analysis of overall heat transfer coefficient and … of overall heat transfer coefficient and...

Journal of Advances in Mechanical Engineering and Science (JAMES)

doi:10.18831/james.in/2015031004 28

Analysis of Overall Heat Transfer Coefficient and Effectiveness in

Split Flow Heat Exchanger using Nano Fluids

*T Aseer Brabin

1,

#S Ananth

2

1Principal, Universal College of Engineering and Technology, Vallioor, Tamilnadu, 627117, India.

2PG Scholar, Department of Mechanical Engineering, Universal College of Engineering and

Technology, Vallioor, Tamilnadu, 627117, India. *[email protected],+919443606238,

#[email protected], +919092585350.

ABSTRACT

Heat exchanger plays a major role in industrial process heating. Heat is transported

among liquids by conduction and convection over the partitions of the heat exchanger. Heat transfer

solutions have little thermal conductivity that significantly bounds the heat exchanger

competence. Many research activities are carried out to improve the thermal properties of fluids

by adding thermally conductive solids into liquids. Nanofluids are suspensions of nanoparticles in the

base fluid which is a new challenge in the field of thermal science provided by nanotechnology. In this

paper an experimental setup of split flow heat exchanger was designed and fabricated. Heat exchanger

performance is analyzed by adding Aluminium oxide (Al2O3) with the base fluid (tap water, distilled

water and a mixture of distilled water and ethylene glycol).The thermal properties like overall heat

transfer coefficient and effectiveness of the heat exchanger were calculated and compared with the

results of base fluid (water).

Keywords: Aluminium oxide, Heat transfer coefficient, Split flow heat exchanger, Effectiveness,

NTU, LMTD.

1. INTRODUCTION

Heat exchangers are devices that

facilitate the exchange of heat between two

fluids that are at different temperatures without

direct contact of fluids. Heat exchangers are

commonly used in practice in a wide range of

applications, from heating and air-conditioning

systems in a household to chemical processing

and power production in large plants. Heat

exchangers differ from mixing chambers in the

way that they do not allow the two fluids

involved to mix with each other. For example,

in a car radiator, heat is transferred from the

hot water flowing through the radiator tubes to

the air flowing through the closely spaced thin

plates attached outside of the tubes. Heat

transfer in a heat exchanger usually involves

convection in each fluid and conduction

through the wall separating the two fluids. In

the analysis of heat exchangers, it is

convenient to work with an overall heat

transfer coefficient U that accounts for the

contribution of all these effects on heat

transfer. The rate of heat transfer between the

two fluids at a location in a heat exchanger

depends on the magnitude of the temperature

difference at that location, which varies along

the heat exchanger. In the analysis of heat

exchangers, it is usually convenient to work

with the logarithmic mean temperature

difference LMTD, which is an equivalent

mean temperature difference between the two

fluids for the entire heat exchanger.

1.1. Classification

The heat exchangers are classified in

to four types, according to the following

Nature of heat exchanger process

Relative direction of motion of

fluids

Mechanical design of heat

exchanger surface

Physical state of heat exchanging

1.2. Nanofluids

Over the past decade, nanofluids,

which are liquids containing suspensions of

nanoparticles have been reported to possess

substantially higher thermal conductivity than

anticipated from the effective medium theories.

This makes them very attractive for the usage

as heat transfer fluids in many applications.

For example, nanofluids would be useful as

coolants in automobile and electronics

industries. However, the reported high thermal

conductivity sometimes cannot be reproduced,


29

and the potential mechanisms leading to the

enhancement are still under scrutiny. By suspending nanophase particles in

heating or cooling fluids, the heat transfer

performance of the fluid can be significantly

improved. The main reasons may be listed as

follows:

1. The suspended nanoparticles increase

the surface area and the heat capacity

of the fluid.

2. The suspended nanoparticles increase

the effective (or apparent) thermal

conductivity of the fluid.

3. The interaction and collision among

particles, fluid and the flow passage

surface are intensified.

4. The mixing fluctuation and turbulence

of the fluid are intensified.

5. The dispersion of nanoparticles

flattens the transverse temperature

gradient of the fluid.

