Influence of Gravity on the Performance Index of Microchannel Heat Exchangers-Experimental Investigations

Transcription

1 Proeedngs of the World Congress on Engneerng 011 Vol III WCE 011, July 6-8, 011, London, U.K. Influene of Gravty on the Performane Index of Mrohannel Heat Exhangers-Expermental Investgatons Thanhtrung Dang, Jyh-tong Teng, and Jann-herng Chu Abstrat Influene of gravty on the heat transfer and flud flow phenomenon of mrohannel heat exhangers was presented expermentally. The effet was determned by two ases: one wth horzontal hannels, the other wth vertal hannels. For vertal hannels, the hot water s flowng upward whh s aganst the gravtatonal feld, whle the old water s flowng downward whh s n the same dreton as the gravtatonal feld. In ths study, the dfferene between the results obtaned from horzontal hannels and those from vertal ones s neglgbly small; the mpat of gravty on the flud flowng through the mrohannel heat exhangers was found to be small, wth the maxmum dfferene between the two ases beng less than 8%. Good agreements were aheved between the results obtaned n the present study and the results obtaned n lteratures. Index Terms mro heat exhanger, gravty, heat transfer rate, pressure drop, performane ndex. I. INTRODUCTION The need for the development of effetve oolng deves has rased muh nterest n mrohannel heat transfer n reent years. A revew on mro heat exhanger related ssues suh as flow physs, fabraton methods, and applatons was done by Bowman and Maynes [1]. Ths revew frstly ntrodued the expermental and numeral nvestgatons of mrohannel flow. Frton and heat transfer measurements of gas flow and lqud flow were dsussed n the paper. The paper ndated that the transton Reynolds number s a funton of surfae roughness and hannel geometry. Moreover, n the paper, the heat exhanger desgns nludng ther omparson and optmzaton were also revewed. Furthermore, several fabraton methods nludng mromahnng, hemal ethng, laser mahnng, eletroplatng, and lamnaton, were dsussed. Manusrpt reeved Marh 18, 011. Thanhtrung Dang s wth Department of Heat and Refrgeraton Tehnology, Ho Ch Mnh Cty Unversty of Tehnal Eduaton, Ho Ch Mnh Cty, Vetnam (e-mal: trungdang@hmute.edu.vn) Jyh-tong Teng, the orrespondng author, s wth Department of Mehanal Engneerng, Chung Yuan Chrstan Unversty, Tawan (e-mal: jyhtong@yu.edu.tw) Jann-herng Chu s wth the Department of Mehanal Engneerng, Chung Yuan Chrstan Unversty, Tawan (e-mal: mr.jhu@gmal.om) Brandner et al. [] desrbed mrostruture heat exhangers and ther applatons n laboratory and ndustry. Several mro heat exhangers were ntrodued: polymer mrohannel heat exhanger wth alumnum separaton fol, eletrally powered lab-sale mrohannel evaporator, eram ounter-flow mrostruture heat exhanger, et. Ameel et al. [3] presented an overvew of the mnaturzaton tehnologes and ther applatons to energy systems. Based on the MEMS tehnologes (slon-based mromahnng, deep X-ray lthography, and the mro mehanal mahnng), proesses were dsussed n the ontext of applatons to flud flow, heat transfer, and energy systems. Revew on expermental results onernng sngle-phase onvetve heat transfer n mrohannels was presented by Morn [4], wth addtonal revew results obtaned for the frton fator, the lamnar-to-turbulent transton, and the Nusselt number n hannels havng a hydraul dameter less than 1 mm. Mathew and Hegab [5] studed on the applaton of effetveness-ntu relatonshp to parallel-flow mrohannel heat exhangers. Besdes, development of nondmensonal parameters (suh as axal dstane, temperature, and heat transfer rate) was arrer out. However, the results were analyzed theoretally only. Studes of effetveness and pressure drop for mro ross-flow heat exhanger