NUMERICAL SIMULATION OF HEAT TRANSFER CHARACTERISTICS IN THE ABSORBER TUBE OF PARABOLIC TROUGH COLLECTOR WITH INTERNAL FLOW OBSTRUCTIONS

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1 ARPN Journal of Engneerng and Appled Scences Asan Research Publshng Network (ARPN). All rghts reserved. NUMERICAL SIMULATION OF HEAT TRANSFER CHARACTERISTICS IN THE ABSORBER TUBE OF PARABOLIC TROUGH COLLECTOR WITH INTERNAL FLOW OBSTRUCTIONS M. Natarajan, R. Thundl karuppa Raj, Y. Raja Sekhar, T. Srnvas and Pranay Gupta School of Mechancal and Buldng Scences, VIT Unversty, Vellore, Inda E-Mal: ABSTRACT Absorber tube performance enhancement by usng passve technques s one of the major topcs of research n the feld of solar thermal power engneerng. Earler studes on ducts revealed that passve augmentaton technques have shown consderable enhancement n heat transfer. Experments were conducted earler on Parabolc Trough Collector (PTC) wth plan absorber tube wthout nserts for dfferent flow rate condtons. It was observed that flud flow rate wth 85 kg/hr has shown hgher temperature dfference for solar flux condton of 850 W/m 2. In the present study, numercal analyss usng CFD s performed by usng nserts of dfferent cross-secton nsde the flud flow path of an absorber tube to study the flow characterstcs. The flud flow and heat transfer phenomena through a 3-dmensonal absorber tube wth varyng heat source are obtaned by solvng the fundamental governng equatons namely: conservaton of mass, momentum and energy. Turbulence s modeled usng SST k-ω model of closure. The heat transfer and pressure drop s calculated from the studes conducted for a mass flow rate of 85 kg/hr usng the ANSYS CFX 12.1 software. The result of the numercal analyss s valdated wth the experments carred out wth the parabolc trough collector. The numercal study s carred out wth trangle, nverted trangle and sem-crcular nserts and compared wth that of plan absorber tube. It has been found that trangle nserton gves optmzed results wth respect to unform heat transfer whch reduces the thermal fatgue even the pressure drop s relatvely hgh compared to the plan absorber tube wthout nserton. Keywords: absorber tube, flow obstructon, trangular turbulator, turbulence heat transfer. INTRODUCTION In Inda, progress of solar thermal research s fast boomng over the last decade. The success of solar concentrators n Inda, especally for communty cookng applcatons has gven motvaton to use these technologes for other applcatons too. Currently, as per the recent report released by MNRE Report, solar power plants are beng planned to setup at a few locatons throughout the country [1]. Parabolc collector trough (PTC) technology has been wdely used n many parts of the world for power generaton actvtes because of ts matured technology. Hgher thermodynamc effcency can be acheved for such type of concentrators. Efforts have been made to ncrease the absorber characterstcs, reducng the heat loss from the collector, extendng heat transfer rato, ncreasng the flud condut aspect rato and optmzng the desgn parameters. Absorber tubes of the PTC system receve the concentrated energy from the reflectve mrrors and delver t to the flowng flud nsde. Mafzul hug and Mchael K. Jensen et al. [2, 3] has revealed that the heat transfer coeffcent of the nternally fnned tube large n the entry regon and then the enhancement n heat transfer n the fully developed regon due the effects of the nternal fns. R. Forrstall and Sane et al. [4, 5] done the modellng and the heat transfer analyss of a parabolc solar trough collector usng fluent and suggested many concepts towards the performance enhancement of the of an absorber tube of a cylndrcal solar parabolc trough collector. Earler many researchers had done numerous smulatons, nvolvng both expermental as well as numercal analyss, and gave a concluson that heat enhancement s possble wth the use of passve augmentaton technques. Inserton of passve elements n the flud flow path has been proved to be a low cost method of mprovng the heat transfer performance, snce the flow characterstcs are performance, snce the flow characterstcs are changed Abu-Khader et al. [6]. The performance of a Solar Collector system s largely dependent on the radaton whch provdes us wth an altogether dfferent set of problems because of the varable nature of the Solar Radaton values, when compared to conventonal heat exchanger systems. Varous radaton models have been suggested by dfferent authors based on the method of estmatng the dffused radaton components. Qu-Wang Wang et al. [7] nvestgated the effect of dfferent