2. LITERATURE REVIEW

Comprehensive reviews of published

papers are available in the open literature

related to the application of nanofluids in heat

transfer.

[1] have carried out an experimental

study on the effect of nanofluid on heat

transfer characteristics of double pipe heat

exchanger with the effect of aluminium oxide

nanofluid. An experimental investigation is

carried out to determine the effect of various

concentrations of Al2O3nano-dispersion mixed

in water as base fluid on heat transfer

characteristics of double pipe heat exchanger

for parallel flow and counter flow

arrangement. The volume concentrations of

Al2O3nanofluid prepared are 0.001 % to 0.01

%. The conclusion derived for the study is that

overall heat transfer coefficient increases with

increase in volume concentration of

Al2O3nano-dispersion compared to water up to

the volume concentration of 0.008 % and then

decreases.

[2] have carried out an experimental

study in order to find out the effects of silicon

nitride nanofluid, having nanoparticles

concentration of 0.1% by volume on heat

transfer, mass flow rate, effectiveness, LMTD

of a shell and tube heat exchanger.

[3] have performed an experimental

and numerical study on-Al2O3/water

nanofluid flowing through the double pipe

counter flow heat exchangers under laminar

flow conditions. They have found that the heat

transfer performance of both the double pipes

increases with increase in the hot and cold

volume flow rates as well as the particle

concentrations and nanofluid inlet temperature

compared to pure water.

[4] have investigated the heat transfer

of a fluid containing nanoparticles of

aluminum nitride with a diameter of about 20

nm, with the water volume fraction (0.1 –0.3)

percent in a horizontal double pipe counter

flow heat exchanger under turbulent flow

conditions. They have found that heat transfer

of nanofluid in comparison with the heat

transfer of fluid is slightly higher than 9%.

[5] have analysed the empirical

correlations associated with the previous

research papers and gave the correlations for

thermal conductivity, density, Nusselt number,

Reynolds’s number and viscosity of the

nanofluid.

[6] studied the fluid flow and heat

transfer characteristics of nanofluids in forced

and free convection flows and potential

applications of nanofluids.

[7] have given the various methods for

preparation of nanofluids and its stability

mechanisms and effects of surfactants on

nanofluids.

[8] studied the various applications of

nanofluids. The fields include heat transfer

applications, automotive applications,

electronics applications, biomedical

applications etc.,

[9] made experimental study on the

forced convective heat transfer and flow

characteristics of a nanofluid consisting of

water and different volume concentrations of

Al2O3nanofluid (0.3–2)% flowing in a

horizontal shell and tube heat exchanger

counter flow under turbulent flow conditions

are investigated.

[10] analysed the performance of a

counter flow micro channel heat exchanger

CFMCHE with a nanofluid as a cooling

medium. Two types of nanofluids used are Cu-

water and Al2O3-water. From the results

obtained, they found that the thermal

performance of CFMCHE increased with

nanofluids as a cooling medium with no extra

increase in pressure drop due to the ultra fine

solid particles and low volume fraction

concentrations.


30

[11] presented an overview of the

recent investigations in the study of the thermo

physical characteristics of nanofluids and their

role in heat transfer enhancement from heat

exchangers. General correlations for the

effective thermal conductivity, viscosity and

Nusselt number of nanofluids are presented.

[12] studied the applications of

nanofluids by focusing on the previous

research papers and concluded the importance

of nanofluids in various fields by providing

some experimental results.

[13] investigated the thermal

performance of a shell and tube heat

exchanger using nanofluids and found that

the effectiveness was increased by a

considerable amount, while the convective and

overall heat transfer coefficient increases even

further with the addition of 3% Al2O3

nanoparticles in water based fluid.

[14] studied the Overall Heat Transfer

Coefficient of Nano Fluids (OHTCNF) in heat

exchangers and the relevant effective

parameters. They reported an improvement in

Heat Transfer (HT) and OHTCNF containing

nanoaluminum oxide with ca. 20 nm particle

sizes and particular volume fraction in the

range of 0.001-0.002. They also studied the

effects of temperature and concentration of

nanoparticles on HT variation as well as

Overall Heat Transfer Coefficient (OHTC) in a

counter current double tube heat exchanger

with turbulent flow.

