.0 200.4 +0.4





Error(0C)

1 50 50.0 50.2 +0.2

2 100 100.0 100.2 +0.2

3 150 150.0 150.3 +0.3

4 200 200.0 200.3 +0.3

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Fig.3.6 Comparison of observed temperature values with set temperature values

The present observed temperature values are validated against

the set temperatures for the calibration of thermocouples (J-type) in

Fig.3.6. It can be observed from Fig.3.6 that both temperatures agree

well, indicating that the thermocouples can be confidently used for

further experimentation.

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3.1.5 Data acquisition and storage system:

The Data acquisition system consists of eight channel

temperature scanner (masibus digital scanner 85 XX). A custom built

software capable of acquiring temperature data as a function of time is

loaded on to a personal computer (P4). This software has a provision to

set the sampling frequency of temperature as low as 0.1 sec. The

storage capacity of the data acquisition system is kept sufficiently

large so that the temperature data can be acquired over large interval

of time.

Fig.3.7. Schematic diagram of data acquisition facility

3.2 Description of the experimental setup

A Schematic diagram and photographic view of the experimental

setup are presented in Fig. 3.8 and Plate.3.1 respectively (please see

Annexure I, for other details). The important components in the

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setup are two stage reciprocating air compressor, rotameter, electric

heater, and control panel. The control panel consists of voltmeter,

ammeter, dimmer stat, and temperature display unit. An aluminum

heater plate rated 500 W and 240 V, insulated on all sides by mica

sheets, is used to heat the printed circuit board (PCB). Five cylindrical

electrical resistors fixed on printed circuit board of diameter 100mm

and 2mm thick are located centrally on the aluminum heater plate. A

chip assembly on PCB is simulated with the electrical resistors which

are 25 mm long and 5 mm in diameter.

The power is supplied to the heater through the dimmerstat to

control the heating rate to the base plate. The current flow and voltage

are measured by ammeter and voltmeter respectively. Teflon coated J-

type thermocouples are used to measure the surface temperatures of

the electronic components (resistors). The location of thermocouples

on the resistor is shown in Fig. 3.9.

The central resistor in the jet array is considered for the

analysis. Two thermocouple leads are inserted into the holes drilled to

the aluminum heater plate. The gap between resistors is filled with

aluminum powder to ensure good thermal contact between the

resistors. One thermocouple is used exclusively to measure the

temperature of the air in the enclosure. All these eight thermocouples

are connected to a temperature display unit through a scanner to

observe the readings and store the values in a personal computer (P4).

56

Fig

. 3.8

. Sch

em

ati

c d

iagra

m o

f an

experi

menta

l setu

p

57

Pla

te 3

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hoto

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of

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experi

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58

The air flow through the nozzles of different diameters located above

the resistors is measured with two types of rotameters. Air at 20-bar is

supplied to the nozzle from a reciprocating air compressor of 160 liter

storage capacity through the rotameters. Provision is made to vary the

distance between the nozzle tip and the test surface. The axis of the

nozzle is always aligned with the central resistor and is normal to the

plane on which heat sources are mounted. The velocity of jet is

measured using a Pitot tube and U-tube Manometer (water) to an

accuracy of 1 %.

Fig.3.9 Location of Thermocouples on resistor surface

59

Fig.3.10 Diagram of wire wound resistor

Specifications of wire wound resistor:

Heat capacity : 5 watt

Resistance : 16 ohms

Tolerance : 1%

60

Plate 3.2 Photographic view of the test section with

500W heater and Aluminum plate

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3.2.1 Range of parameters studied in the experiment

Three different jets are fabricated with 5mm, 8mm, and 10mm

diameters respectively. Resistor with 25mm length and 5mm diameter

are used to generate heat. The ranges of parameters covered are listed

below:

Surface temperature range, Ta, oC 30- 100

Diameter of nozzle, mm 5, 8, and 10

Nozzle-to-electronic components spacing 2 - 10

to nozzle diameter (H/d)

Experimental data are obtained for four different operating conditions

of the jet arrays as shown below.

