Experimental study of heat transfer enhancement in a pipe using twisted tapes

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Experimental study of heat transfer enhancement in a pipe using twisted tapes

  1. 1. INTERNATIONALMechanical Engineering and Technology (IJMET), ISSN 0976 – International Journal of JOURNAL OF MECHANICAL ENGINEERING 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME AND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online) IJMETVolume 4, Issue 2, March - April (2013), pp. 100-111© IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2013): 5.7731 (Calculated by GISI) ©IAEMEwww.jifactor.com EXPERIMENTAL STUDY OF HEAT TRANSFER ENHANCEMENT IN A PIPE USING TWISTED TAPES AND WIRE COILS Dr. A. G. Matani1, Swapnil A. Dahake2 1 Associate Professor, Mechanical Engineering Department, Government College of Engineering, Amravati, (M.S.) India 2 nd M.Tech. [2 Year Thermal Engineering], Government College of Engineering, Amravati, (M.S.) India. ABSTRACT In the proposed work, the heat transfer enhancement using twisted tapes and wire coil on pressure drop, Nusselt number (Nu), friction factor (f) and thermal enhancement index (ߟ) are experimentally determined. The twisted tapes are used as swirl flow generators while wire coil along with twisted tapes used as co-swirl flow generators in a test section. The tests are conducted using the twisted tape with three different twist ratios (y/w = 3.5, 2.66 and 2.25) and wire coil along with twisted tapes, pitch ratio of 1.17 & 0.88 for Reynolds numbers range between 5000 and 18,000 under uniform heat flux conditions. Also double twist generating counter swirl are compared. The experiments using the twisted tape and with wire coil performed under similar operation test conditions, for comparison. The experimental results indicate that the tube with the various inserts provides considerable improvement of the heat transfer rate over the plain tube. The experimental results demonstrate that friction factor (f) and thermal enhancement index (ߟ) increase with decreasing twist ratio (y/w) and Reynolds number. The results also show that the wire coils along with twisted tapes are more efficient than the twisted tapes for heat transfer enhancement. 1. INTRODUCTION Research is going on to investigate the level of heat transfer enhancement that can achieved by forced convection in which air is flow inside horizontal pipe. The comparison between bare tube and enhanced tube configuration are made on the basis of forced convection with air instabilities. These types of inserts increase the heat transfer coefficient 100
  2. 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEMEwith respect to smooth tube. Differences among the enhanced configuration are alsodetermined to observe which of them most stable and unstable one is. These inserts inducedswirl flow heat transfer due to exponentially increasing heat input with exponential periods. In the last decades, significant effort has been made to develop heat transferenhancement techniques in order to improve the overall performance of heat exchangers. Theinterest in these techniques is closely tied to energy prices and, it is expected that the heattransfer enhancement field will go through a new growth phase with the present increase inenergy cost. Although there is need to develop novel technologies, experimental work on theolder ones is still necessary. The knowledge of its performance shows a large degree ofuncertainty which makes their industrial implementation difficult [6]. The efficiency of heattransfer equipment is essential in energy conservation. Furthermore, a more efficient heatexchanger can reduce the size of the heat exchanger, thus reducing the costs associated withboth material and manufacturing of the heat exchanger [2]. Heat transfer enhancementtechnology has been widely applied to heat exchanger applications in refrigeration,automobile, process industries etc. There have been numerous attempts to reduce the size andcost of heat