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Heat Transfer Enhancement Techniques. With Special Attention to Passive Methods of Heat Transfer Enhancement

©2016 Textbook 100 Pages

Summary

Heat exchangers are widely used in the industrial sector, e.g. in the refrigeration, air conditioning, petrochemical, and agricultural food industry. The high cost of energy and material has resulted in an increased effort aimed at producing high performance heat exchanger equipment. Passive methods of heat transfer enhancement do not need external power for enhancement. One of these kinds of passive technique is twisted tape inserts that enhance the performance of heat exchangers. Using multiple twisted tape inserts gives better enhancement than a single twisted tape insert. Using nanofluid gives also better thermal performance than water. Therefore, nanofluid along with twisted tape inserts was used in this study.
For this study, different combinations of multiple twisted tape inserts were designed and fabricated. These different combinations contain dual, triple, and quadruple twisted tapes. Directions of twists are also varied which enables to study the effect of different swirl flow generators. Nanofluid is used with various volume concentrations of 0.07%, 0.14% and 0.21% in order to investigate the effect of nanoparticle concentration on heat transfer enhancement. Experimental investigation was carried out by having a constant heat flux condition and by varying the volume flow rate of flow from 2 to 10 lpm.

Excerpt

Table Of Contents


CHAPTER 6 ­ CONCLUSIONS AND FUTURE SCOPES ... 61
6.1
CONCLUSIONS ... 61
6.2
FUTURE SCOPE ... 62
REFERENCES ... 64
APPENDIX AND ANNEXURES ... 68

7
CHAPTER
1
INTRODUCTION
This chapter presents an introduction to the background of this work throughout the
course, which includes detailed introduction of the heat exchanger, twisted tape inserts and
nanofluids. Then, the methods used for enhancement of heat exchanger performance is
explained and proper method for heat transfer enhancement is suggested in terms of
designing a multiple twisted tape inserts and experimentally investigating its performance.
This work is often followed by the motivation required for this work, objectives set before
carrying out the work. Finally, an organisation of the report is provided in brief.
1.1 INTRODUCTION
A heat exchanger is a device which is constructed to facilitate the heat transfer between
one medium to another medium efficiently. The word "Exchanger" really applied to all types
of equipment in which heat is exchanged but it is often used specially to the equipment in
which heat is exchanged between two process streams that are at different temperature and
are separated by a solid wall and where the two process fluid do not mix with each other.
Heat exchanger is an important and expensive equipment that is used in almost all field of
process such as food and dairy processes, waste heat recovery processes, air conditioning and
refrigeration systems and also plants of oil, petrochemical, sugar, chemical reactors,
pharmaceutical, power generation, etc. Energy recovery is the prime requirement of today to
optimize the energy consumption in industry.
To achieve maximum utilization of thermal energy, several heat transfer enhancement
techniques have been used in many thermal engineering applications such as nuclear reactor,
chemical reactor, chemical process, automotive cooling, refrigeration, and heat exchanger,
etc. Heat transfer enhancement techniques are powerful tools to increase heat transfer rate
and thermal performance as well as to reduce the size of heat transfer system in installing and
operating costs. Heat transfer enhancement in thermal systems can be carried out either by
active or passive methods. In active methods, there is need of supplying external power
source to the fluid or the equipment whereas in passive methods, heat transfer enhancement is
done by turbulence promoters (such as special surface geometries, twisted tape, propeller,
tangential inlet nozzle, snail entry, axial/radial guide vane, spiral fin) or fluid additives (such
as nanofluid), without using any direct external power source. Due to easy

8
installation/operation and cost saving, passive methods are extensively preferred for heat
transfer enhancement.
One important group of devices used in passive method is swirl flow devices which
produce secondary recirculation on the axial flow leading to an increase of tangential and
radial turbulent fluctuation. This allows a greater mixing of fluid inside a heat exchanger tube
and subsequently reduces a thickness of the boundary layer. Among the swirl generators of
tube inserts, twisted tapes have gained great attention and widely used for producing compact
heat exchangers and upgrading the heat transfer rate of the existing heat exchanger due to its
low cost, acceptable thermal performance and ease of manufacture installation. Twisted tapes
are generally equipped along the core tube to generate swirl causing the fluid transfer
between the core tube and near wall tube. This leads to several mechanisms for heat transfer
augmentation by improving flow velocities caused by partial blockage of the tube flow,
which directs toward reducing the hydrodynamic or thermal boundary layer thickness. The
hydraulic diameter reduction results in greater heat transfer coefficient, lengthening flow path
in consequence of a helically twisting fluid motion, improving fluid mixing and thinning
thermal boundary layer. However, more pumping power is required when twisted tapes are
equipped inside the tube. Therefore, economic considerations has to be taken into account by
using twisted tape with a proper geometry.
There are some earlier work, regarding twisted tape inserts, were performed by
researcher. They have investigated effect of single twisted tape inserts on heat transfer
enhancement for both laminar and turbulent flows. Apart from them very few researcher have
worked on the multiple twisted tapes for heat transfer enhancement. Therefore, there is a
large scope for doing research in the field of multiple twisted tapes with modifying different
parameters and investigating the performance.
Now a days there is a trend to use nanofluid in heat exchanger to enhance the thermal
performance. A nanofluid is a fluid prepared by dispersion of metallic or non-metallic
nanoparticles or nanofibers with a typical size less than 100 nm in a liquid. Nanofluids have
attracted huge interest because of their greatly enhanced thermal properties. Nanofluids are
colloidal dilute dispersion of nanoparticles (generally less than 5% in volume) such as metals,
oxides, carbides, or carbon nanotubes in conventional coolants or base fluid such as water,
ethylene glycol, and oil. Miniaturization of electronic and other industrial component has led
to the demand for development if new compact heat exchangers with higher rate of heat

