Effect Of Humidity And Inclination Angle On Microchannel Heat Exchanger Performance

Transcription

1 Purdue Unversty Purdue e-pubs Internatnal Rergeratn and Ar Cndtnng Cnerence Schl Mechancal Engneerng 22 Eect O Humdty And Inclnatn Angle On Mcrchannel Heat Exchanger Perrmance S. M. Sng Unversty Illns at Urbana-Champagn P. S. Hrnjak Unversty Illns at Urbana-Champagn M. H. Km Unversty Illns at Urbana-Champagn C. W. Bullard Unversty Illns at Urbana-Champagn Fllw ths and addtnal wrks at: Sng, S. M.; Hrnjak, P. S.; Km, M. H.; and Bullard, C. W., "Eect O Humdty And Inclnatn Angle On Mcrchannel Heat Exchanger Perrmance" (22). Internatnal Rergeratn and Ar Cndtnng Cnerence. Paper Ths dcument has been made avalable thrugh Purdue e-pubs, a servce the Purdue Unversty Lbrares. Please cntact epubs@purdue.edu r addtnal nrmatn. Cmplete prceedngs may be acqured n prnt and n CD-ROM drectly rm the Ray W. Herrck Labratres at Herrck/Events/rderlt.html

2 R1-2 EFFECT OF HUMIDITY AND INCLINATION ANGLE ON MICROCHANNEL HEAT EXCHANGER PERFORMANCE *Sumn Sng, Research Assstant, 126 W. Green Street, Urbana, IL 6181, USA; Tel.: 217/ ; Fax: 217/ E-Mal: *Authr r Crrespndence P. S. Hrnjak, C-drectr, Ar Cndtnng & Rergeratn Center 126 W. Green Street, Urbana, IL 6181, USA; Tel.: 217/ ; Fax: 217/ E-Mal: pega@uuc.edu Man-He Km, Samsung Electrncs C. 416, Maetan-3Dng Suwn Cty, Kyungk-D , Krea E-Mal: mankm1@samsung.c.kr Clark. W. Bullard, Pressr Mechancal Engneerng, 126 W. Green Street, Urbana, IL 6181, USA; Tel.: 217/ ; Fax: 217/ E-Mal: bullard@uuc.edu ABSTRACT The eect nlet humdty cndtns and nclnatn angle n the ar sde thermal hydraulc perrmance a brazed alumnum heat exchanger has been nvestgated expermentally. The crsslw heat exchanger had lat tubes and lded ns wth n and luver ptch 2.1 mm and 1.4 mm, respectvely. The glycl temperature was held nearly sthermal at 1 C, whle humdty the 12 C nlet ar vared rm 8%. The Reynlds numbers based n luver ptch ranged rm 8 t 4. Ths range peratng cndtns permtted ar-sde heat transer cecents t be determned wthn ±15-2% r dry and wet cndtns, respectvely. Wet-surace heat transer degradatn was substantal and vared wth Reynlds number, but was nly a weak unctn nclnatn angles less than 45 degrees. On the ther hand, the wetted-surace pressure drp penalty was relatvely small, nly 3-14% acrss the range nclnatn angles and Reynlds numbers. The results prvde nsghts nt the ablty t mantan luver-drected lw under nclned and wet-surace dnatns. NOMENCLATURE A: area s: saturatn state C p : specc heat t: temperature C r : capacty rat δ : n thckness D h : hydraulc dameter θ: Inclnatn angle F d : lw depth γ: aspect rat h: heat transer cecent ε: eectveness H: n heght η: Ecency : enthalpy Subscpts k: cnductvty : n L: heat exchanger heght : glycl-sde L p : luver ptch : ar-sde : mass lw rate w: wet-surace r water NTU: Number transer unt 1: nput Q: heat transer rate 2: utput RH: relatve humdty INTRODUCTION The eect nlet humdty cndtns and nclnatn angle n the ar sde thermal hydraulc perrmance a brazed alumnum heat exchanger has been nvestgated expermentally. There are sme publcatns n the eect nclnatn angle and nlet humdty cndtns n the heat transer and pressure drp the heat