exchangers. Hence, there have been continuous attempts to improve theefficiency of heat exchangers by various methods. One of the best methods to achieve this isthe use of augmented heat transfer surfaces. Improved heat transfer can make heat exchangerssmaller and more energy efficient. The tube insert technology is one of the most commonheat transfer enhancement technologies for shell and tube heat exchangers. Tube side enhancement techniques can be classified according to the followingcriteria: (1) additional devices which are incorporated into a plain round tube (twisted tapes,wire coils) and (2) non-plain round tube techniques such as surface modification of a plaintube (corrugated and dimpled tubes) [5] or manufacturing of special tube geometries(internally finned tubes) [6]. The dominant literature study usually mentions five types: wirecoils, twisted tapes, extended surface devices, mesh inserts and displaced elements [7]. Themain advantage of these types in respect to other enhancement techniques such as theartificial roughness by mechanical deformation or internal fin types is that they allow an easyinstallation in an existing smooth-tube heat exchanger. The various single and double twistedtapes are also used to generate swirl/co-swirl [9]. Due to its low cost, the insert devices whichare most frequently used in engineering applications are wire coils and twisted tapes. Heat transfer enhancement techniques have been extensively developed to improvethe thermal performance of heat exchanger systems with a view to reducing the size and costof the systems. Swirl/vortex flow is the one of the enhancement techniques widely applied toheating/cooling systems in many engineering applications. The vortex flows can be classifiedinto two types: continuous swirl and decaying swirl flows. The former represents the swirlingmotion that persists over the entire length of the duct for example helical/twisted tape andcoiled wires inserts while the latter means the swirl created at the duct entrance and thendecays along the flow path such as the tangential injection, the rib/baffle and the wingletvortex generators [10]. Swirl flow has been used in a wide range of applications from variousengineering areas such as chemical and mechanical mixing and separation devices,combustion chambers, turbo machinery to pollution control devices. It is commonly knownthat the swirl flow enhances the heat transfer mainly due to the increased velocity in the swirltube and the circulation of the fluid by forced convection [4]. 101
  3. 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME2. TWISTED TAPES & WIRE COIL TO GENERATE SWIRL The knowledge of heat transfer in a short circular tube with the twisted-tape inserttube is important to investigate the influence of the twist ratio of twisted tape on enhancementof critical heat flux. A lot of the experimental investigations were conducted with the heattransfer characteristics of the swirl flow. It was shown that the twisted-tape insert tubeprovided considerable enhancement of turbulent heat transfer for heating/cooling of fluid.The twisted-tape insert tubes are encountered in a number of important engineering andscience systems such as plasma facing components in fusion experimental facilities, highpower laser systems and etc. We believe that the enhancement of heat transfer for the twisted-tape insert tube would be due to reduction of viscous sub-layer thickness on heated surface oftest tube with an increase in liquid flow velocity from straight flow to swirl one even at afixed mass velocity. Wire coil inserts are devices whose reliability and durability are widely contrasted. Inextreme applications such as the tube-side of fuel pyrotubular boilers with great foulingproblems and with high variations in temperature that produce great dilatations, wires areused without any problem. This is a cheap enhancement technique and it is completely viablefor many industrial applications. This fact hinders a widespread use of wire coil inserts