Effect of Twisted-tae Inserts on Heat Transfer in a Tube Watcarin Nootong, Smit Eiamsa-ard and Pongjet Promvonge, * Deartment of Mecanical Engineering, Faculty of Engineering, King Mongkut s Institute of Tecnology Ladkrabang, Bangkok, 050, Tailand Deartment of Mecanical Engineering, Faculty of Engineering, Maanakorn University of Tecnology, Bangkok 050, Tailand Abstract: Influences of te twisted tae insertion on eat transfer and flow friction caracteristics in a concentric double ie eat excanger ave been studied exerimentally. In te exeriments, te swirling flow was introduced by using twisted tae laced inside te inner test tube of te eat excanger wit different twist ratios, y = 5.0 and 7.0. Te exerimental results revealed tat te increase in eat transfer rate of te twisted-tae inserts is found to be strongly influenced by tae-induced swirl or vortex motion. Over te range investigated, te maximum Nusselt numbers for using te enancement devices wit y = 5.0 and 7.0 are 88% and 59%, resectively, iger tan tat for te lain tube. In addition, te effects of te twisted tae on te eat transfer enancement efficiency are also investigated. Keywords: Twisted Tae, Heat Transfer, Heat Excanger, Swirl Flow, Enancement Efficiency. INTRODUCTION Heat excanger is te aaratus roviding eat transfer between two or more fluids, and tey can be classified according to te mode of flow of fluid or teir construction metods. Heat excangers wit te convective eat transfer of fluid inside te tubes are frequently used in many engineering alication. Augmentation eat transfer, in connection wit fluid mixing or non mixing, is also involved wic most eat excangers ave no contact between te fluids. At resent, te tecnology of te twisted-tae insert is widely used in various industries. Insertion of twisted taes in a tube rovides a simle assive tecnique for enancing te convective eat transfer by introducing swirl into te bulk flow and by disruting te boundary layer at te tube surface due to reeated canges in te surface geometry. It as been exlained tat suc taes induce turbulence and suerimosed vortex motion (swirl flow) causing a tinner boundary layer and consequently resulting in a ig eat transfer coefficient and Nusselt number due to reeated canges in te twisted tae geometry. On consideration of te eat transfer enancement, it can be considered troug bringing te twisted-tae to insert wile te ressure dro inside te tube is iger. Because of low assets and easy setting u, it is widely used, esecially in a comact eat excanger. A eat transfer enancement concet in wic swirl was introduced in te flow was roosed by Kreit and Margolis (959). In tis concet, art of te fluid enters axially wile te remainder is injected tangentially at various locations along te tube axis. Te radial ressure gradient results in tinning of te termal boundary layer wit an accomanying imrovement in eat transfer. Most of te swirl flows were created by insertion of a twisted tae in te tube. Twisted taes, wen inserted into tubes, tend to romote turbulence as well as to intensity mixing of te ot fluid and cold fluid. Tin in turn imroves te eat transfer rocess. Many researces ave investigated te effect of geometry of twisted-taes on eat transfer and friction in a circular or rectangular smoot ie in bot exerimental and numerical studies. Date [], Date and Saa [] numerically redicted te friction and eat transfer caracteristics for laminar flow in a circular tube fitted wit regularly saced twisted-tae elements tat were connected by tin circular rods. Ray and Date [] investigated exerimentally correlations of eat transfer and flow frictions in a square duct wit twisted-tae insert. Kumar and Prasad [4] reorted te imroved solar collectors of water eating tyes by means of twisted taes inserted in te water flow tubes. Sukatme et al. [5] studied exerimentally te effect of te twisted-tae insert on eat transfer rate and ressure dro in laminar flow under uniform eat flux. On te oter and Dulessis and Kroger [6] made te exerimental and numerical studies wit te constant wall temerature in laminar flow. Leina and Bergles [7] rovide a correlation of te eat transfer inside te tube for turbulent flow wic can be utilized in te eat excanger industry. In te resent investigation, te influence of twisted-tae inserts on enancement eat transfer efficiency, Nusselt number and flow friction beaviors in a double ie eat excanger is studied. In te exerimental condition, a twisted-tae was inserted into te inner tube at twist ratios: y = P/D = 5.0 and 7.0, resectively. All of te exeriments were carried out at te same inlet condition wit te Reynolds number of te inner tube, Re = 000 to 000.. METHODOLOGY. Exerimental Aaratus Figures and sow a scematic view of te exerimental setu and a concentric double ie eat excanger fitted wit a twisted tae. Te test section consists of concentric straigt ies made of Plexiglas, wic are joined at regular interval of.0 m by flanges. Te inner diameter of te outer ie is 50 mm and te annulus (flow assage) is of 0 mm in te radial direction