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CHAPTER III

EXPERIMENTAL PROCEDURES

The test set up was designed based on the studies conducted by

various investigators [3, 9, 19, and 28] and keeping in view the

objectives of the present investigation. The details of the test set up

and the ranges of various test parameters are mentioned in tables

3.1and 3.2.

3.1 DETAILS OF THE EXPERIMENTAL SET UP

Experimental set up was designed and fabricated to meet the

requirements of the present investigations. The test assembly was

designed to enable conducting tests in both horizontal and vertical

positioning of the jets. The test apparatus is shown schematically in

Figs 3.1 and 3.2. It consists of a high pressure air compressor, test

chamber and the fluid delivery system. The fluid delivery system has

an air/water reservoir, an auxiliary reservoir, flow control valves,

filter, pressure gauge, filter and piping systems. The auxiliary

reservoir is provided to smooth out the flow fluctuations and steady

flow conditions at the nozzle exit. It also helps in fine control of the

flow rate. The filter has been installed in the flow line prior to the

auxiliary reservoir. Safety valve is provided to prevent excess pressure

build-up in the system.

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The test assembly is shown in Fig 3.3. It consists of a test

chamber, mounting plate, movable nozzle block, base tray and a top

plate. All these components are held together by a vertical support

rod. The test chamber is of stainless steel. It is connected to the

air/water reservoir through a connecting tube. The test chamber

consists of a nozzle block and heater assembly. The nozzle block was

designed and fabricated to enable tests with different types of nozzles.

The nozzle block can be moved vertically using a calibrated screw

thread assembly which is provided along with a circular scale on the

top plate. The nozzle block can be positioned at the desired height

from the test plate by using a calibrated screw head.

Two nozzle blocks having 0.25mm and 0.5mm diameter jets

were used for the investigation. The jet diameters were selected based

on the available literature, mentioned in the table 2.1. The holes are

in a square array of 7X7 and the distance between the holes is 3mm.

The distance between the test plate surface and the jet exit was

maintained at 10mm and 20mm.The heater assembly consists of a

test surface containing a heating element, voltage transformer, two

thermocouples, and a display system. The test surface is a thin copper

plate of 2cmx2cm size and 1mm in thickness fixed onto a Teflon

jacket. Copper is selected due to its high thermal conductivity and

other desirable properties as it has been used by various researchers

in similar investigations. The test plate has been mounted on the

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heater. The test plate represents the surface of a typical electronic

component.

The heater coil consists of 16 gauge Nichrome wire having a

resistance of 2 ohm and capacity of 1 kW. The input power to the

heater element is varied using a variable voltage transformer. The test

plate is insulated from the surroundings using Teflon insulation to

ensure that the power supplied to the heater is dissipated only

through the test plate. Thermocouples were mounted as shown in Figs

3.1 and 3.2 underneath the test surface on the centre line. The

thermocouples show the uniform surface temperature on the test

surface. The thermocouples are connected to the display system.

The following are the functions of the control and display

system: (i) Variation of the heat input to the test surface. (ii) Display

the test surface temperatures, heater input voltage and current.

Digital temperature indicator, voltmeter and ammeter are used for this

purpose and (iii) Limit the maximum test surface temperature and cut

off the power supply to the test surface when the test surface

temperature increases above the set value. Wattmeter having a range

of 1.5kW was used to measure the power supplied to the heater. In

case of multiple air jet experiments, the flow rates through the

multiple jets were measured using a calibrated venturimeter and the

water manometer arrangement. While conducting experiments with

multiple water jet, the water flow rate was measured using a flow

meter and the direct measurements.