3 exchangers. Hwever, mst the publshed data have cnsdered bare-tube banks, hgh-n tube banks and cnventnal nned tube heat exchangers (Grehn, 1983, Mnhert et al., 1986, Mre et al., 1979, Aarde et al. 1993, Chang et al. 1994, Kedzersk, 1997, Mrth et al., 1993, Mrth et al., 1994, Wang et al. 1997, Wang et al., 2). Mrth et al (1993, 1994) shwed that nlet humdty cndtns aected the heat exchanger perrmance. On the ther hand, Wang et al. (1997, 2) reprted that they dd nt nluence sgncantly the sensble heat transer cecents, whle ther eect n the pressure drps depended n the heat exchanger cnguratns, especally the lngtudnal tube ptch. When the lngtudnal tube ptch was 22 mm, the eect nlet cndtns was neglgble, whle r the lngtudnal ptch 19.5 mm the rctn actrs r RH =9% were 5-25% larger than thse r RH=5%. A mcrchannel tube heat exchanger s ne the ptental alternatves r replacng the cnventnal nned tube heat exchangers and has been cnsdered as bth evapratr and gas cler r prttype CO2 arcndtnng systems (Km and Bullard, 2). Many nvestgatrs have studed the ar-sde heat transer and pressure drp characterstcs the luvered n and lat tube heat exchangers (Sahnun et al., 1992, Chang et al., 1996, Chang et al., 1997, Km et al., 2, Chu et al., 1994, Km and Bullard, 22, McLaughln et al., 2). Hwever, nly small amunt publshed data n the eect the nclnatn angle n the perrmance the brazed alumnum heat exchangers s avalable n the pen lterature. Recently, Osada et al. (1999) studed the eect nclnatn n the heat transer and pressure drp characterstcs the luvered n autmtve evapratrs wth larger lw depth (F d =58 and 7 mm) and cnducted cndensate vsualzatn tests. They reprted that bth the leeward and wndward nclnatns mprved heat exchanger perrmance. Km et al. (21) nvestgated the eect nclnatn angle (, ±3, ±45, and ± clckwse) n the heat transer and pressure drp a brazed alumnum heat exchanger wth F d =2 mm under dry and wet cndtns. They und that the heat transer perrmance r bth dry and wet cndtns was nt nluenced sgncantly by the nclnatn angle (- <θ < ), whle the pressure drps ncreased cnsstently wth the nclnatn angle. In ur resdental CO2 prttype system, the ndr heat exchanger was nclned t 67 the vertcal due t the space lmtatns n the wnd tunnel (Beaver et al. 1999). The eect nclnatn n thse experments was nt knwn because the heat exchanger was t large t test at derent angles. The purpse ths study s t prvde expermental data n the eect an nclnatn angle and nlet humdty cndtns n the ar-sde thermal hydraulc perrmance r a smaller brazed alumnum heat exchanger under dry and wet surace cndtns. A seres tests are cnducted r the ar-sde Reynlds number range 8-4 wth varatn the nclnatn angles (, 14, 3, 45 and 67 clckwse) rm the vertcal pstn. The pressure drp characterstcs are als addressed. A mre detaled analyss the wet surace data can be und n Km et al. (22), whch als explres the ssue leeward and wndward nclnatns and luver drectns at the nlet and utlet the heat exchangers. EXPERIMENTAL SET UP Test Apparatus Fgure 1 shws a schematc dagram the apparatus used n the study. It cnssts a ducted arlw system, heat transer lud (glycl) crculatn and data acqustn system. It s stuated n a cnstant temperature and humdty chamber that can mantan temperature wthn ±.5 C and abslute humdty ±2%. The ar nlet cndtns the heat exchanger are mantaned by cntrllng the chamber temperature and humdty. The arsde pressure drp thrugh the heat exchanger s measured usng a derental pressure transducer and the arlw rate s determned rm the nzzle pressure derence. Heat Exchanger The tested heat exchanger has 38 parallel tubes (crcuts) and ns wth 17 luvers, 27 luver angle, lw depth 27.9 mm, n ptch 2.1 mm, n thckness.1 mm, tube ptch 9.9 mm. The luver ptch, luver length and n heght are 1.4 mm, 6.6 mm and 8.3 mm, respectvely and the cre sze s 394 mm x 381 mm. Test Cndtns and Methds Fgure 2 shws smple schematc a heat exchanger nstallatn. The heat exchanger s nstalled n the test sectn, surrunded by nsulatn t prtect t rm heat lss and ar leakage. Fr leeward (θ =, 14, 45, and

