Post on 07-Feb-2018
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CHAPTER 5
CONVECTIVE HEAT TRANSFER COEFFICIENT
5.1 INTRODUCTION
The primary objective of this work is to investigate the convective
heat transfer characteristics of silver/water nanofluid. In order to do this, a
convective heat transfer experimental test facility is designed and constructed.
The test facility consists of a tube-in-tube heat exchanger test section with
high accuracy measuring devices for temperature, flow rate and pressure.
A multi channel data logger is used to record the signals automatically from
these devices. The post processing of the data is done using a MATLAB
programme for the estimation of parameters, such as the heat transfer
coefficient and pressure drop. The test facility is validated using the
experimental results with de-ionized water against the published results and
correlations. This chapter deals with the working principle of the test facility,
measurement devices used in the facility, repeatability, long term stability
study of the nanoparticles in the base fluid, actual experimental procedure,
data reduction and the uncertainty analysis of the study.
5.2 EXPERIMENTAL FACILITY
The test facility consists of two flow circuits, one for the cooling
water and the other for nanofluid as shown in Figure 5.1. The nanofluid
circuit consists of several units such as a pump, a flow meter, a constant
temperature cooling water bath, a nanofluid bath with a temperature
controller.
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Nanofluid Circuit
Cooling Water Circuit
Nanofluid
TankMass flow meter
(nanofluid)
Flow meter
(Cooling water)
Constant
Temperature
Bath
Pump (cooling water)
Pump
(nanofluid)
Flow control valve
(cooling water)
Flow control valve
(nanofluid)
By-pass valve
By-pass valve
TNano outTNano in
TWater in
TWater out
TW 1 TW2TW 3
TW 4 TW5TW 6
TN6TN 5 TN 4
TN 3 TN 2 TN1
Cooling system
P
Differential pressure transmitter
Heater
Heater
Stirrer
N1
N2
N3N4
W1
W2
W3
W4
Figure 5.1 Schematic diagram of the experimental test facility
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The test section is a 2.94 m long counter flow horizontal tube-in-
tube heat exchanger with nanofluid flowing in the inner tube while the
cooling water flows in the annular side. In the test section, the temperature of
the nanofluid and the cooling water are measured at regular intervals. The test
section configuration is explained in section 5.3. In the nanofluid circuit, the
silver/water nanofluid is pumped into the inlet of the test section at different
temperatures and flow rate. It is cooled by the cooling water which flows in
the annulus in the counter flow direction. Similarly, in the cooling water
circuit, the heated water leaving the test section flows into a constant
temperature bath, where the heated water is cooled by an auxiliary cooling
system and pumped back into the test section using a centrifugal pump. The
test section (counter flow horizontal tube-in-tube heat exchanger) is thermally
insulated to its full length by ceramic wool, in order to minimize the heat loss
to the surroundings. The heat infiltration in the test setup for the extreme
conditions of T = 40oC and TWi = 25
oC is theoretically estimated to be 215
W/m2. This is just 4% of the minimum heat flux (5237 W/m
2) encountered in
the study. Thus the possible infiltration could be ignored.
The mass flow rate of the nanofluid is measured using a Coriolis
mass flow meter with ± 0.1 % accuracy. The pressure drop of the nanofluid is
measured between the inlet and exit of the test section using an absolute
piezo-resistive differential pressure transmitter with ± 0.075 % accuracy. The
PT100 thin film temperature detectors (RTD’s) with ± 0.15oC accuracy are
used to measure the temperature at all the necessary points. The mass flow
rate of the cooling water is measured using a suitable glass tube rota-meter
with ± 2 % accuracy. The heat transfer test section which consists of a tube-
in-tube heat exchanger is placed inside an insulated wooden box casing on a
horizontal work bench. The differential pressure transmitter indicator, the
nanofluid and the cooling water temperature controller, and the switching
circuits are placed beside the test section for easy access. The temperature
sensors are carefully drawn out from the test section and are connected to the
data logger (Agilent) which is placed near the test section. A photograph of
the same is shown in Figure 5.2.
