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CHAPTER – 4

HEAT TRANSFER OF CuO NANOFLUIDS IN A PLAIN

TUBE

4.1 INTRODUCTION

Nanofluids are engineered by dispersion of fine metallic and non

metallic particles of nanometer dimension in traditional host liquids

which include water, ethylene glycol, propylene glycol, oil etc. Use of

such nanoparticles in the base fluids increase their thermal

conductivity and heat transfer performance of nanofluids. Nanofluids

are new generation heat transfer fluids and can be used for heat

transfer augmentations. Nanofluids have high heat transport

capability and can replace traditional thermo fluids normally used for

heat transfer applications in heat exchangers, chemical process

plants, manufacturing processes, automotives and cooling of

electronic components. Nanofluids are used in micro channel cooling

without any clogging and sedimentation problems. The nanofluids can

also be employed in high heat flux applications where single phase

pure fluids are not capable of transferring the heat at desired rate.

Nanofluids conserve energy and hence preferred over

conventional base fluids. Heat transfer augmentation using nanofluids

is one of the emerging areas of research. Generally conventional single

phase fluids have low thermal conductivities when compared to

metals and their oxides. The fluids with suspended particles of metals

and metal oxides are supposed to exhibit better heat transfer

properties than the conventional fluids without solid particles.

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Particles clogging, sedimentation and erosion are some of the common

problems associated with the use of micro or millimeter sized solid

particles when suspended in the host fluids. Such problems can be

minimized by replacing micrometer sized particles by nano sized

particles.

Many heat transfer augmentation techniques are reported in

literature. Heat transfer enhancement in fluids can be effected

primarily by two techniques viz. passive heat transfer technique and

active heat transfer technique. Passive heat transfer techniques can

be employed by provision of rough and extended surfaces tubes and

creation of swirl in the flow using inserts of certain geometrical shape.

Active heat transfer techniques include applying of electric/magnetic

fields, inducing vibrations in the heated surface, injection and jet

impingement of fluids etc.

The above techniques can hardly meet the requirements of high

heat transfer performance desired by present day modern heat

exchanger. Compact heat exchangers with higher performance

demand fluids having better heat transfer capabilities. Such devices

results in material saving, energy conservation and hence low cost of

heat exchangers. Nanofluids improve thermal conductivity of host

fluids and now become important area of research attracting the

attention of many researchers across the world. The nanofluids will

quench the thirst of investigators who are in quest to engineer better

heat transfer fluids.

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Heat transfer coefficient and friction factor are two important

parameters associated with thermo fluids. Many experimental as well

as theoretical investigations have been carried out to study heat

transfer and pressure drop characteristics of pure fluids. Use of two

phase nanofluids for heat transfer enhancement has boosted the

research interest among many research groups across the globe.

Literature confirmed that nanofluids give higher heat transfer

coefficient compared to the base fluid. The investigation results on

nanofluids indicated that heat transfer coefficient increases with the

increase of nanoparticle concentration in the base fluid.

Most of the research works done so far on nanofluids are

experimental studies and confined either to laminar or turbulent flow

conditions. The host or base fluid is water in majority of the cases. In

severe cold climatic conditions glycols are added to water in different

proportions to reduce the freezing point of heat transfer liquids. Glycol

based fluids are used in base board heaters, automobile radiators and

process plants particularly in cold countries where the ambient

temperatures are below zero degree Celsius.

4.2 HEAT TRANSFER EXPERIMENTAL SET UP AND PROCEDURE

The aim of the present experimental investigation is to estimate

heat transfer coefficient and friction factor of CuO nanofluids. To

carryout the experiments, three different CuO nanofluids in the

volume concentration of 0.025%, 0.1% and 0.5% are carefully

prepared using an anti-freezing water and propylene glycol (70:30 by

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volume percent) blend as the base fluid. The photographic view of

base fluid and CuO nanofluids are shown in the Plate.4.1. Stable

nanofluids in required quantity are prepared.

Plate 4.1 Photographs showing preparation of base fluid and CuO

nanofluids

The experimental set up is designed and fabricated to conduct

experiments using the base fluid as well as CuO nanofluids of all the

three volume concentrations and by allowing the fluids to flow in a

circular plain tube. The objective of the present experimental

investigation is to study heat transfer coefficient and friction factor

characteristics of base fluid and CuO nanofluids both in laminar flow

and transition flow regimes under constant heat flux boundary

conditions. The schematic diagram of the experimental setup is

represented in the Fig.4.1. The photographic view of the experimental

setup is shown in Plate.4.2. The experimental setup constitutes a flow

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Fig.4.1 Schematic diagram of the experimental setup

Plate.4.2 Photographic view of the experimental setup

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loop which includes different parts viz. temperature, pressure and flow

rate measuring sections. It is also provided with heating and cooling

section. A control valve in the loop regulates the fluid flow in the test

section. The test section is 1.7m long and the test section tube is

made of copper material and having the dimensions of 0.0140 m

internal diameter, 2 m long, 1 mm tube thickness.