[15] investigated the thermal

characteristics of turbulent nanofluid flow in a

rectangular pipe. Four different types of

nanoparticles Al2O3, ZnO, CuO and SiO2 at

different volume fractions of nanofluids in the

range of 1% to 5% are considered in this

investigation.

[16] investigated the enhancement of

heat transfer coefficient and Nusselt number of

a nanofluid containing nanoparticles (𝛾-

AL2O3) with a particle size of 20 nm and

volume fraction of 0.1%–0.3% (V/V). Effects

of temperature and concentration of

nanoparticles on Nusselt number changes and

heat transfer coefficient in a double pipe heat

exchanger with counter turbulent flow are

investigated.

In [17] experimental investigations

were made on heat transfer and pressure drop

characteristics of water based CuO nanofluid

inside a horizontal tube. The upper limitation

of the particle volume fraction with respect to

heat transfer performance was also found.

CuO-water nanofluids with volume fractions

of 0.5%, 1%, 2%, and 4% were prepared by

dispersing the CuO nanoparticles with an

average diameter of 33 nm into deionised

water.

[18] investigated turbulent forced

convection of 𝛾-Al2O3/water nanofluid in a

concentric double tube heat exchanger using

mixture two-phase model. Nanofluids are used

as coolants flowing in the inner tube while hot

pure water flows in the outer tube. The studies

are conducted for Reynolds numbers ranging

from 20,000 to 50,000 and nanoparticle

volume fractions of 2, 3, 4, and 6 percent.

In [19] the forced convective heat

transfer in a water based nanofluid has been

experimentally compared to pure water in an

automobile radiator. Five different

concentrations of nanofluids in the range of

0.1-1 vol. % have been prepared by the

addition of

TiO2 nanoparticles into the water. Results

demonstrate that increasing the fluid

circulating rate can improve the heat

transfer performance. Meanwhile, application

of nanofluid with low concentrations can

enhance heat transfer efficiency up to 45% in

comparison with pure water.

[20] reported the theoretical study on

the heat transfer and flow characteristics of

nano fluids consisting of water flowing in a

horizontal shell and tube heat exchanger. The

Al2O3 nano particles are used in the present

study. The result show that the heat transfer

coefficient of nano fluid is slightly higher

than that of the base liquid at same mass flow

rate and at same temperature.

In [21] literature survey was made

which gives the techniques for implementation

of nanofluids in a car radiator for cooling the

engine.

[22] showed the review of various

research papers in nanofluids and concludes

that nanofluids have great potential for heat

transfer enhancement and are highly suited

for application in heat transfer processes.

[23] presented a short review on the

heat transfer enhancement of a car radiator by

using nano fluids.

[24] studied the enhancement of heat

transfer in a solar collector using nano-fluid.

They also presented a review of various

research papers on solar collectors.


31

[25] studied the heat transfer

characteristics of nanofluids mixed with base

fluids in a certain proportion flowing in a tube,

under constant heat flux boundary conditions

in laminar and transition flow regimes.

Experiment shows that the 3% volume

concentration gives highest value of %

increment in convective heat transfer

coefficient when compared to 2% and 4%

volume concentration.

The review of exising literature reveal

the facts related to the application of nanofluid

in heat exchangers for the enhancement of heat

transfer rate. They can be summarized as

follows. Nanofluids are relatively practiced

recently to enhance the heat transfer rate.

There are different types of nanofluids which

can be used to enhance the heat transfer rate

and many researchers have carried out

numerical and experimental analysis on the

application of nanofluid in the enhancement of

heat transfer rate under different conditions.

Extensive experiments have been carried out

on double pipe heat exchanger to determine the

effect of different type of nanofluids (having

nanoparticles of Al2O3 and CuO.) and

concentration of nanoparticles in nanofluid

during parallel and counter flow condition. The

present work is focused on the analysis of

overall heat transfer coefficient and

effectiveness in split flow heat exchanger using

nano fluids.