trougout. Water is sulied to te ie from te cilled water loo of caacity 0. m, an electrical eater controlled by adjusting te voltage, a stirrer, and a cooling coil immerged inside a storage tank. Hot air from a 7.5 kw blower was directed troug te inner tube, wile cold water was umed troug te annulus. In te test run, te taes were laced in two different twisted ratios: y = 0.6 and 0.8 as deicted in Fig.. Twisted taes were made from stainless steel stris of tickness.0 mm and widt 9.5 mm. Tey were fabricated by twisting a straigt tae, about its longitudinal axis, wile being eld under tension. All te ressure readings are taken under isotermal conditions. Inclined and U-tube manometers wit manometric fluid (SG = 0.86) as working fluid is used for measuring te ressure dro at eac robe location. Te least count of te ressure measurement manometer is 0.5 mm water is used as te working fluid for entire study. Liquid and air flow meters were used to measure bot water and air flow rates. Te volumetric Corresonding autor: kongje@kmitl.ac.t
flow rates of te ot air and cold water were adjusted by control valves, situated before te inlet orts. Te inlet air was eated by an adjustable electrical eater. Bot te inlet and outlet temeratures of te ot air and te cold water were measured by multi-cannel wit iron-constantan termocoule (tye K). It was necessary to measure te temerature at 5 stations altogeter at te outer surface of te inner tube for finding out te average Nusselt number. PC Comuter Data Logger wit Termocoule tye K Concentric Tube Heat Excanger Rotameter Rotameter Ciller U-Manometer wit Water Water Reservoir Termostat wit Electrical Heater Centrifugal Pum Blower Fig. Scematic diagrams of exerimental aaratus. Data reduction equations For fluid flows in a concentric tube eat excanger, te eat transfer rate of te ot fluid (air) in te inner tube can be exressed as: wile te eat transfer of te cold fluid (water) for te outer tube is wereas, and Q = MC (T T ) () o i Q = A(T ~ T ) () w b Tb = (To + Ti ) / () T ~ w = Tw / 5 (4) Were T w is te local wall temerature and evaluated at te outer wall surface of te inner tube. It must be measured at te det from te outer surface of 0.5 mm. Te average wall temeratures are calculated from 5 oints, lined between te inlet and exit of te inner tube. Te average eat transfer coefficient is calculated from te energy balance are estimated as follows: = MC (T T ) / A(T ~ o i w Tb ) (5) Te mean value of te Nusselt number is calculated based on te mean wall temerature w T and te local mean bulk fluid temerature.
D Nu = (6) k Were, and k are te mean eat transfer coefficient and mean termal conductivity of te fluid, resectively, at all termocoule location. Te local termal conductivity k of te fluid is calculated from te fluid roerties at te local mean bulk fluid temerature. Te Reynolds number is calculated based on te difference rate Q troug te test section. Re = VD / v (7) Were v is te kinematics viscosity of te working fluid. Friction factor can be written as follows: Dc D f = L D P V ρ Outer tube Inner tube (8) L Fig. Te inner tube fitted wit twisted tae at different twist ratios (y = 5.0 and 7.0). RESULTS AND DISCUSSION In te case of te twisted tae inserts, te means Nusselt numbers (of te inner tube or ot air) increased about 88% for y = 5.0 and 59% for y = 7.0, wen comared wit te lain tube. It was sown tat te smaller of te twist ratio, te iger in mean Nusselts number as seen in Fig. (a). From te exerimental results, it is clear tat te twisted tae inserts caused swirl and ressure gradient in te radial direction. Te boundary layer along te tube wall would be tinner wit te increase of radial swirl and ressure resulting inn more eat flow troug te fluid. Moreover, swirl caused te flow to be turbulent, wic led to even better convection eat transfer. From te figure, it is deicted tat te effect of te twisted tae inserts decreased at low Reynolds numbers due to te weak swirl and low flow velocity. Tus, te increase in Nusselt number was low at smaller Reynolds number, wile it became greater at te iger Reynolds numbers. Tis enomenon is related to te seed of te swirl-flow of te ot air and te results of te destruction of te boundary layer level. Trougout te exerimental results, it is obvious tat te narrow twist ratio (y = 5.0) yielded te iger values of eat transfer tan tose were at te greater twist ratio (y = 7.0). Tis was due to te more violently swirl wit te narrow twist ratio. Te friction factor in te inner tube varied wit Reynolds number as sown in Fig. (b). It can be seen tat te frictions are similar in trend for bot te lain tube and te tube fitted wit twisted tae inserts. It was observed tat te twisted tae inserts caused swirling flow into te tube and leaded to ig friction of.7 and.94 times of te lain tube, for y = 5.0 and 7.0, resectively in comarison wit te lain tube. It can be seen tat for te y = 5.0 gave smaller value of friction tan tat y = 7.0 because te intervals gave lower value of swirling flow tan tose in te case of small twist ratio.
