Review of passive heat transfer augmentation techniques

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Review of passive heat transfer augmentation techniques A Dewan1 , P Mahanta1, K Sumithra Raju1 and P Suresh Kumar2 1 Department of Mechanical Engineering, Indian Institute of Technology, Guwahati, India 2 Department of Ocean Engineering and Naval Architecture, Indian Institute of Technology, Kharagpur, India

Abstract: Heat transfer augmentation techniques (passive, active or a combination of passive and active methods) are commonly used in areas such as process industries, heating and cooling in evaporators, thermal power plants, air-conditioning equipment, refrigerators, radiators for space vehicles, automobiles, etc. Passive techniques, where inserts are used in the flow passage to augment the heat transfer rate, are advantageous compared with active techniques, because the insert manufacturing process is simple and these techniques can be easily employed in an existing heat exchanger. In design of compact heat exchangers, passive techniques of heat transfer augmentation can play an important role if a proper passive insert configuration can be selected according to the heat exchanger working condition (both flow and heat transfer conditions). In the past decade, several studies on the passive techniques of heat transfer augmentation have been reported. The present paper is a review on progress with the passive augmentation techniques in the recent past and will be useful to designers implementing passive augmentation techniques in heat exchange. Twisted tapes, wire coils, ribs, fins, dimples, etc., are the most commonly used passive heat transfer augmentation tools. In the present paper, emphasis is given to works dealing with twisted tapes and wire coils because, according to recent studies, these are known to be economic heat transfer augmentation tools. The former insert is found to be suitable in a laminar flow regime and the latter is suitable for turbulent flow. The thermohydraulic behaviour of an insert mainly depends on the flow conditions (laminar or turbulent) apart from the insert configurations. The present review is organized in five different sections: twisted tape in laminar flow; twisted tape in turbulent flow; wire coil in laminar flow; wire coil in turbulent flow; other inserts such as ribs, fins, dimples, etc. Keywords:

heat transfer augmentation, thermohydraulic performance, twisted tape, wire coil

NOTATION di do f f0 h H H1, H2, H3, H4 k n Nu Nu0 Pr

internal diameter of the tube (m) outer diameter of the tube (m) friction factor with an insert friction factor without any insert convective heat transfer coefficient pitch length (m) different pitch lengths in descending order thermal conductivity exponent Nusselt number with an insert Nusselt number without any insert fluid Prandtl number

The MS was received on 30 March 2004 and was accepted after revision for publication on 19 May 2004 Corresponding author: Institute of Fluid Mechanics (LSTM), FriedrichAlexander University of Erlangen-Nuremburg, Cauerstrasse 4, D-91058 Erlangen, Germany.

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Re SW S t TT y

Reynolds number swirl parameter spacing between two twisted tapes insert thickness (m) twisted tape twist ratio

a n f

molecular diffusivity of heat molecular diffusivity of momentum correction factor

1

INTRODUCTION

Heat exchangers have several industrial and engineering applications. The design procedure of heat exchangers is quite complicated, as it needs exact analysis of heat transfer rate and pressure drop estimations apart from issues such as long-term performance and the economic aspect of the equipment. The major challenge in designing a heat exchanger is Proc. Instn Mech. Engrs Vol. 218 Part A: J. Power and Energy

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A DEWAN, P MAHANTA, K SUMITHRA RAJU AND P SURESH KUMAR

to make the equipment compact and achieve a high heat transfer rate using minimum pumping power. Techniques for heat transfer augmentation are relevant to several engineering applications. In recent years, the high cost of energy and material has resulted in an increased effort aimed at producing more efficient heat exchange equipment. Furthermore, sometimes there is a need for miniaturization of a heat exchanger in specific applications, such as space application, through an augmentation of heat transfer. For example, a heat exchanger for an ocean thermal energy conversion (OTEC) plant requires a heat transfer surface area of the order of 10 000 m2/MW. Therefore, an increase in the efficiency of the heat exchanger through an augmentation technique may result in a considerable saving in the material cost. Furthermore, as a heat exchanger becomes older, the resistance to heat transfer increases owing to fouling or scaling. These problems are more common for heat exchangers used in marine applications and in chemical industries. In some specific applications, such as heat exchangers dealing with fluids of low thermal conductivity (gases and oils) and desalination plants, there is a need to increase the heat transfer rate. The heat transfer rate can be improved by introducing a disturbance in the fluid flow (breaking the viscous and thermal boundary layers), but in the process pumping power may increase significantly and ultimately the pumping cost becomes high. Therefore, to achieve a desired heat transfer rate in an existing heat exchanger at an economic pumping power, several techniques have been proposed in recent years and are discussed in the following sections.

ment ratio is defined as (Nu=Nu0 ) (f =f0 )1=3 where Nu, f, Nu0 and f0 are the Nusselt numbers and friction factors for a duct configuration with and without inserts respectively. The friction factor is a measure of head loss or pumping power. 2.3

The Nusselt number is a measure of the convective heat transfer occurring at the surface and is defined as hd/k, where h is the convective heat transfer coefficient, d is the diameter of the tube and k is the thermal conductivity. 2.4

IMPORTANT DEFINITIONS

In this section a few important terms commonly used in heat transfer augmentation work are defined.

2.1

Thermohydraulic performance

For a particular Reynolds number, the thermohydraulic performance of an insert is said to be good if the heat transfer coefficient increases significantly with a minimum increase in friction factor. Thermohydraulic performance estimation is generally used to compare the performance of different inserts such as twisted tape, wire coil, etc., under a particular fluid flow condition.

2.2

Overall enhancement ratio

The overall enhancement ratio is defined as the ratio of the heat transfer enhancement ratio to the friction factor ratio. This parameter is also used to compare different passive techniques and enables a comparison of two different methods for the same pressure drop. The overall enhanceProc. Instn Mech. Engrs Vol. 218 Part A: J. Power and Energy

Prandtl number

The Prandtl number is defined as the ratio of the molecular diffusivity of momentum to the molecular diffusivity of heat, n=a. 2.5

Pitch

Pitch is defined as the distance between two points that are on the same plane, measured parallel to the axis of a twisted tape. 2.6

2

Nusselt number

Twist ratio, y

The twist ratio is defined as the ratio of pitch to inside diameter of the tube y ¼ H/di, where H is the twist pitch length and d is the inside diameter of the tube. 3

HEAT TRANSFER AUGMENTATION

In general, some kind of inserts are placed in the flow passage to augment the heat transfer rate, and this reduces the hydraulic diameter of the flow passage. Heat transfer enhancement in a tube flow by inserts such as twisted tapes, wire coils, ribs and dimples is mainly due to flow blockage, partitioning of the flow and secondary flow. Flow blockage increases the pressure drop and leads to increased viscous effects because of a reduced free flow area. Blockage also increases the flow velocity and in some situations leads to a significant secondary flow. Secondary flow further provides a better thermal contact between the surface and the fluid because secondary flow creates swirl and the resulting mixing of fluid improves the temperature gradient, which ultimately leads to a high heat transfer coefficient. Twisted tape generates a spiral flow along the tube length. A wire coil insert in a tube flow consists of a helical coiled spring which functions as a non-integral roughness. Figure 1 shows three typical configurations of twisted tape with 1808 A04804 # IMechE 2004

REVIEW OF PASSIVE HEAT TRANSFER AUGMENTATION TECHNIQUES

Fig. 1

Example of (a) full-length twisted tape, (b) regularly spaced twisted tape and (c) smoothly varying (gradually decreasing) pitch full-length twisted tape

twisted pitch, and Fig. 2 shows a typical configuration of a wire coil. In a turbulent flow, the dominant thermal resistance is limited to a thin viscous sublayer. The wire coil insert is more effective in a turbulent flow compared with a twisted tape, because wire coil mixes the flow in the viscous sublayer near the wall quite effectively, whereas a twisted tape cannot properly mix the flow in the viscous sublayer. For a laminar flow, the dominant thermal resistance is limited to a thicker region compared with a turbulent flow.