were presented by Kang and Tseng [6]. At the same effetveness, heat transfer rate and pressure drop were expressed as a funton of average temperature. However, n ther study, they dd not study for the ases wth varyng mass flow rates at eah sde. Chen and Chen [7] presented a numeral study of the effet of nlet/outlet arrangement on the performane of mrohannel heat snk. Sx types of heat snk were studed wth the best performane beng the V-type. Beause that f the mrohanels have the same ross-seton area and wdth of mrohannel, the depth of mrohannel obtaned from V-shaped mrohannel s deeper than that obtaned from retangular-shaped one. So t s not easy to desgn a heat exhanger wth the subtrate thkness from 1. to mm usng V-type mrohannels. Fol et al. [8] studed numerally on the heat flux, heat transfer rate, and pressure drop n hannels wth numerous aspet ratos. However, the results n Ref. [8] were presented wthout experments. A study on the smulatons of a trapezodal shaped mro heat exhanger was presented by Dang et al. [9]. Usng the geometr dmensons and the flow ondton assoated wth the mro heat exhanger, a heat flux of 13.6 W/m was evaluated by numeral method. Besdes, for the ISBN: ISSN: (Prnt); ISSN: (Onlne) WCE 011

2 Proeedngs of the World Congress on Engneerng 011 Vol III WCE 011, July 6-8, 011, London, U.K. mrohannel heat exhanger, behavors of the temperature and veloty profles were determned. Effet of flow arrangement on the heat transfer related behavors of a mrohannel heat exhanger was presented by Dang et al. [10, 11]. For all ases done n the study, the heat flux and performane ndex obtaned from the ounter-flow arrangement are always hgher than those obtaned from the parallel-flow one: the values obtaned from the ounter-flow are 1.1 to 1. tmes of those obtaned from the parallel-flow. Dang and Teng [1] studed effet of the substrate thkness of ounter-flow mrohannel heat exhangers on the heat transfer behavors. It was found that the atual heat transfer rate vares nsgnfantly wth the substrate thknesses varyng from 1. to mm. However, the results obtaned n [1] only mentoned the heat transfer behavors of the heat exhangers, whle the flud flow behavors of the heat exhangers were not dsussed. Dang et al. [13] presented an expermental study of the effets of gravty on heat transfer and pressure drop behavors of a mrohannel heat exhanger. However, the results n [13] were presented only for a mrohannel heat exhanger evaluated under the ondton of rsng the nlet temperature for the hot sde. To summarze, t s goal of ths paper to study expermentally for the effets of gravty on the heat transfer and flud flow behavors of mrohannel heat exhangers. In the followng seton, two mrohannel heat exhangers wll be dsussed under the ondton of rsng mass flow rate for the old sde. II. METHODOLOGY A. Expermental set-up Three major parts are used n the expermental system: the test seton (the mrohannel heat exhanger), syrnge system, and overall testng loop, as shown n Fg. 1. In ths study, two mrohannel heat exhangers were tested. The heat transfer proess of these deves s arred out between two lquds whh are hot water and old water; the hot and old fluds are flowng n the opposte dretons. Fg. shows the dmensons of the test setons. The substrate materal used for the heat exhangers s alumnum, wth the thermal ondutvty of 37 W/(mK), densty of,700 kg/m 3, and spef heat at onstant pressure of 904 J/(kgK). For eah mrohannel heat exhanger, the top sde for the hot water has 10 mrohannels and the bottom sde for the old water also has 10 mrohannels. The length of eah mrohannel s 3 mm. Mrohannels have retangular ross-seton wth the wdth and the depth beng W and D, respetvely. In a mrohannel