9
removal form cooling fluid. Hence multiple twisted tape inserts along with nanofluid fulfils
this demand. Therefore, this work is also deals and decides the feasibility of use of multiple
twisted tape inserts along with nanofluid in tubular heat exchanger.
1.2 MOTIVATION
A high cost of energy and material has resulted in an increased effort aimed at producing
high performance heat exchanger equipment. The methods of improving convective heat
transfer in the tubes of heat exchangers have been widely investigated by many researchers.
Still there is a wide scope and challenging task for performance enhancement of heat
exchanger. A lot of research in the area of improvement of design parameters such as fin
thickness, fin spacing, tube diameter, tube spacing, coil width, etc. is being carried out for
heat transfer enhancement from a quite long time.
The heat transfer enhancement techniques have been classified into two main categories
such as active and passive. An active techniques which require external power for heat
transfer augmentation, and passive technique needs no such external power for enhancement.
One of the passive technique is use of twisted tape inserts in order to enhance the
performance of heat exchanger, recently research is being started by use of nanofluid in heat
exchangers. Therefore, in this work, use of nanofluid along with twisted tape inserts are being
carried out.
1.3 OBJECTIVES
The main objective of this investigation is to study the performance of multiple twisted
tape inserts with nanofluid in tubular heat exchanger. The proposed work includes the
determination of
i. Overall heat transfer coefficient of water and nanofluid with/without multiple twisted
tape inserts.
ii. The effect of single/dual/multiple twisted tape inserts on the overall heat transfer
coefficient.
iii. The effect of single/dual/multiple twisted tape inserts on frictional pressure loss.
iv. Effect of Reynolds number on Thermal performance factor.
v. Effect of Nusselt number and Reynolds number on the heat transfer coefficient.
vi. Deciding the feasibility of multiple twisted tape inserts with and without nanofluid for
enhancing the heat transfer rate.

10
1.4 PROPOSED WORK
It is proposed to study experimentally the effect of multiple twisted tape on heat transfer
coefficient, pressure drop and thermal performance factor of tubular heat exchanger with
water and Al
2
O
3
nanofluid.
In this work, aluminium oxide as a nanoparticle and SLS (Sodium Lauryl Sulphate) as a
surfactant is used in water for preparation of Al
2
O
3
nanofluid. Three different volume
percentage of aluminium oxide nanoparticles are added in the water such as 0.07%, 0.14%
and 0.21%). Experimental investigations are carried out as per standard test procedure with
constant heat flux condition at different mass flow rates.
1.5 ORGANISATION OF REPORT
Chapter 2 presents the background information on literature review related to proposed
work which includes single and multiple twisted tape inserts along with and without
nanofluid.
Chapter 3 presents the theory of heat transfer enhancement techniques, heat exchanger
classification, basics of twisted tape inserts, design of tubular heat exchanger with various
assumptions and the parameter considerations.
Chapter 4 deals with experimental work carried out for this work which includes
different devices used in setup, test procedure adopted and preparation and stability of
nanofluid, stability evaluation method, and various properties of nanofluid obtained from
existing correlations. This chapter also includes testing and measured data analysis.
Chapter 5 includes result and discussions with the help of graphical representation. It
includes comparison of experimental result obtained from various combinations of twisted
tapes with water and various concentrations of nanofluid.
Chapter 6 concludes the present research study and also provides future scope.