Thus, a wire coil insert is not effective in a laminar flow because it cannot mix the bulk flow well, and the reverse is true for a twisted tape insert. Hence, twisted tapes are generally preferred in laminar flow. Performance and cost are the two major factors that play an important role in the selection of any passive technique for the augmentation of heat transfer. Generally, twisted tape and wire coil inserts are more widely applied and have been preferred in the recent past to other methods, probably because techniques such as the extended surface insert suffer from a relatively high cost and a mesh insert suffers from a high pressure drop and fouling problems.

3.1

Fig. 2 A04804 # IMechE 2004

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Example of wire coil insert

Laminar and turbulent flow through a circular tube

For fully developed laminar flow in a circular tube without any insert, the Nusselt number has a constant value under the condition of constant wall temperature, Nu ¼ 3.657, and Proc. Instn Mech. Engrs Vol. 218 Part A: J. Power and Energy

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the friction factor for this flow is given as f ¼ 16/Re. For fully developed turbulent flow in a smooth circular tube, the Nusselt number can be predicted by the Dittus – Boelter correlation, Nu ¼ 0.023Re 0.8Pr 0.4, and the friction factor can be obtained by the relation f ¼ 0.079Re 20.25 4

CLASSIFICATION OF AUGMENTATION TECHNIQUES

Generally, heat transfer augmentation techniques are classified in three broad categories: (a) active method, (b) passive method, (c) compound method. The active and passive methods are described with examples in the following subsections. A compound method is a hybrid method in which both active and passive methods are used in combination. The compound method involves complex design and hence has limited applications. 4.1

Active method

This method involves some external power input for the enhancement of heat transfer and has not shown much potential owing to complexity in design. Furthermore, external power is not easy to provide in several applications. Some examples of active methods are induced pulsation by cams and reciprocating plungers, the use of a magnetic field to disturb the seeded light particles in a flowing stream, etc. 4.2

Passive method

This method does not need any external power input and the additional power needed to enhance the heat transfer is taken from the available power in the system, which ultimately leads to a fluid pressure drop. The heat exchanger industry has been striving for improved thermal contact (enhanced heat transfer coefficient) and reduced pumping power in order to improve the thermohydraulic efficiency of heat exchangers. A good heat exchanger design should have an efficient thermodynamic performance, i.e. minimum generation of entropy or minimum destruction of available work (exergy) in a system incorporating a heat exchanger. It is almost impossible to stop exergy loss completely, but it can be minimized through an efficient design. 5

REVIEW

An extensive literature review of all types of heat transfer augmentation technique with external inserts up to 1985 has been discussed by Bergles [1]. In the following subsections, literature involving recent work on passive heat transfer augmentation techniques by employing twisted tapes, wire coils, dimples, ribs and fins as an insert has been reviewed. Proc. Instn Mech. Engrs Vol. 218 Part A: J. Power and Energy

5.1

Twisted tape in laminar flow

A summary of important investigations of twisted tape in a laminar flow is presented in Table 1. Twisted tape increases the heat transfer coefficient at a cost of rise in pressure drop. Several researchers have studied various configurations of twisted tape, such as full-length twisted tape, short-length twisted tape, full-length twisted tape with varying pitch and regularly spaced twisted tape. This section discusses which configuration of twisted tape is suitable for laminar flow. Whitham [2] studied heat transfer enhancement by means of a twisted tape insert way back at the end of the nineteenth century. Date and Singham [3] numerically investigated heat transfer enhancement in laminar, viscous liquid flows in a tube with a uniform heat flux boundary condition. They idealized the flow conditions by assuming zero tape thickness, but the twist and fin effects of the twisted tape were included in their analysis. Saha et al. [4, 5, 12] reported experimental data on a twisted tape generated laminar swirl flow friction factor and Nusselt number for a large Prandtl number (205 , Pr , 518) and observed that, on the basis of a constant pumping power, short-length twisted tape is a good choice because in this case swirl generated by the twisted tape decays slowly downstream which increases the heat transfer coefficient with minimum pressure drop, as compared with a full-length twisted tape. Regularly spaced twisted tape decreases the friction factor and reduces the heat transfer coefficient because the spacing of the twisted tape disturbs the swirl flow. Hong and Bergles [6] reported heat transfer enhancement in laminar, viscous liquid flows in a tube with uniform heat flux boundary conditions, but their correlation has limited applicability as it is valid for a high Prandtl number (approximately 730). The circumferential temperature profile for swirl flow is related to tape orientation. Tariq et al. [7] found that in a laminar flow the introduction of turbulent promoters, such as an internally threaded tube, is not efficient compared with a twisted tape insert on the basis of the overall efficiency. Depending upon the flowrate and tape geometry, the enhancement in heat transfer is due to the tube partitioning and flow blockage, the large flow path and secondary fluid circulation. Manglik and Bergles [8] considered all these effects and developed laminar flow correlations for the friction factor and Nusselt number, including the swirl parameter, which defines the interaction between viscous, convective inertia and centrifugal forces. These correlations pertain to the constant wall temperature case for fully developed flow, based on both previous data and their own experimental data. The heat transfer correlation as proposed by them is 0:14 m 9 0:391 3:385 0:2 Nu ¼ 4:162½6:413 10 (SWPr ) m w

where SW is the swirl parameter and is defined as pffiffiffi SW ¼ Re= y. Based on the same data, a correlation for A04804 # IMechE 2004


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Table 1 Summary of important investigations of twisted tape in laminar flow Authors

Fluid

Configuration of twisted tape

Type of investigation

1 Saha and Dutta [4]

Water with (205 , Pr , 518)

(a) Short length Experimental in a (b) Full length circular tube (c) Smoothly varying pitch (d) Regularly Spaced

2 Bergles and Hong [6]

Water (3 , Pr , 7) (83 , Re , 2460) Ethylene Glycol (84 , Pr , 192) (13 , Re , 390)

Full-length twisted tape

3 Tariq et al. [7]

Air (1300 , Re , 104)

No insert

Observations

(1) Friction and Nu low for Friction factor and short length tape Nusselt number (2) Short length tape requires increase with small pumping power insertion of any (3) Multiple twist and single passive augmentation twist has no difference on technique thermohydraulic Of several passive performance techniques studied in (4) Uniform pitch last decade, it was (Fig. 1a) performs better observed that twisted than gradually decreasing tape is effective in pitch laminar flow Experiment in (1) Nu is function of twist Further, twisted tape has circular tube ratio, Re and Pr been used as full(2) Friction is affected by length twisted tape, tape twist only at high Re half-length twisted (3) Nu is 9 times that of tape and varyingempty tube pitch twisted tape Experiment in (1) Efficiency lower for It was observed that internally threaded threaded tube than short-length twisted tube twisted tape, but better tape is better than fullthan some fluted length one on basis of geometries thermohydraulic (2) Good promoter of performance turbulence (3) Heat transfer coefficient in internally threaded tube approximately 20 per cent higher than that in smooth tube