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Fig 3.1: Schematic diagram of multiple water jet experimental setup

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Fig 3.2: Schematic diagram of multiple air jet experimental setup

37

H

D

Fig 3.3: Details of the test assembly

A: Test plate

E

G

A

B

B : Heating Element

C : Nozzle block

D : Power supply

E : Vernier scale

F : Fluid Inlet

G : Adjustable screw

H : Teflon jacket

I : Base plate

C

F

H

D

I

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Plate 3.1: Experimental arrangement for multiple jet experiments in

Vertical position

Transformer

Display unit

Watt meter

Test chamber Power supply

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Plate 3.2: Experimental arrangement for multiple jet experiments in

horizontal position

Display unit

Watt meter

Test chamber

Transformer

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Plate 3.3: Test stand assembly

Fluid inlet

Drain out

Test

plate

Nozzle

block

Positioning screw with graduated scale

Base tray

Power

supply

Teflon insulation

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Table 3.1: Test parameters for multiple water jet experiments

Sl. No Parameters Ranges

1. Test plate size (cm2) 2 x 2

2. Heat flux ,q (W/cm2) 25 to 200

3. Working fluid Water

4. Jet exit to test plate surface,

Z (mm) 10 and 20

5. Number of jets (7 x 7) 49, square array

6. Distance between jets, S(mm) 3

7. Pitch to

diameter ratio (S/d) 6 and 12

8. Jet diameter, d (mm) 0.5 and 0.25

9. Flow discharge, Q (ml/min)

1320,1440,1560,1740 and 2040

with d=0.5mm

840,1020,1140 and 1320 with

d= 0.25 mm

10. Reynolds number, (Re)

1137,1240,1560,1740 and2040

with d=0.5mm

1450,1760,1965 and 2276 with

d= 0.25 mm

11. Positioning of jet Vertical and horizontal

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Table 3.2: Test parameters for multiple air jet experiments

Sl. No Parameters Ranges

1 Test plate size (cm2) 2 x 2

2 Heat flux ,q (W/cm2) 25 to 200

3 Working fluid Air

4 Jet exit to test plate surface,

Z (mm) 10 and 20

5 Number of jets (7 x 7) 49, square array

6 Distance between jets, S (mm) 3

7 Pitch to

diameter ratio (S/d) 6 and 12

8 Jet diameter, d (mm) 0.5 and 0.25

9 Reynolds number, (Re)

1290,1570 and1816

with d=0.5mm

2573,3638 and 4455

with d=0.25mm

10 Positioning of jet Vertical and horizontal

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3.2 PROCEDURE FOR CONDUCTING EXPERIMENTS

Before starting the experiments the test plate surface was

cleaned to remove the dust and residual adhesive stains on the

surface. Compressed air is passed through the tube connecting the

nozzle block and the reservoir to remove any dust particles in the

nozzle block. Repeatability tests were conducted in order to check the

quality of the experimental data.

3.2.1 Multiple Water Jet Impingement

Following are the steps adopted while conducting the

experiments with multiple water jets.

• Keep the test assembly which consists of nozzle block and

heater assembly in vertical/horizontal positions.

• Adjust the distance from the test surface to the nozzle block by

10mm.

• Start the flow of water jet to the test chamber.

• Fix the constant flow rate.

• Supply heat input to the test plate through wattmeter.

• Once the test plate reaches the steady state condition, note

down the Wattmeter, thermocouple and water flow rate

readings.

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• Increase the heat input to the next value.

• Maintain a constant flow rate.

• After the test plate reaches steady state condition note down the

readings.

• Repeat the experiment for different heat inputs, keeping the flow

rate constant.

• Repeat the experiments with different flow rates.

• Repeat the procedures for different flow rates, different jet

diameters and both vertical and horizontal positioning of the

jets.

3.2.2 Multiple Air Jet Impingement

Following are the steps adopted while conducting the

experiments with multiple air jets.

• Keep the test assembly which consists of nozzle block and

heater assembly in vertical/horizontal positions.

• Adjust the distance from the test surface to the nozzle block by

10mm.

• Start the flow of air jet to the test chamber.

• Fix the constant flow rate.

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• Supply heat input to the test plate through wattmeter.

• Once the test plate reaches the steady state condition, note

down the Wattmeter, thermocouple and manometer readings.

• Increase the heat input to the next value.

• Maintain a constant flow rate.

• After the test plate reaches steady state condition note down the

readings.

• Repeat the experiment for different heat inputs, keeping the flow

rate constant.