(a) Circular nozzle with different Reynolds number and nozzle-to-

target heater spacing.

(b) Rectangular nozzle with different Reynolds number and nozzle-to-

target heater spacing.

(c) Square nozzle with different Reynolds number and nozzle-to- target heater spacing.

(d) Different Radial locations with circular, rectangular and square nozzles.

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3.2.2 Types of nozzles used in the present investigation:

There are three different types of nozzles used in the present

investigation. They are Circular, Square and Rectangular nozzles.

(a) (b) (c)

Plate3.3 Photographic view of different circular nozzles: (a) d =5 mm, (b) d = 8mm (c) d= 10mm

68

Plate 3.4 Photographic view of

square nozzle, de = 11.28mm

Plate3.5 Photographic view of

rectangular nozzle, de = 13.3mm

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Table 3.3

Geometry and dimensions of the nozzles

S.No

Type of

nozzles

Height

(mm)

Breadth

(mm)

*Equivalent diameter

(de) (mm)

Hydraulic

diameter (d*) (mm)

1

Circular 5,8 and

10 5,8 and

10 5,8 and 10

5,8, and 10

2

Square 10 10 11.28 10

3

Rectangular 5 20 13.3 8

*Equivalent diameter (de) is defined on the basis of area of the nozzle

For square nozzle, Area = 10 x 10 mm2

100d4

2

e

de = 11.28mm.

For rectangular nozzle, Area = 5 x 20 mm2

de = 13.3mm.

Hydraulic diameter (d*) is defined as = perimeterwetted

)area(4

For rectangular nozzle = 205x2

20x5x4

= 8mm.

64

Fig. 3.11 Schematic line diagram of different nozzles

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3.3 Experimental procedure

The air jet issuing from the nozzle and impinging on the

resistors is depicted as free jet and wall jet regions respectively. Five

cylindrical electrical resistors fixed to an insulating plate (PCB) of

diameter 100mm and 2mm thick located centrally on an aluminum

heater plate is shown in Fig.3.12. Power is supplied to the resistors

through a step down transformer and for the aluminum plate through

a dimmer stat. The heat input to the aluminum plate is adjusted with

the help of dimmer stat. The temperatures at all the thermocouple

positions are recorded until steady state is reached. These data are

utilized for the calculation of steady state heat convection heat

transfer rate. The jet array is kept in three different geometric

orientations as mentioned above, and steady state temperatures are

noted for each orientation of the jet array.

Fig.3.12 Schematic diagram of flow emanating from the nozzle

impinging on resistors surface

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The volumetric energy generation due to heating of the resistors

using AC current is assumed to be uniform. The temperature of the

resistors is allowed to rise up to 950 C and then cooled by forced

convection mainly from the top surface by the air stream flowing in

the wall jet region. The surface temperatures of the resistors are

recorded till they attain 450C. The procedure is repeated at different

flow rates of air with temperature values recorded in the different

Reynolds numbers. The heat loss from the resistors towards the

heater plate is assumed to be negligibly small. Experimental data as

mentioned above are obtained for jet arrays having five resistors.

3.4 Method and model calculation

A model calculation is presented below for the case of a vertical

jet array. The values of various parameters and the calculation

procedure are given below.

Ambient air temperature, Ta = 300 C

Thermal conductivity of the fluid, kf = 0.026 W/mK

Surface temperature of the electronic components =82 0 C

Kinematic viscosity of the fluid, f = 15.89 x 10-6 m2 /s

Prandthal number, Pr = 0.71

Density of the fluid, = 1.106 kg/m3

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1. Velocity of the air on the surface of the electronic components

Reynolds number, Re = 5850 (arbitrary chosen)

Diameter of the nozzle, d = 5mm

But

dUORe (3.1)

5850 =UOx5x10-3

15.89x10-6

UO= 18.59 m/sec

16.1

1000x

100

hhx81.9x2x98.0U 21O

(3.2)

h1 h2 = 8.74cm

2. Mass flow rate:

For a constant above value of (h1-h2), the air is supplied on to the

surface of electronic components. Thus the required mass flow rate is

obtained and indicated by the Rotameter.