4 Saha and Bhunia Servotherm medium oil [5] (205 , Pr , 512, 45 , Re , 840)

5 Manglik and Bergles [8]

Water (3.5 , Pr , 6.5) and ethylene glycol (68 , Pr , 100)

6 Ray and Date [9] Water (100 , Re , 3000, Pr 4 500)

Twisted tape (twist ratio 2.5 4 Y 4 10)

Comments

Experiment in circular tube

(1) Heat transfer characteristics depend on twist ratio, Re and Pr (2) Uniform pitch performs (Fig. 1a) better than gradually decreasing pitch (Fig. 1c)

Three different twist Experiment in ratios: 3, 4.5 and 6 isothermal tube

(1) Proposed correlation for friction and Nusselt number (2) Physical description of enhancement mechanisms

Full-length twisted tape with width equal to side of duct

Numerical work for square duct

(1) Proposed correlations for friction and Nu (2) Higher hydrothermal performance for square duct than circular one (3) Local Nusselt number peaks at cross-sections where tape aligned with diagonal of duct

7 Lokanath and Misal [10]

Water (3 , Pr , 6.5 and lube oil (Pr 418)

Twisted tape

Experiment in plate heat exchanger and shell and tube heat exchanger

(1) Large value of overall heat transfer coefficient produced in water-towater mode with oil-towater mode

8 Shah and Skiepko [11]

Water

No insert

Experiment in heat exchanger

(1) Irreversibility minimum for energy conversion process thermal system and for heat exchanger irreversibility should be intermediate

Thermohydraulic performance for square tube with twisted tape is better than that of twisted tape in circular tube Uniform pitch twisted tape (Fig. 1a) performs better than gradually varying pitch twisted tape (Fig. 1c) Pinching of twisted tape gives better results compared with connected thin rod

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Table 1 Continued Fluid

Configuration of twisted tape


Fluids with 205 4 Pr 4 518

Twisted tape (regularly spaced)


10 Al-Fahed and Chakroun [13]

Oil

Twisted tape with twist ratios 3.6, 5.4, 7.1 and microfin

Experiment in single (1) For low twist ratio shell and tube heat resulting low pressure exchanger drop, tight fit will increase more heat transfer (2) For high twist it is different (3) Microfins are not used for laminar

11 Liao and Xin [14]

(1) Water (2) Ethylene glycol, (3) Turbine oil 5.5 , Pr , 590 80 , Re , 50000

Segmented twisted tape and threedimensional extended surfaces

Experiment in tube flow

(1) In a tube with threedimensional extended surfaces and twisted tape increases average Stanton number up to 5.8 times compared with empty smooth tube

12 Ujhidy et al. [15]

Water

Twisted tape

Experiment in channel

(1) Explained flow structure (2) Proved existence of secondary flow in tubes with helical static elements.

13 Suresh Kumar et al. [16, 17]

Water

Twisted tape

Experiment in large-diameter annulus

(1) Observed relatively large values of friction factor (2) Measured heat transfer in annulus with different configurations of twisted tapes

14 Wang and Sunden [18]

Water (0 , Re , 2000) (0.7 , Pr , 30)

Twisted tape

Experiment in circular tube flow

(1) Both inserts effective in enhancing heat transfer in laminar region compared with turbulent flow (2) Twisted tape has poor overall efficiency if pressure drop is considered

15 Saha and Chakraborty [19]

Water (145 , Re , 1480) (4.5 , Pr , 5.5)

Twisted tape (regularly spaced) (1.92 , Y , 5.0)


(1) Larger number of turns may yield improved thermohydraulic performance compared with single turn

16 Lokanath [20]

Water (240 , Re , 2300) (2.6 , Pr , 5.4)

Full-length and half-length twisted tapes

Experimental in horizontal tube

(1) On unit pressure drop basis and on unit pumping power basis, half-length twisted tape is more effective than full-length twisted tape

Authors 9 Saha et al. [12]

the friction factor was developed by Manglik and Bergles [8] and is given as (fRd )sw ¼ 15:767

p þ 2 2t=di p 4t=di

2

(1 þ 106 SW2:55 )1=6


Observations

Comments

(1) Pinching of twisted tape gives better results than connecting thin rod for thermohydraulic performance (2) Reducing tape width gives poor results; larger than zero phase angle not effective

where, di and t are the tube inner diameter and the thickness of the twisted tape respectively. Date and Ray [9] derived a correlation for the friction factor and Nusselt number for a square duct from the predicted data. They compared the correlation for the friction factor with experimental data and the agreement was found to be within +10 per cent. A04804 # IMechE 2004


Lokanath and Misal [10] studied the performance of a plate heat exchanger and augmented shell and tube heat exchanger for different fluids. They found that twisted tapes of tighter twists are expected to give higher overall heat transfer coefficients in the augmented shell and tube heat exchanger. Shah and Skiepko [11] investigated the existence of entropy generation extrema and a relationship between the extrema and the heat exchanger effectiveness. Saha et al. [12] found that pinching (placing of a twisted tape exactly at the centre of the tube) of twisted tape in a tube performs better than a twisted tape inserted by a loose fit. They further showed that a non-zero phase angle in-between the segmented twisted tape gives poor results because the swirl will break easily in-between the two segmented twisted tapes. Furthermore, reduction in the width of the twisted tape is not effective, compared with a twisted tape of width equal to the inside diameter of the tube. AI-Fahed et al. [13] observed that, for a low twist ratio (Y ¼ 5.4) and high pressure drop, a loose fit is recommended for design of the heat exchanger, since it is easier to install and to remove for cleaning purposes. Other than this twist ratio, a tight-fit twisted tape provides better performance than a loose-fit twisted tape. Liao and Xin [14] reported experimental data on the compound heat transfer enhancement technique and concluded that the enhancement of heat transfer in a tube with threedimensional internal extended surfaces by replacing continuous twisted tape with almost segmented twisted tape inserts results in a decrease in the friction factor but with a comparatively small decrease in the Stanton number. The Stanton number is defined as the ratio of heat transfer rate to the enthalpy difference and is a measure of the heat transfer coefficient. Ujhidy et al. [15] have studied laminar flow of water in coils and tubes containing twisted tapes and helical static elements and proposed a modified Dean number that takes into account the curvature of the spherical line cut out by the helical element from the tubular housing. The Dean number is a measure of the magnitude of the secondary flow. Suresh Kumar et al. [16, 17] investigated the thermohydraulic performance of twisted tape inserts in a large hydraulic diameter annulus. The thermohydraulic performance in laminar flow with a twisted tape is better than the wire coil for the same helix angle and thickness ratio. This is probably due to the fact that, in laminar flow, the dominant thermal resistance is not limited to a thin wall region but extends over the entire cross-section. Thus, a twisted tape insert mixes the bulk flow and is probably effective. Considering the overall enhancement ratio, twisted tape is effective for small Prandtl number fluids and wire coil is effective for high Prandtl number fluids (Pr . 30), as discussed by Wang and Sunden [18] who reported correlations for ethyl glycol and polybutene (1000 4 Pr 4 7000). Compared with the data of Manglik and Bergeles [8], their data are valid for relatively larger values of the Prandtl number. A04804 # IMechE 2004

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Saha and Chakraborty [19] observed laminar flow heat transfer and pressure drop characteristics in a circular tube fitted with regularly spaced twisted tape and concluded that there is a drastic reduction in the pressure drop, more than the corresponding reduction in heat transfer. Thus, it appears that, on the basis of a constant pumping power, a large number of turns may yield improved thermohydraulic performance compared with a single turn on twisted tape. Lokanath [20] represented experimental data on laminar flow of water through a horizontal tube under uniform heat flux condition and fitted with half-length twisted tape. He found that, on the basis of unit pressure drop and unit pumping power, half-length tapes are more effective than full-length tapes.