Five K-type RTD thermocouples are soldered on the outer

surface of the tube along the test section with an equal distance

between the thermocouples. The thermocouple leads are properly

insulated. The tube is wrapped with a thin fiber-glass sheet to

electrically isolate it from the heater coils. Nichrome heater coils are

wound around the test section tightly with the help of fiber-glass

insulation material. The maximum rating of the heater coil is 1000 W.

The test section of the tube is heated by flexible electrical heater coils

which give uniform heat flux boundary conditions. The electrical input

to the test section is regulated by a variable transformer to give a

constant heat flux along the length of the test section. To prevent heat

losses, the tube is covered with an insulating tape and then wound

with an asbestos rope of 3 mm diameter. The test section is kept in a

square casing and the space between the test section and casing is

stuffed compactly with rock wool insulating material to prevent heat

transfers to the surrounding atmosphere. Two more thermo couples

are fitted one at each end of the tube to measure inlet and outlet

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temperature of the fluids. The thermocouples used have a resolution

of 0.1°C.

The aspect ratio of the test section is long enough to give a

hyderodynamically developed flow. The experimental set up is

equipped by a centrifugal pump. The fluid after passing through

heated test section is discharged into the chiller tank .The chiller tank

aids fluids to attain a steady state condition at faster rate. The fluid

from the chiller is then falls in the storage tank due to gravity. The

centrifugal pump is operated to pump the fluid in the test section. The

flow is regulated by a dimmer stat. The mass flow rate of fluid is

measured based on the time taken to collect the known amount of

fluid. The fluid under investigation flows in a loop continuously. Two

pressure taps are inserted at the ends of the test section. The two

ends of pressure taps are connected to the two ends of the U-tube

manometer with the help of flexible tubes. The manometer is filled

with carbon tetra chloride for the laminar flow and mercury in the

used for turbulent flow conditions.

The energy balance is made between the electrical energy

supplied to the heating coil and the heat absorbed by CuO nanofluids

using Equation (4.1) and Equation (4.2). The experiments are repeated

till satisfactory values are obtained for electrical and heat energies.

The deviations between the electrical and heat energy is found

negligible. The accuracy and reliability of the data generated by

experimental set up is checked by conducting the experiment first

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with distilled water. The average experimental heat transfer coefficient

of different fluids used in the present work is calculated by Eq. (4.3).

The average Nusselt number is estimated by using Eq. (4.4). The

properties of CuO nanofluids are calculated at mean bulk temperature

and used for estimation of nanofluid experimental Nusselt number.

Electrical energy supplied to the heating coils = IVQ (4.1)

Heat energy absorbed by the fluids = ip TTCmQ 0 (4.2)

Experimental heat transfer coefficient of single phase fluid

meanwall

ExpTTA

Qh

(4.3)

Where LDA ; 5

TTwall ;

2

io

mean

TTT

k

DhNuExp (4.4)

4.3 NANOFLUID HEAT TRANSFER MEASUREMENT

The thermocouples were calibrated before conducting

experiments, using ice and boiling water which corresponds to 0 mv

and 4.27 mv respectively. Zero error was established for all the

thermocouples from the linear fit. The nanofluids inside the test

section are heated by giving a constant heat flux 8030 W/m2

calculated based on the test section outer diameter and electrical

power input supplied to the heating coils. The fluid attains a steady

state condition after about one hour time for a laminar flow and in

about 45 minutes for turbulent flow. Under the steady state

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conditions, the amount of electrical energy supplied to the test section

is assumed to be equal to the heat energy absorbed by the working

nanofluids. The local average temperature of tube wall is measured by

five thermocouples brazed on the test section outer surface. The inlet

and outlet temperature of nanofluids are measured by two

thermocouples inserted in tube inlet and outlet. The properties of

working fluids are evaluated at mean bulk temperature of concerned

fluids. The temperature data of all the CuO nanofluid concentrations

is recorded by a data logger and retrieved later and used for

estimation of heat transfer coefficients of nanofluids.