3. EXPERIMENTAL SETUP

The setup consists of an inner tube and

an outer shell. The inner tube is made of

copper and it has a length of 400mm and a

diameter of 10mm. The outer shell is made of

stainless steel and it has a length of 300mm

and diameter of 50mm. Figure B1 shows the

fabricated split flow heat exchanger and figure

B2 shows the experimental set up used for the

analysis. Four digital temperature sensors are

used to measure the inlet and outlet

temperature of the hot and cold water. Flow

control valve is used to control the flow of

fluids. Two submersible pumps are immersed

in water tanks to pump the hot fluid in to the

tube and cold fluid in to the shell.

3.1. Shell

Shell is the outermost part of the heat

exchanger in which the cold fluid carrying

nanoparticles flows and it consists of an inner

port and an outer port. The cold fluid enters the

input port and receives heat from the hot fluid

and exits via the outer port. The inner port and

outer port has a diameter of 12mm and it is

fitted in the centre part of the shell. The

material of the shell is stainless steel.

3.2. Tube

The inner tube rests inside the shell

and it gives heat to the cold fluid. The hot fluid

from the water tank enters in to the tube and

exits via the exit port. The inlet and outlet

temperatures are recorded by the digital

temperature sensor by means of a probe

attached at the inlet and outlet ports. The tube

has a diameter of 10mm. The material of the

inner tube is copper.

4. ANALYSIS OF HEAT EXCHANGERS

[26] used two methods for the analysis

of heat exchangers viz the Logarithmic Mean

Temperature Difference (LMTD) and NTU-

Effectiveness Method. The analysis is shown

by means of equations from (4.1) to (4.22)

Using first law of thermodynamics, the

rate of heat transfer from the hot fluid is equal

to the rate of heat transfer to the cold one. That

is,

( ) (4.1)

( ) (4.2)

where the subscripts c and h stands for

cold and hot fluids respectively.

, – mass flow rates of cold and hot

fluids in kg/s.

, – Specific heat rates for cold and hot

fluids in

, – inlet temperatures in

, – outlet temperatures in

It is often convenient to combine the

product of the mass flow rate and the specific

heat of a fluid into a single quantity in heat

transfer analysis. This quantity is called the

heat capacity rate and is defined for the hot and

cold fluid streams as

= (4.3)

= (4.4)

Therefore, heat transfer rate in terms

of heat capacity rate becomes

( ) (4.5)

( ) (4.6)


32

In a heat exchanger the rate of heat

transfer can also be expressed in an analogous

manner to Newton’s law of cooling as

(4.7)

where is the overall heat transfer

coefficient in

, is the heat transfer

area in , is an appropriate average

temperature difference between the two fluids.

4.1. Logarithmic Mean Temperature

Difference(LMTD)

The heat transfer rate equation is given

by

(4.8)

where

( ) (4.9)

For parallel flow heat exchangers

(4.10)

(4.11)

For counter flow heat exchangers

(4.12)

(4.13)

For cross flow heat exchangers

= (4.14)

is known as the log mean

temperature difference, which is the suitable

form of the average temperature difference for

use in the analysis of heat exchangers. Here

and represent the temperature

difference between the two fluids

4.2. The Effectiveness – NTU method

It is easy to use in heat exchanger

analysis when the inlet and outlet temperatures

of the hot and cold fluids are known or can be

determined from an energy balance when we

discuss the method in log mean temperature

difference (LMTD). Once ∆Tm, the mass flow

rates, and the overall heat transfer coefficient

are available, the heat transfer surface area of

the heat exchanger can be determined from

(4.15)

Therefore, the LMTD method is very

suitable for determining the size of a heat

exchanger to realize the prescribed outlet

temperatures when the mass flow rates and the

inlet and outlet temperatures of the hot and

cold fluids are specified. With the LMTD

method, the task is to select a heat exchanger

that will meet the prescribed heat transfer

requirements. The procedure to be followed by

the selection process is as follows:

1. Select the type of heat exchanger

suitable for the application.

2. Determine any unknown inlet or outlet

temperature and the heat transfer rate

using an energy balance.

3. Calculate the log mean temperature

difference ∆Tm and the correction

factor F, if necessary. Obtain (select or

calculate) the value of the overall heat

transfer coefficient U.