inindustrial heat exchangers. The twisted tapes are made of mild steel and have tape width (w) of 10 mm, 15 mm &20mm shown in Figure 1, tape thickness (d) of 0.8 mm, and tape length (l) of 900 mm. Alltapes were prepared with different twist ratios, y/w = 3.5, 2.66 and 2.25 respectively wheretwist ratio is defined as twist length (l) to tape width (w). The double twisted tapes aremanufactured by combining two single twisted tapes, which generate counter swirl in pipeshown in Figure 2. On the other hand, to avoid an additional friction in the system that mightbe caused the thicker tape. To produce the twisted tape, one end of a straight tape wasclamped while another end was carefully twisted to ensure a desired twist length. Anotherinsert, wire coil is made up of aluminum wire of 2 mm diameter having pitch of 30mm & 40mm shown in Figure 3. These twisted tapes are fixed one by one inside the pipe having wirecoil to generate co-swirl. Figure 1: Single twisted tapes (y/w = 3.5, 2.66 & 2.25) 102
  4. 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME Mar Figure 2: Double twisted tape (y/w = 2.66) Figure 3: Wire coil (p/d = 0.88)3. TEST SECTION The test section is surrounded by nichrome heating wire, which is wrapped around thetest section with a pitch distance of 5 mm. This pitch is good enough to provide a nearlyuniform heating on the outer surface of the test section tube. The heating wire was poweredby a variable AC power supply. The overall electrical power added to the heating section, Q,was calculated by measuring the voltage (0 200 V) and the electrical current (0 A). To (0–200 (0–2control the convection losses from the test section and other components, foam insulation andglass wool used. Four thermocouples are to be embedded on the test section to measuresurface temperature of pipe and two thermocouples are placed in air stream at entrance andexist of test section to measure air temperature. To avoid floating voltage effects, thethermocouple bead is insulated from the electrically heated tube wall surface with a very thinsheet of mica between the thermocouple and the tube surface so as not to be effected fromelectricity. Fig. 2 shows the schematic view of experimental set-up. icity. set Figure 4: Experimental set-up 103
  5. 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME4. DATA REDUCTION In the experiments, the heat transfer rate in the tube is taken into account under auniform heat flux (UHF) condition by using air as the test fluid. The heat transfer given bythe hot surface to fluid (i.e. air) at any Reynolds number isRate at which air is heated Qa = mCp (To - Ti)The convection heat transfer from the test section is given asQc = hAs(Ts - Tm)At steady state condition, the heat transfer is assumed to be equal to the heat loss from thetest section that can be drawn asQa = QcThe mean temperature of the fluid in the test tube is given byTm = (To + Ti) / 2Ts is the mean temperature of surface wall temperature of the test tube. The average walltemperature is calculated from 4 points of local wall temperatures lined between the inlet andthe exit of the test tube. The average heat transfer coefficient (h) and the mean Nusseltnumber (Nu) are estimated byh = mCp (To - Ti) / hA(Ts - Tm)The Nusselt number in terms of average heat transfer coefficient is defined asNu = hD / kThe Reynolds number is written asRe = ρUD / µThe experiment pressure losses, ∆p across the test tube are arranged in non-dimensional formby using the following equation ∆ܲ ݂ൌ ‫ܷ ܮ‬ଶ ቀ‫ ܦ‬ቁ ሺ ሻ 2in which U is mean velocity in the test tube and L is the test tube length. All of thermo-physical properties of the air are determined at the overall mean air temperature (Tm).5. EXPERIMENTAL RESULTS AND DISCUSSIONS In this section, the pressure drop, friction factor characteristics, heat transfer andthermal enhancement index in a tube fitted with twisted tapes, double twisted tape and wirecoil (counter/co-swirl tape) are presented. The experiments are performed in the range of Reynolds number between 5000 and18,000. The results obtained for the tube fitted with the single twisted tapes (ST), wire coiland the empty tube are used as the reference data for the performance evaluation of themodified tubes. 104