The uncertainty analysis of the parameters involved in the

present investigations is carried out by following the prescribed

procedure as outlined by Beckwith.

Gnielinski (1976) has developed a correlation for prediction of

Nusselt number for single phase fluids in the transition and turbulent

flow regimes and is given by the following Equation.

1Pr2

7.121

Pr1000Re2

32

5.0f

f

Nu , where 282.3Reln58.1

f (4.5)

The above equation (4.5) is valid for the values of

6105Re2300 X and 2000Pr5.0 .

Dittus-Boelter (1930) also developed a correlation for estimation

of Nusselt number for pure water and is given by the Eq. (4.6).

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4.08.0 PrRe023.0Nu (4.6)

The above equation (4.6) is valid for the values for 410Re and

200Pr6.0 .

4.4 NANOFLUID FRICTION FACTOR ESTIMATION

The pressure drop in the fluid flowing across the test section is

estimated based on the height of manometric liquid column in the U-

tube manometer. The experiment is conducted for different mass flow

rates of nanofluids. The resolution in the flow meter reading is ±0.1

lt/min. The difference in the height of the barometric liquid columns

in the U-tube manometer is a measure of pressure drop in the fluid.

The relationship between friction factor and Reynolds number is

established by the pressure drop in the flow and the average fluid

velocity V. The pressure drop in the test section is estimated in terms

of friction factor, the tube dimensions and fluid flow velocity using the

following equation (Eq.4.7).

2

2V

D

LCghP

i

f

(4.7)

fCf 4 Where Cf Darcy friction coefficient

Friction factor is now defined by the following equation (Eq. 4.8)

f =22

1

V

D

L

P i

(4.8)

The average velocity V of the fluid is calculated using the relation V =

m/A. where ‗m‘ is the discharge or mass flow rate of fluids and ‗A‘ is

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nominal area of cross–section, given by A =4

2

iD. Based on the tube

nominal diameter the Reynolds number is defined by the Eq. (4.9).

i

eD

mR

4 (4.9)

Blasius (1908) developed a correlation for friction factor which is

valid in the flow range varying from Reynolds number 300 to 510 and is

given by the Eq. (4.10).

25.0Re31640 /.f (4.10)

Moody‘s (1944) equation for single phase fluid for flow in a tube

is given by

Re

64f (4.11)

4.5 RESULTS AND DISCUSSION

The Nusselt number and friction factors of the base fluid and CuO

nanofluids are calculated and explained in the following sections.

4.5.1. Nusselt number of the base and CuO nanofluids in plain

tube

The energy balance between the heat supplied to the test

section and heat absorbed by the fluid flowing in the test sections is

made. The experiments were then conducted for base as well as CuO

nanofluids and the temperature recorded by all the thermocouples

was noted. Before conducting the experiments for estimation of

Nanofluid heat transfer and hence the Nusselt number, the reliability

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of the fabricated experimental setup is checked by conducting

experiments with pure water. The experimental heat transfer

coefficient is estimated by using Eq. (4.3) and the experimental

Nusselt number is then computed using the Eq. (4.4)

The experimental Nusselt number results obtained for water are

compared with the Nusselt correlations of Gnielinski given by Eq. (4.5)

and correlation of Dittus–Boelter given by Eq. (4.6), as shown in the

Fig.4.2. The result clearly shows that the experimental Nusselt

numbers of the present work are closely matching with both the

Nusselt correlations. This indicates that the fabricated experimental

setup is a reliable one and can be used to generate experimental data.

After ensuring the experimental reliability, experiments are

carried out with the base fluid as well as CuO nanofluids of all the

three concentrations under investigation one after the other in the

Reynolds number ranging from 1000 to 10000 under. The CuO

nanofluids are allowed to flow in a circular plain tube with a constant

heat flux as the boundary conditions. After ensuring steady condition,

the temperatures are noted. The average experimental connective heat

transfer coefficients and experimental Nusselt number for all the CuO

nanofluids are estimated using the thermo physical properties of

nanofluids taken at bulk mean temperature. The experimental for

Nusselt number at different mass flow rates are shown in the Fig. 4.3.

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Fig.4.2 Comparison of experimental Nusselt number of water with

correlations

Fig.4.3 Experimental Nusselt number of CuO Nanofluid Vs

Reynolds number for different volume concentrations

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It is observed from the results that Nusselt number increases

with increase of Reynolds number and also with increase in the CuO

nanoparticle volume concentration in the base fluid. The

enhancement in the heat transfer as predicted by Prasher et al (2006)

can be attributed to high thermal conductivity of nanofluids.