4. Calculate the heat transfer surface area

.

The task is completed by selecting a

heat exchanger that has a heat transfer surface

area equal to or larger than . A second kind

of problem encountered in heat exchanger

analysis is the determination of the heat

transfer rate and the outlet temperatures of the

hot and cold fluids for prescribed fluid mass

flow rates and inlet temperatures when the type

and size of the heat exchanger are specified.

The heat transfer surface area A of the heat

exchanger in this case is known, but the outlet

temperatures are not. Here the task is to

determine the heat transfer performance of a

specified heat exchanger or to determine if a

heat exchanger available in storage will do the

job.

For this alternative problem, the

LMTD method could still used. It is not

practical because the procedure would require

tedious iterations. In 1955, Kays and London

came up with a method called the

effectiveness-NTU method, which greatly

simplified heat exchanger analysis in an

attempt to eliminate the iterations from the

solutions of such problems.

This method is based on a

dimensionless parameter called the heat

transfer effectiveness , defined as the ratio of

actual heat transfer rate to the maximum

possible heat transfer rate.

(4.16)

The actual heat transfer rate in a heat

exchanger can be determined from an energy

balance on the hot or cold fluids and can be

expressed as

( ) ( )

(4.17)

To determine the maximum possible

heat transfer rate in a heat exchanger, we first

recognize that the maximum temperature

difference in a heat exchanger is the difference

between the inlet temperatures of the hot and

cold fluids. That is,

(4.18)


33

The heat transfer in a heat exchanger

will reach its maximum value when (1) the

cold fluid is heated to the inlet temperature of

the hot fluid or (2) the hot fluid is cooled to the

inlet temperature of the cold fluid. These two

limiting conditions will not be reached

simultaneously unless the heat capacity rates of

the hot and cold fluids are identical (i.e.,

).When , which is usually the

case, the fluid with the smaller heat capacity

rate will experience a larger temperature

change, and thus it will be the first to

experience the maximum temperature, at

which point the heat transfer will come to a

halt. Therefore, the maximum possible heat

transfer rate in a heat exchanger is

( ) (4.19)

where is the smaller of

= and .

Effectiveness relations of the heat

exchangers typically involve the dimensionless

group . This quantity is called the

number of transfer units NTU and is expressed

as

(4.20)

where the overall heat is transfer

coefficient and is the heat transfer surface

area of the heat exchanger. Note that is

proportional to .Therefore, for specified

values of and , the value of is a

measure of the heat transfer surface area . Thus, the larger the , the larger is the heat

exchanger.

In heat exchanger analysis, it is also

convenient to define another dimensionless

quantity called the capacity ratio c as

(4.21)

The effectiveness of the heat

exchanger for cross flow heat exchanger with

both fluids unmixed is given by the equation

* , ( ) + (4.22)

5. RESULTS AND DISCUSSION

The experimental analysis is carried

out in split flow heat exchanger using tap

water, distilled water and a mixture of distilled

water and ethylene glycol. The measured

temperatures of hot water outlet and cold water

outlet by keeping constant temperature hot

water inlet are presented in table 1, 2 &3.

Table A1, A2 & A3 shows calculated results of

mass flow rate, heat transfer rate, LMTD,

overall heat transfer coefficient, NTU and

effectiveness. From tables A1, A2 and A3 it is

found that heat transfer rate, LMTD, overall

heat transfer coefficient, NTU and

effectiveness decreases with decrease in

temperature. From these tables it is also found

that the effectiveness of heat exchanger

increases for a mixture of ethylene glycol and

distilled water compared to tap water and

distilled water. Thus it is evident from these

results that the use of nano fluids increases the

effectiveness of heat exchanger. Table 1.Tap water as a cooling fluid

Table 2.Distilled water as a cooling fluid

Table 3.Mixture of Ethylene glycol and Distilled

water as a cooling fluid

Time taken for

One litre water

collection(litre

s/sec)

42 44 45 46

Hot water inlet

( ) 90 80 70 60

Cold water

inlet ( ) 30 30 30 30

Hot water

outlet

( )

78 70 63 55

Cold water

outlet

( )

37 36 34 33

Time taken for

One litre water

collection (litres/sec)