heat exhanger, all hannels are onneted by manfolds for the nlet and outlet of hot water and for those of old water, respetvely. The manfolds of the heat exhangers are of the same ross-setons: havng a retangular shape wth a wdth of 3 mm and a depth of 300 m. Water tank Pump Buffer tank Exhaust ar valve Pre-heater Pump Heater Buffer tank Fg. 1. Shemat of the test loop for mrohannel heat exhangers Fg. shows the dmensons of the test seton. In ths study, two mrohannel heat exhangers were desgned and manufatured, wth ther dmensons lsted n Table 1. Fg. 3 shows a photo of the mrohannel heat exhanger. These test setons were manufatured by preson mromahnng [3]. Eah nlet hole or outlet hole of the heat exhangers has a ross-setonal area of 9 mm. The four sdes of the heat exhanger were thermally nsulated by the glass wool wth a thkness of 5 mm. To seal the mrohannels, two layers of PMMA (polymethyl metharylate) are bonded on the top and bottom sdes of the substrate by UV (ultravolet) lght proess, as ndated n Fg. 3. The physal propertes of the PMMA and the glass wool are lsted n Table [14]. P Heat exhanger Balane Table 1. Geometral parameters of mrohannel heat exhangers No. Dmensons of the substrate (mm) T T P Dmensons of the hannel (m) L W T W D T T Balane ISBN: ISSN: (Prnt); ISSN: (Onlne) WCE 011

3 Proeedngs of the World Congress on Engneerng 011 Vol III WCE 011, July 6-8, 011, London, U.K. 1. Thermoouples, T-type. Pump, Model PU-087, made by Jaso 3. Pump, VSP-100, made by Tokyo Rkakka 4. Heater, Model AXW-8, made by Medlab 5. Dfferental pressure transduer, Model PMP4110, made by Duk 6. Mro eletron balane, Model TE-14S, made by Sartorous. Table 3. Unertanty data for measured parameters Parameter Temperature Pressure Mass flow rate Channel heght Channel wdth Channel length Unertantes 0.1 C 0.05% FS g 7 m 10 m 70 m Fg.. Dmensons of the test seton PMMA Test sample In order to study the effets of gravty on heat transfer and flud flow behavors of the heat exhangers, all expermental ondtons for the two mrohannel heat exhangers were kept the same. Throughout the paper, the expermental ondtons of testng were dsussed: the ase s studed under ondton of nreasng the mass flow rate of the old sde. Further detals of the ase are as follows: The nlet temperature and the mass flow rate of the hot sde were fxed at 70 ºC and g/s, respetvely; at the old sde, the nlet temperature was fxed at.5 ºC and the mass flow rates were varyng from to g/s. Fg. 3. A photo of the mrohannel heat exhanger Table. The physal propertes of the PMMA and the glass wool Materal Densty kg/m 3 Thermal ondutvty W/(mK) PMMA Glass wool Expermental data for the mrohannel heat exhanger were obtaned under the onstant room temperature of 5 ºC. For ths study, DI water (deonzed water) was used as the workng flud. Eah nlet or outlet of the heat exhanger has a set of two thermoouples to reord temperature values. So, there are eght thermoouples n total. At eah sde, a dfferental pressure transduer was used to measure the pressure drop. To assess the auray of measurements presented n ths work, the unertanty values for measured parameters are lsted n Table 3. In addton, the unertantes on the dmensons of mrohannel evaluated by usng a sannng laser made by Mtaka/Ryokosha model NH-3. The unertantes of the sannng laser were estmated to be ± 0.03 µm. Equpments used for the experments are lsted as follows [13]: B. Data analyss In the followng analyses, the major assumptons were made: - The flud s a lamnar flow - The flud flow s nompressble and ontnuum - Heat transfer s steady - Neglgble radaton heat transfer. For the experments arred out n ths study, the effets on the heat transfer and flud flow suh as heat flux, effetveness, pressure drop, and performane ndex of the heat exhangers wll be dsussed as follows. The maxmum heat transfer rate, Q max s evaluated by Q max = (m) mn (T h, T, ) (1) The heat transfer rate of the heat exhanger, Q s alulated by Q = m (T,o T, ) () The effetveness (NTU method) s determned by Q Q max (3) Q Heat flux s alulated by q A Or q = k = Tlm R T lm m (T,o - T nl W, ) (4) (5) ISBN: ISSN: (Prnt); ISSN: (Onlne) WCE 011

