[revised copy] investigations on the cooling of a...
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32
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
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