Increased surface areas of nanoparticle, intense forced convection

accompanied by Brownian motion of nanoparticles in the vicinity of

tube wall are other reasons for heat transfer enhancement.

The experimental data obtained is subjected to the regression

analysis and a correlation equation to predict the Nusselt number of

glycol based CuO nanofluids flowing in a circular plain tube is

developed and is given by Eq. (4.12 )

2307.04.059106.0

Re 1PrRe1168.0 gNu (4.12)

The Eq. (4.12) is valid in the Reynolds number range of

,10000Re1000 5.00 %, 97.18Pr56.11 .

A parity graph is drawn between the predicted regression

equation given by Eq.(4.12) and the experimental Nusselt number and

is shown in Fig. 4.4. For the transition flow of CuO nanofluids in the

present experimental work, the Nusselt correlation has an average

deviation of 4.72% and standard deviation of 5.64 %.

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Fig.4.4 Comparison of predicted and experimental Nusselt number of

CuO nanofluids for different volume concentrations

4.5.2 CuO nanofluid Friction factor in a plain tube

The reliability of the present experiment is also tested for

friction factor calculations. The experiment is conducted using pure

water and the experimental friction factor of water is calculated by Eq.

(4.8) and is compared with Blasius equation for friction factor given by

Eq.(4.10) and Moody‘s friction factor equation given by Eq. (4.11). The

Reynolds number is calculated by Eq. (4.9), based on the mass flow

rate of fluids. The experimental friction factor results of pure water are

found to be in closer agreement with the with the friction factor

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correlations of Blasius and Moody and the same can be observed in

the Fig.4.5. This ensures reliability and accuracy in the

measurements of experimental data for CuO nanofluid friction factor.

Fig.4.5 Comparison of experimental friction factor of water with

Moody and Blasius equations for experimental reliability verification

Experiments are then conducted with the water-propylene glycol

base fluid and CuO nanofluids one after the other. The friction factor

of base fluid and CuO nanofluids are computed using Eq. (4.8). The

friction factor value diminishes as mass flow rate of nanofluids

increases and the same is evident from Fig.4.6, which shows variation

of CuO nanofluids friction factor with Reynolds number. It can also be

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Fig.4.6 Variation of experimental friction factor of CuO nanofluid with

Reynolds Number for different volume concentrations

observed from the results that a little increment in the friction factor

of nanofluids over the base fluid is observed. The friction factor with

0.5% volume concentration is slightly high because of increase in the

density of the Nanofluids fluids over other concentrations

considered.However the magnitude of nanofluid friction factor is

negligible. The variation of friction factor with Reynolds number in

laminar flow is higher when compared to the friction factor in

transition flow. Hence two different regression equations are developed

to predict CuO nanofluid friction factor in laminar as well as

transition flow regimes. The experimental friction factor and

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regression friction factor are in perfect linear relationship for the

laminar and transition flow and is shown by the Fig.4.7 and Fig.4.8

respectively.

Fig.4.7 Comparison of predicted and experimental friction factor of CuO nanofluids for laminar flow

Based on the frictional factor data obtained in the experiment

for the base fluid and CuO Nanofluids flowing in a circular plain tube,

a regression equation to predict friction factor in laminar flow

conditions is developed and is given by Eq. (4.13).

1720.08456.0

Re 1Re08.24

gf

(4.13)

The Eq. (4.13) is valid for 2500Re1000 , 5.00

A parity graph is drawn between the predicted regression

equation given by Eq.(4.13) and the experimental friction factor and is

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shown in Fig. 4.7. For the laminar flow of CuO nanofluids in a circular

plain tube, the Nusselt correlation has an average deviation (AD) of

3.86% and standard deviation (SD) of 4.54%.

Similarly, based on the experimental data obtained for frictional

factor in the transition regimes, a regression equation is developed to

predict the friction factor and is given by Eq. (4.14).

2129.02279.0

Re 1Re2753.0

gf (4.14)

The equation (4.14) is valid for transition flow for the values in the

range of 10000Re2500 , 2.6Pr4.4 , and 5.00 .

Fig. 4.8 Comparison of predicted and experimental friction factor

and CuO nanofluids for transition flow

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A parity graph is also drawn between the predicted correlation

for friction factor given by Eq. (4.14) and the experimental friction

factor and the results are shown in Fig. 4.8. For the transition flow of

CuO nanofluids in a plain circular tube, the friction factor correlation

has an average deviation of 2.15% and standard deviation (SD) of

2.74%.