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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.
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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
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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
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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
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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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49
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
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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
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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
2. Calibration values for thermo-couple (TC-02)
S.No Set
Temperature(0C)
Actual
Temperature(0C)
Observed
Temperature(0C) Error(0C)
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
3. Calibration values for thermo-couple (TC-03)
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
4. Calibration values for thermo-couple (TC-04)
S.No Set
Temperature(0C)
Actual
Temperature(0C)
Observed
Temperature(0C) Error(0C)
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
5. Calibration values for thermo-couple (TC-05)
S.No Set Temperature(0C)
Actual Temperature(0C)
Observed Temperature(0C)
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
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6. Calibration values for thermo-couple (TC-06)
S.No Set Temperature(0C)
Actual Temperature(0C)
Observed Temperature(0C)
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
7. Calibration values for thermo-couple (TC-07)
S.No Set
Temperature(0C)
Actual
Temperature(0C)
Observed
Temperature(0C) Error(0C)
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
8. Calibration values for thermo-couple (TC-08)
S.No Set Temperature(0C)
Actual Temperature(0C)
Observed Temperature(0C)
Error(0C)
1 50 50.0 50.2 +0.2
2 100 100.0 100.2 +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).
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Fig
. 3.8
. Sch
em
ati
c d
iagra
m o
f an
experi
menta
l setu
p
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Pla
te 3
.1 P
hoto
gra
ph
ic v
iew
of
an
experi
menta
l setu
p
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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
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Fig.3.10 Diagram of wire wound resistor
Specifications of wire wound resistor:
Heat capacity : 5 watt
Resistance : 16 ohms
Tolerance : 1%
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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.
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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
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= 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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69
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.
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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
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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