18

CHAPTER- 2

LITERATURE REVIEW

With the desired objectives as explained in the preceding chapter,

literature survey is carried out to ascertain the progress in the field of

heat transfer enhancement. The need to save energy and to reduce

dimensions and cost of apparatus has stimulated the search for various

techniques of heat transfer enhancement. Heat transfer augmentation

technology has been developed and applied to heat exchanger

applications in the past few decades. Several attempts were made to

reduce the size and cost of the heat exchangers till date.

The literature in enhanced heat transfer is growing faster than that

for the engineering literature as a whole. At least fifteen percent of the

heat transfer literature is directed towards the techniques of heat

transfer augmentation now.

In the past decade, several studies on passive and active

techniques of heat transfer augmentation have been reported. Literature

review on active and passive techniques of heat transfer augmentation is

given in the following sections.

19

2.1 ACTIVE TECHNIQUES OF HEAT TRANSFER AUGMENTATION

Eid and Gomaa [24] conducted experimental investigations on

enhancement of heat transfer rate from thin planner fins by vibration.

The specimen, which had the thin planner fins, was heated by an electric

heater and was vibrated vertically with a frequency between 12.5 Hz to

50 Hz. They concluded that vibrations of a rather high frequency were

preferable to increase the percentage of heat transfer enhancement rates.

Experimental investigations on the flow induced vibration

characteristics of conical-ring turbulators on heat transfer enhancement

in heat exchangers were done by Yakut and Sahin [42] with Reynolds

numbers ranging from 8000 to 18000. The conical-rings, with 10, 20 and

30 mm pitches, were inserted in a pipeline in which air was used as the

working fluid. It was found that with the increase of Reynolds number,

Nusselt number also increased and the smallest pitch arrangement

resulted in maximum enhancement of heat transfer rate.

Kim et al. [20] examined the effects of mechanical vibrations on

critical heat flux (CHF) at atmospheric pressure in vertical annulus tube

under electrically heated condition. CHF was increased by mechanical

vibration up to 16.4%. An empirical correlation was suggested for the

prediction of CHF enhancement.

Ni et al. [71] reviewed the concept of mixing enhancement through

pulsation and oscillation. Cheng et al. [85] found that the vibration

induced by the pulsation flow at the low flow velocity could significantly

20

increase the convective heat transfer coefficient. The mechanical designer

has to ensure that the use of such intense vibrations would not result in

the damage or failure of the equipment. Hence, from the literature

review, it is understood that the use of vibrations to enhance the heat

transfer rates is not a practical alternative to other methods.

Guo and Dhir [30] investigated about the tangential injection in

single and two-phase flows. With tangential injection, average heat

transfer coefficient was observed to increase by 4 times in the range of

experimental parameters considered.

Esmaeilzadeh et al. [26] studied the application of EHD

(Electro Hydro Dynamic) actuator on local heat transfer enhancement by

using wire-plate electrodes in laminar and turbulent duct flow. The

obtained results for the flows with Re≤1000 showed that single wire-plate

electrode was suitable for local enhancement.

Nakabe K. et al. [67] conducted flow velocity measurements and

heat transfer experiments for a jet obliquely discharged into a duct. The

obtained velocity data revealed that the inclined jet was effective in

generating longitudinal vortices in the cross flow.

From the literature review, it is observed that, although active

techniques are effective in obtaining considerable heat transfer

enhancements, practical applications are limited, because of the

difficulty of reliably providing mechanical or electrical effect. They are not

given much focus due to their design, which is complex in nature, and

21

external power is not so easily available for all applications. Hence many

researchers preferred passive heat transfer enhancement techniques for

their simplicity and applicability for many applications. More over, they

do not need any external power input except for pump or blower power to

move the fluid. Hence, the present work is carried out using one of the

passive heat transfer augmentation techniques.

2.2 PASSIVE TECHNIQUES OF HEAT TRANSFER AUGMENTATION

The majority of commercially attractive enhancement techniques

are passive techniques. Literature review on passive techniques of heat

transfer augmentation is given below.

2.2.1 Treated Surfaces

These are primarily applicable to two-phase heat transfer and they

consist of a variety of structured surfaces (continuous or discontinuous

integral surface roughness or alterations) and coatings. Though the

treatment provides a roughness to the surface, it is not large enough to

influence single-phase heat transfer.

2.2.2 Rough Surfaces

The use of surface roughness in turbulent flow is one of the

simplest and highly effective techniques. It essentially disturbs the

viscous laminar sub-layer near the wall to promote higher momentum

and heat transport. Rough surfaces have been employed to enhance heat

22

transfer in single-phase flows both inside and outside the tubes.

Experimental investigations on the heat transfer enhancement in

horizontal tubes with spirally corrugated wall with Ethylene glycol as the

working fluid were performed in the Reynolds number range of

90 < Re < 800 by Rainieri and Pagliarini [90].

Vicente et al. [81] presented the experimental results for dimpled

tubes under laminar and transition flow regions .The experimental

results of laminar flow showed dimpled tube friction factors 10% to 30%

higher than the smooth tube ones.

Pedro et al. [126] conducted experiments using dimpled tubes in a

horizontal tube from laminar to transition regimes with water and

ethylene glycol as working fluids with Rayleigh number varying from

106–108. The friction factors were 10% and 30% higher than the smooth

tube ones.

The hydraulic behavior of dimpled tubes was observed to depend

mainly on dimple height. The heat transfer enhancement of transverse

ribs in circular tubes was experimentally investigated by San and Huang

[39] with air as working fluid in the Reynolds number range of

4608–12,936. The mean heat transfer and friction data were obtained for

airflow. They observed that the effect of rib height was stronger than that

of rib pitch on the Nusselt number obtained.