11
CHAPTER 2
LITERATURE REVIEW
This chapter gives a reviews of the literature related to this work. It gives the survey of
literature on twisted tape inserts and application of nanofluid in heat exchanger. This chapter
also covers the concluding remarks on the summary of literature reviewed.
2.1 INTRODUCTION
Twisted tape inserts are the very cost effective method for heat transfer enhancement in
heat exchanger. Hence, they have attracted considerable research attention. Many researcher
have studied experimentally and numerically the performance of single twisted tapes for heat
transfer enhancement. Very few researcher have studied the effect of multiple twisted tape on
heat exchanger performance. Researcher have also attempted to investigate heat transfer
enhancement in various type of heat exchanger by using new passive heat transfer
enhancement techniques such as use of nanofluids. Paper reviewed for this work are
categorized under the heat transfer enhancement by twisted tape inserts without nanofluid and
heat transfer enhancement by twisted tape inserts along with nanofluid.
2.2 STATE OF ART OF REVIEW OF THE HEAT TRANSFER ENHANCEMENT BY
TWISTED TAPE INSERTS WITHOUT NANOFLUID
M.M.K. Bhuiya et al.
[1]
studied experimentally, the influences of triple twisted tapes on
heat transfer rate, friction factor and thermal enhancement efficiency. The investigations were
conducted using the mild steel triple twisted tapes with four different twists under uniform
heat flux condition. The experimental results demonstrated that the Nusselt number, friction
factor and thermal enhancement efficiency increased with decreasing twist ratio. The results
indicated that the presence of triple twisted tapes led to a higher increase in the heat transfer
rate over the plain tube. Correlations were developed based on the data gathered during this
work for predicting the heat transfer, friction factor and thermal enhancement efficiency
through a circular tube fitted with triple twisted tape inserts in terms of twist ratio, Reynolds
number and Prandtl number.
Xiaoyu Zhang et al.
[2]
numerical analysis, the heat and fluid-flows through a round tube
fitted with triple or quadruple twisted tapes of different clearance, with the aim to verify the
thought of core flow heat transfer enhancement and investigate the effect of multi
longitudinal vortex on the flow, heat transfer and friction loss behaviour. The contour plots of
predicted velocity, streamline and temperature are also presented. The obtained results show

12
that, a maximum increase of 171% and 182% are observed in the Nusselt number by using
triple and quadruple twisted tapes. And the friction factors of the tube fitted with triple and
quadruple twisted tapes are around 4-7 times as that of the plain tube and the results verify
the theory of the core flow heat transfer enhancement. Physical quantity synergy analysis is
performed to investigate the mechanism of heat transfer enhancement.
C. Thianpong et al.
[3]
investigate experimentally, the influences of twin-counter/co-
twisted tapes on heat transfer rate, friction factor and thermal enhancement index. The twin
counter twisted tapes are used as counter-swirl flow generators while twin co-twisted tapes
are used as co-swirl flow generators in a test section. The tests are conducted using the twin
counter twisted tapes and twin co-twisted tapes with four different twists for different
Reynolds numbers under uniform heat flux conditions. The experiments using the single
twisted tape are also performed under similar operation test conditions, for comparison. The
experimental results demonstrate that Nusselt number, friction factor and thermal
enhancement index increase with decreasing twist ratio. The results also show that the twin
counter twisted tapes are more efficient than the twin co-twisted tapes for heat transfer
enhancement. In addition, the empirical correlations of the heat transfer, friction factor and
thermal enhancement index are also reported.
S. Eiamsa-ard et al.
[4]
numerically analysis, the heat and fluid-flows through a round
tube fitted with twisted tape, with the aim to investigate the effect of tape clearance ratio on
the flow, heat transfer and friction loss behaviors. A finite volume method with the standard
k­ turbulence model, the Renormalized Group k­ turbulence model, the standard k­
turbulence model, and the Shear Stress Transport k­ turbulence model, is used in the
simulation. The computations show that predicted results by Shear Stress Transport k­
turbulence, are in good agreement with the measurements than other models. The contour
plots of predicted velocity vector, static pressure, temperature, and turbulent kinetic energy
are also presented. The obtained results show that, the mean heat transfer rates for the tube
with twisted tape inserts are higher than that for the plain tube. The thermal performance
factor of the twisted tape is influenced by the clearance ratios and the best thermal
performance factor at constant pumping power is found at zero clearance ratio i.e. tight-fit
twisted tape.
M.M.K. Bhuiya et al.
[5]
explored the effects of the double counter twisted tapes on heat
transfer and fluid friction characteristics in a heat exchanger tube. The double counter twisted

13
tapes were used as counter-swirl flow generators in the test section. The experiments were
performed with double counter twisted tapes of four different twist ratios (y = 1.95, 3.85,
5.92 and 7.75) using air as the testing fluid in a circular tube turbulent flow regime where the
Reynolds number was varied from 6950 to 50,050. The experimental results demonstrated
that the Nusselt number, friction factor and thermal enhancement efficiency were increased
with decreasing twist ratio. The results also revealed that the heat transfer rate in the tube
fitted with double counter twisted tape was significantly increased with corresponding
increase in pressure drop. In the range of the present work, heat transfer rate and friction
factor were obtained to be around 60 to 240% and 91 to 286% higher than those of the plain
tube values, respectively. The maximum thermal enhancement efficiency of 1.34 was
achieved by the use of double counter twisted tapes at constant blower power. In addition, the
empirical correlations for the Nusselt number, friction factor and thermal enhancement
efficiency were also developed, based on the experimental data.
Halit Bas and Veysel Ozceyhan
[6]
experimentally investigated the flow friction and
heat transfer behaviour in a twisted tape swirl generator inserted tube. The twisted tapes are
inserted separately from the tube wall. The effects of twist ratios (y/D = 2, 2.5, 3, 3.5 and 4)
and clearance ratios (c/D = 0.0178 and 0.0357) are discussed in the range of Reynolds
number from 5132 to 24,989, and the typical one (c/D = 0) is also tested for comparison.
Uniform heat flux is applied to the external surface of the tube wall. The air is selected as a
working fluid. The obtained experimental results from the plain tube are validated by using
well known equations given in literature. The using of twisted tapes supplies considerable
increase on heat transfer and pressure drop when compared with those from the plain tube.
The Nusselt number increases with the decrease of clearance ratio and twist ratio, also
increase of Reynolds number. For all investigated cases, heat transfer enhancement tends to
decrease with the increase of Reynolds number and to be nearly uniform for Reynolds
number over 15,000 and y/D lower than 3.0. The highest heat transfer enhancement is
achieved as 1.756 for c/D = 0.0178 and y/D= 2 at Reynolds number of 5183. Consequently,
the experimental results present that the best operating regime of all investigated twisted tape
swirl generator inserts is detected at low Reynolds number, leading to more compact heat
exchanger. The empirical correlations based on the experimental results of the present study
are also given for prediction the heat transfer, friction factor and heat transfer enhancement.