fn profles on the absorber tube flow characterstcs of an nternally fnned tube. O. García-Valladares and N. Velázquez [8] conducted numercal analyss to determne whether the use of concentrc crcular heat exchangers mproved the functonng of parabolc trough solar collector. Munoz and Abanades et al. [9] mentoned the heterogeneous nature of flux dstrbuton on the absorber wall and ndcated how the use of nternally fnned tubes would results n homogenzed temperature dstrbuton on the absorber wall. The use of these fns also resulted n decrease n outer surface temperature and thermal losses. Matthew Roesle et al. [10] conducted numercal analyss for heat loss from the absorber tube of a parabolc trough solar collector. Sekhar et al. [11] performed expermental smulaton to study the heat transfer enhancement usng Al 2 O 3 nanofluds n a plan tube wth twsted tape nserts. Thundl et al. [12] had earler performed expermental and numercal analyss to study the performance of the 674

2 ARPN Journal of Engneerng and Appled Scences Asan Research Publshng Network (ARPN). All rghts reserved. absorber tube for a concentrc tube nserton confguraton and t was observed that there s slght ncrease n outlet temperature wth a huge penalty n pressure drop. Ths study s ntended to estmate the heat transfer performance under unform heat flux boundary condton n the turbulent flow range for varous cross-sectons of nsertons usng CFD analyss. EXPERIMENTAL SETUP A prototype PTC system s nstalled at the terrace of SMBS buldng, VIT Unversty Vellore campus as shown n Fgure-1. The dmensons of the same parabolc trough collector and absorber tube are consdered for the present analyss as shown n Table-1. Absorber tube materal used s made of copper. Fgure-1. Solar parabolc trough collector. Table-1. Dmensons of parabolc trough collector. Wdth of collector (W) 0.91 m Length of collector (L) 1.50 m Outer dameter of absorber tube (D o ) m Inner dameter of absorber tube (D ) m Reflectvty of recever (K) 0.85 Rm angle (Ø r ) 90 0 Focal length (f) m Concentraton rato (C) Materal propertes of the tube are consdered as mentoned below: Densty = 8978 kg/m 3 Specfc heat capacty = 381 J/kgK Thermal conductvty = W/mK Water s taken as the workng flud n the analyss and the propertes are shown n Table-2. Table-2. Propertes of the workng flud (water). Propertes of flud (water)@ 20 0 C Value Densty of flud (ρ) kg/m 3 Specfc heat capacty (C p ) 4182 J/kg K Thermal conductvty (k) W/m K Dynamc vscosty (µ) kg/m-s METHODOLOGY OF CFD SIMULATION The numercal analyss usng CFD s carred out wth plan absorber tube and tube nserted wth dfferent nserton cross sectons are as shown n Fgures 2-4, respectvely. The flud flow smulaton s accomplshed usng commercal CFD software Fluent Meshng of the Model has been done usng ICEM CFD software V Some assumptons have been made durng the conduct of expermental smulaton and are lsted as follows: It has been assumed that the tube wll be subjected to two boundary condtons- upper part of wll be exposed to ambent drect radaton and lower part 90 0 wll be exposed wth concentrated radaton. The upper part of the tube s assumed to receve a constant drect beam radaton of W/m 2 as measured manually at VIT Unversty on 19 th January 2013 usng Pyrhelometer an d the remanng 90 0 receve constant concentrated radaton of W/m 2 calculated as per the equaton reported by Cheng et al. [8]; (1) The flux dstrbuton s assumed to be unform over the surface of the tube; (2) The flud flow s assumed to be fully developed; Assumptons (1) and (2) have been arrved at by consderng nformaton avalable n exstng lterature [8, 9, 11]. The flud flow has been consdered a s 85 kg/hr and the temperature of flud at nlet of the absorber tube has been consdered based on an expermental values. Mesh generaton of the 3-D model of the absorber tube s then done usng the pre-processor ICEM CFD meshng tool. The entre model s dscretzed usng hexahedral mesh elements whch are accurate and nvolve less solver effort and requres hgh skll of meshng algorthm compared to tetrahedral elements. The hexahedral meshng of the flud domans of sem-crcular nserton and nverted trangle nserton are shown n Fgures 4 and 5, respectvely. The Y + value s mantaned less than 1 near the bottom wall surface where the heat flux s hgher. Ths Y+ value s requred to capture the turbulence physcs exerted by the SST K - ω model of closure. Fne control on the hexahedral mesh near the wall surface allows capturng the thermal and velocty boundary layers accurately. The dscredted model s checked to have a mnmum angle of 79.6 and a mnmum determnant qualty of 0.9. The nverted trangle nserton flud doman s meshed wth , and hexahedral elements and t has been found that the numercal soluton 675