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Figure 5.2 Photographic view of the experimental test facility
Computer
Data logger
Mass flow meter
Rota-meterPump
Differential
pressure
transmitter
PID Controllers
Nanofluid tank
Constant
temperature bath
Condensing unit
TEST SECTION
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5.3 TEST SECTION
The test section consists of a tube-in-tube counter flow heat
exchanger made up of bright annealed hardened copper tubes. The inner tube
has an inner diameter of 4.3 mm and an outer diameter of 6.3 mm, while the
outer tube has an inner diameter of 10.5 mm and an outer diameter of
12.7 mm. The total effective length of 2.94 m is arranged horizontally into
five equal subsections as shown in Figure 5.3.
Figure 5.3 Photographic view of the heat transfer test section kept
inside the wooden box before packing with insulation
material
A differential pressure transmitter and two RTDs are mounted at
both ends of the test section to measure the pressure drop between the inlet
and outlet, and the bulk temperatures of the nanofluid, respectively. Six RTDs
are mounted at different locations on the inner and outer tube surface of the
wall to measure the temperature distributions along the length of the heat
exchanger test section. The locations for the temperature measurement of the
nanofluid and cooling water are shown schematically in Figure 5.4 and a
photographic view of the same is shown in Figure 5.5. At the beginning of
every subsection, the cooling water temperature (TW) and outer wall
temperature of the inner tube (TN) carrying the nanofluid are measured using
PT100 RTD sensors. The entire length of the test section is initially wound
Nanofluid tube
Cooling water tube
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with 2” of asbestos rope insulation of thermal conductivity k = 0.1 W m-1
K-1
.
The insulated test section is then mounted inside a wooden box of (300 cm ×
30 cm × 30 cm). Thermocole insulation of k = 0.5 W m-1
K-1
with 5 cm
thickness is provided along the inner periphery of the box. The remaining
space inside the box is packed with high quality glass wool and ceramic wool
of k = 0.3 W m-1
K-1
as shown in Figure 5.6. This ensures that heat lost by the
hot fluid is equal to the heat gained by the cold fluid and there is no heat
interaction with the atmosphere. Both the inner and outer tubes are thoroughly
cleaned for any traces of dirt in it. To ensure that the test section is leak free,
high pressure nitrogen test and vacuum test are performed.
Cooling water
TN2
(Measured)
TW1 (Measured)
0.42 m
Nanofluid
TW2 (Measured)
TN1
(Measured)
TN
Figure 5.4 Locations of temperature measurement for nanofluid and
cooling water
Figure 5.5 Photographic view of the temperature sensor locations
Nanofluid RTD
Cooling water RTD
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Figure 5.6 Photographic view of the glass wool insulated test section
5.4 NANOFLUID CIRCUIT
The nanofluid tank with a 0.5 kW heater controlled with a
proportional integral differential controller (PID-C) is used, in order to keep
the nanofluid temperature constant. The controlled flow rate of the
silver/water nanofluid at required temperature is used, to provide the desired
heat load to the cooling water for a particular operating condition. The
nanofluid is heated in an insulated cylindrical tank 16 cm long and 12 cm in
diameter. This tank is deliberately made small (1 litre) so that the volume
concentration of nanoparticles can be varied from 0.3% to 0.9% with a
maximum nanofluid mass of 93 grams. The cost involved in procuring the
nanoparticles is a constraint in deciding the tank size. The temperature inside
the nanofluid tank is measured using a sheathed thermocouple with ± 1oC
accuracy, which gives the necessary feedback signals to the temperature
controller. The heated nanofluid is tapped out from the bottom of the tank and
pumped into the test section in the counter flow direction to the cooling water.
The nanofluid flow rate is measured using a Coriolis type mass flow meter
connected in the circuit just after the pump. In the test section the nanofluid
loses heat to the cooling water and is returned to the tank at a lower
temperature where it is again heated to the required temperature level. The
nanofluid entry temperature is varied from 50oC to 90
oC during the
Glass wool insulation
to the test section
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experimentation. The flow rate of the nanofluid is varied by using a flow
control valve, which is placed prior to the mass flow meter. All the inter
connecting tubes in the nanofluid circuit are provided with necessary
insulation to avoid heat loss to the surroundings.