14
2.3 STATE OF ART OF REVIEW OF THE HEAT TRANSFER ENHANCEMENT BY
TWISTED TAPE INSERTS ALONG WITH NANOFLUID
W.H. Azmi et al.
[7]
have undertaken the experiments with TiO
2
for flow of nanofluid in
a tube and with tape inserts for the determination of heat transfer coefficients and friction
factor in the turbulent range of Reynolds numbers. The nanofluid heat transfer coefficients in
the Reynolds number range of 8000 and 30,000 increased with volume concentration up to
1.0% and decreased at higher concentrations for flow in a tube. For flow over tape insert of
twist ratio 5, the maximum enhancement in heat transfer coefficient is 81.1% compared to
water and 48.1% with nanofluid at 1.0% concentration. A comparison of heat transfer
enhancements for flow of nanofluid in a tube with twisted tape insert is made considering
pressure drop with Advantage Ratio. For flow with twisted tape inserts, the Advantage Ratio
increases with twist ratio for both water and nanofluid.
E. Esmaeilzadeh et al.
[8]
carried out an experimental study to investigate heat transfer
and friction factor characteristics of -Al
2
O
3
/water nanofluid through circular tube with
twisted tape inserts with various thicknesses at constant heat flux. The twist ratio of twisted
tape remained constant while the thicknesses were changed through three values. The
experiments were performed in laminar flow regime of Reynolds numbers. Results indicated
that twisted tape inserts enhanced the average convective heat transfer coefficient, and also
more the thickness of twisted tape is more the enhancement of convective heat transfer
coefficient is. Also, the highest enhancement was achieved at maximum volume
concentration. Results showed that nanofluids have better heat transfer performance when
utilized with thicker twisted tapes. At the same time, the increase in twisted tape thickness
leads to an increase in friction factor. In the end, the combined results of these two
phenomena result in enhanced convective heat transfer coefficient and thermal performance.
Smith Eiamsa-ard and Kunlanan Kiat kittipong
[9]
investigate experimentally, the
thermal performance characteristics in a heat exchanger tube by multiple twisted tapes in
different arrangements and TiO
2
nanoparticles with different concentrations as the working
fluid. The tube inserted the multiple twisted tapes showed superior thermal performance
factor when compared with plain tube or the tube inserted a single twisted tape, due to
continuous multiple swirling flow and multi-longitudinal vortices flow along the test tube.
The higher number of twisted tape inserts led to an enhancement of thermal performance that
resulted from increasing contact surface area, residence time, swirl intensity and fluid mixing
with multi-longitudinal vortices flow. Moreover, arrangement of twisted tapes in counter