3 ARPN Journal of Engneerng and Appled Scences Asan Research Publshng Network (ARPN). All rghts reserved. obtaned do not vary sgnfcantly beyond hexahedral elements. Smlar grd ndependence study s carred out for the geometry consdered n ths analyss. Fgure-5. Hexahedral Mesh of the absorber tube wth nserton. Fgure-2. Trangular nserton. Contnuty equaton: ( u ) = 0 x ρ (1) Fgure-3. Inverted trangular nserton. Momentum equaton: p ( ρu u ) = + ( µ + µ ) j 2 u ) 3 l l ( µ t + µ ) δj + ρg Energy equaton: t u u j + j (2) µ µ t T ( ρ ut ) = + Sr x x + pr T (3) σ K equaton: Fgure-4. Semcrcular nserton. The followng assumptons are made for CFD analyss: a. Steady state heat transfer s consdered so that the heat flux at the wall does not change. b. The contact thermal resstance between the wall and the flud s not consdered. c. Thermal conductvty of the absorber tube materal s unform and constant. d. The radaton heat transfer from the absorber tube s neglected. µ T k σk ( ρu k) = µ + + Gk ρε ε equaton: ( ρu ε ) = µ T ε µ + + σε k ε ( C ρε ) 1Gk C 2 Where the turbulent vscosty µ t and the producton rate Gk are gven by: 2 ( ρ )( k ) Cµ u t = (6) ε (4) (5) Pre-processng The flud flow s n turbulent and n steady condton. So the governng equatons for contnuty, momentum, energy and standard k-ε turbulence model can be expressed as follows: 676

4 ARPN Journal of Engneerng and Appled Scences Asan Research Publshng Network (ARPN). All rghts reserved. u Gk = µ t * (7) u u j j + j In order to mprove the qualty of the analyss second-order upwnd analyss has been done so as to obtan more accurate results. The SIMPLE algorthm has been used to ensure couplng between velocty and pressure. RESULTS AND DISCUSSIONS Experments were conducted n the month of January 2013, on the prototype PTC system wth water as workng flud usng plan absorber tube. The expermental setup used n the earler analyss s a closed crcut confguraton. The parameters measured durng the expermentaton are nlet temperature, outlet temperature, ambent temperature, nlet mass flow rate and the pyrhelometer readng. The beam radaton values were contnuously measured and the varaton of the radaton durng the duraton of the experment s wthn the allowable lmt (±50 W/m 2 ). The results of the experment conducted for dfferent flow rates confrm that for water mass flow rate of 85 kg/hr the temperature dfference between outlet and nlet flud temperature s maxmum as shown n Table-3. Hence, usng the same flow parameters CFD smulatons have been carred out for absorber tube wth nserton of dfferent cross secton to analyze the flow characterstcs. The nsertons havng the cross sectons of trangular, nverted trangle and semcrcular are consdered n the analyss. The boundary condtons assumed for the model are ambent temperature as K and pressure at outlet s Pa, respectvely. The values of the boundary condtons lke operatng temperature, mass flow rate of water are taken from the earler expermental work [13]. Other boundary condtons lke densty, specfc heat, thermal conductvty and other materal propertes are consdered as constants throughout the analyss. Intally the absorber tube wthout nserton s numercally solved and valdated aganst the same boundary condtons as that of the expermental setup and a very good correlaton s obtaned between the numercally predcted results and the expermental setup as reported n our prevous work [13]. upon ganng confdence wth valdaton the CFD tool s further extended to analyze numercally absorber tube wth trangle, nverted-trangle and sem-crcle nsertons. The dscretzed flud doman s mported to ANSYS CFX 12.1 pre processor and the boundary condtons and turbulence model are gven. The top and bottom of the absorber tube surfaces are gven constant wall fluxes of W/m 2 and W/m 2, respectvely. In the case of the model wth the nserton, the wall of the nserton s assumed to be adabatc. The water nlet temperature s set at a temperature of K whch s obtaned from expermental data. The solver s made to have a convergence of 10E-06 for mass, momentum and turbulence and set at 10E-08 for energy. Table-3. Temperature data of expermental set up. Mass flow rate (kg/hr) Average nlet temperature ( C) Average outlet temperature ( C) The heat transfer takes place from the wall surface of the absorber tube to the flud. The nlet of the absorber tube s gven Mass Flow Inlet Boundary Condton wth the mass flow rate set at kg/s, as used n the expermental analyss. The results of the