5.5 COOLING WATER CIRCUIT
A controlled flow rate of cooling water and at required temperature
is used to provide the desired cooling load to the nanofluid for a particular
flow condition. The water is cooled using an auxiliary cooling system which
includes a 0.5 TR capacity condensing unit and a 1 kW heater coupled with a
PID controller in order to keep the temperature of the cooling water constant
in an insulated rectangular vessel of dimension 50 cm × 50 cm ×
40 cm. The temperature inside the bath is measured using a sheathed PT100
RTD with ± 0.15oC accuracy, which gives necessary signals to the
temperature controller. A stirrer motor operating at 1500 rpm is mounted to
ensure uniform temperature inside the cooling water bath. The cooling water
is tapped out from the bottom of the tank and pumped through a glass tube
rota-meter into the test section in the counter current direction to the
nanofluid. The cooling water gains heat from the nanofluid and returns to the
bath at a higher temperature. The flow rate of the cooling water can be varied
by using a flow control valve, and the diversion valve that returns the excess
cooling water back into the cooling water bath. However, in the present study,
the cooling water flow rate of 16 g s-1
is kept for all the testing conditions.
The maximum testing temperature of the cooling water during
experimentation is maintained below 35oC. All the interconnecting tubes in
the cooling water circuit are provided with necessary insulation to avoid heat
infiltration into the system.
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5.6 MEASUREMENT DEVICES
The accuracy of the heat transfer coefficient depends on the
accuracy level of the measuring instruments. The complete details of the
devices are listed in Appendix 2. The details of the measuring devices are
explained below.
5.6.1 Temperature sensors
To measure the temperature, 2 wire calibrated PT100 thin film
temperature sensors of 2 m wire length are used. The sensors are fixed to the
surface of the copper tube with a suitable tape, properly glued and wound
with a Teflon tape to secure its location from moisture entry, if any. The level
of uncertainties in the sensors according to the manufacturer is ± 0.15oC.
5.6.1.1 Accuracy confirmation test
The temperature is one of the main inputs for the estimation of the
heat transfer coefficient. Hence, it is necessary to measure the global residual
error of all the sensors. To achieve this, the following procedure is used:
(i) The system is brought to a steady state operating condition as
explained in section 5.7.
(ii) The test section valves N3, N4, W3, and W4 (Figure 5.1) are
closed to arrest the flow of cooling water and nanofluid across
the test section. The test facility is completely switched OFF.
(iii) The temperature values are observed in the data logger
through a computer interface to see whether all the
temperature values from the sensors are uniform (i.e.
TW = TWO) after waiting for steady state.
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(iv) The temperatures are recorded from this point for a span of
2 hours. The mean deviation of temperature indicated by all
the sensors with respect to time, is found out.
After 2 hours, the test revealed that the maximum and minimum
deviation of TW and TWO is ± 0.08oC and ± 0.13
oC respectively. This confirms
the accuracy of the temperature measuring system to be within ± 0.15oC.
5.6.2 Differential pressure transmitter
To measure the pressure difference across the nanofluid heat
transfer test section, piezo-resistive differential pressure transmitter (0-2bar)
is used. The pressure transmitter gives the differential pressure between the
inlet and outlet of the test section with ±0.075 % accuracy and the output from
the pressure transmitter is 4 - 20 mA.
5.6.3 Mass flow meter
To measure the mass flow rate of the nanofluid in the circuit
accurately, a Coriolis mass flow meter (3–50 g s-1
) of ± 0.1 % accuracy is
used. This meter is capable of measuring the mass flow rate and density of the
liquid that passes through it. The output from the meter is 4 - 20 mA and the
diameter of the coriolis tube is 2mm. The inlet and outlet of the meter are
flared to the circuit tubing through a VCO coupling.
5.6.4 Cooling water flow meter
A calibrated glass tube rota-meter of ± 2 % accuracy is used to
measure the water flow rate in the cooling water circuit. The Pyrex glass tube
rota-meter with an inner diameter of 12.7 mm and a stem length of 300 mm is
used in the present study.
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5.7 PROCEDURE TO START THE EXPERIMENTAL
FACILITY
To bring the system to a steady state condition for the
experimentation, it is necessary to follow a certain procedure. This is given
below:
The required quantity of the silver/water nanofluid with the
chosen volume concentration is prepared and fed into the
nanofluid bath and is heated there to the required temperature.