15
current was superior energy saving devices for the practical use, particularly at low Reynolds
number. This was especially the case for quadruple counter tapes in the cross directions
where heat transfer enhancement with relatively low friction loss penalty was deserved. This
arrangement gives highest thermal performance factor with TiO
2
nanoparticle as a working
fluid than using pure water.
M.T. Naik et al.
[10]
investigated experimentally, the heat transfer and friction factor of
CuO nanoparticles dispersed in water/propylene blend in a plain tube with and without
twisted tape inserts. Considerable enhancement in the Nusselt number is observed with CuO
nanofluids over the base fluids and heat transfer enhancement is linearly proportional to the
nanoparticle volume concentration in the base fluid. The increase friction factor of
Nanofluids over the base fluid is not significant in a plane tube. The use of twisted tape
inserts in CuO nanofluids enhances the heat transfer coefficient with little increment of
friction factor and transfer enhancement is proportional to the number of twists on inserts.
Correlations are developed to predict Nusselt number and friction factor for the flow of CuO
nanofluids in a tube with and without twisted tape inserts.
S. Eiamsa-ard and K. Wongcharee
[11]
investigated experimentally, the combined
effects of nanofluids, dual twisted-tapes and a micro-fin tube on the heat transfer rate, friction
factor and thermal performance factor characteristics. The authors conducted experiments
using the micro-fin alone as well as the micro-fin equipped with a single twisted tape for
comparison. The experimental results revealed that the heat transfer rate increased with
increasing nanofluid concentration. At similar operating conditions, the micro-fin tube
equipped with dual twisted-tapes consistently gave superior thermal performance factor to the
one equipped with a single twisted-tape as well as the micro-fin tube alone. For all cases,
thermal performance factors were apparently above unity. This indicates the beneficial effect
for the energy saving by the uses of the combined techniques.
L. SyamSundar et al.
[12]
studied experimentally, the turbulent convective heat transfer
and friction factor characteristics of magnetic Fe
3
O
4
nanofluid flowing through a uniformly
heated horizontal circular tube with and without twisted tape inserts. Experiments are
conducted in the particle different volume concentrations, twisted tape inserts of different
twist ratios and Reynolds number range of 3000 to 22000. Heat transfer and friction factor
enhancement of Fe
3
O
4
nanofluid in a plain tube with twisted tape insert is 51.88% and 1.231
times compared to water flowing in a plain tube under same Reynolds number. Generalized

16
regression equation is presented for the estimation of Nusselt number and friction factor for
both water and Fe
3
O
4
nanofluid in a plain tube and with twisted tape inserts under turbulent
flow condition.
L. Syam Sundar and K.V. Sharma
[13]
experimentally determined the thermo physical
properties like thermal conductivity and viscosity of Al
2
O
3
nanofluid at different volume
concentrations and temperatures and validated. Convective heat transfer coefficient and
friction factor data at various volume concentrations for flow in a plain tube and with twisted
tape insert is determined experimentally for Al
2
O
3
nanofluid. Experiments are conducted in
the Reynolds number range of 10,000­22,000 with tapes of different twist ratios in the range
of 0 < H/D < 8.3. The heat transfer coefficient and friction factor of 0.5% volume
concentration of Al
2
O
3
nanofluid with twist ratio of five is 33.51% and 1.096 times
respectively higher compared to flow of water in a tube. A generalized regression equation is
developed for the estimation of Nusselt number and friction factor valid for both water and
nanofluid in plain tube and with inserts under turbulent flow conditions.
Y. Raja Sekhar et al.
[14]
conducted heat transfer experiments in a pipe under low
Reynolds number range using water and water based nanofluids. Heat transfer coefficient and
friction factor for nanofluid in the flow path enhanced compared to water. The experimental
data is compared with the data of literature and are found to be in good agreement. The
increase in heat transfer coefficient in plain tube with use of nanofluids is greater by 8-12%
compared to the flow of water in a plain tube. The nanofluid of 0.5% particle concentration is
having highest friction factor compared to water. The Nusselt number and friction factor
increases with increase of particle concentration. But, friction factor decreases with increase
of Reynolds number of flow whereas the Nusselt number increases. Using nanofluid with a
high heat exchange can help in reduce the size of the heat exchanger or without increasing the
size of the heat exchanger efficiency of the system can be improved. Further, using twisted
tapes and nanofluids in the pipe flows is advantageous since it is visible from the results that
the energy gained with heat exchange is more than the energy spent on pumping power.
Finally, it was concluded that heat transfer enhancement in a horizontal tube increases with
Reynolds number of flow and nanoparticle concentration.

17
2.4 CONCLUDING REMARKS
The final remarks made from the literature review reveals that there is a large scope for
investigating the performance of multiple twisted tape inserts along with nanofluid at various
parameters which are summaries as,
1. A number of experimental studies have been reported to investigate the effects of
various inserts for performance evaluation.
2. Among the reported research works on different types of inserts performed by the
different researchers, it is clear that a few research works were presented on multiple
twisted tape inserts.
3. Most of research works has done on enhancement of heat transfer by nanofluid but
heat transfer enhancement by using twisted tape together with nanofluids are limited
explored.
4. Experimental study of heat transfer enhancement by using multiple twisted tapes
together with Al
2
O
3
Nanofluid is not completely available in literature.
From above it is clear that number of experimental studies have been reported for
nanofluids for enhancement of heat transfer. Heat transfer enhancement by inserting different
swirl flow devices in flow path is main point of interest for most of research person in recent
year. However, there is need to study experimentally the combine effect of multiple twisted
tapes and nanofluid. Keeping this aspect in the mind nanofluid with the multiple twisted tape
inserts in place of single twisted tape is been selected for this work.
2.5 CLOSURE
Based upon the experimental and numerical studies reported in preceding section, it is
evident that lot of work is carried out in order to understand the reason of heat transfer
enhancement by single twisted tape insert. Very little work is carried out on multiple twisted
tape inserts along with nanofluid. Therefore, there is a lot of scope to investigate the
performance of multiple twisted tape inserts along with nanofluid.