4 67 clckwse) nclnatns, a seres tests r wet cndtns are perrmed n the range the Reynlds number based n luver ptch 8-4. As shwn n Fg. 3, the parttn abve the heat exchanger allws the ar t turn and enter almst nrmal t the cl, and then t s then turned agan thrugh the duct Hwever r θ = 67, an upstream duct was added t prevent ar leakage, and may have rced the ar t enter the heat exchanger at a mre blque angle. The nlet ar temperature and relatve humdty ranges are 12 C and -8%, respectvely, and glycl nlet temperatures are ~2.5 C. DATA REDUCTION The ttal heat transer rate used n the calculatn s the arthmetc average ar- and glycl-sde heat transer rate (Q). The data reductn prcess s the same as Km et al. (21), s nly a bre descrptn s gven here. The ε-ntu equatn r bth luds unmxed s.22 NTU.78 ε = 1 exp { exp( C NTU ) 1} (1) r Cr Assumng zer glycl-sde ulng resstance and wall resstance, the ar-sde heat transer cecent r dry-surace cndtns can be btaned rm the llwng equatns Q UA C p (2) ε =, NTU =, Cr = C ( t t 1) C C p = + UA h A η h A p p Assumng zer glycl-sde ulng resstance and wall resstance, the ar-sde heat transer cecent r wet-surace cndtns can be btaned rm the llwng equatns Q U w Aw b (4) ε =, NTU =, C r = ( ) C 1 s, 1 w p 1 b b = + (5) U w Aw h A ηwhw Aw Where b and b are the slpe the ar saturated curve at the mean glycl temperature and the mean w external surace temperature. Fr the heat transer cecents n the glycl-sde, tw-dmensnal duct lw was apprxmated snce the duct aspect rat (γ = 22.4) s extremely large (Shah and Lndn, 1978). Dh Nu = Re Pr (6) L The surace eectveness and the n ecency r the dry surace (Kuehn and Threlkeld, 1998) are A η = 1 (1 η ) (7) A tanh( m * l) η =, * m l m 2h δ 1 + F * =, k δ d l = H 2 δ (8) The slutns r ecency dry ns als apply r ecency wet ns we substtute h r the wet w n n place h r the dry n. The verall heat transer cecent r the wet surace h w s 1 hw = (9) C p + yw k w b h w (3)

5 Where h s the sensble heat transer cecent r the wet surace, and y w s the thckness cndensatn water lm, whch s neglected here. Accuntng r all nstrument errrs, prperty uncertantes, uncertantes r the heat transer cecents were estmated t be ±15-2% r dry and wet cndtns, respectvely. And uncertantes r the ar-sde pressure drps were abut ±1% (Mat, R.J., 1988). RESULTS AND DISCUSSION Fgures 3-6 present the results r the heat transer and pressure drp. Fgure 3 shws hw the ar-sde heat transer cecents r dry surace vary wth ace velcty and nclnatn angle. As expected, heat transer cecents ncrease wth ace velcty. The heat transer cecents r the dry cndtns were nt aected sgncantly by the nclnatn angles at lw Reynlds number. The eect nclnatn ncreases as Reynlds number ncreases. The heat transer cecents deterrated substantally at 67, especally when the Reynlds number was hgher. Fgure 4 shws hw the sensble heat transer cecents r wet surace vary wth ace velcty, nclnatn angle, and nlet