  6. 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME5.1 Heat transfer result Experimental results of the Nusselt number (Nu) in plain tubes combined with atwisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66) and wire coil (p/d =1.17, 0.88) are presented in Figure 5. The Nusselt numbers for the plain tube acting alone arealso plotted for comparison. The data show that the Nusselt number (therefore, the heattransfer coefficient) increases with increasing Reynolds number for the conventionalturbulent tube flow. This is the most likely caused by a stronger turbulence and better contactbetween fluid and heating wall. It is noted that the increasing Nusselt number in the plaintube in common with a twisted tapes and wire coil is caused by the generating of pressuregradient along the radial direction, and this leads to redeveloping of thermal/hydrodynamicboundary layer. The higher increase of the Nusselt number in this style of both turbulenceand swirl flows is a consequence of the higher reduction of boundary layer thickness andincrease of resultant velocity. The variations of Nusselt number with Reynolds number for single and double twistedtapes with wire coil of pitch ratio (p/d = 1.17) shown in Figure 6 and with wire coil of pitchratio (p/d = 0.88) shown in Figure 7. Nusselt number increases with the decrease of twist ratioand the increase of Reynolds number. The highest Nusselt number is achieved for twist ratio(y/w = 2.25) and pitch ratio (p/d =0.88). However, the heat transfer enhancement by twisted tapes is less efficient than thatoffered by wire coil indicated by the lower Nusselt number depending on operating condition. smooth tube 90 STT(y/w)=3.5 80 STT(y/w)=2.6 70 6 STT(y/w)=2.2 60 5 DTT(y/w)=3.5 Nu 50 DTT(y/w)=2.6 6 40 WC(p/d)=1.1 7 30 20 5000 7000 9000 11000 13000 15000 17000 ReFigure 5: Single twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66)and wire coil (p/d = 1.17, 0.88) 105
  7. 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME STT(p/d)=1.17,(y/w)= 110 3.5 100 STT(p/d)=1.17,(y/w)= 2.66 90 STT(p/d)=1.17,(y/w)= 2.25 80 DTT(p/d)=1.17,(y/w)= 3.5 70 DTT(p/d)=1.17,(y/w)= Nu 2.66 60 50 40 30 5000 10000 15000 ReFigure 6: Single and double twisted tapes along with wire coil of pitch ratio (p/d = 1.17) STT(p/d)=0.88,(y/w)=3. 120 5 110 STT(p/d)=0.88,(y/w)=2. 66 100 STT(p/d)=0.88,(y/w)=2. 25 90 DTT(p/d)=0.88,(y/w)=3. 80 5 DTT(p/d)=0.88,(y/w)=2. 70 Nu 66 60 50 40 30 5000 7000 9000 11000 13000 15000 17000 Re Figure 7: Single and double twisted tapes with wire coil of pitch ratio (p/d = 0.88)5.2 Friction factor results Experimental results of the friction factor (f) characteristics in plain tubes combined witha twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66) and wire coil (p/d= 1.17, 0.88) are presented in Figure 5. The friction factors of the plain tube acting alone arealso plotted for comparison. Figure shows the influence of a plain tube combined with atwisted tape and wire coil on pressure loss, which indicates the friction in a heat exchanger. 106
  8. 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME The relationship between pressure loss in terms of friction factor and Reynolds numberfor the tube with twisted tape (STT & DTT) and wire coil inserted and also for the plain tubeis presented in Figure 5. It is found that using twisted tape and wire coil gives higher frictionfactor values than those from the plain tube as expected. The friction factor decreases withthe increase of twist ratio and Reynolds number. The variations of friction factor with Reynolds number for single and double twistedtapes with wire coil of pitch ratio (p/d = 1.17) shown in Figure 9 and with wire coil of pitchratio (p/d = 0.88) shown in Figure 10. The highest friction factor (or pressure loss) is obtainedin case p/d = 0.88 & y/w = 2.66. 0.3 smooth tube STT 0.25 (y/w)=3.5 STT Friction factor (f) 0.2 (y/w)=2.66 STT 0.15 (y/w)=2.25 DTT (y/w)=3.5 0.1 DTT (y/w)=2.66 0.05 0 5000 7000 9000 11000 13000 15000 17000 Re Figure 8: Single twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66) and wire coil (p/d = 1.17, 0.88) 0.5 STT (p/d)=1.17,(y/w)=3.5 0.45 STT (p/d)=1.17,(y/w)=2.66 0.4 STT 0.35 (p/d)=1.17,(y/w)=2.25 Friction factor (f) DTT 0.3 (p/d)=1.17,(y/w)=3.5 0.25 0.2 0.15 0.1 5000 7000 9000 11000 13000 15000 17000 ReFigure 9: Single and double twisted tapes along with wire coil of pitch ratio (p/d = 1.17) 107