18
CHAPTER 3
THEORY AND DESIGN OF TUBULAR HEAT EXCHANGER WITH
TWISTED TAPE INSERTS
This chapter covers the theory of heat transfer enhancement techniques, theory of twisted
tapes, classification of heat exchangers and brief description of twisted tape inserts. Also this
chapter includes the design procedure of tubular heat exchanger.
3.1 THEORY OF HEAT TRANSFER ENHANCEMENT TECHNIQUES
Several heat transfer enhancement (HTE) techniques have been used in many
engineering applications such as nuclear reactor, chemical reactor, chemical process,
automotive cooling, refrigeration, and heat exchanger, etc. HTE techniques are powerful
tools to increase heat transfer rate and thermal performance as well as to reduce of the size of
heat transfer system in installing and operating costs. However, the need for miniaturization
of thermal equipment has shifted the focus to the development of new high performance
fluids with thermal conductivities higher than those of the conventional liquids. These high
performance fluids can contribute to the evolution of space-saving yet cost effective thermal
equipment with higher competitiveness in the global market.
Heat transfer enhancements can be achieved through active and passive methods as
suggested by Ahuja
[15]
and Bergles
[16]
. Active heat transfer enhancement is achieved by the
application of external energy on the fluid. Passive enhancement is attained by increasing the
fluid surface area, providing artificially roughed surface, by turbulence promoter (such as
special surface geometries, twisted tape, propeller, tangential inlet nozzle, snail entry,
axial/radial guide vane, spiral fin) or fluid additives (such as nanofluid), of the passive
methods, the heat transfer enhancement with nanofluid is highly encouraging. Due to its easy
installation/operation and cost saving, passive method has drawn great attention.
Generally speaking, tube flow can be divided into two parts
[17]
the boundary flow and
the core flow. The boundary flow is a fluid region near the wall in the tube, beyond which in
the tube the core flow is defined. Heat transfer enhanced tubes such as
[18-21]
spiral grooved
tube, longitudinal troughed tube, corrugated tube, inner-finned tube, spiral-ribbed tube,
micro-ribbed tube and so on, are mainly considered to effectively design and improve heat
transfer surface in the boundary flow. Moreover, those improved surfaces dominate
convective heat transfer between the fluid and the tube wall. Therefore, this kind of methods
can be called surface-based heat transfer enhancement or heat transfer enhancement in the

19
boundary flow. On the contrary, the heat transfer enhancement in the core flow can be called
fluid-based heat transfer enhancement. The surface-based heat transfer enhancement is the
common method to enhance heat transfer in the tube. While these measures are effective for
heat transfer, however, intensifying fluid disturbance in the boundary flow will result in more
dissipation of fluid momentum, and enlarging continuously extended surface will cause more
frictional resistance and viscosity dissipation. Thus, the flow resistance will be increased by
adopting these techniques. If the flow resistance is overlarge, the fluid velocity will become
small, which may weaken convective heat transfer between the fluid and the surface.
3.2 THEORY OF TWISTED TAPES
One important group of devices used in passive method is swirl flow devices which
produce secondary recirculation on the axial flow leading to an increase of tangential and
radial turbulent fluctuation. This allows a greater mixing of fluid inside a heat exchanger tube
and subsequently reduces a thickness of the boundary layer
[22-27]
.Among the swirl generators
of tube inserts, twisted tapes have gained great attention and widely used for producing
compact heat exchangers and upgrading the heat transfer rate of the existing heat exchanger
due to its low cost, acceptable thermal performance and ease of manufacture installation
[28]
.Twisted tapes are generally equipped along the core tube to generate swirl causing the fluid
transfer between the core tube and near wall tube. This leads to several mechanisms for heat
transfer augmentation by improving flow velocities caused by partial blockage of the tube
flow, which directs toward reducing the hydrodynamic or thermal boundary layer thickness.
The hydraulic diameter reduction results in greater heat transfer coefficient, lengthening flow
path in consequence of a helically twisting fluid motion, improving fluid mixing and thinning
thermal boundary layer. However, more pumping power is required when twisted tapes are
equipped inside the tube. Therefore, economic consideration has to be taken into account by
using twisted tape with a proper geometry.
3.3 CLASSIFICATION OF HEAT EXCHANGERS
In order to meet the widely varying applications, several types of heat exchangers have
been developed which are classified on the basics of nature of heat exchange process, relative
direction of fluid motion, design and construction features and physical state of fluids.