23

2.2.3 Extended Surfaces

Extended or finned surfaces are most widely used techniques that

include finned tube for Shell & tube exchangers, plate fins for compact

heat exchanger and finned heat sinks for electronic cooling. Enhanced

heat transfers from finned surfaces for buoyancy driven natural or free

convection has been considered primarily for cooling of electrical and

electronic devices and for hot water base - board room heaters. By using

segmented or interrupted longitudinal fins inside circular tubes, heat

transfer can be increased by periodically disrupting and restarting the

boundary layer on the finned surface and perturbing the bulk flow field.

Lawson et al. [63] investigated the use of delta winglets to enhance

heat transfer rates on the tube surface of louvered fin heat exchangers. It

was observed that, delta winglets placed on louvered fins could augment

the heat transfer along the tube wall by 47% with a corresponding

increase in pressure loss of 19%.

Heat transfer and pressure drop experiments were conducted

using CD 15W/40 lubricant oil in the punched HPD

(High Pressure Direction) type steel offset strip fin arrays by Guo et al.

[49]. Mass flow rate of oil was varied from 100 to 1000 kg/h, and oil inlet

temperature was maintained at 90°C. The Colburn factor and the

Fanning friction factor for different samples of offset strip fins were

obtained in the Reynolds number range of 30 to 500. The authors

24

observed that, with the increase of fin height and width, the Colburn

factor and the friction factor increased at the same Reynolds number.

Bergles et al. [7] conducted experimental investigations with

internally finned tubes. The heat transfer and friction characteristics of

eight internally finned tubes were determined under turbulent flow

region. The tubes showed a heat transfer performance improvement of

25 to 170% for the same pumping power.

Zeitoun and Hegazy [137] presented an analysis for fully developed

laminar convective heat transfer in a pipe provided with internal

longitudinal fins. The fins were arranged in two groups of different

heights. They concluded that, use of different fin heights in internally

finned pipes enables the enhancement of heat transfer at reasonably low

friction coefficients.

Al-Sarkhi et al. [4] conducted numerical investigations on the

convection heat transfer of a vertical internally finned tube. The

governing equations were solved numerically using the control volume

technique. They found that the height and number of the radial fins

would determine the velocity and temperature distributions inside the

tube.

2.2.4 Displaced Enhancement Devices

Several types of inserts which are categorized as displaced

enhancement devices include static mixer elements, metallic mesh,

discs, wire matrix inserts, rings or balls which tend to displace the fluid

25

from the core of the channel to its heated or cooled wall and vice versa,

keeping the heat transfer surface unaltered. Wire matrix tube insert

fitted inside a circular tube is shown in figure 2.1.

Fig. 2.1 Wire matrix tube insert

Fig. 2.2: Conical Ring inserts in circular tubes

a:- Diverging Ring b:- Converging Ring

c:- Converging and Diverging Rings l:- length of cone L:- length of tube

26

Different types of conical ring inserts used in circular tubes are

shown in figure 2.2. Inserts, as the earliest approach to the tube-side

enhancement, are convenient and the cheapest way to upgrade the

efficiency of an already existing apparatus. These inserts do not alter

heat transfer surface and provide a lot of scope for inter-mixing of the

fluid particles causing enhancement of convective heat transfer. Heat

transfer enhancement is mainly due to flow blockage and partitioning of

the flow. But the flow blockage in turn increases the pressure drop also.

Blockage increases the flow velocity in some situations and leads to a

significant secondary flow. The secondary flow creates a swirl in the fluid

that provides a better thermal contact between the fluid and the surface.

The mixing of fluid improves the temperature gradient, which ultimately

leads to enhancement of heat transfer.

2.2.5 Swirl Flow Devices

Swirl flow devices generally consist of a variety of tube inserts,

geometrically varied flow arrangements and duct geometry modifications

that produce flows. Twisted tape inserts are the most widely used swirl

flow devices for single-phase flows. These inserts increase the heat

transfer coefficient with a relatively small increase in pressure drop.

Twisted tape insert is shown in figure 2.3.

27

Fig.2.3 Twisted Tape insert

Twisted tapes can be used in the existing shell and tube heat

exchangers to upgrade their heat duties. If they are employed in a new

exchanger for a specified heat duty, significant reduction in size can be

obtained. The ease of fitting multiple bundles with tape inserts and their

removal makes them useful in fouling situations, where frequent tube-

side cleaning may be required.

2.2.5.1 Inserts

Inserts are used in both the above categories namely: Displaced

enhancement devices and swirl flow devices. Many different types of

inserts have been evaluated and examined by various investigators in

order to enhance the heat transfer rates in heat exchangers. The thermo

hydraulic performance of an insert is determined with reference to its

ability to enhance the heat transfer coefficient with a minimum increase

in friction factor.

Experiments on enhancement of heat transfer in a tube using

inserts have been widely reported. Heat exchanger tube inserts have

been used for many years as reliable means for heat transfer

28

enhancement and fouling mitigation in petroleum refineries and chemical

plants. In the heating mode operation, the rotating flow is noted to have

favorable centrifugal convection effect, which can increase the heat

transfer coefficient. Types of inserts are:

Twisted tape inserts

Longitudinal strip inserts

Mesh inserts

Brush inserts

Louvered strip inserts

Conical ring inserts

Wire coil inserts etc.

2.2.5.2 Optimization

Usually, an increase in heat transfer is obtained by an increase in

pressure drop. In some cases, where heat transfer coefficients are

increased by about 4 times, friction factors are increased to about 50

times. An increased friction factor implies increased power requirement

for pumping the fluid. Hence it is equally important to reduce the friction

loss while increasing the heat transfer rate. The best technique is the one

that results in an optimum value of both the parameters.