relatve humdty. Wet-surace heat transer degradatn was substantal and vared wth Reynlds number. Fr the same nlet humdty (8% RH), heat transer cecents has a mnmum when θ =. Ths may be due t the gravtatnal rce eect n the prmtn r cndensate dranage. Recently, Km et al. (21) als reprted a mdest nclnatn angle prmtes dranage s heat transer cecents ncrease. The heat transer cecents decrease wth the ncrease ar nlet humdty, snce hgher nlet humdty wll cause mre cndensate accumulatn n the cl surace and ths cndensate acts as anther thermal resstance r lw Reynlds number lws studed here (Km and Bullard, 2). Hwever, the nlet humdty eect n the heat transer cecent s nt sgncant r the small nclnatn angles (θ 45 ). As shwn n Fgure 4, ts eect ncreases wth nclnatn angle and r θ = 67, the heat transer cecents decrease sgncantly wth the ncrease ar nlet humdty. Fgure 5 presents ar-sde pressure drps vs. ace velcty wth varatn nclnatn angle. As expected, pressure drps r bth dry and wet cndtns ncrease systematcally wth ace velcty and nclnatn angle. The pressure drps r wet cndtns are 3-14 % larger than thse r dry cndtns at the same ace velcty. As shwn n Fgure 5, r θ = 67 a sgncant pressure drp ncrease was ccurred. Ths result s smlar t that by Km et al. (21) wh reprted pressure drps ncreased sgncantly when θ. Furthermre, n case θ = 67, there s an upstream duct whch wll causes the addtnal upstream lsses asscated wth blque ar entrance t the heat exchanger. Fgure 6 shws the eect ar nlet humdty n the pressure drps. The nlet humdty des nt nluence sgncantly the pressure drps, a result smlar t that r the cnventnal nned rund tube heat exchangers wth ully wet surace (Wang et al.1997). On the ther hand, the prevus test data wth the mcr-channel heat exchanger wth smaller n and luver ptch rat (F p / L p = 1.4/1.7) and larger lw depth (F d = 41.8 mm) shwed that the ar nlet humdty aected systematcally the ar-sde pressure drps (Bewe et al.1999) Ths derence prbably s due t the derence heat exchanger gemetry. The heat exchanger used n ths study has larger n and luver ptch rat (F p / L p = 2.1/1.4) and smaller lw depth (F d = 27.9 mm), and s the eect cndensate amunt n the surace may be smaller cmpared t the heat exchanger wth smaller n ptch and larger lw depth, suggestng the nlet humdty eect n the pressure drps depends n heat exchanger cnguratn, especally n ptch. Under wet-cl cndtns, data were nt repeatable when the cl was vertcal. We beleve that the luvers n mcrchannel heat exchangers can becme brdged wth cndensate under sme cndtns (e.g. at lw ace velctes n vertcal rentatn), s t appears as a lat n and has degraded perrmance. T see the water dranage eect n ar-sde pressure drp and heat transer cecent, we ran sme tests at angle attack, 8% humdty, and 9 cm where ace velcty s abut 4 m/s. The results were nt repeatable, but a systematc relatnshp was bserved. Fgure 7 shws that the ar-sde heat transer cecents decrease wth the ar sde pressure drp, perhaps due t cndensate brdgng between ns, whch decreases bth by changng luverdrected t duct-drected lw. Upn clse nspectn the expermental prcedures, t was und that the hgh ar pressure drp and ar-sde heat transer cecent ccurred when the evapratr was ntally dry and the 9