5.2

Twisted tape in turbulent flow

The important investigations of twisted tape in turbulent flow are summarized in Table 2. In turbulent flow, the dominant thermal resistance is limited to a thin viscous sublayer. The following section discusses the performance of twisted tape inserts in turbulent flow. A tube inserted with a twisted tape performs better than a plain tube, and a twisted tape with a tight twist ratio provides an improved heat transfer rate at a cost of increase in pressure drop for low Prandtl number fluids. This is because the thickness of the thermal boundary layer is small for a low Prandtl number fluid and a tighter twist ratio disturbs the entire thermal boundary layer, thereby increasing the heat transfer with increase in the pressure drop, as discussed by Royds [21]. Smithberg and Landis [22] gave an analytical model of the tape-generated swirl mechanism. Because of swirl, the ratio of maximum velocity to mean velocity is smaller in a tube with a twisted tape compared with that in a straight flow (i.e. without a twisted tape). This creates a centrifugal force and aids convective heat transfer [23 –25]. Twisted tape is also effective in high Prandtl number fluids because for such fluids it provides high heat transfer with less pressure drop compared with other inserts [26]. Lopina and Bergles [27] observed that the difference between isothermal and heated flow friction factors for the swirl flow of liquids is substantially less than the corresponding difference for a plain tube. In turbulent flow, insertion of a twisted tape increases the heat transfer, but the pressure drop also increases significantly [28, 29]. Short-length twisted tape (25 –45 per cent of the tube length) performs better than full-length twisted tape [30 –33]. Date [34] reviewed available friction factor and Nusselt number correlations for flow in a tube containing a twisted tape and pointed out that existing correlations deviate from measurements by 30 per cent. Studies of Klepper [35] and Kidd Jr [36] suggest that short-length twisted tape is more useful in a gas-cooled nuclear reactor compared with full-length tape. Date [37] formulated and solved numerically the problem of fully developed, uniform Proc. Instn Mech. Engrs Vol. 218 Part A: J. Power and Energy

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Table 2

Summary of important investigations of twisted tape in turbulent flow Configuration of tape


Water

(a) Full length (b) Full length with tighter twist ratio

Experimental in circular tube

Smithberg and Landis [22]

Water

Full length twisted Analytical tape

Proposed mathematical model for tape generated swirl mechanism

3

Cresswell [23]

Water

Twisted tape


Ratio of maximum velocity to mean velocity is smaller in swirl flow compared with straight flow

4

Kreith and Margolis [24]

Water

Twisted tape


Centrifugal force aids convection when fluid is heated up

5

Thorsen and Landis Water [25]

Twisted tape


Centrifugal force aids convection when fluid is heated up and inhibits convection when fluid is cooled

Authors

Fluid

1

Royds [21]

2

6

Observations

Comments

Tighter twist ratio results in greater heat transfer and pressure drop

In turbulent flow, shortlength twisted tape is more effective than fulllength twisted tape (Fig. 1a) based on thermohydraulic performance

Gambill and Bundy Water [26]

Twisted tape


Twisted tape also important for high Prandtl number

7

Lopina and Bergles [27]

Water

Twisted tape


Isothermal friction factor for swirl flow of liquids is substantially less than for a plain tube

8

Colburn and King [30]

Water

Inserts like baffled Experiment in tube and circular tube short-length twisted tape

Short-length twisted tapes more effective than full-length twisted tapes

9

Seigel [28]

Water

Twisted tape

Experiment in horizontal tube

In horizontal tube inserted twisted tape increases heat transfer coefficient with increase in pressure drop

10 Koch [29]

Water

Twisted tape


Increase in heat transfer coefficient accompanied by a comparable increase in pressure drop

11 Seymour [31]

Water

Twisted tape


Short-length twisted tape performs better than full-length twisted tape

12 Kreith and Sonju [32]

Water

Twisted tape


Short-length (25–45 per cent of tube length) tapes perform better than full-length tapes

13 Date [34]

Water

Twisted tape


Friction and Nu for flow in tube containing tape deviate 30 per cent

14 Klepper [35]

Water

Short-length tape


Usefulness of tape in gas-cooled nuclear reactor

15 Kidds Jr [36]

Nitrogen

Short-length tape

Tube flow

Effectiveness of twisted tape in gas cooled nuclear reactor

16 Date [37]

Water

Twisted tape

Numerical

Solved numerically problem of fully developed flow

As Prandtl number increases, heat transfer coefficient is enhanced by twisted tape All configurations (Figs 1a to c) of twisted tape lead to high friction factor, which is due to fact that twisted tape disturbs entire flowfield Overall enhancement ratio increases with tighter twist ratio and decreases with increase in Reynolds number If pressure drop is not important, twisted tape is good option to use in turbulent flow

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Table 2 Continued Authors

Fluid

Configuration of tape


Observations

17 Bolla et al. [38]

Water

Transverse ribs and twisted tape

Experiment in circular tube with both ribs and tape

Transverse rib performs better than tape

18 Zozulya and Shkuratov [39]

Water

Twisted tape


Smooth decrease in pitch of twisted tape has significant influence on heat transfer

19 Huang and Tsou [40]

Water

Twisted tape


Studied free swirl flow

20 Blackwelder and Kreith [41]

Water

Twisted tape


Recommended that optimum design of heat exchanger with tape-induced swirl flow must consider combination of continuous and decaying swirl flow

21 Backshall and Landies [42]

Water

Twisted tape


Studied boundary layer characteristics of incompressible twisted tape generated swirl flow

22 Watanable et al. [43]

Water

Twisted tape


Maximum thermal stress appears near piping that connects parts of cover plate of plate fin heat exchanger channels

23 Genis and Rautenbach [44]

Water

Twisted tape


Studied thermohydraulic characteristics of high-velocity water flow in short tubes

24 Budov and Zamyatin [45]

Water

Twisted tape

Experiment in cylindrical channel

Loss of head due to hydraulic resistance in twisted tape generated swirl flow

25 Beckarmann and Water Goldschimd [46]

Twisted tape

Flue way of water heater

Hot cross tapes in flue way of water heater play important role in heat transfer to tube wall

26 Yamada et al. [47]

Water

Wall radiation

Experiment in shell and tube heat exchanger

Made similar observation as above

27 Gupte and Date [48]

Air

Twisted tape

Numerical study in annulus

Semi-empirically evaluated friction and heat transfer data for tape-generated swirl flow in annulus