Inserts generating swirl flows are highly attractive because the

convective heat transfer coefficient is greatly increased by the centrifugal

effect of swirl flow. An increase in the heat transfer inevitably causes an

29

increase in pressure drop, which, some times may be larger than the

increase of heat transfer. On the other hand, even with lower heat

transfer augmentation, the insert, which does not generate swirl flow,

may also be an eligible augmentative device. Therefore, it is necessary to

select appropriate inserts to optimize the heat transfer augmentation.

Literature review on inserts used in different geometries like

horizontal tube, double pipe, shell and tube heat exchangers and other

geometries is given in the following sections.

2.3 HEAT TRANSFER ENHANCEMENT USING NUMERICAL METHODS

Nasr et al. [34] in their study presented the application of

Artificial Neural Networks (ANNs) to determine the performance of helical

wire coil inserts inside the horizontal tube. Experimental investigations

were also carried out to determine the heat transfer enhancement and

pressure drop in the presence of wire coil inserts with Reynolds numbers

ranging from 4200 to 49,000.

The performance of a heat exchanger in the presence of helical

baffles was studied numerically and experimentally by Gang Lei [134].

Computational fluid dynamics method was used to simulate the heat

exchanger with helical baffles. The experiment was conducted with hot

oil on shell side and with cold water on tube side. It was concluded that

CFD predictions could be safely relied upon in the design and

optimisation of a new geometric configuration.

30

Numerical and experimental investigations were conducted on the

heat transfer characteristics of a helically baffled heat exchanger by

Zhang et al. [140]. The commercial CFD program Fluent 6.0 was used to

simulate the performance of the heat exchanger.

Numerical and experimental investigations on heat transfer and

pressure drop in a tube with circular cross sectional rings were carried

out by Ozceyhan et al. [125]. Numerical analysis was performed with

FLUENT 6.1.22 code. They observed that, with increase in Reynolds

number, there was an increase in Nusselt number and decrease in the

friction factor.

Numerical and experimental investigations on heat transfer in

helically coiled heat exchangers were carried out by Jayakumar et al.

[36]. They found that, specification of conjugate heat transfer conditions

was appropriate compared to specification of constant temperature or

constant heat flux boundary condition for the heat exchanger in

modeling. They fabricated experimental setup to estimate the heat

transfer characteristics of helically coiled heat exchanger. Using the CFD

package FLUENT 6.2, they compared the experimental results with the

CFD calculation results and correlations were developed for calculation

of the inner heat transfer coefficient of the helical coil based on the

experimental results.

Numerical simulation for the heat transfer enhancement in a tube

fitted with right-left helical inserts under laminar flow conditions was

31

performed by Nagarajan and Sivashanmugam [66] using Fluent 6.2.16

version.

Numerical simulation for the heat transfer enhancement in a tube

fitted with regularly spaced helical inserts under laminar flow conditions

was discussed by Sivashanmugam et al. [111] using Fluent version

6.2.16. The results of simulation matched with the literature value for

plain tube with the deviation of less than ± 9% for Nusselt number and

± 15 % for friction factor.

Krishna et al. [108] carried out experimental and numerical

investigations on heat transfer characteristics of circular tube fitted with

half twist insert. Masoud Rahimi [59] reported numerical and

experimental investigations on the friction factor and Nusselt number of

a tube equipped with the classic and modified twisted tape inserts.

Conte and Peng [19] studied numerically and experimentally regarding

convective heat transfer through small and compact coiled pipe heat

exchangers.

A CFD package (PHOENICS 3.3) was used to numerically study the

heat transfer characteristics of a double-pipe helical heat exchanger by

Timothy Rennie et al. [122]. A computational study was made by

Sreenivasulu and Prasad [116] on convective heat transfer in an

annulus with its inner cylinder wrapped by a helical wire within a

Reynolds number range of 20000 to 180000. It was observed that the

32

performance ratio for the wrapped wire annulus was much greater than

one.

2.4 TURBULENT FLOW HEAT TRANSFER ENHANCEMENT IN HORIZONTAL TUBES USING INSERTS

An experimental study was performed by Promvonge [87] to

investigate the airflow friction and heat transfer characteristics in a

round tube fitted with coiled square wire turbulators in the turbulent

regime with Reynolds numbers ranging from 5000 to 25,000. The

evaluated performance of coil wires was quite similar: at Re = 5000 and

25000, heat transfer enhancement efficiency (η) was 1.2–1.3 and 1.1–

1.15 respectively. He concluded that the coiled square wire should be

used in place of conventional round one to obtain higher heat transfer

rates.

Sivashanmugam and Nagarajan [109] conducted experimental

investigations to estimate the enhancement of heat transfer in a circular

tube fitted with right-left helical screw inserts of different twist ratios.

A maximum performance ratio of 2.97 was obtained in the presence of

helical screw tape inserts.

Ahmet [1] focused on the investigation of forced convection and

entropy generation characteristics of air in a circular pipe under

turbulent flow with baffle inserts. Experimental investigations were

conducted on nine baffle-inserted tubes with Reynolds numbers varying

from 3000 to 20,000.

33

Lopina and Bergles [50] reported a detailed experimental study on

heat transfer and friction factor characteristics in the presence of twisted

tape inserts with water as working fluid. Due to the increased circulation

and tape fin effect, 100% heat transfer enhancement was obtained.

Smith and Promvonge [113] conducted experiments on heat

transfer enhancement using diamond shaped inserts. Due to the flow

blockage, the inserts caused high frictional losses.

Dean and Peter [21] used stainless steel pall rings for insertion into

a copper tube of 20 mm inside diameter. The pall rings were easily

inserted into the tube to create roughness at the tube wall. They

postulated that for small ring spacing, the fluid would rapidly become

mixed upon entering the packed length.