  9. 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME STT 0.6 (p/d)=0.88,(y/w)=3.5 0.55 STT (p/d)=0.88,(y/w)=2.66 0.5 STT 0.45 (p/d)=0.88,(y/w)=2.25 DTT Friction factor (f) 0.4 (p/d)=0.88,(y/w)=3.5 DTT 0.35 (p/d)=0.88,(y/w)=2.66 0.3 0.25 0.2 0.15 0.1 5000 7000 9000 11000 13000 15000 17000 Re Figure 10: Single and double twisted tapes with wire coil of pitch ratio (p/d = 0.88)5.3 Performance factor results Siva Rama Krishna, Govardhan Pathipaka, P. Sivashanmugam [3] proposed aperformance evaluation analysis for the same pumping power and this method used forpresent study, the performance ratio is defined as ே௨௜/ே௨௣ߟൌ ೑೔ బ.మవభ ሺ ሻ ೑೛ The performance analysis was made by above Eq. and the results are shown in Figure11, 12, 13. In the present work, a thermal performance factor is evaluated since the factor isan important parameter indicating the potential of a twisted tape for practical applications.The thermal evaluation is considered under constant pumping power for each twisted tapewith respect to the case without twisted tape (plain tube). The thermal performance factorsfor single twisted tape, double twisted tape and wire coil are presented in Figure 11.Apparently, a thermal performance factor decreases with increasing Reynolds number for alltape inserts. A larger pressure loss at a higher Reynolds number is responsible for thementioned result. This suggests that the geometry of wire coil is more appropriate forpractical use than the others in the view point of energy as well as operating cost savings. 108
  10. 10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME STT (y/w)=3.5 1.35 STT 1.3 (y/w)=2.66 STT 1.25 (y/w)=2.25 Performance factor DTT (y/w)=3.5 1.2 DTT 1.15 (y/w)=2.66 WC(p/d)=1.17 1.1 1.05 1 0.95 5000 7000 9000 11000 13000 15000 17000 ReFigure 11: Single twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66) and wire coil (p/d = 1.17, 0.88) STT 1.65 (p/d)=1.17,(y/w)=3.5 STT 1.55 (p/d)=1.17,(y/w)=2.66 STT (p/d)=1.17,(y/w)=2.25 1.45 DTT Performance factor (p/d)=1.17,(y/w)=3.5 1.35 DTT (p/d)=1.17,(y/w)=2.66 1.25 1.15 1.05 0.95 5000 7000 9000 11000 13000 15000 17000 Re Figure 12: Single and double twisted tapes along with wire coil of pitch ratio (p/d = 1.17) 109
  11. 11. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME 1.75 STT (p/d)=0.88,(y/w)=3.5 1.65 STT 1.55 (p/d)=0.88,(y/w)=2.66 Performance factor STT 1.45 (p/d)=0.88,(y/w)=2.25 DTT 1.35 (p/d)=0.88,(y/w)=3.5 1.25 1.15 1.05 0.95 5000 10000 15000 Re Figure 13: Single and double twisted tapes with wire coil of pitch ratio (p/d = 0.88)6. CONCLUSIONS Thermal characteristics in a tube fitted with twisted-tapes in co-swirl arrangement withwire coil are presented in the present study. The work has been conducted in the turbulent flowregime, Reynolds number from 5000 to 18,000 using air as the test fluid. The findings of thework can be drawn as follows: For the inserted tube, the pressure drop tends to increase with the rise in mass flow rate while the friction factor and performance factor give the opposite trends. The compound enhancement devices of the tube and the co-swirl show a considerable improvement of heat transfer rate and thermal performance relative to the smooth tube acting alone, depending on twist ratios. The co-swirl tube yields higher friction factor and performance factor than the smooth tube at low Reynolds number. The various inserts in pipe mixes the bulk flow well and therefore performs better in laminar flow. The result also shows twisted tape insert is more effective, if no pressure drop penalty is considered. Twisted tape in turbulent flow is effective up to a certain Reynolds number range. Swirl flow heat transfer is higher than non swirling flow.NOMENCLATUREd wire diameterf Friction factorߟ Performance factorNu Nusselt numberp pitchp/d pitch ratioRe Reynolds numberw Width of tapey Twist length(y/w) twist ratio 110