The auxiliary cooling system is switched ON to cool the water
to the required temperature in order to let it flow in the
annulus of the test section in the counter current direction to
the nanofluid.
The stirrer motor is switched ON to ensure a uniform
temperature inside the cooling water bath.
The pump in the nanofluid circuit is switched ON and the flow
rate of the nanofluid is set by adjusting the flow control valve.
Throughout the test section loop, all valves except the bypass
valves are opened.
Similarly, the pump in the cooling water circuit is switched
ON and the flow rate of the cooling water is set by adjusting
the flow control valve for all the testing conditions.
The PID temperature controllers in the nanofluid and the
cooling water circuits are switched ON. The inlet temperatures
of the nanofluid and the cooling water are set by using the PID
controllers as per the required conditions. The PID controllers
in both the circuits are programmed to automatically regulate
the heater and cooler based on the initial set conditions.
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The test facility is made to run for a sufficient time until the
steady state condition is reached. The steady state condition is
manually confirmed, by checking the uniformity in the
temperature indicated by all the temperature sensors in the test
section, in accordance with the test conditions. Then the
required parameters are logged into the data logger and saved
in the computer for data processing.
5.8 EXPERIMENTATION PROCEDURE
The system is brought to a steady state condition as described in the
previous section. The parameters which are varied in the experimentation are
the mass flow rate, and the inlet temperature of the nanofluid and cooling
water. The mass flow rate and the temperature at the entry of the test section
are the major design parameters, which are varied to set the desired operating
condition in the setup.
Thus, to achieve a certain condition, the nanofluid with a specific
concentration is first filled in the nanofluid tank. The pump for the nanofluid
and that for the cooling system are started. The inlet temperature of the
nanofluid is varied from 50oC to 90
oC, whereas the inlet temperature of the
chilled water is varied from 25oC to 35
oC. Similarly, the mass flow rate of the
nanofluid is varied from 2 g s-1
to 23 g s-1
, while the cooling water flow rate
of 16 g s-1
is kept constant for all the test conditions. The heated nanofluid
passing through the heat exchanger test section is cooled by the cooling water.
The test section temperature distribution and pressure drop are obtained after
the system reaches the steady state condition. The tests are repeated for
different inlet temperatures of the nanofluid, mass flow rates and volume
concentrations of the silver nanoparticles as shown in the test matrix in
Table 5.1. These flow conditions are selected so that the study could cover a
full laminar-turbulent regime heat transfer. For this experiment the steady
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state condition is arrived after 1½ hours. During the experimental runs, the
pressure difference of the nanofluid and the tube wall temperature at different
positions along the axial direction of the test section are measured. Each
experiment is repeated at least two times to get the average values. The
measured temperatures and pressures are used to calculate the heat transfer
coefficients. The same procedure is repeated with different mass flow rates,
inlet temperatures and volumetric concentrations of the silver nanoparticles. It
is noticed that a certain level of fluctuation in the parameter to be measured is
usually present in the measuring instruments. In the case of the mass flow rate
of the nanofluid (mN), a fluctuation of ± 0.01 g s-1
is observed. For example, if
the required mN = 8 g s-1
, then the readings are logged between 7.99 g s-1
to
8.01 g s-1
, which is considered as a steady state condition for mN = 8 g s-1
.
Similarly, the fluctuation in the differential pressure of the nanofluid is
observed to be within ± 0.1 kPa. However, a fluctuation in the nanofluid and
cooling water temperature are practically not seen.