Sibel et al. [106] conducted experiments using equilateral triangle

cross-sectioned coiled wire inserts. Due to the presence of inserts, both

heat transfer and pressure drop increased over the smooth tube.

Maximum overall enhancement efficiency obtained was 36.5%.

Chang et al. [104] devised a broken twisted tape insert with twist

ratio in between 1 and 2.5. It was tested for its thermal performance with

Reynolds numbers ranging from 1000 to 40,000. Maximum Nusselt

number increase obtained was 2.4 times that of the continuous twisted

tape insert. An elaborative literature survey about all types of heat

transfer enhancement techniques with external inserts was given by

Bergles et al. [10].

34

Sivashanmugam and Suresh [110] performed experimental

investigations to estimate the enhancement of heat transfer in a circular

tube fitted with helical screw elements of different twist ratios. The data

obtained from experimental investigations was compared with the data

obtained from plain tube to assess the performance of the insert.

Betul and Bali [11] conducted experimental investigations to

determine the heat transfer and friction factor characteristics in a

horizontal pipe by the insertion of vortex generators with Reynolds

numbers ranging from 5000 to 30000. Nusselt numbers were increased

from 18% to 163% compared to smooth pipe.

Experimental investigations were conducted to determine the

combined effect of C-nozzle turbulators together with snail entry on heat

transfer rate and pressure drop characteristics in a uniform heat flux

tube with air as the working fluid. Reynolds numbers ranged from 8000

to 18000 during the investigations conducted by Promvonge and Eiamsa

[88]. At smaller pitch ratios, considerable enhancement in heat transfer

rate was obtained over the plain tube.

Experimental investigations were conducted to determine the

combined effect of V-nozzle turbulators together with snail entry inside a

uniform heat flux tube with air as the working fluid by Promvonge and

Eiamsa [86]. A set of converging-diverging nozzles like a venturi

structure, were placed inside the test tube and the snail was mounted at

the entrance of the tube to create a swirl flow.

35

Hsieh et al. [103] conducted experimental investigations for

determining friction factor in a horizontal tube fitted with longitudinal

and crossed strip inserts. Air was used as the working fluid with

Reynolds numbers ranging from 6500 to 19500. They observed that

friction factor rise due to the presence of inserts was typically between

1.1 and 1.5 with respect to bare tube.

Hsieh et al. [102] conducted experimental investigations on heat

transfer enhancement inside a horizontal circular tube fitted with

longitudinal and crossed strip inserts. Air was used as the working fluid

with Reynolds numbers ranging from 6500 to 19500. The heat transfer

enhancement of the tube in the presence of inserts was about two to four

times that of bare tube.

An experimental study was carried out by Pongjet [84] to determine

the friction and heat transfer characteristics in a round tube fitted with

snail entry and coiled wire turbulators. Reynolds numbers ranged from

5000 to 25,000 during the investigations. Maximum Nusselt number

ratio obtained was 3.9.

Gul and Evin [29] performed experimental investigations in the

Reynolds number range of 5000 to 30,000 to determine the heat transfer

and friction characteristics in the presence of short helical tape placed at

the entrance of the test section. A maximum enhancement of 300% in

local heat transfer coefficient was obtained.

36

Promvonge et al. [73] in his experiments used conical-ring and

twisted-tape swirl generator to enhance the heat transfer rates. Twisted-

tape was placed at the core of the conical-ring to create turbulence in the

test section. Air was used as working fluid with the Reynolds numbers

ranging from 6000 to 26,000. They observed that the tube fitted with

conical-ring together with twisted-tape inserts provided 4 to 10% higher

Nusselt number and enhancement efficiency values compared to those of

conical-ring alone.

Yakut and Sahin [41] conducted experimental investigations in the

Reynolds number range of 5000 to 35000, to determine the flow-induced

vibration, heat transfer and performance characteristics of coiled wire

turbulators. The performance ratios were higher at lower Reynolds

numbers than those at higher Reynolds numbers.

Experimental investigations with trapezoidal cut twisted tape

inserts for twist ratios of 6 and 4 were carried out by Murugesan

et al. [65] in turbulent flow region. They observed that Nusselt number

and friction factor increased with the decrease in twist ratio.

Performance ratio was greater than one; hence, they reported that

enhancement using trapezoidal cut twisted tape was competent in the

point of energy savings.

To assess the benefit of using enhancement techniques like twisted

tape, wire coiled and helically coiled ribbons in smooth pipes, extended

performance evaluation criteria was implemented by Zimparov and

37

Penchev [141]. A reduction in the temperature driving force reduces the

entropy generation and increases the second law efficiency [92].

Eiamsa et al. [25] conducted experimental investigations in the

Reynolds number range of 4600 to 20,000 on heat transfer enhancement

in a tube fitted with combined devices i.e., twisted tape and wire coil. Air

was used as working fluid. It was observed that, compared to each

enhancement device alone, the compound devices further increased the

heat transfer rates.

Rahai and Wong [91] conducted experimental investigations on

turbulent jets from round tubes with coil inserts and compared the

results with the corresponding results for a smooth tube. They concluded

that, a coil insert with a proper configuration could be used to enhance

the mixing process and could be an effective mixer in an HVAC air

delivery system.

Angirasa [22] conducted experiments with Reynolds numbers

ranging from 17,000 to 29,000 by using metallic fibrous materials with

two different porosities of 97% and 93%. The improvement in the average

Nusselt number was about 3-6 times in comparison with the case when

no porous material was used.

