62

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.

63

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

64

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.

65

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

66

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

67

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

68

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

69

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.

70

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.

71

(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.

72

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.

73

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

74

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

75

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.

76

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

77

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)

78

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.

79

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

80

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.