6 cm test was run bere any lwer ar lw tests. It ndcates that the cndensate brdgng between ns wll nt happen there s n brdgng ntally and the ar-lw rate s hgh enugh. The lw ar pressure drp and ar-sde heat transer cecent happened when the 9 cm experment was ran ater sme lwer ar lw rate experments under wet surace cndtns. It suggests that there s brdgng between ns when the ar lw rate s lw. And surprsngly, the brdgng stll perssts as the lw rate s ncreased gradually t 9 cm. CONCLUSIONS The eect leeward nclnatn n the heat transer cecents and pressure drps a mult-luvered n heat exchanger r dry and wet surace cndtns has been nvestgated expermentally wth varatn ar nlet humdty. Wet-surace heat transer degradatn was substantal and vared wth Reynlds number. The heat transer characterstcs are nluenced sgncantly by the nclnatn angle, especally r θ > 45. The pressure drps r wet cndtns are 3-14% larger than thse r dry cndtns, and ncrease cnsstently wth nclnatn angle. The eect ar nlet humdty n the heat transer and pressure drp s neglgble n case the larger n ptch heat exchanger studed here. We cncluded, tentatvely at least, that a mdest angle attack prmtes dranage. Prbably that s why the aut ndustry rutnely tlts ts lat plate-lded luvered n evapratrs abut 1 degrees the vertcal. Apparently t s als benecal r the thnner (16mm) heat exchangers used n ur resdental prttype. We als beleve that the luvers n mcrchannel heat exchangers can becme brdged wth cndensate under sme cndtns (e.g. at lw ace velctes n vertcal rentatn). Surprsngly, they reman brdged as arlw rate s ncreased. Hwever, the arlw s ntally large and the cl then begns t cndense msture n respnse t rsng humdty, the dranage appears t reman unmpared. REFERENCES Aarde, D.J. and Krger, D.G., 1993, Flw lsses thrugh an array A-rame heat exchangers, Heat Transer Engneerng, Vl. 14, N. 1, pp Beaver, A.C., J. M. Yn, C. W. Bullard, P. S. Hrnjak Expermental and mdel study the heat pump/ar cndtnng systems based n transcrtcal cycle wth R744, IIR Cngress, Sdney, Australa. Bewe, D.E., McEnaney, R.P., Park, Y.C., Yn, J.M., Bullard, C.W. and Hrnjak, P.S., 1999, Cmparatve expermental study subcrtcal R134a and transcrtcal R744 rergeratn systems r mble applcatns, ACRC CR-17, Unversty Illns at Urbana-Champagn. Chang, W.R., Wang, C.-C., and Chang, Y.J. 1994, Eect an nclnatn angle n the heat transer and pressure drp characterstcs a wavy nned-tube heat exchanger, ASHRAE Trans., Vl. 1, Pt. 2, pp Chang, Y. and Wang, C., 1996, "Ar sde perrmance brazed alumnum heat exchangers," Jurnal Enhanced Heat Transer, Vl. 3, N. 1, pp Chang, Y. and Wang, C., 1997, "A generalzed heat transer crrelatn r luvered n gemetry," Int. J. Heat Mass Transer, Vl. 4, N. 3, pp Chu, C.B., Wang, C.C., Chang, Y.J. and Lu, D.C., 1994, Expermental study heat transer and lw rctn characterstcs autmtve evapratrs, ASHRAE Trans., Vl. 1, Pt. 2, pp Frank, P. I. And Davd, P.D., 1996, Fundamentals Heat and Mass Transer 3rd Edtn, JOHN WILEYJhn Wley & SONSSns, INC. Grehn, H.G., 1983, Heat transer and lw resstance yawed tube bundle heat exchangers, Heat Exchangers: Thery and Practce, Hemsphere Publshng C., pp Kedzersk, M.A., 1997, Eect nclnatn n the perrmance a cmpact brazed plate cndenser and evapratr, Heat Transer Engneerng, Vl. 18, N. 3, pp Km, M.-H., Yn, J.M., Bullard, C.W. and Hrnjak, P.S., 2, Develpment a mcr-channel evapratr mdel r a CO2 mble ar-cndtner, Prceedngs the ASME, AES-Vl. 4, Advanced Energy Systems Dvsn, pp Km, M.-H. and Bullard, C. W, 2, Ar-sde thermal perrmance mcr-channel heat exchangers under dehumdyng cndtns, Prceedngs the 2 Internatnal Rergeratn Cnerence at Purdue, pp Km, M.-H., B. Yun and C.W. Bullard, 21,"Eect nclnatn n the ar-sde perrmance a brazed alumnum heat exchanger under dry and wet cndtns," Int. J. Heat Mass Transer, Vl.44, N. 24, pp