Mehmet and Kuzay [61] studied the enhanced heat transfer in

round tubes filled with rolled copper mesh with Reynolds numbers

ranging from 5000 to 19,000. Water was used as the energy transport

38

fluid. The increase in heat transfer coefficient was ten times compared to

that of plain tube.

2.5 LAMINAR FLOW HEAT TRANSFER ENHANCEMENT IN HORIZONTAL TUBES USING INSERTS

Hsieh and Huang [101] conducted experimental investigations with

Re<=4000 to determine the heat transfer and friction factor

characteristics of water in horizontal tube in the presence of square,

rectangular and crossed strip inserts with aspect ratio varying from

1 to 4. They reported that enhancement of heat transfer at the same

Reynolds number was 16 times with a friction factor rise of only 4.5

times when compared to plain tube.

The results of the heat transfer enhancement and pressure drop in

the Reynolds number range of 100 to 4030 in small tubes with an inside

diameter of 2.0 mm inserted with different inserts were presented by

Wen [129].

Saha and Langille [94] presented the experimental data for friction

factor and Nusselt number in a horizontal tube under uniform wall heat

flux condition fitted with longitudinal full-length strip, short-length strip

and regularly spaced strip elements.

Liao [48] conducted experiments to study the heat transfer and

friction characteristics for water, ethylene glycol and ISOVG46 turbine

oils with Reynolds numbers ranging from 80 to 50,000. The above fluids

were flowing inside four tubes with three-dimensional internal extended

39

surfaces. They found that for VG46 turbine oil, the Stanton number and

friction factor could be increased up to 5.8 times and 6.5 times

respectively compared to plain tube.

Gosselin and Alexandre [51] considered three types of thermal

enhancers i.e. internal fins, porous medium fillings and insertion of high

conductivity solid particles for their study. Their objective was to

maximize the heat transfer rate from the pipe to the cold fluid.

Bali and Ayhan [123] showed that the propeller type swirl

generator provided considerable enhancement of convective heat transfer

coefficients. Comparison of the Nusselt numbers for swirl flow with those

of smooth pipe revealed that the Nusselt number increased considerably

in the presence of swirl generator.

Garcia et al. [3] conducted experimental investigations in laminar

and transition regimes on three wire coils of different pitches inserted in

a smooth tube. They reported that, wire inserts performed better than

twisted tapes in the Reynolds number range of 700 to 2500.

Garcia [27] carried out experimental analysis in laminar and

transition regimes with wire coil inserts using hydrogen bubble

visualization and PIV techniques. They observed that at Reynolds

numbers lower than 500, the flow in tubes with wire inserts was similar

to the flow in a smooth tube.

A new method was postulated by Sarma et al. [99] to determine the

heat transfer coefficients in a tube fitted with twisted tape inserts. To

40

enhance the heat transfer rates from the tube wall, the wall shear and

the temperature gradients were modified.

Garcia et al. [28] conducted experimental analysis with six wire

coils inserted in a smooth tube with Reynolds numbers ranging from

80 to 90,000, covering laminar, transition and turbulent regimes.

Hong and Bergles [31] tested the influence of twisted tapes on

enhancement of heat transfer with water and ethylene glycol as test

fluids in the laminar flow region. They obtained a 1000% enhancement in

Nusselt number.

Experimental investigations with short-length and full-length

twisted tape inserts were carried out by Saha and Dutta [93].

Short-length twisted-tapes up to 33 percent length of the tube were

found to perform better than the full-length twisted-tapes on the basis of

constant pumping power and heat duty.

Experimental investigations on heat transfer and flow friction

characteristics of a tape generated swirl flow inside a 25 mm inside

diameter circular tube were presented by Patil A.G. [79]. In order to

reduce excessive pressure drops associated with full width twisted tapes,

with less corresponding reduction in heat transfer coefficients, reduced

width twisted tapes of widths ranging from 11.0 to 23.8 mm, which were

lower than the tube inside diameter were used. Reduced width twisted

tape inserts gave 18% to 56% lower isothermal friction factors than full

width tapes. Nusselt numbers were decreased only slightly by 5% and

41

25%, for tape widths of 19.7 and 11.0 mm, respectively. They reported

that, tapes of width 19.7 mm performed more or less like full width tapes

on the basis of constant pumping power. The reduced width tapes offered

20% to 50% savings in the tape material as compared to the full width

tapes. They reported that, use of reduced width tapes was beneficial in

terms of economy also.

Experiments were conducted to determine the heat transfer

enhancement in a round tube fitted with wire coil as a turbulator by

Yukitsugu Shoji et al. [135]. The test fluid used was water.

Hantsch et al. [5] performed experiments using fiber array inserts

made of Silicon Carbide and stainless steel. For both long and short

externally heated tubes, heat transfer and friction factor characteristics

were studied. Depending on the mass flow rate and wall temperature,

heat transfer with SiC fibers was 100% more compared to without fibers.

They could also observe that by using stainless steel fibers, the

enhancement was only little lower.

Pavel and Mohamad [13] conducted experimental investigations in

the Reynolds number range of 1000 to 4500 to determine the effect of

metallic porous inserts in a pipe subjected to constant heat flux.

Maximum increase in the Nusselt number in the presence of porous

inserts was 5.2 times in comparison with the plain tube with a highest

pressure drop of 64.8 Pa.

42

Numerical and experimental investigations in a tube fitted with

metallic porous materials, on the rate of heat transfer were investigated

by Pavel and Mohamad [14]. Experiments were carried out at constant

wall heat flux with Reynolds numbers ranging from 1000 to 4500. They

concluded that, higher heat transfer rates could be obtained using

porous inserts at the expense of a reasonable pressure drop.