40 42 43 44

Hot water inlet

( ) 90 80 70 60

Cold water inlet ( ) 30 30 30 30

Hot water outlet

( ) 85 77 68 59

Cold water outlet

( ) 33 32 31 31

Time taken for

One litre water

collection

(litres/sec)

38 39 39 40

Hot water inlet

( ) 90 80 70 60

Cold water inlet

( ) 30 30 30 30

Hot water

outlet

( )

83 75 66 58

Cold water

outlet

( )

34 34 33 32


34

Graph B1 shows the relation between

temperature and heat transfer rate. It is found

that heat transfer rate is high in the mixture of

ethylene glycol and distilled water compared

to tap water and distilled water. It also found

that heat transfer rate is decreasing with

decrease in temperature.


temperature and overall heat transfer

coefficient. It is found that overall heat

transfer coefficient is high in the mixture of

ethylene glycol and distilled water compared

to tap water and distilled water. It is also

found that overall heat transfer coefficient is

decreasing with decrease in temperature.


temperature and effectiveness. It is found that

effectiveness is high in a mixture of ethylene

glycol and distilled water compared to tap

water and distilled water. It is also found that

effectiveness is decreasing with decrease in

temperature.

6. CONCLUSION

In this work, convective heat transfer

of tap water, distilled water and a mixture of

ethylene glycol and distilled water is analysed

with various parameters such as mass flow

rate, heat transfer rate, overall heat transfer

coefficient and effectiveness with constant

temperature in split flow heat exchanger. The

results show that the heat transfer

characteristics is increased/enhanced for a

mixture of ethylene glycol and distilled water

as a cooling fluid compared to clean water.

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36

APPENDIX A

Table A1.Heat transfer characteristics for a tapwater

Table A2.Heat transfer characteristics for a distilled water

Temperature

( ) 90 80 70 60

Mass flow rate

(kg/sec) 0.0250 0.0238 0.0232 0.0227

Heat transfer

rate (k )

Eq (4.6)

0.602 0.344 0.223 0.109

LMDT ( )

Eq ( 4.14) 54.88 45.57 38.49 28.50

Overall heat

transfer

coefficient

(k )

Eq (4.7)

0.0877 0.0603 0.0463 0.0305

NTU

Eq (4.20) 0.105 0.0756 0.0518 0.0349

Effectiveness

(%)

Eq (4.22)

3.84 3.51 3.20 2.92

Temperature ( ) 90 80 70 60

Mass flow rate

(kg/sec) 0.0263 0.0256 0.0256 0.0250

Heat transfer rate

(k )

Eq (4.6)

0.770 0.535 0.428 0.313

LMDT ( )

Eq ( 4.14) 54.48 45.50 36.50 27.50

Overall heat

transfer

coefficient

( )

Eq (4.7)

0.1130 0.0940 0.0938 0.0910

NTU

Eq (4.20) 0.112 0.0953 0.0951 0.0945

Effectiveness (%)

Eq (4.22) 3.92 3.75 3.78 3.77


37

Table A3.Heat transfer characteristics for a mixture of ethylene glycol and distilled water

Temperature ( ) 90 80 70 60

Mass flow rate

(kg/sec) 0.0238 0.0227 0.0222 0.0217

Heat transfer rate

(k )

Eq (4.6)

0.944 0.750 0.513 0.358

LMDT ( )

Eq ( 4.14) 50.45 41.96 34.47 25.98

Overall heat

transfer

coefficient

(k )

Eq (4.7)

0.150 0.143 0.119 0.110

NTU

Eq (4.20) 0.2384 0.2382 0.2027 0.1917

Effectiveness (%)

Eq (4.22) 5.24 5.23 4.92 4.84


38

APPENDIX B

Figure B1.Fabricated split flow heat exchanger

Figure B2.Experimental setup of split flow heat exchanger used for analysis


39

Graph B1.Temperature v/s heat transfer rate

Graph B2.Temperature Vs Overall Heat transfer coefficient


40

Graph B3.Temperature Vs Effectiveness

analysis of overall heat transfer coefficient and … of overall heat transfer coefficient and...

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