20
Fig. 3.1 Classification of Heat Exchanger
3.3.1 Tubular Heat Exchanger
Tubular heat exchangers are generally build of circular tubes, although elliptical,
rectangular or round/flat twisted tubes. There is considerable flexibility in design of tubular
heat exchanger because of core geometry can be varies easily by changing the tube diameter,
length and arrangement. Tubular exchanger can be designed for high pressure relative to
environment and high pressure differences between the fluids. Tubular exchangers are used
primarily for liquid to liquid and liquid to phase change (Condensing and evaporating) heat
transfer application. They are also used for gas to liquid and gas to gas heat transfer heat
transfer application primarily when operating pressure and temperature is very high or
fouling is a severe problem on at least one fluid side and no other types of exchanger can be
used. These tubular heat exchangers may be classified as shell and tube type, double pipe
type, and spiral tube type heat exchangers
[29]
.
Heat
Exchanger
Recupurative
Direct Contact
type
In Direct
Contact type
Tubular
Double type
Spiral type
Shell type
Plate
Gasket Plate
Spiral Plate
Lamella
Extended
Surface
Plate Fin
Tube Fin
Regenerative
Fixed Matrix
Regenerator
Rotary
Regenerator
Drum Type
Disc Type

21
3.4 DESIGN OF TUBULAR HEAT EXCHANGER
The design of heat exchanger consist of the specification of the geometry (cross sectional
area and length) that transfer the required heat load within the metallurgic limitation of
material. Depending upon the heat exchanger design methodology, there are a set of
geometric parameters that needs to be specified before the start of design.
3.4.1 Assumption in Design of Tubular Heat Exchanger
1. Properties of fluid are considered as constant, at an average value of inlet and outlet
temperatures with little loss inaccuracy.
2. Constant heat flux boundary condition is considered.
3. Flow through heat exchanger is fully developed, steady and constant.
4. Fluid stream experiences little or no change in their velocities and elevations hence the
kinetic energy and potential energy changes are negligible.
5. Outer surface of heat exchanger is assumed to perfectly insulated.
6. There is no fouling in heat exchanger.
3.4.2 Design of Test Section
The test section of this experimental setup consist of tube, heater and insulation as shown
in fig. 3.2
Fig. 3.2 Schematic Diagram of Test Section
3.4.2.1 Tube Selection
Tube of test section is selected in such a way that, it provides minimum thermal
resistance to flow of heat. For this purpose it is necessary to use material having high value of
thermal conductivity. Among all engineering materials copper is most suitable one.
To get the best results from experiments flow through the tube must be hydro
dynamically fully developed. For fully developed flow length of test section must be
sufficiently larger. For this purpose 1 m long copper tube is chosen for experimentation. Test

22
section must sustain pressure forces at higher temperature of fluid. Thickness of 3 mm is
considered as safe value from previous literature. Specification of test section tube is given in
table 3.1
3.4.2.2 Heater Selection
Heater is to be wound on outer periphery of the test section tube. Heater is selected in
such a way that it can provide constant heat flux condition with desired heat flux. To
calculate the capacity of heater required, it is essential to know mass flow rate of fluid, heat
transfer rate, and losses through test section. Minimum temperature difference of 1
0
C is
considered for heater design.
Mass flow rate through the test section is calculated as,
Assuming turbulent flow through test section with R
e
=25000,
R
e
=
25000 =
(
. )( )( . )
.
V
= 0.692 m/sec
Mass flow rate:
= ( ) (V) (A
c
)
=
(988.1)
(0.692)(
0.02 )
= 0.859 kg/s
Capacity of heater is calculated by,
Heat transfer rate
Q = C
p
Table 3.1 Specifications of Test Section Tube
Material Copper
Length 1
m
Inner Diameter
20 mm
Outer Diameter
23 mm

23
= (
0.859)(4181.1)(1)
=
3.59
kW
Assuming, losses of 4%
Q = (1.04) (3.59) =3.74 kW
=4 kW
Hence at least heater of 4 kW capacity is required to fulfil the conditions of experiment.
From the safety point of view, heater of capacity 6 kW is selected. Specification of heater
used is given in table 3.2
Table 3.2 Specifications of Heater
Heater type
Nichrome wire heater
Capacity 6
kW
Diameter of wire
1mm (20 Gauge)
Electric supply
240V; 5A
Surface Temperature
550
0
C
3.4.2.3 Insulation Selection
To avoid heat loss to the atmosphere it is necessary to cover test section with appropriate
layer of insulation. Insulation should sustain temperature of heater i.e. 550
0
C with minimum
heat loss to atmosphere. Keeping this aspect in mind "Ceramic wool" is selected for
insulation purpose.
Thickness of insulation must be selected from heat transfer point of view to achieve
minimum heat loss to atmosphere. For proper selection of insulation, critical thickness of
insulation is important to know, which is given by,
r
c
=
=
.
= 0.012
t= r
c
-r
o
=0.012-0.0115 =0.005 m = 5mm
Any thickness of insulation more than 5mm will reduce heat loss. From heat transfer
point of view, thickness of 25 mm is selected. Specification of insulation is given in table 3.3