5.8.1 Test matrix
Based on the literature survey the variables for the experimentation
are fixed and the test matrix is shown in Table 5.1. The survey has revealed
that 4.3 mm tubes are often used in the small size heat exchangers in thermal
industries for building heating, industrial process heating and for solar heating
applications. The nanoparticles concentration is limited to be less than 1% in
order to avoid excessive pressure drop due to viscosity rise. The mass flow
rate chosen corresponds to the flow condition from the laminar, transition and
turbulent regimes for the 4.3 mm diameter tube that pertain to the Reynolds
number ranging from 800 to 12,000. The temperature of cooling water in
normal conditions range from 25oC to 35
oC and hence 25
oC, 30
oC and 35
oC
are considered as the cooling water inlet temperature with a flow rate of 16 g
s-1
for all conditions. The nanofluid inlet temperature is to be studied for 50oC
to 90oC range. However, since the nanofluid returning from the test section is
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also collected in the same tank (1 litre capacity) the nanofluid entry
temperature to the test section is also indirectly influenced by the temperature
of cooling water circulated. Thus the temperature of the nanofluid at steady
state is noted for all combinations of mass flow rates and cooling water
temperatures. The observed steady state nanofluid inlet temperatures ranged
from 40oC to 65
oC for the cooling water temperatures of 25
oC, 30
oC and 35
oC
respectively. Needless to say, that the heaters in both the tanks are also
regulated to achieve these conditions. Thus, the parameters in the test matrix
are rationally fixed based on the application temperature and the test facility
considerations.
Table 5.1 Test matrix for experimentation
Volume
Concentration
(%)
T(N1)
(oC)
T(W1)
(oC)mN (g s-1) Observations
Derived
Parameters
0.3
0.6
0.9
48 25 2
3
4
5
8
11
14
17
20
23
1.Temperature variations along
the test section (TN, TW)
2.Pressure drop across the inlet
and outlet of the test section
P)
1.Heat transfer
coefficient
2.Nusselt number
3.Heat flux
4.Friction factor
57 30 do
65 35 do
5.8.2 MATLAB – EXCEL interface for Data Reduction
The wall temperature of the nanofluid (TN), the cooling water
temperature (TW) measured across each subsection, the pressure difference at
the inlet and exit of the test section ( P), and the mass flow rate of the
working fluids (mN and mW) are noted. The data reduction procedure to
estimate the heat transfer coefficient and pressure drop is shown in Figure 5.7.
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START
W W W W1 W2Q = m C (T - T )
o i
i i N i W o
ln(d /d )1 1 1= + +
U A h A 2 kL h A
N
i i
i o i W o
1h =
2 kA L A1 1- -
U ln(d d ) h A
STOP
i W iU = Q A LMTD
0.8 0.3
W
kh = 0.023 Re Pr
D
Figure 5.7 Flow chart for data reduction
The heat load is calculated by estimating the change in internal
energy of the cooling water and balancing the energy transfer exchange
between the nanofluid and the cooling water. Then the overall heat transfer
coefficient is calculated by the logarithmic mean temperature difference
between the nanofluid and the cooling water. As the test section is insulated,
the cooling load applied to a known length of the test section is assumed to be
reflected proportionally in the change in temperature of the nanofluid across
the same length of the test section. The annular side heat transfer coefficient is
calculated from the Dittus-Boelter correlation (1962). Hence, finally, the heat
transfer coefficient of the nanofluid is calculated. For every test condition, the
temperature and pressure data are logged in continuously to check for
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attaining steady state operation. Once the test section stabilizes the data are
logged in for further processing. The experiment for each condition is
repeated for at least 2 times. The average results of the scans are taken as the
steady state reading for a given condition. The output spread sheet from the
interface is programmed to estimate and store the required results, such as the
heat transfer coefficient, the Reynolds number, heat flux, the Nusselt number
and the pressure drop etc. The MATLAB routine is also programmed to
compare the experimental results with the well known correlations, and
present the statistical information. Thus, a versatile software interface is
established between the experimental database, the MATLAB and EXCEL.
5.9 UNCERTAINTY ANALYSIS
The accuracy of the heat transfer coefficient depends on the
accuracy level of the measuring instruments. Therefore, the uncertainty of the
estimated heat transfer coefficient is to be ascertained. The error analysis for
the heat transfer coefficient is carried out by applying the uncertainty analysis
suggested by Moffat (1988). The heat flux (q) and its uncertainty are
estimated by using equations (5.1) and (5.2). The mass flow rate and
temperature of the nanofluid are the parameters considered for the uncertainty
analysis. The uncertainty of the heat transfer coefficient is calculated using
equations (5.3) and (5.4). For the operating conditions shown in Table 5.1, the
range of uncertainty is shown in Table 5.2. A sample of the model calculation
for uncertainty analysis is given in the Appendix 2.