• Repeat the experiments with different flow rates.

• Repeat the procedures for different flow rates, different jet

diameters and both vertical and horizontal positioning of the

jets.

3.3 DATA ACQUISITION

The test plate was allowed to reach a steady state. The test data

on air/water flow rate, velocity, power dissipation and temperatures

was acquired. Prior to the recording of the heat transfer data for

analysis, experiment was conducted to obtain the time required to

reach the steady state. It was found that the average test plate

temperature was within 0.10C of its steady state value within 10

minutes of required power to the test plate.

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Test surface temperature measurements were recorded using

thermocouples which are mounted underneath of a test surface. The

fluid inlet temperature is recorded using a 1.5mm diameter type T

thermocouple. The thermocouples were calibrated prior to installation

and measurements were compared. The wattmeter was used to

measure the heat flux input to the test plate.

The flow rate of water was measured using a flow meter and

also checked by direct measurements during the multiple water jet

experiments. Venturimeter was used to measure the air flow rate in

the case of multiple air jet experiments. The U-tube water manometer

is connected to the inlet and throat of the venturimeter to measure the

pressure drop between these two points and hence the volumetric flow

rate.

3.4 DATA REDUCTION

Heat flux was calculated from the electrical power supplied to

the heated test surface. Heat flux is determined using the following

relation:

q = P/A (3.1)

The heat transfer coefficient [h] is calculated from the heat flux

and to the temperature difference between fluid inlet and test plate

surface temperature (∆t).

h = q/∆t (3.2)

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The jet Reynolds number were calculated by the relation

Re = ρV d/µ (3.3)

Where ρ = Fluid density (kg/m3), V= Velocity (m/s) and µ= Viscosity

(Ns/m2)

The Nusselt number is calculated from the heat transfer

coefficient (h), jet diameter [d] and thermal conductivity [k] of the fluid

using thermo physical properties at the film temperature.

Nu = hd/k (3.4)

Test plate temperature is the temperature measured by the

thermocouples embedded in the test plate. This temperature is

measured at two locations on the test plate. These two surface

temperatures are within ±20C. The test plate is heated using a

resistance heater embedded uniformly below test plate. The material

of the test plate is copper. With the present arrangement it is assumed

that the temperatures measured by the thermocouples represent the

average value. The total heat dissipated from the total area of the plate

is used for the calculation of average value of heat flux. Although the

heat flux is not uniform over the total area, the non-uniformity may

not be significant. Table below shows the estimates of heat loss from

the test plate due to conduction through the insulation and radiation

for some typical cases.

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Estimation of heat losses

Sl no

Total heat input

Heat loss due to conduction

Heat loss due to radiation

Percentage heat losses due to conduction

Percentage heat losses due to radiation

Sum of losses in percentage

1 800W 1.6W 0.135W 0.2% 0.017% 0.217%

2 500W 1.44W 0.068W 0.288% 0.0137% 0.30%

3 400W 1.28W 0.053W 0.32% 0.0132% 0.332%

4 200W 0.48W 0.038W 0.24% 0.019% 0.259%

It is found that the total heat losses are within 0.5%. This

aspect has been considered in the calculation of heat transfer

coefficients.

3.5 UNCERTAINTY ANALYSIS OF THE EXPERIMENTAL DATA

The uncertainty analysis was carried out using the standard

single sample method recommended by Kline and McClintock. The

uncertainties of the various parameters are listed in table 3.3.The

sample calculations were given in appendix.

Table 3.3: Uncertainties of relevant parameters for jet impingement

SL. No. Parameters Uncertainty

1. Heat flux, q (W/cm2) ± 1.0 %

2. Exit jet velocity, V (m/s) ± 1.0 %

3. Flow discharge, Q (ml/s) ± 1.0 %

4. Reynolds number, Re ± 1.5 %

5. Heat transfer coefficient, h (W/cm2 0C) ± 3.5 %

6. Nusselt number, Nu ± 3.5 %

7. Temperature differences, (∆t) ± 0.10C