28 Filipak [49]

Water

Twisted tape

Experiment in vertical tube

Made experimental study of tape-generated swirl flow in vertical section

29 Donevski and Kuleshza [50]

Water

Twisted tape


Predicted frictional losses coupled with additional vortex mixing effect

30 Rao [51]

Water

Twisted tape

Experiment in axial duct flow

Performed experimental study on augmentation of heat transfer in axial ducts of electrical machines

31 Fomina et al. [52]

Water

Twisted tape fins

Economizer

Checked first pilot plant having economizer with twisted tape fins

Comments

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Fluid



Observations

32 Algifri and Bharadwaj [53]

Water

Twisted tape (Short length)


Presented series of solutions to governing equations for short-length twisted tape generated decaying swirl flow

33 Algifri et al. [54]

Water

Twisted tape

Experiment in circular pipe

Presented series of solutions to governing equations for short-length twisted tape generated decaying swirl flow

34 Burfoot and Rice [55]

Water

Twisted tape


Found that surface roughness of tape insert affects thermohydraulic characteristics

35 Kumar and Bharadwaj [56]

Water

Twisted tape


Obtained theoretically heat transfer and pressure drop correlations

36 Kumar and Prasad [57]

Water

Twisted tape

Experiment in solar water heater

Investigated performance of tape inserted solar water heater

37 Fujita and Lopez [58]

Water

Twisted tape and teflon tape


Did not find any evidence of tape fin effects in heat transfer characteristics of snugly fitted stainless steel tape insert when compared with Teflon tape insert

38 AI-Fahed and Chakroun [59]

Water

Full-length twisted tape

Experiment in horizontal isothermal tube

There is optimum tape width, depending on twist ratio and Re, for best thermohydraulic performance

39 Saha et al. [60]

Water

Regularly spaced tape

Experimental study in circular tube

On basis of constant heat flux, regularly spaced tape does not perform better than full-length twisted tape

40 Rao and Sastri [61]

Water

Twisted tape

Experimental study in rotating twisted tape

Enhancement of heat transfer offsets rise in friction factor due to rotation

41 Sivanshanmugam and Sunduram [62]

Water

Twisted tape


Studied thermohydraulic characteristics of tape-generated swirl flow

42 Agarwal and Raja rao [63]

Water

Twisted tape


Studied thermohydraulic characteristics of tape-generated swirl flow

43 Peterson et al. [64]

Water

Twisted tape

Heat exchanger

Experimented with highpressure water as test liquid in turbulent flow with low heat fluxes and low wall–fluid temperature differences typical of liquidto-liquid heat exchanger

44 Naumov and Semashko [65]

Water

Twisted tape


Used mathematical model for adiabatic cross-section to study pulsed asymmetric heating of pipe by external heat flux and calculated thermophysical parameters of cooled pipes containing tapes within collector of seviated ions in T-15 tokamak injection system

Comments

(Table continued ) Proc. Instn Mech. Engrs Vol. 218 Part A: J. Power and Energy

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Table 2 Continued Fluid



Observations

45 Naumov et al. [66]

Water

Twisted tape


Same as above (finite difference method)

46 Naumov et al. [67]

Water

Twisted tape


47 Yokoya et al. [68]

Water

Twisted tape

Experimental study

Same as above (finite difference method) with modification Demonstrated novel use of tapes in controlling flow in continuous casting mould and refining process

48 Klaczak [33]

(1300 , Re , 8000)

Shor-length twisted tape


Found usefulness of shortlength twisted tapes

49 Hijikata et al. [69]

Water

Experiment in vertical test section

Radiation between pipe wall and tape increase heat transfer rate by 50 per cent

50 Chung and Sung [70]

Air

Transverse curvature

Annulus pipe flow (direct numerical simulation)

Turbulent structure more effective near outer wall compared with inner wall

51 Yang and Hawang [71]

Air (1 104 , Re , 5 104)

Porous baffles and Numerical in solid baffles rectangular channel

Porous type baffle is good for thermohydraulic performance, performance does not depend on baffle height

52 Manglik and Bergles [72]

Water (3.5 , Pr , 6.5) and ethylene glycol (68 , Pr , 100)

Three different twist ratios 3, 4.5, 6.

(1) Proposed correlation for friction and Nusselt number (2) Physical description of enhancement mechanism

Authors

Experiment in isothermal tube

property flow in a tube containing a twisted tape. He compared existing experimental data with his own numerical prediction. Bolla et al. [38] observed that in the turbulent flow region a transverse rib performs better than a twisted tape insert in a duct flow. Zozulya and Shkuratov [39] found that a smooth decrease in pitch of a twisted tape results in an improved heat transfer rate. Huang and Tsou [40] studied free swirl flow in a pipe. Blackwelder and Kreith [41] recommended that an optimum design of heat exchanger with twisted tape induced swirl flow should consist of continuous and decaying swirl flow. Backshall and Landies [42] studied the boundary layer characteristics of twisted tape generated incompressible swirl flow. Watanable et al. [43] observed that the maximum thermal stress appears near the piping connecting parts of the cover plate of the plate fin heat exchanger channels. Genis and Rautenbach [44] studied the thermohydraulic characteristics of a high-velocity water flow in short tubes with twisted tape inserts. Budov et al. [45] predicted a loss of head owing to hydraulic resistance in a twisted tape generated swirl flow. Beckermann and Goldschmid [46] and Yamada et al. [47] observed that, in addition to convection, radiative heat transfer from the relatively hot (1260 8C) cross-twisted tape in the flue-way of a water heater plays an important role in the heat transfer to the tube wall and this phenomenon cannot be neglected. A04804 # IMechE 2004

Comments

Gupte and Date [48] evaluated semi-empirically the heat transfer coefficient and friction factor for twisted tape generated swirl flow in an annulus. Twisted tape generated swirl flow in a vertical test section (contrary to the usual practice of a horizontal test section) was experimentally studied by Filipak [49]. Donevski and Kulesza [50] predicted frictional losses from the combined effects of axial and tangential boundary layer flow coupled with an additional vortex mixing effect. Rao [51] performed an experimental study of augmentation of heat transfer in the axial ducts of electrical machines. Fomina et al. [52] verified the first pilot plant having an economizer with twisted tape fins. Algifri and Bharadwaj [53] and Algifri et al. [54] presented a series of solutions to governing equations for short-length twisted tape generated decaying swirl. Burfoot and Rice [55] found that the surface roughness of a tape insert affects the thermohydraulic characteristics. Kumar and Bharadwaj [56] obtained theoretically the heat transfer and pressure drop correlations using the Kreith and Sonju [32] solution for the velocity vector, which decays along the axis of the tube. Kumar and Prasad [57] investigated experimentally the performance of a tape-inserted solar water heater. Fujita and Lopez [58], in an experimental investigation, found no evidence of tape fin effects in the heat transfer characteristics of a snugly fitted stainless Proc. Instn Mech. Engrs Vol. 218 Part A: J. Power and Energy