0 00 (a) Nusselt number 80 60 40 0 0 0.5 (b) Friction factor 0.4 0. 0. y=6.0 y=8.0 Plain tube 0. 0 (c) Enancement efficiency.5.5 η =0.86Re 0.7 y -0. 0 000 6000 9000 000 5000 Reynolds number Fig. Nusselts number, friction factor and enancement efficiency versus Reynolds number for te tube fitted wit twisted tae at different twist ratios (y) A useful comarison between swirl and axial flow can be made by comaring eat transfer coefficients at equal uming ower, since tis is relevant to te oeration cost. For constant uming ower; ( V P ) (V& P ) t and te relationsi between friction and Reynolds number can be exressed as: & = (9) ) t ( f Re ) = ( f Re (0) 4
Te enancement efficiency (η ) at constant uming ower is te ratio of te convective eat transfer coefficient of te tube wit twisted tae to te lain tube wic can be written as follows: Te enancement efficiency for twisted taes can be written as: t η = () 0.7 0.8 η = = 0.86 Ret y () t Trougout te results of te exerimentation, it aears tat use of te small twist ratio (y) leads to iger enancement efficiency tan tat of te larger twisted ratio (y) as can be seen in Fig. (c). It can be seen tat te enancement efficiency increases as te twisted ratio (y) decreases wic generally iger at ig Reynolds numbers for all twist ratios. Tis suggests tat twisted taes are feasible in terms of energy saving at iger Reynolds numbers. It is assumed tat te erformance imrovement in te intervals of Reynolds number and te sizes of te twisted tae is rogressively develoing. Tus it is ossible to use te devices wit te various styles of taes inserts to reduce te ressure dro and ossibly increase te eat transfer. 4. CONCLUSION Exerimental investigation of enancement efficiency, eat transfer and friction factor caracteristics of circular tube fitted wit twisted tae inserts of different twist ratios as been studied. It can be observed tat te swirl flow els decrease te boundary layer tickness of te ot air flow and increase residence time of ot air in te inner tube. Te enancement efficiency and Nusselt number increases wit decreasing te twist ratio and friction factor also increases wit decreasing te twist ratio. Te artitioning and blockage of te tube flow cross-section by te tae, resulting in iger flow velocities. Secondary fluid motion is generated by te tae twist, and te resulting twist mixing imroves te convection eat transfer. 5. REFERENCES [] Date, A.W. (974) Prediction of fully develoed flow in a tube containing a twisted tae, International Journal Heat Mass Transfer, 7,. 845-859. [] Date, A.W. and Saa, S.K. (990) Numerical rediction of laminar flow and eat transfer in a tube fitted wit regularly saced twisted-tae elements, International Journal Heat Fluid Flow,,. 46-54. [] Ray, S. and Date, A.W. (00) Friction and Heat Transfer Caracteristics of Flow troug Square Duct wit Twisted Tae Insert, International Journal Heat and Mass Transfer, 46,. 889-90. [4] Kumar, A. and Prasad, B.N. (000) Investigation of twisted tae inserted solar water eaters-eat transfer, friction factor and termal erformance results, Renewable Energy Journal, 9,. 79-98. [5] Sukatme, S.P., Gaitonde, U.N., Sidore, C.S. and Kuncolienkar, R.S. (987) Forced convection eat transfer to visous liqiuid in laminar flow in a tube wit a twisted tae, Proceding Nint Natl. Heat Mass Transfer Conference, (Paer no. HMT 7-87) Indian Institute of Science, Bangalore, India, Part B, -7. [6] Dulessis, J.P. and Kroeger, D.G. (98) Numerical rediction of laminar flow wit eat transfer in tube wit a twisted tae insert, Proceeding of te International Conference on Numerical Metods in Laminar and Turbulent Flow,. 775-785. [7] Leina, R.F. and Bergles, A.E. (969) Heat transfer and ressure dro in tae-generated swirl flow of single-ase water, ASME Journal Heat Transfer, 9,. 44-44. 5