CFD analyss for plan tube s compared wth the expermental results and found to be n good agreement wth a devaton of less than 5%, thus valdatng the present CFD analyss. The wall temperature dstrbuton along the length of the absorber tube for plan tube and tube wth nsertons s shown n Fgures 6-9, respectvely. It can be observed from Fgure-6 that the radal temperature dstrbuton between the top and bottom layers of the tube s not unform from the nlet to outlet secton. Also the wall temperature s maxmum near the nlet regon, on the wall surface recevng concentrated radaton. The wall temperature decreases along the axs of the tube on the same surface due to heat transfer between the wall surface and the flud. The wall temperature s lower n the case of the absorber tube wth nserton than that of the absorber tube wthout nserton. Fgure-6. Wall temperature dstrbuton of absorber tube wthout nserton. The Fgures show more unform temperature dstrbuton between the top and bottom layers of the tube along ts entre length for the absorber tube wth trangle nserton compared to plan tube and tube wth other type of nsertons. The maxmum temperature n case of plan tube s K and mnmum s K 677

5 ARPN Journal of Engneerng and Appled Scences Asan Research Publshng Network (ARPN). All rghts reserved. and hence there s no unform temperature dstrbuton as shown n Fgure-10. Because of ths huge temperature dfference the thermal loadng wll be more and hence the lfe tme of the plan tube would be lesser compared to other type of nsertons. Even though the temperature dfference n case of sem-crcular nserton s low, but the heat absorbng capacty s relatvely low compared to plan and other type of absorber tubes wth nsertons and the maxmum temperature of the wall s lmted to around 335 K as shown n Fgure-9. The pressure drop of the semcrcular nserton s 274 Pa as shown n Fgure-17 whch s relatvely hgher for any type of absorber tube consdered n ths analyss. The pressure drop for the trangular and nverted trangle are almost the same wth 147 Pa whch s relatvely hgher compared to plan absorber tube wth 48 Pa as seen n the Fgures 14 to 16. Even though the pressure drop wth trangular nserton s hgh compared to plan absorber tube t s preferred because of unform temperature dstrbuton. Compared to trangular and nverted trangular nsertons absorber tube both have the same pressure drop and heat gan characterstcs, but the thermal loadng s more unform for trangular nserton compared to nverted trangle absorber tube. bottom layers of the tube experence hgh thermal loadng compared to the top layers due to whch the tube may easly fal due to non-unform thermal loadng and the product lfe could be too short. If tube wth nsertons are consdered the thermal loadng s almost unform except for nverted trangular nserton where the bottom layers experence some non unformty. But, the magntude of non unformty s mnmzed. Also, t can be observed from Fgures 11 and 12 that the average outlet temperature s almost constant. Fgure-9. Wall temperature dstrbuton of absorber tube wth semcrcular nserton. Fgure-7. Wall temperature dstrbuton on the absorber tube wth trangular nserton. Fgure-10. Outlet temperature dstrbuton for absorber tube wthout nserton. Fgure-8. Wall temperature dstrbuton of absorber tube wth nverted trangular nserton. Fgures show the temperature dstrbuton at the outlet secton for plan as well as tube wth dfferent nsertons. It can be observed from Fgure-10 that the 678

6 ARPN Journal of Engneerng and Appled Scences Asan Research Publshng Network (ARPN). All rghts reserved. Fgure-11. Outlet temperature dstrbuton for absorber tube wth Trangular nserton. The varaton n magntude of temperature dstrbuton around the nserton could be due to the nsde convecton currents occurrng durng the flud flow. The temperature dstrbuton at the outlet for semcrcular nserton shown n Fgure-13 confrms that the bottom flud layers of the tube have very low temperature as compared to other nsertons. Ths s because the low heat flux n the top surface of the absorber tube s zero and hence the total heat source to the semcrcle nserton type s much lesser than the other confguratons. It can also be observed from Table-4 that consderable dfference n outlet temperatures s not observed for plan tube and tube wth nsertons. Fgure-12. Outlet temperature dstrbuton for absorber tube wth nverted Trangular nserton. CONCLUSIONS 1) The 3-dmensonal numercal analyss s able to predct the flud flow and heat transfer characterstcs through the absorber tubes wth and wthout nsertons. 2) No apprecable changes n temperature are obtaned between plan absorber tubes and absorber tubes wth dfferent types of nsertons. 3) The thermal stresses on the absorber tube wth nserton s lower than that on the absorber tube wthout nserton and t s found that trangular nserton s better compared to other types of nsertons. Table-4. Temperature Detals of Plan absorber tube and wth varous nsertons. S. No. Type of absorber tube T n (K) T out (K) 1 Plan tube Trangle nserton Inverted trangle Sem-crcle Fgure-13. Outlet Temperature Dstrbuton for Absorber Tube wth sem crcular nserton. 679