2.6 DUCTS AND CHANNELS USING INSERTS FOR HEAT TRANSFER ENHANCEMENT

Saha and Mallick [95] experimentally investigated the heat transfer

and pressure drop characteristics in rectangular and square ducts fitted

with twisted tape inserts under laminar flow conditions. They reported

that regularly spaced twisted tape elements performed significantly better

than the full length twisted tapes.

Lu and Jiang [52] numerically and experimentally investigated the

effect of different angled ribs in a rectangular channel. The numerical

results indicated that the heat transfer coefficients were largest with the

60° ribs, but the channel with the 20° ribs had the best overall thermo

hydraulic performance considering the heat transfer and pressure drop.

Experimental investigations on the convective heat transfer in a channel

fitted with perforated ribs were presented by Buchilin [15] for the

Reynolds numbers ranging from 30000 to 60000. Jordan [38]

investigated the steady-state statistics of the turbulence inside a rib-wall

circular duct by the large-eddy simulation methodology. Wang et al. [127]

43

experimentally investigated the use of filament insert to enhance the heat

transfer rates in a square channel.

2.7 DOUBLE PIPE HEAT EXCHANGERS USING INSERTS FOR

HEAT TRANSFER ENHANCEMENT

Twisted tape inserts fitted inside the pipe act as turbulence

promoters. Experimental investigations were conducted by Anil Singh

Yadav [6] to evaluate the heat transfer performance of double pipe heat

exchanger using full length twisted tapes fitted in the inner pipe of

double pipe heat exchanger. Hot oil was introduced into the inner pipe

with cold water flowing on the shell side. Experiments were conducted for

different mass flow rates of oil. The presence of insert could increase the

heat transfer coefficient by 60% compared to that of conventional heat

exchanger.

Zio [33] performed experimental investigations by heating streams of

air, water and glycerin in tubes of different diameters furnished with

perforated baffles of various types. The results were presented as a

correlation between Nusselt and Prandtl numbers. A method was

discussed regarding momentum transfer and fluid flow field in the

system in relation to baffle parameters. A detailed review on the

application of twisted tape inserts in concentric heat exchangers in

improving their performance was presented by Manglik and Bergles [53].

Zhang et al. [139] experimentally investigated the performance of a

helically baffled single tube heat exchanger. The tested tubes included

44

one smooth tube and five petal-shaped fin tubes with different

geometrical parameters for improving the heat transfer on the shell side.

Smith Eiamsa-ard and Pongjet Promvonge [114] experimentally

investigated heat transfer and flow friction characteristics of a helical

screw tape with or without core rod in the Reynolds number range of

2000 to 12000 in a concentric double tube heat exchanger. Water was

used as the working fluid. They concluded that the increase in Nusselt

number of using helical tape with and without core rod was 230% and

340% respectively compared to plain tube.

Heat transfer and friction characteristics were investigated

experimentally by using louvered strip inserts inserted in the inner tube

of a concentric tube heat exchanger in the Reynolds number range of

6000 to 42000 by Smith et al. [115]. Water was used as the working

fluid. Experimental results were obtained for louvered strips with forward

and backward arrangement for various inclined angles (15°, 25° and

30°). Highest heat transfer rate was obtained for the backward inclined

angle of 30°. The increases in average Nusselt number and friction loss

for the inclined forward louvered strip were 284% and 413% while those

for the backward louvered strip were 263% and 233% respectively over

the plain tube.

The heat transfer characteristics and the pressure drop of the

horizontal double pipe with coil wire insert in the Reynolds number

range of 8000 to 24000 was experimentally investigated by Paisarn [76].

45

He concluded that coil wire insert could significantly enhance heat

transfer in laminar flow region.

Experimental investigations were conducted on the effect of star

shaped internal aluminum fin inserts in the Reynolds number range of

1900 to 47000 in a concentric tube heat exchanger by

Leonard et al. [46]. Water was used as working fluid. Overall heat

transfer coefficient was increased by 51% due to the presence of insert.

Paisarn Naphon [77] experimentally investigated the heat transfer

characteristics and pressure drop in a horizontal double pipe heat

exchanger in the presence of twisted tape insert. The twisted tape insert

showed significant effect on enhancing the heat transfer rates.

Paisarn Naphon et al. [78] experimentally investigated the heat

transfer characteristics and pressure drop in a horizontal double pipe

heat exchanger in the presence of helical tape and the results were

compared with those of without helical ribs.

Watcharin et al. [128] experimentally investigated the effect of twisted

tape inserts at different twist ratios on the heat transfer and flow friction

characteristics in a double pipe heat exchanger. Maximum Nusselt

numbers obtained by using twisted tape inserts were 188% higher than

those of plain tube.

Pahlavanzadeh et al. [74] experimentally investigated the effect of

wire coil and wire mesh inserts on the heat transfer and pressure drop

characteristics. Water was used as the working fluid. The heat transfer

46

rate and pressure drop increased by 22 - 28% and 46% respectively for

wire coil and 163 -174% and 500% respectively for wire mesh over the

plain tube values.

2.8 SHELL AND TUBE HEAT EXCHANGERS USING INSERTS FOR HEAT TRANSFER ENHANCEMENT

Nasiruddin and Kamran Siddiqui [69] conducted numerical

investigations on the heat transfer enhancement in a heat exchanger in

the presence of vortex generators. Air was used as the working fluid.

With the decrease of Reynolds numbers from 20,000 to 5000,

Nusselt number increased by more than a factor of two.

Simin Way [107] suggested that the sealers installed inside the

shell-and-tube heat exchanger could effectively block the baffle-shell gap

and decrease the circular leakage flow. He opined that, sealers are

convenient to install, guarantying safe and long lasting operation.