4 Proeedngs of the World Congress on Engneerng 011 Vol III WCE 011, July 6-8, 011, London, U.K. The overall thermal resstane R s determned by R = R ond + R onv (6) The log mean temperature dfferene s alulated by Tmax Tmn Tlm (7) Tmax ln Tmn where m s mass flow rate (subsrpts h and stand for the hot sde and old sde, respetvely), n s number of mrohannels, s spef heat, T h,, T h,o, T, and T,o are nlet and outlet temperatures of the hot and old sdes, respetvely, q s heat flux, A s heat transfer area, k s overall heat transfer oeffent, Rond s ondutve thermal 1 1 resstane, Ronv s onvetve thermal resstane, hh h h h and h are the onvetve heat transfer oeffents of the hot sde and the old sdes, respetvely, s thkness of heat transfer, s thermal ondutvty, and T s the log lm mean temperature dfferene. The Reynolds number s alulated by: wdh m Re (8) W D 4A where Dh s the hydraul dameter, w s veloty n P the z-dreton, s dynam vsosty, s densty, A s ross-setonal area, and P s wetted permeter. The total pressure drop of the heat exhanger s gven by pt ph p (9) where p and h p are pressure drops of hot and old sdes, respetvely. The performane ndex,, s determned by Q m (T,o - T ), (10) pt ph p The expermental unertantes were estmated, followng the method desrbed by Holman [15]; the fnal expressons for unertantes were gven as follows: 1/ U Q m T, o T, (11) Q m T, o T, By usng the estmated errors of parameters lsted n Table 3, the maxmum expermental unertantes n determnng Q, Re, and were,1%, 3.1%, and 3.3%, respetvely, for all ases beng studed. III. RESULTS AND DISCUSSION For the expermental system, the nlet temperature and the mass flow rate of the hot sde were fxed at 70 ºC and g/s, respetvely; at the old sde, the nlet temperature was fxed at.5 ºC and the mass flow rates were varyng from to g/s. In ths study, nfluene of gravty was determned by two ases: one wth horzontal hannels, the other wth vertal hannels. For vertal hannels, the hot water s flowng upward whh s aganst the gravtatonal feld, whle the old water s flowng downward whh s n the same dreton as the gravtatonal feld. Two mrohannel heat exhangers T1 and T were tested: these two mrohannel heat exhangers have the same physal onfguratons for ther substrates, manfolds, and lengths of hannels; only the ross-setonal areas of mrohannels are dfferent. The mrohannels of T1 have a retangular ross-seton wth wdth of 500 m and depth of 300 m; the mrohannel of T3, wdth of 500 m and depth of 180 m. Parameters of the heat exhangers (T1 and T) are lsted n more detal n Table 1. Outlet temperature of hot sde, 0 C T1-vertal T-horzontal T-vertal Mass flow rate of old sde, g/s Fg. 4. Comparson of the outlet temperatures of hot sde U Re Re m m m T U m T ph p ph p, o W W T T D D 1/ (1), o, (13), 1/ Fg. 4 shows a omparson of the outlet temperature of hot sde of two mrohannel heat exhangers under the effet of gravty. It s observed that the outlet temperatures of hot sde obtaned from horzontal hannels and those from the vertal ones are neglgbly small. A omparson of the outlet temperatures of old sde of two mrohannel heat exhangers s shown n Fg. 5. The outlet temperatures (for both the hot and the old sdes) are funtons of the mass flow rate of old sde; the outlet temperatures derease as the mass flow rate of the old sde nreases. ISBN: ISSN: (Prnt); ISSN: (Onlne) WCE 011