Required mass flow rate = 8LPM

Resolution of the manometer which is 1mm water column.

h1 = 19.96cm and h2 = 11.22cm

Required mass flow rate = 8 LPM

The mass flow rate of air is calculated making use of the following

equation:

Vm O (3.3)

Where, OO

OO

TR

P

= 0.95 287x303

68

= 1.09 kg/m3

m 1.09x8 LPM

= 1.09x8x10-3

60 = 2.725x10-4 kg/sec

= 0.98kg/hr.

3. Local value of heat transfer coefficient ( h)

The local value of heat transfer coefficient (h) is obtained from the

following equation:

33.0618.0 PrRe193.0Nu = (3.4)

0.193(5850)0.618 (0.71)0.33 = hx5x10-3

26.3x10-3 h =193 W/m2 K.

4. Heat transfer rate

The heat transfer rate (Q) is obtained from the following equation

Q = h AS (TS Ta ) (3.5)

= 193x (2.5x0.5x 10-4) x (82-30)

= 1.254 W.

5. Local Nusselt number

The local Nusselt number of the electronic component is calculated as

follows

Nu= (3.6)

= 193x5x10-3 0.026

= 37.11

airk

dh

airk

dh

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6. Recovery factor (rf)

The recovery factor is defined by ratio of the difference of recovery

temperature (Trt) and jet total temperature (Tjt) to the jet dynamic

temperature (Tdt).

dt

rtf

T

TjtTr

(3.7)

= 80-28 71.8

= 1.24

7. Effectiveness ( )

The effectiveness is defined by the difference of adiabatic wall

temperature and recovery temperature to the difference of jet total

temperature and ambient temperature.

ajt

rtaw

TT

TT

(3.8)

= 0.72 In the present experimental investigation the heat sources (electronic

components) are mounted on a printed circuit board. For all practical

purposes the printed circuit board may be assumed as an adiabatic

wall. In the present experimentations the reference temperature is

taken as the adiabatic wall temperature for calculations.

70

(a) Nozzle-to-electronic resistor spacing to nozzle

diameter for circular nozzle

(b) Nozzle-to-electronic resistor

spacing to nozzle equivalent diameter with square nozzle

(c) Nozzle-to-electronic

resistor spacing to nozzle equivalent

diameter for rectangular nozzle

(d) Dimensionless radial locations with circular, square and

rectangular nozzles

Fig. 3.13. Jet array in different orientations.

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3.5 Regression equations

The following equations are obtained from the experimental

results by nonlinear regression analysis for Nu0, stagnation Nusselt

number for theoretical and experimental analysis. Heat fluxes q (the),

q(exp) is evident that they can be used to calculate Q(the), Q(exp) of

the jet array in different orientations as a function of system

parameters.

3.5.1. Different jet Reynolds number (Red) and the nozzle- to resistor

spacing with circular nozzle

(i) For 5mm diameter of the nozzle:

06.0

33.05.0

dCorrd

HPrRe2.0Nu

= (3.81)

with an average deviation of (AD) = 8% and standard deviation of

(SD) = 9.9% .Eq.(3.81) is valid in the range 5850 < Red < 10000, Pr =

0.71, and 2 < H/d < 6.

33.06.0dO PrRe193.0Nu = (3.82)

(ii) For 8mm diameter of the nozzle:

012.0

4.04.0

dCorrd

HPrRe296.1Nu

= (3.83)

over the ranges 7325 < Red < 12200, 0.69 < Pr

72

with an average deviation of (AD) = 9% and standard deviation of (SD)

= 10.5% .Eq.(3.85) is valid in the range 5850 < Red < 12200, 0.70

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fluid measurement