  12. 12. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEMEREFERENCES1. Shou-Shing Hsieh, Feng-Yu Wu, Huang-Hsiu Tsai, “Turbulent heat transfer and flow characteristics in a horizontal circular tube with Shou-Shing Hsieh, Feng-Yu Wu,Huang-Hsiu strip-type inserts”, International Journal of Heat and Mass Transfer (2003),Vol. 46, pp. 823- 835.2. Leonard D. Tijing, Bock Choon Pak, Byung Joon Baek, Dong Hwan Lee, “A study on heat transfer enhancement using straight and twisted internal fin inserts”, International Communications in Heat and Mass Transfer (2006), Vol. 33, pp. 719-726.3. Siva Rama Krishna, Govardhan Pathipaka, P. Sivashanmugam, “Heat transfer and pressure drop studies in a circular tube fitted with straight full twist”, Experimental Thermal and Fluid Science (2009), Vol. 33, pp. 431–438.4. H. Gül a, D. Evinb, “Heat transfer enhancement in circular tubes using helical swirl generator insert at the entrance”, International Journal of Thermal Sciences (2007), Vol. 46, pp. 1297- 1303.5. Chinaruk Thianpong, Petpices Eiamsa-ard, Khwanchit Wongcharee, Smith Eiamsa-ard, “Compound heat transfer enhancement of a dimpled tube with a twisted tape swirl generator”, International Communications in Heat and Mass Transfer (2009), Vol. 36, pp. 698–704.6. Alberto Garcia, Pedro G. Vicente, Antonio Viedma, “Experimental study of heat transfer enhancement with wire coil inserts in laminar-transition-turbulent regimes at different Prandtl numbers Experimental study of heat transfer enhancement with wire coil inserts in laminar- transition-turbulent regimes at different Prandtl numbers”, International Journal of Heat and Mass Transfer (2005), Vol. 48, pp. 4640-4651.7. Alberto Garcia, Juan P. Solano, Pedro G. Vicente, Antonio Viedma, “Enhancement of laminar and transitional flow heat transfer in tubes by means of wire coil inserts”, International Journal of Heat and Mass Transfer (2007), Vol. 50, pp. 3176-3189.8. Kumbhar D.G., Dr. Sane N.K., “Heat Transfer Enhancement in a Circular Tube Twisted with Swirl Generator: A Review”, Proc. of the 3rd International Conference on Advances in Mechanical Engineering, SVNIT Surat, January 4-6, 2010.9. Panida Seemawute, Smith Eiamsa-ard, “Thermohydraulics of turbulent flow through a round tube by a peripherally-cut twisted tape with an alternate axis”, International Communications in Heat and Mass Transfer (2010), Vol. 37, pp. 652–659.10. Pongjet Promvonge, Somsak Pethkool, Monsak Pimsarn, Chinaruk Thianpong, “Heat transfer augmentation in a helical-ribbed tube with double twisted tape inserts”, International Communications in Heat and Mass Transfer (2012), Vol. 39, pp. 953-959.11. S. Eiamsa-ard, C. Thianpong, P. Eiamsa-ard, “Turbulent heat transfer enhancement by counter/co-swirling flow in a tube fitted with twin twisted tapes”, Experimental Thermal and Fluid Science (2010), Vol. 34, pp. 53–62.12. Halit Bas, Veysel Ozceyhan, “Heat transfer enhancement in a tube with twisted tape inserts placed separately from the tube wall”, Experimental Thermal and Fluid Science (2012), Vol. 41, pp. 51–58.13. S. Eiamsa-ard , P. Seemawute, “Decaying swirl flow in round tubes with short-length twisted tapes”, International Communications in Heat and Mass Transfer (2012), Vol. 39, pp 649– 656.14. Er. Pardeep Kumar, Manoj Sain and Shweta Tripathi, “Enhancement of Heat Transfer using Wire Coil Insert in Tubes” International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 2, 2012, pp. 796 - 805, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. 111

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