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steel tape insert when compared with a Teflon tape insert. AI-Fahed et al. [59] observed that there is an optimum tape width, depending on the twist ratio and Reynolds number, for the best thermohydraulic characteristics. Saha et al. [60] have shown that, for a constant heat flux boundary condition, regularly spaced twisted tape elements do not perform better than full-length twisted tape because the swirl breaks down in-between the spacing of a regularly twisted tape. Rao and Sastri [61], while working with a rotating tube with a twisted tape insert, observed that the enhancement of heat transfer offsets the rise in the friction factor owing to rotation. Sivashanmugam and Sundaram [62] and Agarwal and Rao [63] studied the thermohydraulic characteristics of tape-generated swirl flow. Peterson et al. [64] experimented with high-pressure (8 – 16 MPa) water as the test liquid in turbulent flow with low heat fluxes and low wall –fluid temperature differences typical of a liquid – liquid heat exchanger. Naumov et al. [65, 66] used a mathematical model for an adiabatic cross-section to study the pulsed asymmetric heating of pipes by an external heat flux and calculated the thermophysical parameters of cooled pipes containing twisted tapes within the collector of deviated ions in the T-15 tokamak injection system. Yokoya et al. [68] demonstrated a novel use of twisted tapes in controlling the flow in a continuous casting mould and refining process. Hijikata et al. [69], while carrying out experiments in a vertical test section, observed that the radiation between the pipe wall and the twisted tape increases the heat transfer rate by about 50 per cent. Chung and Sung [70] performed a direct numerical simulation for turbulent heat transfer in a con-

centric annulus, and they observed that the thermal structure is more effective near the outer wall than near the inner wall. Yang and Hwang [71] observed that a porous type of baffle is good for thermohydraulic performance and there is an optimum height of the baffle. Manglik and Bergles [72] developed correlations for both turbulent flow and laminar flow. For an isothermal friction factor, the correlation describes most available data for laminar, transitional and turbulent flows within 10 per cent. However, a family of curves is needed to develop correlation for the Nusselt number on account of the non-unique nature of laminar – turbulent transition. Their correlations are as follows 1:75 0:079 p p þ 2 2t=di 1:25 Re0:25 p 4t=di p 4t=di 2:752 1 þ 1:29 y 0:8 p p þ 2 2t=di 0:2 Nu ¼ 0:023Re0:8 Pr 0:4 f p 4t=di p 4t=di

f¼

where f is the fluid property variation for liquids and is given as f ¼ (m=mw )n , where n ¼ 0.18 for heating and n ¼ 0.3 for cooling, m is the dynamic viscosity and mw denotes the dynamic viscosity at the wall temperature. 5.3

Wire coil in laminar flow

The important investigations of wire coil in a laminar flow are shown in Table 3. Inaba and Ozaki [73] showed that

Table 3 Summary of important investigations of wire coils in laminar flow Authors

Fluid

Configuration of wire coil


1

Inaba and Ozaki [73]

Water

Wire coil

Experiment in circular pipe flow

2

Ujhidy et al. [15]

Water

Wire coil

Experiment in channel flow

3

Wang and Sunden Water [18]

Wire coil


Both inserts are effective in enhancing heat transfer rate in region other than turbulent flow and tape has poor overall efficiency if pressure drop is considered

4

Inaba and Haruki [74]

Water

Wire coil


Effect of wire coils on pipe friction and heat transfer coefficients elucidated under various values of flow velocity, wire coil diameter, pitch and length

5

Oliver and Shoji [75]

Non-Newtonian fluid 30 , Pr , 9020 , Re , 2000

Wire coil

Experiment in round tube

Heat transfer enhancement by factor of 4


Observations

Comments

High heat transfer and low Performance of wire coil pressure loss obtained by (Fig. 2) in laminar flow leading edge effect near tube depends on Prandtl number. inlet and turbulent flow If Pr ¼ 0.7, performance is downstream of wire coil not good based on overall enhancement ratio. Overall Explained flow structure enhancement ratio increases (helical static element with Prandtl number produces secondary flow)

A04804 # IMechE 2004


the turbulent flow induced by a wire coil enhances heat transfer even downstream of the wire coil. They developed empirical relations for the Nusselt number as a function of the Prandtl number. The pressure drop was found to be proportional to the length of the wire coil. They observed a high heat transfer coefficient and a small pressure loss owing to the leading edge effect near the tube inlet and turbulent flow downstream of the wire coil. Ujhidy et al. [15] found that the helical static element produces secondary flow. Wang and Sunden [18] found that a wire coil insert performs effectively in enhancing the heat transfer in the turbulent flow region, whereas tape has a poor overall efficiency. Inaba and Haruki [74] investigated heat transfer enhancement of flowing water in a tube with flow drag reduction additives by inserting wire coils. They found that a reduction in the flow resistance resulted in a reduced heat transfer coefficient in the tube. Oliver and Shoji [75] performed experiments in a tube by inserting wire coils in a tube using a non-Newtonian fluid and found that heat transfer is enhanced by a factor of 4 and the relative pressure drop caused by the wire coil is by factor of 5.

5.4

Wire coil in turbulent flow

The important investigations of wire coil in a turbulent flow are shown in Table 4. Ravigururajan and Bergles [76] have

521

developed correlations for the friction factor and heat transfer coefficient that are widely applied. By incorporating the roughness type and Prandtl number, their heat transfer data from the correlation were in good agreement with the correlation of Petukov and Popov [77]. Rahai and Wong [78] predicted that wire coil with a large pitch spacing increases the mixing, turbulent kinetic energy and half-width but decreases the maximum mean velocity. Kim et al. [79] observed that the slug rise velocity and void fraction in a vertical round tube is higher for a wire coil insert than a smooth tube and studied the flow pattern. Sams [80] found that a vortex flow can be created through the wire coil. Novozhilov and Migai [81] have proposed heat transfer correlations for a tube with wire coil in turbulent flow. Kumar and Judd [82] proposed correlations for heat transfer. They concluded that the range of wire diameter they tested (e/d ¼ 0.1,0.13,0.15, where e/s the rib height and d is the inside diameter) had a negligible effect on the rate of heat transfer. Their correlation underpredicts by nearly 50 per cent. Rahai et al. [83] studied the effects of a wire coil insert with p/D ¼ 1 (where p is the pitch spacing and D is the inside diameter) on the mixing enhancement of a turbulent jet from a Bunsen burner. Their results show a significant increase in the mixing process in the near discharge region of the jet. Furthermore, they studied the effect of various coil pitch spacings on the mixing process. Arici and Asan [84] studied the enhancement of turbulent

Table 4 Summary of important investigations of wire coils in turbulent flow Authors

Fluid

Configuration of wire coil


1

Ravigururajan and Bergles [76]

Water

Wire coil

2

Rahai and Wong [78]

Air

3

Kim et al. [79]

4

Observations

Comments

Experimental study

Developed correlations for friction and heat transfer

Wire coil (Fig. 2) in turbulent flow performs better than any other insert

Coil insert


Coil with large pitch spacing increases mixing, turbulent kinetic energy and halfwidth but reduces maximum mean velocity

Gas and liquid

Wire coils

Experiment in vertical round tube

Flow pattern, slug rise velocity and void fraction are higher for wire coil insert than smooth tube

Increase in pitch length of wire coil increases overall enhancement ratio. However, overall enhancement ratio decreases with increase in Reynolds number from 5000 to 45 000

Sams [80]

Air

Wire coil

Experiment in tube

Vortex flow can be created through coiled wire. Developed correlation for friction factor and Nusselt number

5

Novozhilov and Migai [81]

Air

Wire coil

Experiment in tube

Proposed correlations for heat transfer

6

Kumar and Judd [82]

Water 4 , Pr , 5

Wire coil

Experiment in tube

Proposed correlations for heat transfer. Correlations underpredicts by 50 per cent

7

Rahai et al. [83]

Air

Wire coil


Investigated effect of various coil spacings on mixing process

8

Arici and Asn [84]

Water

Wire coil

Experiment in tube flow

Increase in pitch of wire coil increases heat transfer

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flow by means of a wire coil insert. They found that an increase in the pitch of the wire coil at a constant Reynolds number decreases the heat transfer for the wall-attached wire coil inserts. In the case of displaced wire coils, an opposite phenomenon was observed where an increase in the pitch resulted in an increased heat transfer.