Xing Xiaokai [130] developed an innovative EAF

(Electromagnetic Anti-Fouling technology) for heat transfer enhancement

in a heat exchanger. A series of experiments with and without an EAF

device were performed.

Numerical and experimental analyses were carried out by

Yu-Wei Chiu et al. [136] to study thermo-hydraulic characteristics of

airflow inside a circular tube with different tube inserts, including

longitudinal strip inserts and twisted-tape inserts with different twisted

angles.

47

For the applications involving heat transfer over the combustion

chamber, heat transfer augmentation for air was obtained by using swirl

flow path by Kevat et al. [44]. They carried out experimental study for

different swirl pitches from 40, 60, 80, 100 and 120 mm incorporating

for each pitch, a combination of various heat inputs and flow rates of air.

Based on their work, they proposed correlations to predict the heat

transfer and pressure drop for Reynolds numbers ranging from 5000 to

30,000.

2.9 HEAT TRANSFER ENHANCEMENT IN OTHER TYPES OF HEAT EXCHANGERS

Jolly et al. [37] carried out research that had led to the

development of a computer program for predicting thermo fluid

properties including the heat transfer and pressure drop characteristics

of bayonet tube heat exchangers. Heat transfer augmentation was

identified as a possible inclusion in the code using empirical techniques.

The content of the code provides acceptable consideration of the effects of

conduction, convection and radiation for a variety of shell-side gases

using the relevant and accepted methods and equations.

Heat transfer and friction factor characteristics for a helically

baffled heat exchanger combined with petal shaped finned tubes was

experimentally investigated by Zhang et al. [138]. Oil (ISO VG-32) was

used as hot fluid and with water used as coolant in the heat exchanger.

48

To find the suitable correlations for designing the coiled finned-

tube heat exchangers used in cryogenic applications, experimental

investigations were carried out by Gupta et al. [85]. Experiments were

conducted in the range of effective Reynolds numbers varying from 500

to 1900.

Pesteei et al. [83] conducted exhaustive experimental study on the

use of winglet pairs in finned tube heat exchangers. They concluded that

the enhancement in heat transfer coefficient and pressure drop were

46% and 18% respectively.

2.10 HEAT TRANSFER ENHANCEMENT IN OTHER GEOMETRIES

Manglik and Bergles [54, 55 and 56] presented the mechanistic

parameters in their papers to identify the effect of swirl on the flow field.

Rabas et al. [89] proposed a new correlation scheme to predict the heat

transfer coefficient and friction factor for the spirally- grooved tubes.

Pei-Xue et al [82] investigated the forced convection heat transfer

of water and air in sintered porous plate channels. The effects of the

heights and widths of the hexagonal fins on the heat transfer and

pressure-drop characteristics were investigated using the

Taguchi experimental-design method by Yakut et al. [132]. Junkhan et

al. [40] and Bergles et al. [9] experimentally investigated the influence of

three inserts in fire-tube boilers i.e. two bent-strips and one twisted tape.

At the Reynolds number of 10,700, the heat transfer enhancements for

49

these three inserts were 125%, 157% and 65% respectively compared to

plain tube.

Nirmalan et al. [72] conducted visual studies on seven different

bent-strip types of inserts in order to identify the effect of the inserts on

the flow characteristics in a fire-tube boiler. Chang [17] conducted

experimental investigations with air as working fluid in the Reynolds

number range of 600 to 10000 with 450 ribs in a reciprocating duct. They

concluded that ribbed wall increases heat transfer by 260–300%

compared to smooth wall.

Martemianov et al. [58] their study showed that, traditional

correlations for swirl flows are insufficient. Mehmet et al. [62] studied the

effect of the geometry of the deflecting element on the heat transfer and

fluid friction characteristics. The heat transfer rates in a drag-reducing

flow were examined by Yeo et al. [133] for surfactant concentrations of

70, 80 and 90 parts per million, at Reynolds numbers of 7,000, 12,000

and 16,200 respectively.

Experimental investigations were carried out on the heat transfer

enhancement and pressure drop characteristics in the presence of

twisted tape inserts, during the flow boiling of R-134a, inside a horizontal

evaporator by Akhavan et al. [2].

Li Xiao et al. [47] conducted experiments on water tunnel using

dye injection in a longitudinal vortex flow with two kinds of ribs of

different widths. Moawed et al. [64] experimentally investigated the heat

50

transfer and pressure drop characteristics of a sinusoidal pipe fitted

inside a straight pipe.

Kenan et al. [43] carried out experimental investigations for tapes

with double-sided delta-winglets under different geometrical and flow

parameters. Paisarn Naphon et al. [75] investigated the effect of

curvature ratios on the heat transfer and flow developments in the

horizontal spirally coiled tubes.

Jaisankar et al. [35] experimentally studied about tubes fitted with

Left - Right twisted tape inserts of various twist ratios to enhance the

convective heat transfer rates for thermo- syphon solar water heater

system. The swirl flow was induced by Left-Right twisted tape in

clockwise and counterclockwise direction inside the riser tube, which

enhanced the heat transfer rates. The various parameters such as

Nusselt number, friction factor and thermal efficiency were studied.

Xuelei Chen and Sutton [131] proposed a combined

convective – radiative heat transfer computation scheme to solve

combined problems, with and without the porous core. In the presence of

insert, the radiative heat transfer and convective heat transfer both were

enhanced. With porous insert, the convective and radiative Nusselt

numbers increased up to 35% and 105% respectively.

51

2.11 HEAT TRANSFER ENHANCEMENT USING NANO FLUIDS AND INSERTS

Experimental investigations were performed to determine the

enhancement of heat transfer coefficient of steam in a horizontal tube

fitted with helical twisted tape inserts by Sarma et al. [98]. The results

indicated substantial enhancement in the condensation heat transfer

coefficients using the tape inserts. The ratio of pitch to diameter ranged

from 2.5 to 10. Liquid Reynolds number varied from 100 to 1000 and

that of vapor from 900 to 100000.