24
Table 3.3 Specifications of Insulation
Material Ceramic
wool
Max temperature sustain
1260
0
C
Density 80-100
kg/m3
Thermal conductivity
0.12 W/m K
Thickness of insulation
25 mm
3.4.2.4 Pump Selection
Pump is selected in such a way that it should provide required mass flow rate of flow
through test section. Power required, to pump fluid from water tank to different components
of experimental setup is given by,
P
h
= Q g H
=
(1000)
(9.81)(50)
=183.4
W
Assuming pump efficiency = 0.6
P = =
.
.
= 305.67
P =
.
.
= 0.41
= 0.5
Hence, a single stage centrifugal pump of capacity 0.5 HP is selected.
3.4.2.5 Selection of Sensors
To measure different parameters such as temperature, pressure drop, flow rate during
experimentation, it is essential to install suitable sensors at required location. All sensors
should have sufficient accuracy to predict performance of experimental setup.
Temperature sensor should have temperature range of 0-600
0
C. It should have resolution
of minimum 0.1
0
C. With keeping this aspect in mind, K-Type thermocouple is selected. It has
temperature range of 0-1260
0
C with sensitivity of 40-55 µV/
0
C and a resolution of 0.1
0
C.
Due to turbulent flow in heat exchanger, flow meter should have range of 2-12 lpm. It
should have least count of minimum 1 lpm. With keeping this aspect in mind, rotameter is
selected for flow rate measurement. It has range of 2-20 lpm with a least count of 1 lpm.

25
Pressure sensor should be accurate enough to measure pressure drop across test section.
With keeping this aspect in mind, digital type differential pressure transducer is selected. It
has range of 0-9999.9 Pa with a least count of 0.1 Pa.
3.5 DUAL/TRIPLE/QUADRUPLE TWISTED TAPES
The schematic view and details of the single, dual, triple and quadruple twisted tapes with
different arrangements are shown in fig. 3.3 and Table 3.4. All twisted tapes were made of
aluminium strip with a thickness of 1 mm, which is a minimum twisting operation, and a
length of 1000 mm. To fabricate a twisted tape, one end of a straight tape was clamped while
another end was carefully twisted to achieve a desired twist length.
Fig. 3.3 Configurations of Multiple Twisted Tapes
Single twisted tape was 19 mm in width while dual, triple and quadruple twisted tapes
were 8 mm in width. The tapes were formulated at constant twist ratio (y/W) of 5 where twist
ratio is defined as twist length (180
o
/ twist length) to tape width (W). For dual, triple and
quadruple twisted tapes, each tape was individually twisted and subsequently welded
together. In the experiment, the swirl direction corresponding to tape arrangement was
designed as: (i) co-swirl flow; all tapes were aligned to be twisted in the same direction. In
this case, dual, triple and quadruple twisted tapes were assigned as Co-DTs/Co-TTs/Co-QTs,
respectively, (ii) counter-swirl flow; this arrangement was designed for dual and quadruple

26
twisted tapes. In the case of dual twisted tapes, two tapes were aligned to be twisted in
opposite directions and assigned as C-DTs. In the case of quadruple twisted tapes, two tapes
were aligned to be twisted in the same direction which was opposite to that of other two
tapes. In addition, the quadruple counter tapes consisting of two pairs of tapes were in two
different arrangements, to produce (1) parallel counter-swirl flow and (2) cross counter-swirl
flow. For parallel counter-swirl flow, the tapes in each pair produced swirl flow in the same
direction; in this case the quadruple counter tapes were assigned as PC-QTs. For cross
counter-swirl flow, the tapes in each pair produced swirl flow in the opposite directions. The
quadruple counter tapes were assigned as CC-QTs.
Table 3.4 Configurations of Multiple Twisted Tapes
Twisted tape
ST
Co-DTs
C-DTs
Co-TTs
C-TTs
Co-QTs
PC-QTs CC-QTs
(a) Number of tape
1
2
2
3
3
4
4
4
(b) Tape width (W)
19 mm 8 mm
8 mm
7.5 mm 7.5 mm
7 mm
7 mm
7 mm
(c) Tape pitch length (y) 90 mm 40 mm
40 mm
37.5
mm
37.5
mm
35 mm
35 mm
35 mm
(d) Twist ratio (y/W)
5
5
5
5
5
5
5
5
(e) Tape thickness ()
1 mm
1 mm
1 mm
1 mm
1 mm
1 mm
1 mm
1 mm
(f) Material
Al
Al
Al Al Al Al Al
Al
(g) Swirl type
S­S
Co D-Ss
Counter
D-Ss
Co
T-Ss
Counter
D-Ss
Co
Q-Ss
Counter
Q-Ss in P-A
Counter
Q-Ss in C-A
After completing theory, design and selection of components for complete experimental
setup, basics and preparation methods of nanofluids along with experimental methods are
discussed in neat chapter.

Details

Pages
Type of Edition
Erstausgabe
Year
2016
ISBN (PDF)
9783960675495
ISBN (Softcover)
9783960670490
File size
12 MB
Language
English
Institution / College
G.H. Raisoni College of Engineering
Publication date
2016 (June)
Keywords
Heat Transfer Enhancement Heat Exchanger Twisted Tape Insert Nanofluid Tubular Heat Exchanger Friction Loss Thermal Performance Agricultural food industry Refrigeration Air conditioning
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Title: Heat Transfer Enhancement Techniques. With Special Attention to Passive Methods of Heat Transfer Enhancement
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