5.5

Other techniques

There are several other techniques to enhance the heat transfer, such as ribs, dimples, additives, fins, etc., and these are summarized in Table 5. Chang [85] proposed an empirical correlation for the spatial time averaged heat transfer for combined effects of reciprocation and rib-induced flow and also investigated the heat transfer mechanism in a reciprocating square duct fitted with 458 crossed ribs on two opposite walls. It was observed that a ribbed wall increases the heat transfer by 260 –300 per cent compared with a smooth wall, and the pulsating force and convective inertia affect the local heat transfer. Mahmood and Lingrani [86] reported experimental data and flow visualization in a dimpled channel and found that the vortex periodically shed from the dimples becomes stronger as the nondimensional channel height H/D ratio (H ¼ channel height, D ¼ dimple print diameter) decreases, and such behaviour leads to local Nusselt number augmentation at these same locations. Leung et al. [87] investigated the possibility of enhancing the forced convection from a turbulent flow in a triangular duct by imposing artificial roughness on its internal surface. A significantly higher heat transfer ensues by transverse ribs uniformly fixed along its axial length, and for maximum heat transfer performance the optimum size of the rib is 6.35 mm and the optimum roughness is 3 mm. Kumar et al. [88] carried out an experimental investigation to augment the heat transfer rate by enhancing the heat transfer coefficient during the condensation of pure steam and R-134a over horizontal finned tubes. Spines were found to be more effective in the bottom side of the circular integral tube. Liu et al. [89] developed a neural network to evaluate and predict boiling heat transfer enhancement using additives. Their model can select the additives for particular working fluid. Golriz and Grace [90] experimentally show that the addition of an angled deflector to the fin region of circular membrane water – wall heat exchanger surfaces in circulating fluidized beds can lead to a significant increase in the local heat transfer. Karwa et al. [91] studied experimentally the flow of air in rectangular ducts of various aspect ratios with repeated transverse rib roughness. They found that the highest friction factor and heat transfer coefficient occur for 158 chamfered ribs. The presence of ribs doubles the friction by comparison with that of a smooth tube. Kulankara and Herold [92] found that the surface tension of lithium bromide is affected by surfactants. Taslim et al. [93] observed that the effectiveness of surface ribs in heat transfer augmentation depends on Proc. Instn Mech. Engrs Vol. 218 Part A: J. Power and Energy

geometric factors of the ribs such as the height, pitch, angle of attack, orientation, shape and arrangement. They found that 458 angled ribs show the highest thermal performance compared with 908, V-shaped and discrete ribs when the friction loss is considered. Fann et al. [94] showed that the stagnant vortices behind the 908 transverse ribs result in a rise in the fluid temperature in the recirculating zone and the wall temperature near the rib location, and local heat transfer improvement occurs near the reattaching point between the ribs. Chen et al. [95] studied heat transfer enhancement in tubes using six different dimple depth tubes. They found that the enhancement of the heat transfer coefficient is 1.25 –2.37 times that of a smooth tube, and the tube with the largest ratios of dimple depth to tube inside diameter, dimple depth to pitch and dimple depth to dimple diameter is good for heat transfer on the tube side on the basis of a constant Reynolds number or pumping power. Mochizuki et al. [96] observed that these surface ribs enhance the turbulent fluctuations and could considerably modify the near-wall flow structures by creating secondary flows and vortices. They penetrate the sublayers of flow to cause periodic redevelopment of the boundary layers. The overall rib effects on heat transfer are improved heat transmission and modified spatial distributions. They further showed that the force field established in the ribbed duct flow, such as the centrifugal force in the bend, could modify the rib-induced flows. Chang et al. [97] observed that, in a duct fitted with transverse ribs, the flow cells behind the 908 ribs are no longer stagnant but periodically shed when the duct reciprocates. The typical zigzag streamwise heat transfer variation along the ribbed wall in a stationary system yields a large wavy pattern in the reciprocating duct. Okajima et al. [98] showed that interaction of the oscillating mainstream with angled rib flow triggers a double pair of dynamic counter-rotating vortical cells, with one attached on the angled rib and the other shed away from the rib. According to Mahmood et al. [99], the use of such dimples produces augmented surface heat transfer levels compared with smooth surfaces because of reattachment of the shear layer forming across the top of each dimple and because of the vortex structures and vortical fluid shed from each individual dimple indentation which then advects over the flat surface just downstream. Afanasyev et al. [100] studied different surfaces shaped by a system of spherical cavities in a turbulent flow and found that such shaping of the heating surface has no appreciable effect on the hydrodynamics of flow but results in considerable (up to 30– 40 per cent) heat transfer enhancement.

6 6.1

OBSERVATIONS Twisted tape in laminar flow

A twisted tape insert mixes the bulk flow well and therefore performs better in a laminar flow than any other insert, A04804 # IMechE 2004


523

Table 5 Summary of important investigations of ribs and some other inserts in a flow Configuration of insert


Air 600 , Re , 10 000

Ribs (458)

Mahmood and Ligrani [86]

Air

Dimples

3

Leung et al. [87]

Air 4000 , Re , 15 000

4

Kumar et al. [88]

5

Authors

Fluid

Observations

Comments

1

Chang [85]

Experiment in reciprocating duct

(1) Ribbed wall increases heat transfer by 260–300 per cent compared with smooth wall (2) Pulsating force and convective inertia affects local heat transfer

Experimental study in channel

(1) Vortex periodically shed as H/D ratio decreases. At same location, heat enhancement takes place

There are some other techniques (other than twisted tape and wire coil) such as ribs, dimples, additives and deflectors used for enhancement of heat transfer

2

Ribs

Triangular duct

(1) Enhancement of forced convection by ribs is better than that by roughness of wall (2) Optimum size of rib 6.35 mm, roughness 3 mm

Steam

Fins

Horizontal finned tube

(1) Fins enhance heat transfer during condensation of pure steam (2) Spines found to be more effective in bottom side of circular integral tube

Liu et al. [89]

Water

Additives

Neural network study

(1) Developed neural network is developed for boiling heat transfer enhancement due to additives (2) This model can select additives for particular working fluid

6

Golriz and Grace [90]

Water

Deflectors

Experiment in circulating fluidized bed

(1) Addition of angled deflector improves heat transfer

7

Kulankara and Herolld [92]

Water and lithium bromide

Additive


(1) Surface tension of lithium bromide is affected by surfactants (2) Additive in vapour has stronger effect on surface tension than in liquid

8

Taslim et al. [93]

Air

Ribs

Experiment in channel

(1) 458 angled rib shows the highest thermal performance

9

Fann et al. [94]