5 Proeedngs of the World Congress on Engneerng 011 Vol III WCE 011, July 6-8, 011, London, U.K. Outlet temperature of old sde, 0 C T1-vertal 40 T-horzontal T-vertal Mass flow rate of old sde, g/s Fg. 5. Comparson of the outlet temperatures of old sde Total pressure drop, Pa 6000 T1-vertal 5000 T-horzontal T-vertal Re number of old sde Fg. 7. Comparson of the total pressure drops 30 Heat transfer rate, W T1-vertal T-horzontal T-vertal Performane ndex, W/kPa T1-vertal T-horzontal T-vertal Mass flow rate of old sde, g/s Fg. 6. Comparson of the heat transfer rates Re number of old sde Fg. 8. Comparson of the performane ndes The outlet temperatures of hot sde obtaned from T1 s hgher than those obtaned from T; however, the outlet temperatures of old sde obtaned from T1 s lower than those obtaned from T. As a result, the heat transfer rate obtaned from T s hgher than that obtaned from T1, as shown n Fg. 6. The results obtaned from the present study are n good agreement wth those obtaned from [8]. Fol et al. [8] ndated that under the onstant mass flow rate ondton, the hgher the heat flux, the lower the aspet rato (defned as the rato of the mrohannel heght to ts wdth). It s shown from Fg. 6 that the heat transfer rates obtaned from horzontal hannels and those from the vertal ones are neglgbly small. The heat transfer rate of the heat exhangers s a funton of the mass flow rate of old sde: t nreases from 4.8 to 9.9 W wth the mass flow rate of old sde rsng from to g/s (for the heat exhanger T). Beause that the hydraul dameter of hannel n T s smaller than that of hannel n T1, ths results n the veloty n the hannel of T to be hgher than that of T1, leadng to a hgher total pressure drop n T than that n T1, as shown n Fg. 7. Besdes, the Fgure shows that the total pressure drop s a funton of Reynolds number of old sde; the total pressure drop nreases as rsng the Re number of old sde. Expermental results for effets of gravty on the behavor of pressure drop for the mrohannel heat exhanger are also shown n Fg. 7. It s observed that the hange of pressure drop between the two ases (horzontal hannels and vertal hannels) s neglgbly small; the maxmum hange n pressure s 7.% for a pressure drop from 1060 to 044 Pa. It was found that the pressure drop of T s tmes hgher than that of T1, whle the heat transfer rate of T s 1.06 tmes hgher than that of T1. As a result, the performane ndex (defned as the rato of the heat transfer rate to the pressure drop n the heat exhanger) obtaned from T1 s hgher than that obtaned from T, as shown n Fg. 8. For ISBN: ISSN: (Prnt); ISSN: (Onlne) WCE 011

6 Proeedngs of the World Congress on Engneerng 011 Vol III WCE 011, July 6-8, 011, London, U.K. heat exhanger T1, a performane ndex of 1.68 W/kPa was aheved for water from the hot sde havng an nlet temperature of 70 C and a mass flow rate of g/s and for water from the old sde havng an nlet temperature of.5 C and mass flow rate of g/s. It s also observed that the hange of performane between the two ases (horzontal hannels and vertal hannels) s neglgbly small; the maxmum hange n performane s 5.5%, out of a performane ndex from to 1.68 W/kPa. In summary, t s onluded that the mpat of gravty on the flud flowng through the mrohannel heat exhanger an be gnored as ndated n [8, 9, 13, 16]. IV. CONCLUSION An expermental work was done on two mrohannel heat exhangers to arry out the evaluaton of ther performane for the varyng the mass flow rates of the old sde. These two mrohannel heat exhangers have the same physal onfguratons for ther substrates, manfolds, and lengths of hannels; only the ross-setonal areas of mrohannels