7 Km, M.-H and C.W. Bullard, 22, "Ar-sde thermal hydraulc perrmance mult-luvered n alumnum heat exchangers," Internatnal Jurnal Rergeratn, Vl. 25, N. 3, pp Km, M.-H., S.M. Sng and C.W. Bullard, 22, "Eect nlet humdty cndtn n the ar-sde perrmance an nclned brazed alumnum evapratr," In press, Internatnal Jurnal Rergeratn. Kuehn, T.H. and Ramsey, J.W., 1995, Thermal Envrnmental Engneerng 3rd Edtn, Prentce Hall. McLaughln, W.J. and Webb, R.L., 2, Wet ar sde perrmance luver n autmtve evapratrs, SAE Techncal Paper Seres, Mrth, D.R. and Ramadhyan, S., 1993, Predctn clng-cl perrmance under cndensng cndtns, Int. J. Heat and Flud Flw, Vl. 14, N. 4, pp Mrth, D.R. and Ramadhyan, S., 1994, Crrelatns r predctng the ar-sde Nusselt numbers and rctn actrs n chlled-water clng cls, Expermental Heat Transer, Vl. 7, pp Mre, F.K. and Rstrcell, J.R., 1979, Turbulent lw and pressure drp lsses behnd blque hgh-drag heat exchangers, Int. J. Heat Mass transer, Vl. 22, pp Mnhet, M. and Frem, J., 1986, Eect tube bank nclnatn n the thermal hydraulc perrmance ar cled heat exchangers, Prc. 8th Int. Heat Transer Cnerence, pp Osada, H., Ak, H., Ohara, T., and Kuryanag, 1999, Expermental analyss r enhancng autmtve evapratr n perrmance, Prceedngs the Internatnal Cnerence n Cmpact Heat Exchangers and Enhancement Technlgy r the Prcess Industres, R.K. Shah, Ed., pp Sahnun, A. and Webb, R.L., 1992, "Predctn heat transer and rctn r the luver n gemetry," J. Heat Transer, Vl. 114, pp Mat, R.J., 1988, Descrbng the uncertantes n expermental results, Expermental Thermal and Flud Scence, Vl. 1, pp Wang, C., Hseh, Y. And Ln Y., 1997, Perrmance plate nned tube heat exchangers under dehumdyng cndtns, Jurnal Heat Transer, Vl. 119, pp Wang, C., Ln, Y. And Lee, C., 2, Heat and mmentum transer r cmpact luvered n-and-tube heat exchangers n wet cndtns, Int. J. Heat Mass transer, Vl. 43, pp Warren, M.R. James, P.H. and Yung, I.C., 1998, Handbk Heat Transer 3rd Edtn, McGraw-Hll Fgures B Hu Sp RH Indr Chamber Dpea Dpen H TC W TG IC TG N Ten mg Tg Tg Sc CH Glycl B - Blwer, CH- Glycl Chller, H Heater, Hu Humder, IC Indr Cl, mg Glycl Mass Flw Meter, N Nzzle, Sc Cndensate Scale, Sp Speed Cntrller and Tachmeter, TC Temperature Cntrller, TG Thermcuple Grd, W Watt Transducer Indces: a ar, e evapratr, g glycl, nlet, n nzzle, utlet Fgure 1: Schematc dagram test apparatus.

8 Ar lw Ar lw HX (9 θ) 4 HX (9 θ) 4 Fg. 3(a) θ =, 14, and 45 Fg. 3 (b) θ = 67 Fgure 2: Schematc dagram a heat exchanger nstallatn [unt length: mm] h (W/m 2 -K) /dry 14 /dry 45 /dry 67 /dry Face velcty (m/s) Fgure 3: Angle eect n h under dry cndtn h (W/m 2 K) /hum7% /hum8% 14 /hum7% 14 /hum8% 45 /hum7% 45 /hum8% 67 /hum7% 67 /hum8% Face velcty (m/s) Fgure 4: Angle and nlet humdty eect n h under wet cndtn

9 Arsde pressure drp (Pa) /d r y 1 4 /d r y 4 5 /d r y 6 7 /d r y /h u m 8 % 1 4 /h u m 8 % 4 5 /h u m 8 % 6 7 /h u m 8 % Face velcty (m/s) Fgure 5: Angle eect n ar-sde pressure drp 8 Arsde pressure drp (Pa) /hum % 45 /hum 7% 45 /hum 8% 45 /hum 9% ace velcty (m/s) Fgure 6: Inlet humdty eect n ar-sde pressure drp 1 14 h (W /m 2 K) Ar sde pressure drp (P a) Fgure 7: Water dranage eect