Syam and Sharma [117] conducted experiments with longitudinal

strip inserts to enhance the heat transfer rates. The strips considered for

heat transfer behavior in a circular tube were of rectangular and square

cross sections with different aspect ratios (AR). The test section was

maintained constant wall heat flux boundary condition. The Reynolds

number ranged from 4000 to 10,000. Highest heat transfer rates and

pressure drop were obtained for the longitudinal strip insert of aspect

ratio, AR = 1. The enhancement of heat transfer coefficient as compared

to a conventional bare tube at the same Reynolds number was found to

be about a factor of 22 at Re = 10,000, while the friction factor rise was

about a factor of 3.5 at the same Reynolds number.

A numerical study of heat transfer and friction factor

characteristics of Al2O3 nanofluid in a circular tube was carried out by

Syam and Sharma [118]. At the same Reynolds number of 10000 and

52

30,000, Nusselt number for 0.1% Al2O3 nano fluids increased by 1.24

and 1.22 times respectively over the base fluid. They observed that heat

transfer coefficient of nanofluid increased with increase in the volume

concentration of nanofluid and Reynolds number. Pressure losses

increased with the increase in volume concentration of the nanofluid.

Convective heat transfer of nano fluids inside a circular tube with

twisted tape inserts was experimentally investigated by Sarma et al. [97].

Experiments were carried out in the transition flow regime with Al2O3

nano fluid with and without tape inserts. They observed that the use of

Al2O3 nano particles as a dispersed phase in water could significantly

enhance convective heat transfer, and the enhancement increased with

the Reynolds number as well as particle concentration. Heat transfer

enhancement was further increased by using twisted tape insert inside

the circular tube.

2.12 COMPOUND HEAT TRANSFER ENHANCEMENT

Bergles et al. [8] conducted experimental investigations to quantify

the effect of surface roughness and twisted tapes on the heat transfer

enhancement. The effect of combining these two techniques was

investigated in the turbulent region. The combination provided further

improvement as expected.

Megerlin et al. [60] experimentally studied the effect of brush and

mesh type inserts. They observed that both inserts gave up to 1000

53

percent improvements in heat transfer coefficients while the pressure

drop penalty was high up to 20 times, as compared to plain tubes.

An experimental investigation of tube side flow and heat transfer

under laminar flow conditions for internally finned tube and for the tube

fitted with twisted tape insert was reported by Marner and Bergles [57].

Polybutene 20, was used as the test fluid.

Chang et al. [105] conducted experiments on compound heat

transfer enhancement in a tube fitted with serrated twisted tape in the

Reynolds number range of 5000 to 25000. The heat transfer

enhancement due to the presence of serrated twisted tape was obtained

as 250 to 480% when compared to plain tube.

Some of the passive techniques for single phase heat transfer were

discussed by Dewan [23], Zimparov [124]. They reported that, compound

augmentation method was one of the most promising heat transfer

augmentation techniques.

Experimental investigations on heat transfer and friction

characteristics of spirally corrugated tubes combined with twisted tape

inserts were performed by Zimparov [144] with the Reynolds numbers

varying from 3000 to 60000. Water was used as the working fluid.

Eiamsa and Promvonge [112] experimentally investigated the heat

transfer and flow friction characteristics in a circular wavy-surfaced tube

fitted with a helical-tape insert. They observed that the heat transfer rate

54

from using the wavy surface combined with helical tape insert was

significantly higher than that of plain tube.

Bharadwaj et al. [12] experimentally investigated the combined

effect of spirally grooved tube with and without twisted tapes. Water was

used as working fluid. Zimparov [142,145] created a mathematical model

to predict the friction factors and heat transfer coefficients for a spirally

corrugated tube combined with twisted tape insert.

2.13 SCOPE OF THE PRESENT WORK

From the literature review, it was found that, experimental

investigations using longitudinal strip inserts of aspect ratio AR=1 and 4

were carried out in turbulent region where as numerical investigations

using the same are scarce. Hence, in the present work, preliminary

numerical investigations on heat transfer enhancement in turbulent

region with longitudinal strip inserts of aspect ratio AR=1 and AR=2.5

were carried out followed by experimental investigations using the

experimental setup. Each of the two strips was inserted inside the

horizontal circular tube of the experimental setup through which air

from the blower was forced to flow. Heat transfer and friction factor

characteristics were investigated employing these inserts. The results are

compared with those of plain tube to estimate the enhancement of heat

transfer rate in the presence of inserts.

55

Secondly, from the literature review, it was found that, Pavel and

Mohamad [13] focused on the heat transfer enhancement inside tubes

using mesh inserts under laminar flow region. Numerical and

experimental investigations on heat transfer enhancement using mesh

inserts in turbulent region are not reported till date. Hence, the present

work attempts to address this deficiency by performing numerical and

experimental investigations on turbulent flow heat transfer enhancement

using mesh inserts in the horizontal circular tube.

The results of the above two numerical and experimental

investigations have indicated that, although heat transfer

enhancements are obtained, frictional resistances are also increased

with the presence of inserts inside the tubes. Therefore, the present

study is taken up adopting a novel concept in augmenting the heat

transfer rate by using louvered strip inserts with circular, square and

trapezoidal geometries which are developed in the laboratory and

experimentally investigated. Finally, performance comparison of all the

inserts is made and the optimum geometry is recommended.

Literature survey is carried out up to the year 2010. With this in-

depth study of Literature Review, objectives and methodology adopted

in this research work are presented in Chapter- 3.