are dfferent. For heat exhanger T1, a performane ndex of 1.68 W/kPa was aheved for water from the hot sde havng an nlet temperature of 70 C and a mass flow rate of g/s and for water from the old sde havng an nlet temperature of.5 C and mass flow rate of g/s. The mpat of gravty on the flud flowng through the mrohannel heat exhanger was found to be small, wth the maxmum dfferene between the results of horzontal and vertal hannels beng less than 8%. In addton, n ths study, good agreements were aheved between the results obtaned from the present study and the results obtaned from the lteratures. external heat transfer, Internatonal Journal of Thermal Senes, Volume 49, Issue 1, 010, pp [6] S.W. Kang and S.C. Tseng, Analyss of effetveness and pressure drop n mro ross-flow heat exhanger, Appled Thermal Engneerng, Volume 7, Isuue 5-6, 007, pp [7] R. Chen and J. Chen, Numeral study of the nlet/outlet arrangement effet on mrohannel heat snk performane, Internatonal Journal of Thermal Senes, Volume 48, Issue 8, 009, pp [8] K. Fol, T. Okabe, M. Olhofer, Y. Jn, and B. Sendhoff, Optmzaton of mro heat exhanger: CFD, analytal approah and mult-objetve evolutonary algorthms, Internatonal Journal of Heat and Mass Transfer, Volume 49, Issue 5-6, 006, pp [9] T.T. Dang, Y.J. Chang, and J.T. Teng, A study on the smulatons of a trapezodal shaped mro heat exhanger, Journal of Advaned Engneerng, Volume 4, Issue 4, 009, pp [10] T.T. Dang, J.T. Teng, and J.C. Chu, Effet of flow arrangement on the heat transfer behavors of a mrohannel heat exhanger, Proeedngs of the Internatonal MultConferene of Engneers and Computer Sentsts 010, Hongkong, 010, pp [11] T.T. Dang and J.T Teng, Influene of flow arrangement on the performane ndex for an alumnum mrohannel heat exhanger, IAENG Transatons on Engneerng Tehnologes Volume 5, the Ameran Insttute of Physs (AIP), Vol. 185, 010, pp [1] T.T. Dang and J.T. Teng, Effet of the substrate thkness of ounter-flow mrohannel heat exhanger on the heat transfer behavors, Proeedngs of the nternatonal symposum on omputer, ommunaton, ontrol and automaton 010, Tawan, 010, pp [13] T.T. Dang, J.T. Teng, and J.C. Chu, A study on the smulaton and experment of a mrohannel ounter-flow heat exhanger. Appled Thermal Engneerng, Volume 30, Issue 14-15, 010, pp [14] COMSOL Multphyss verson 3.5 Doumentaton, Sept 008 [15] J.P. Holman, Expermental methods for engneers, MGraw-Hll, New York, 1984 [16] S.G. Kandlkar, S. Garmella, D.Q. L, S. Coln, and M.R. Kng, Heat transfer and flud flow n mnhannels and mrohannels. Elsever, 006. ACKNOWLEDGMENT The supports of ths work by (1) the projet (Projet No. NSC 99-1-E ) sponsored by Natonal Sene Counl of the Republ of Chna n Tawan and () the projet (under Grant No. CYCU-98-CR-ME) sponsored by the spef researh felds at Chung Yuan Chrstan Unversty, Tawan, are deeply appreated. REFERENCES [1] W.J. Bowman and D. Maynes, A revew of mro-heat exhanger flow physs, fabraton methods and applaton, Proeedngs of ASME IMECE 001, New York, USA, Nov 11-16, 001, HTD-480, pp [] J.J. Brandner, L. Bohn, T. Hennng, U. Shygulla, and K. Shubert, Mrostruture heat exhanger applatons n laboratory and ndustry, Proeedngs of ICNMM006, ICNMM , Lmerk, Ireland, 006, pp [3] T.A. Ameel, R.O. Warrngton, R.S. Wegeng, and M.K. Drost, Mnaturzaton tehnologes appled to energy systems, Energy Converson and Management, Volume 38, 1997, pp [4] G.L. Morn, Sngle-phase onvetve heat transfer n mrohannels: a revew of expermental results, Internatonal Journal of Thermal Senes, Volume 43, Issue 7, 004, pp [5] B. Mathew and H. Hegab, Applaton of effetveness-ntu relatonshp to parallel flow mrohannel heat exhangers subjeted to ISBN: ISSN: (Prnt); ISSN: (Onlne) WCE 011