N pN Nm C T
q = D L
(5.1)
2 222
N TN1 TN2
N N1 N2 N1 N2
Um U UUq
q m T T T T (5.2)
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NN WI
qh
T T (5.3)
WI
2 2 2
TN N N
N N N WI
UUh h hUq
h h q h T (5.4)
Table 5.2 Uncertainties of the measured and derived quantities
Parameters Uncertainty
Mass flow rate (mN) ± 0.1 %
Temperature (TN) ± 0.15 °C
Heat flux (qN) ± 1.18 % to ± 4.86 %
Heat transfer coefficient (hN) ± 4.44 % to ± 7.72 %
5.9.1 Repeatability test
To check the repeatability of the experimental setup, tests are
performed as prescribed in Ross et al (1987). Many repeatability tests are
conducted to further ascertain the credibility of the test facility, and the results
are compared on a day to day basis, as performed by Ross et al (1987).
A comparison of the silver/water nanofluid with 0.6 % volume fraction for
three mass flow rate conditions, namely, 14, 17 and 20 g s-1
is shown in
Figure 5.8. The deviation in the heat transfer coefficient between day1 and
day 2 lies within ± 1.5 %. This confirms that the repeatability and accuracy of
the test facility are acceptable.
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5
6
7
8
9
10
5 6 7 8 9 10
HT
C (
kW
m-2
K-1
)
HTC (kW m-2K-1)
Day 2
Day 1+10%
-10%
Figure 5.8 Repeatability test for 0.6 % volume concentration of
silver/water nanofluid
5.10 LONG TERM STABILITY TEST
A long term stability test is conducted to ensure that the
nanoparticles are stably suspended in the base fluid for a longer period. The
test is conducted for the mass flow rates of 14, 17 and 20 g s-1
with 0.6 %
volume fraction of silver/water nanofluid. The tests are repeated for the same
operating conditions at regular intervals. After conducting the experiment
with 0.6 % volume concentration of the silver/water nanofluid, the test facility
is switched OFF and kept for 15 days without any disturbance. After 15 days,
the experiments are repeated for the same test conditions. Then, the
experiments are repeated for the same condition after 30 days and 45 days.
Further, the measurements are taken continuously one after the other without
leaving any time delay between the first and the second reading for a given
inlet test conditions. Finally, the results from the initially conducted
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experiments (immediately after the preparation of the silver/water nanofluid
sample) are compared with those taken continuously, after 15 days after
30 days and after 45 days. The variations observed in the heat transfer
coefficient against the Reynolds number over this period are shown in
Figure 5.9.
5
5.5
6
6.5
7
7.5
8
6000 6500 7000 7500 8000 8500
HT
C
(k
W m
-2K
-1)
Reynolds number
Initial
After 15 Days
After 30 Days
After 45 Days
Continuous1
Continuous2
Figure 5.9 Long term stability test for 0.6 % volume concentration of
silver/water nanofluid
The results show that a deviation of 4.5 % is observed in the heat
transfer coefficients between the initially conducted experiments and the
reading taken after 15 days. Similarly, a deviation of 5.4% is observed for
readings taken after 30 days when compared with those of the initial ones and
4.7% deviation is observed for reading taken after 45 days and 1.07 %
deviation is observed in the continuously taken readings. The deviation
observed is less than 6%, which is normally accepted due to experimental
errors. Thus, from the obtained results it is clearly seen that the nanoparticles
81
are stably suspended in the base fluid over a longer period of time and hence
the heat transfer characteristics are also stable.
Based on the uncertainty analysis, repeatability and long term
stability tests, it is confirmed that the experimental facility has been fabricated
as per the standards and the procedure adopted for conducting the
experiments is also acceptable. Therefore, the experimental observation
results from this facility are considered for further data reduction to evolve the
heat transfer coefficient correlation, which could be used as a tool for
designing heat exchangers. A detailed discussion of the obtained results from
the experiments, comparisons of the measured data with the published results
and the mechanisms involved in the enhancement of heat transfer are
presented in chapter 7.