Air

Ribs

Experiment in serpentine passage

(1) Local heat transfer rates on ribbed surface develops at locations where fluid is relatively cool

10

Chen et al. [95]

Water

Dimples

Experiment in tube

(1) Best dimpled tube is with large ratio of depth to tube inside diameter, ratio of dimple depth to pitch and ratio of dimple depth to dimple diameter for better heat transfer on tube side

11

Mochizuki et al. [96]

Air

Ribs

Experiment in tube

(1) External force field established in ribbed duct flow, such as centrifugal force in bend

600 Re , 11 000

Ribs having 458 angle show good thermal performance Performance of dimple basis of ratio of channel height to dimple print has been studied in literature and, as ratio of channel height to dimple print decreases, shedding of vortex becomes stronger and this leads to augmentation in heat transfer

(Table continued )

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524



Fluid

Configuration of insert


12

Chang et al. [97]

Air

Ribs

Experiment in reciprocating duct

(1) External force field established in ribbed duct flow, such as rotating or reciprocating force

13

Okajima et al. [98]

Air

Ribs

Experiment on flat plate

(1) Detailed growth and shedding process of vortices depends on oscillating frequency of mainstream

14

Mohmood et al. [99]

Air

Dimples

Experiment in dimpled channel

(2) Friction factor and pressure drop usually not increased appreciably, because no form drag is produced by objects protruding into flow

15

Afanasyev et al. [100]

Air

Spherical cavities

Experiment on flat plate

(1) Surface shaped by spherical cavities enhances heat transfer up to 30–40 per cent

Observations

Comments

number, the thickness of the thermal boundary layer is small compared with the hydrodynamic boundary layer and the wire coil breaks this boundary layer easily. Therefore, both heat transfer and pressure drop are large.

because in a laminar flow the thermal resistance is not limited to a thin region. However, twisted tape performance also depends on the fluid properties such as the Prandtl number. If the Prandtl number is high, say Pr . 30, the twisted tape will not provide good thermohydraulic performance compared with other inserts such as wire coil. On the basis of a constant pumping power, short-length twisted tape is better than full-length twisted tape. In the design of a compact heat exchanger for laminar flow, twisted tape can be used effectively to enhance the heat transfer.

Wire coil enhances the heat transfer in turbulent flow efficiently. It performs better in turbulent flow than in laminar flow. The thermohydraulic performance of wire coil is good compared with twisted tape in turbulent flow.

6.2

6.5

Twisted tape in turbulent flow

Twisted tape in turbulent flow is effective up to a certain Reynolds number range but not over a wide Reynolds number range. Compared with wire coil, twisted tape is not effective in turbulent flow because it blocks the flow and therefore the pressure drop is large. Hence, the thermohydraulic performance of a twisted tape is not good compared with wire coil in turbulent flow. Therefore, it may be concluded that, for compact heat exchanger design, wire coil is a good choice in turbulent flow. However, a short-length twisted tape yields good thermohydraulic behaviour compared with full-length twisted tape in turbulent flow.

6.3

Wire coil in laminar flow

Wire coil in laminar flow enhances the heat transfer rate significantly. However, the performance depends on the Prandtl number. If the Prandtl number is high, the performance of the wire coil is good because, for a high Prandtl Proc. Instn Mech. Engrs Vol. 218 Part A: J. Power and Energy

6.4

Wire coil in turbulent flow

Other passive techniques

There are several passive techniques other than twisted tape and wire coil to enhance the heat transfer in a flow, such as ribs, fins, dimples, etc. These techniques are generally more efficient in turbulent flow than in laminar flow.

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5 Saha, S. K. and Bhunia, K. Heat transfer and pressure drop characteristics of varying pitch twisted-tape-generated laminar smooth swirl flow. In Proceedings of 4th ISHMT– ASME Heat and Mass Transfer Conference, India, 2000, pp. 423– 428 (Tata McGraw-Hill, New Delhi). 6 Hong, S. W. and Bergles, A. E. Augmentation of laminar flow heat transfer in tubes by means of twisted-tape inserts. Trans. ASME J. Heat Transfer, 1976, 98, 251– 256. 7 Tariq, A., Kant, K. and Panigrahi, P. K. Heat transfer enhancement using an internally threaded tube. In Proceedings of 4th ISHMT – ASME Heat and Mass Transfer Conference, India, 2000, pp. 277– 281 (Tata McGraw-Hill, New Delhi). 8 Manglik, R. M. and Bergles, A. E. Heat transfer and pressure drop correlations for twisted tape insert in isothermal tubes. Part 1: laminar flows. Trans. ASME, J. Heat Transfer, 1993, 116, 881– 889. 9 Ray, S. and Date, A. W. Friction and heat transfer characteristics of flow through square duct with twisted tape insert. Int. J. Heat and Mass Transfer, 2003, 46, 889– 902. 10 Lokanath, M. S. and Misal, R. D. An experimental study on the performance of plate heat exchanger and an augmented shell and tube heat exchanger for different types of fluids for marine applications. In Proceedings of 5th ISHMT– ASME Heat and Mass Transfer Conference, India, 2002, pp. 863– 868 (Tata McGraw-Hill, New Delhi). 11 Shah, R. K. and Skiepko, T. Entropy generation extrema and their relationship with heat exchanger effectiveness-number of transfer unit behaviour for complex flow arrangements. In Proceedings of 5th ISHMT– ASME Heat and Mass Transfer Conference, India, 2002, pp. 911– 919 (Tata McGrawHill, New Delhi). 12 Saha, S. K., Dutta, A. and Dhal, S. K. Friction and heat transfer characteristics of laminar swirl flow through a circular tube fitted with regularly spaced twisted-tape elements. Int. J. Heat and Mass Transfer, 2001, 44, 4211– 4223. 13 Al-Fahed, S., Chamra, L. M. and Chakroun, W. Pressure drop and heat transfer comparison for both micro-fin tube and twisted-tape inserts in laminar flow. Exp. Thermal and Fluid Sci., 1999, 18, 323– 333. 14 Liao, Q. and Xin, M. D. Augmentation of convective heat transfer inside tubes with three-dimensional internal extended surfaces and twisted tape inserts. Chem. Eng. J., 2000, 78, 95 – 105. 15 Ujhidy, A., Nemeth, J. and Szepvolgyi, J. Fluid flow in tubes with helical elements. Chem. Engng and Processing, 2003, 42, 1 – 7. 16 Suresh Kumar, P., Mahanta, P. and Dewan, A. Study of laminar flow in a large diameter annulus with twisted tape inserts. In Proceedings of 2nd International Conference on Heat Transfer, Fluid Mechanics, and Thermodynamics, Victoria Falls, Zambia, 2003, paper KP3. 17 Suresh Kumar, P., Mahanta, P. and Dewan, A. Study of heat transfer and pressure drop in a large hydraulic diameter annulus. 17th National Heat and Mass Transfer Conference and 6th ISHMT/ASME Heat and Mass Transfer Conference, Indira Gandhi Centre for Atomic Research, Kalpakkam, India, 2004, pp. 62 – 66. 18 Wang, L. and Sunden, B. Performance comparison of some tube inserts. Int. Commun. Heat Transfer, 2002, 29, 45 – 56. 19 Saha, S. K. and Chakraborty, D. Heat transfer and pressure drop characteristics of laminar flow through a circular tube A04804 # IMechE 2004

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