7 ARPN Journal of Engneerng and Appled Scences Asan Research Publshng Network (ARPN). All rghts reserved. Fgure-14. Pressure drop profle of the absorber tube wthout nserton. Fgure-15. Pressure drop profle of the absorber tube wth nverted trangular nserton. 4) Even though the pressure drop n the absorber tube wth trangular nserton s 147 Pa whle that n the absorber tube wthout nserton s 48 Pa, the absorber tube wth trangular nserton s preferred due to more unform temperature dstrbuton whch leads to better lfe tme of the product. Fgure-17. Pressure drop profle of the absorber tube wth sem crcular nserton. 5) The transent numercal study can be further extended to establsh the real physcs takng place n the absorber tubes before t reaches equlbrum whch would gve more nsght for optmzng other types of nsertons. Nomenclature Ib Drect normal rradance (W/m 2 ) W Wdth of the parabolc trough collector (m) L Length of the absorber tube (m) D Dameter of the absorber tube (m) m Mass flow rate (kg/s) T Temperature (K) u, v, w x, y, z velocty components (m/s) C Concentraton rato K Reflectvty of collector system x, y, z Cartesan co-ordnate system Greek symbols Μ Dynamc vscosty (kg/m-s) µ t Turbulent vscosty (kg/m-s) K Thermal conductvty (W/mK) Ε σ T Turbulent dsspaton rate Turbulent Prandtl number Ν Knematc vscosty (m 2 /s) Ρ Densty (kg/m 3 ) REFERENCES [1] Desgn and mplementaton of new fnancng mechansm and nstruments for promoton of solar water heatng systems n Inda. Report of MNRE. Posted on 23 October 2013, New Delh, Inda. Fgure-16. Pressure drop profle of the absorber tube wth trangular nserton. [2] Mafzul Huq and A.M. Azz-ul Huq Expermental measurements of heat transfer n an nternally fnned tube. Int.comm.Heat and Mass Transfer. 25:

8 ARPN Journal of Engneerng and Appled Scences Asan Research Publshng Network (ARPN). All rghts reserved. [3] Mchael K Jensen and Alex Vlakancc Techncal Note - Expermental nvestgaton of turbulent heat transfer and flud flow n nternally fnned tubes. Internatonal Journal of Heat and Mass Transfer. 42: [4] R. Forrstall Heat Transfer Analyss and Modellng of a Parabolc Trough Solar Recever Implemented n Engneerng Equaton Solver. NREL/TP , October. [13] R. Thundl Karuppa Raj, Tangellapall Srnvas, M. Natarajan, Arun Kumar K, Ansh Chengappa,and Amay Deoras Expermental and Numercal Analyss usng CFD Technque of the Performance of the Absorber Tube of a Solar Parabolc Trough Collector wth and wthout Inserton. IEEE transactons. pp [5] Z.D. Cheng, Y.L. He, J. Xao, Y.B. Tao and R.J. Xu Three-dmensonal numercal study of heat transfer characterstcs n the recever tube of parabolc trough solar collector. Internatonal Communcatons n Heat and Mass Transfer. 37: [6] Mazen M. Abu-Khader Further understandng of twsted tape effects as tube nsert for heat transfer enhancement. Heat and Mass transfer. 43: [7] Qu-Wang, Me Ln and Mn Zeng Effect of lateral fn profles on turbulent flow and heat transfer performance of nternally fnned tubes. Appled Thermal Engneerng. 29: [8] O. García-Valladares and N. Velázquez Numercal smulaton of parabolc trough solar collector: Improvement usng counters flow concentrc crcular heat exchangers. Internatonal Journal of Heat and Mass Transfer. 52: [9] D.R. Waghole, G.V. Parshwad, R.M. Warkhedkar, N.K. Sane and V.S. Kulkarn Heat Transfer analyss of recever/absorber Tube of Parabolc Trough collector. Proceedngs of the 37 th Natonal and 4 th Internatonal Conference on Flud Mechancs and Flud Power. December 16-18, IIT Madras, Chenna, Inda. [10] Javer Mun oz and Alberto Aba nades A techncal note on applcaton of nternally fnned tubes n solar parabolc trough absorber ppes. Solar Energy 85: [11] Matthew Roesle, Volkan Coskun and Aldo Stenfeld Numercal Analyss of Heat Loss from a Parabolc Trough Absorber Tube wth Actve Vacuum System ASME, Journal of solar Energy Engneerng. 133(031015): 1-5. [12] Y. Raja Sekhar, K.V. Sharma, R. Thundl Karuppa raj and C. Chranjeev Heat Transfer Enhancement wth Al 2 O 3 Nanofluds and Twsted Tapes n a Ppe for Solar Thermal Applcatons. Proceda Engneerng. 64: