Effect of Louver Angle on Performance of Heat Exchanger With Serpentine Fins and Flat Tubes in Periodic Frosting

Transcription

1 Purdue Univerity Purdue e-pub Interntionl Rerigertion nd Air Conditioning Conerence School o Mechnicl Engineering 212 Eect o Louver Angle on Perormnce o Het Exchnger With Serpentine Fin nd Flt Tube in Periodic Froting Predrg S. Hrnjk peg@illinoi.edu Ping Zhng Chri Rennel Follow thi nd dditionl work t: Hrnjk, Predrg S.; Zhng, Ping; nd Rennel, Chri, "Eect o Louver Angle on Perormnce o Het Exchnger With Serpentine Fin nd Flt Tube in Periodic Froting" (212). Interntionl Rerigertion nd Air Conditioning Conerence. Pper Thi document h been mde vilble through Purdue e-pub, ervice o the Purdue Univerity Librrie. Plee contct epub@purdue.edu or dditionl inormtion. Complete proceeding my be cquired in print nd on CD-ROM directly rom the Ry W. Herrick Lbortorie t Herrick/Event/orderlit.html

2 251, Pge 1 Eect o louver ngle on perormnce o het exchnger with erpentine in nd lt tube in roting Peg Hrnjk 1,3 *, Ping Zhng 1,2, Chri Rennel 1 1 Univerity o Illinoi, Deprtment o Mechnicl Science nd Engineering ACRC, Urbn, IL, USA lo 3 Cretive Therml Solution, Inc. Urbn, IL, USA 2 Zhejing Voctionl College o Commerce, Hngzhou, Binwen Rod 47, Chin E-mil: peg@illinoi.edu ABSTRACT Thi pper preent the reult o n experimentl tudy on the ir-ide preure drop nd overll het trner coeicient chrcteritic or erpentine-louvered-in, microchnnel het exchnger in periodic roting. It ocue on quntiiction o the eect o louver ngle on het trner nd preure drop nd on derot nd rerot time. Nine het exchnger diering in louver ngle nd in pitch (i.e. louver ngle to nd in pitch 15 to 18 pi) re tudied. The ce velocity w 3.5 m/ nd inlet ir reltive humidity o 7% nd 8%. Eect o in pitch nd louver pitch on initil Colburn j ctor nd Fnning riction ctor during the trt o the irt roting cycle re reported, nd compred to the prediction by the Chng nd Wng (1997). 1. INTRODUCTION Microchnnel het exchnger re becoming quite populr compct lterntive to bulkier in-nd-tube het exchnger when ued motly condener. Chrge reduction i n dditionl beneit. However, microchnnel het exchnger re till not widely ued outdoor coil in het pump ytem. Specil concern re relted to condente removl, dringe, in the dehumidiiction mode nd in deroting, beide rerigernt ditribution tht i not object o thi pper. It i well ccepted tht louver directed low i importnt or incree in het trner coeicient ince Achichi nd Cowell (1988) nd lter other publiction. In ddition to conventionl deteriortion eect rot buildup on the louver trongly ect ir-ide perormnce by ltering the low rom louver directed low to duct directed low. Thu, to identiy geometry tht will be more rot tolernt nd quntiy eect, it i importnt to hed more light on the eect o the geometricl prmeter on the ir low during roting. Severl tudie o rot propertie nd rot growth mechnim on the round-tube-plte-in het exchnger hve been reported. Kondepudi nd O Nel (1987) reviewed the literture on the eect o rot ormtion on inned tube het exchnger perormnce, nd Kondepudi nd O Nel (1989) conducted rot growth reerch on louver-in-roundtube het exchnger. Mchielen nd Kerchbumer (1989) conducted reerch on the eect o roting nd deroting on het exchnger perormnce. Yn et l. (25) invetigted the perormnce o roted inned-tube het exchnger with plin in, ingle-bnk louvered in, nd multi-louvered in, nd ound tht the het trner rte, the overll het trner coeicient, nd the preure drop or multi-louvered in were higher thn or other. Only ew tudie deling with the eect o rot on the lt tube louvered compct het exchnger hve been publihed in the open literture to dte. Kim nd Groll (23) reported comprion between microchnnel nd intube het exchnger when ued n outdoor coil in het pump ytem. The tudy included both cooling nd heting tet. The uthor reported roting/deroting time nd the heting cpcity o the het pump with both coil. The eect o other vrible uch het exchnger inclintion nd in per inch were lo tudied. The uthor concluded tht microchnnel het exchnger hve horter roting time compred to in-tube het exchnger nd tht the roting time decreed even urther with ech cycle due to reidul wter retined t the end o ech derot cycle. Xi et l. (26) tudied the eect o rot, derot, nd rerot on the ir-ide thermlhydrulic perormnce o louvered-in, lt tube het exchnger which reembled microchnnel het exchnger. An overll het trner coeicient w obtined or the het exchnger or relitic rnge o temperture nd low rte. Frot thickne w meured uing imge cptured with CCD cmer nd rot weight w obtined by uing high preciion cle. They developed numericl model, which w lo experimentlly vlidted, to predict the rot thickne nd blockge rtio. Additionl tudie include Crlon nd Hrnjk [21] who ocued on undertnding how environmentl condition, ir temperture nd reltive humidity, rerigernt temperture, nd ir nd rerigernt temperture glide, ect the depoition nd ditribution o rot on het exchnger, peciiclly o

3 251, Pge 2 thoe ound in rerigerted diply ce. Snkr Pdhmnbhn et l [28] compred rot nd derot cycling perormnce between microchnnel het exchnger with louvered in nd in-tube het exchnger with tright in employed outdoor coil o het pump ytem. Locl urce temperture nd weight o the coil were tken in rel time during the experiment. The m o rot ccumultion during heting tet w obtined by uing lod cell. Zhng nd Hrnjk (29, 21, 21b) nlyzed nd compred ir-ide perormnce o three type o het exchnge in roting condition. They tudied eect o ome geometric prmeter on perormnce o PF2 het exchnger in periodic roting nd compred to erpentine louvered in geometry. However, there i very little h been publihed to ddre rot ormtion nd deroting on erpentine-louvered-in het exchnger nd little bi vilble in the open literture or the deign o microchnnel het exchnger. Thi pper meured the eect o louver ngle on the ir-ide het exchnger perormnce in rnge o typicl operting condition in erch or good vlue o louver ngle prmeter. In ddition uthor hope tht thi tudy will provide dditionl inight into the perormnce o microchnnel het exchnger under cyclicl roting condition. 2. EXPERIMENTAL FACILITY AND PROCEDURE The experimentl cility ued to tet the het exchnger i hown in Figure 1. A ingle phe econdry rerigernt, % ethyl lcohol, i cooled by n R44A chiller ytem nd pumped through the experimentl loop. The lcohol m low rte i controlled by djuting the pump peed. A Corioli-eect m low meter (±.1%, rnge: -218kg/h) i ued to meure the m low rte through the het exchnger. The lcohol temperture i controlled uing PID1 to regulte power upplied to Heter 1 nd i meured uing immerion thermocouple probe (±.2 C) t the inlet nd exit o the het exchnger. Figure 1: Experimentl cility The environmentl chmber houe pre-cooler, heter, tem inlet, two chilled-mirror enor, nd the wind tunnel. The pre-cooler i ued to cool the chmber to the tet condition. A 3.2 kw heter (Heter 2) i ued to mintin contnt ir temperture within the chmber, blncing the cooling eect o the pre-cooler. It i connected to PID2, which control the tet het exchnger ir inlet temperture bed on reding o type-t thermocouple (±.2 C) plced ner the entrnce to the het exchnger. Stem i ued to mintin contnt dew point within the environmentl tet chmber. It goe through mnully djuted vlve nd then olenoid vlve, which i opened nd cloed by uing PID3, controlled by Generl Etern model D-2-SR chilled-mirror dew-point enor (±.2 C). Two chilled-mirror enor o the me model re ued to meure dew point uptrem nd downtrem o the tet het exchnger. The wind tunnel conit o plexigl ection, duct ection, nd blower (Figure 2). The tet het exchnger i plced in the plexigl ection o obervtion rom the ide i poible. The plexigl ection lo ct n irlow trightener by utilizing wire meh in the necery poition becue o the gret pect rtio o the duct. A

4 251, Pge 3 nozzle i locted in the duct ection to meure the ir m low rte. Three preure tp re plced t the downtrem ide o the het exchnger nd our preure tp re plced t both ide o the nozzle. Preure dierence cro the nozzle nd tet het exchnger re meured by preure trnducer Setr model 239 (±.14% FS, rnge: -249). The blower ued during the tet run t contnt peed throughout ll experiment. Air temperture re meured uing two grid o thermocouple t the inlet nd outlet o the het exchnger. Clibrtion provide ccurcy o ±.2 K. T dp,in T ir,in T re,in T ir,out T dp,out Air out Plexigl Air in Plexigl ection Duck ection Nozzle View Blower T re,out dp ir dp nozzle The detil o the wind tunnel inlet Tble 1 Tet condition T, T r, in Figure 2: The open wind tunnel inide the environmentl chmber in t d T d dp [ C] [ C] [ %] [m 1 ] [ min] [ C] kp -11 7%; 8% dp The dt logger mpled vlue t 1 intervl, nd ix meurement re verged to provide the reult in 1min intervl. The dt re written into text ile or ubequent nlyi. The experiment re conducted t contnt: ir-inlet temperture, rerigernt-inlet temperture, ir inlet humidity, rerigernt m low rte, nd ir low rte. The tet condition re given in Tble 1. Prior to tking dt, lcohol low only to the pre-cooler until the chmber i cooled to the deired temperture. Once the tet condition i reched, the experiment i initited; the econdry rerigernt low to the tet het exchnger nd dt collection begin. When the ir-ide preure drop cro the tet het exchnger incree to ive time it initil vlue, the derot cycle i initited nd the econdry rerigernt t 2 C i pumped through the het exchnger to melt the rot. Ater derot, vlve re witched nd the rerigertion cycle begin. The ytem goe through multiple cycle o derot nd rerot. 3. THE GEOMETRY OF HEAT EXCHANGERS EXPLORED B B.42 mm 19. mm L p Section A-A Figure 3: Structure o the tet het exchnger Section B-B θ

5 251, Pge 4 Fig. 3 indicte geometricl conigurtion nd terminology o the tet het exchnger. The het exchnger re multi-louvered in nd lt tube het exchnger. Totl o nine het exchnger mple re ued or the tet. Tble 2 how peciiction o the tet het exchnger. The geometricl dierence between thee het exchnger re the in pitch (12 pi to 18 pi) nd louver ngle ( to ): ee Tble 1. The tet het exchnger conit o one microchnnel tube brzed to one row o louvered in hown in Figure 3. A heet o plexigl i ued in plce o the econd tube in order to photogrph the het exchnger during the roting nd reroting period. A CCD cmer w ued to viully cpture the roting, deroting nd reroting procee nd to help explin the reoning behind the dierence between het exchnger perormnce. The irlow nd the tube re verticl plne, with rerigernt entering rom the top nd leving t the bottom. Tble 2: Smple geometrie (B1, B2 nd B3 preented in thi pper) HX Nme F p / F L p [pi]/[mm] [mm] [ o ] B1 12/ B2 12/ B3 12/ B4 15/ B5 15/ B6 15/ B7 18/ B8 18/ B9 18/ DATA REDUCTION PROCEDURE The ultimte objective o dt reduction i to obtin the overll conductnce UA nd the overll het trner coeicient U in time. Bic dt nlyi include determintion o the cpcity o the het exchnger, which i clculted rom both the rerigernt ide nd the ir ide blnce, hown by Eq. (1) nd (2): qr m rc pr ( Tr, out Tr, in ) (1) nd q m ( h, in h, out ) q, q, l, (2) with q, m ( C p, int, in C p, outt, out ) (2) q l q q m,, rhg (2b) ( q r q ) q 2 (3) qr q q q (4) Two energy blnce re ued to veriy proper opertion or ech tet run. Eqution (4) deine the dierence between the two energy blnce clculted or ech dt point. The dierence in energy blnce remined below 5% or ll tet when pproching tedy condition. The -NTU method w ued or obtining overll het trner coeicient U. Thi eqution pplie to cro low HX, with both luid unmixed (Ky nd London, 1984)..22 NTU *.78 1 exp [ exp( C NTU ) 1] * C q / q mx (5) (6)

6 251, Pge 5 q mx ( mc p ) min ( T in Tr, UA ( mc UA U A ) p min NTU, in ) ( mc p) C ( mc ) p min mx (7) (8) (9) (1) 5. RESULTS AND DISCUSSION A mentioned, thi pper preent i ocued on the eect o the louver ngle, illutrted in Figure 3(b). Three louver ngle re explored:,, nd where mximl perormnce i expected to be in the typicl ce low velocitie. Nine het exchnger diering in the louver ngle nd in pitch re preented o totl o 15 originlly tudied to ee geometric eect (ee Tble 2). In the tet conducted to explore the eect o the louver ngle on the perormnce o the het exchnger (preented in thi pper), louver pitch i kept contnt (1.4 mm). All nine het exchnger hve the me louver pitch, ce re, in height, in depth nd were teted under the me condition o inlet ir temperture nd reltive humidity, rerigernt inlet temperture, nd ir nd rerigernt m low rte (Tble 1 nd 2). It i importnt to undertnd the eect the louver ngle h on the perormnce o the het exchnger not only during roting (irt cycle), but lo in reroting condition. Froting i the growth o rot lyer on clen, dry het exchnger where reroting i the growth o rot on het exchnger tht h been deroted nd h wter being retined on the urce o the in. Reroting i more common during het exchnger opertion ince roting only occur when the het exchnger i irt intlled or when ll o the wter rom the melting rot lyer ter derot h been removed. Thu it i the mot importnt or the opertion o the het exchnger in relity to undertnd the reroting cycle tht i repetitive other re jut trnient. 5.1 The perormnce t the beginning o the irt roting cycle Eect o louver ngle on the initil preure drop nd overll het trner coeicient during the irt roting cycle Air preure drop [p] () U [W m -2 K -1 ] pi 15 pi 7 18 pi (b) 12 pi 15 pi 18 pi θ [ ] θ [ ] Fig. 4: Vrition o ir-ide preure drop nd overll het trner coeicient with louver ngle t the beginning o the irt roting cycle (clen urce) or three dierent in pitche t ce velocity: 3.5 m/ Figure 4. preent the preure drop dt nd overll het trner coeicient t the beginning o the irt roting cycle, or the multi-louvered in het exchnger with dierent louver ngle, 15, 27 nd 39, nd the in pitche, 12 pi, 15 pi, nd 18 pi (in pcing 2.12, 1.41, nd 1.69 mm). Even thee tet were conducted t 8% reltive humidity uing the tet condition tted in Tble 1., grph indicte opertion in dry condition. Preure drop incree with louver ngle or ll in pitche, but the grdient i reduced louver ngle incree (ee Fig. 4()). For the in with 18pi the preure drop t the beginning o the irt roting cycle or the louver ngle 27 i bout 21% higher thn 15 nd i bout 8% lower thn 39. The overll het trner coeicient incree with louver ngle or mll louver ngle (louver ngle le thn 27 in thi ce), but or the greter louver ngle it eect on the overll het trner coeicient vrie with in pitch,

7 251, Pge 6 hown in Fig. 4(b). For 12 nd 15 pi, the overll het trner coeicient t the beginning o the irt roting cycle incree with louver ngle but or Fp = 18 pi, decree with louver ngle greter thn 27. Thi indicte tht or given geometry nd operting condition mximum het trner perormnce occur t round 27 louver ngle. The overll het trner coeicient t the beginning o the irt roting cycle or louver ngle 27 with in pitch 18 pi i bout.7% higher thn 39 nd bout 8 % higher thn 15. Thi my be due to the lower low eiciency, ee Webb nd Truger (1991). A the low louver ngle, ome o the low trem bype the louver pge nd low duct low, between the in chnnel. The het exchnger perormnce i then cloe to tht o plin duct, nd the louver re reltively ineective. A the louver ngle incree, the riction ctor nd preure drop long the duct low pth incree, cuing more low in the louver direction. Similrly, the in pcing decree, the hydrulic reitnce in the louver direction decree with repect to the xil direction, o more o the low will p through the louver direction, which implie higher low eiciency. Figure 5 preent to cle drwing o three geometrie with in pitch 18 pi nd louver pitch 1.4 mm explored to cilitte undertnding o the trend hown in Figure 4(b). It look like t lower louver ngle 15 geometry vor duct directed low while t 39geometry vor tronger louver directed low. Structure with middle louver ngle (27) eem to be more blnced nd tht i wht correlte well with the het trner perormnce hown erlier. ) Louver ngle 15 b) Louver ngle 27 c) Louver ngle 39 Fig. 5: Louvered-in rry or in pitch 18 pi (in pcing 1.41mm), louver pitch 1.4 mm, louver ngle 15, 27, Eect o louver ngle on initil Colburn nd Fnning riction j ctor during the irt roting cycle nd comprion to the prediction by the Chng nd Wng (1997) The dt reduction proce or the ir-ide convective het trner coeicient, h, during the irt roting cycle, i the me Xi et l. (26), which i developed bed on Xi nd Jcobi (24, 25), nd Zhng nd Hrnjk (21), o only brie decription i given here. q, 1 1 FTlm h A hr Ar ( q, / q ) (11) A A h ( q with r k h ( q k / q r, / r ) k r A A / r A 2 F h tnh ( k / 2 k / q, ) Fh r 2 r / 2 [ k r h ( q h ( q / q / q,, ) ) r )( / 2) / k r r (12), (13) k r ] (14)

8 251, Pge 7 Where ΔTlm i the log men temperture dierence determined under the umption o counter low condition, nd F i the cro low correction ctor to ΔTlm. The vlue or F re obtined uing the plot o logrithmic men temperture dierence correction ctor or cro low het exchnger developed by Bowmn et l. (194). For the initil vlue during the irt rot-growth cycle, η, nd η, cn be expreed ollowing: A, 1 (1, ) A (15) tnh[ 2h ( q / q ) /( k )( F / 2)],, h 2h ( q / q, ) /( k )( Fh / 2) (16) Flow inide the tube w lminr (4<Re<7) nd the lt tube rerigernt-ide het trner coeicient w determined bed on the Shh correltion (1975). For RerPrrDr/TL 33.3 (here in the rnge o 38< RerPrrDr/TL <55), tht i: 1/ D.953 Re Pr r r Nu r r r TL w (17) Then, the rerigernt-ide convective het trner coeicient i obtined by Nurkr hr Dr (18) The het trner nd preure drop cn be expreed in term o the Colburn ctor, j, nd the Fnning riction ctor,. The ir i treted n incompreible luid, nd the denity o ir i treted contnt ccording to the verge ir temperture. A 2dp 2 Atot (19) h j GC.8.7 p 2 / 3 Pr 12pi 15pi 18pi pi 15pi 18pi (2) ctor.6.5 () j ctor θ [ ] θ [ ] Fig. 6: Eect o louver ngle j nd ctor t the beginning o the irt roting cycle (b) Figure 6 diply the eect o louver ngle on the initil Colburn j ctor nd the initil Fnning riction ctor during the irt cycle. Figure 6() how tht the Fnning riction ctor,, incree the louver ngle get lrger nd the in pcing get mller. Figure 6(b) how tht the j ctor hve the me trend the initil overll het trner coeicient: or 12 nd 15 pi, the j ctor incree with louver ngle but or Fp=18 pi, the j ctor incree with louver ngle when louver ngle i le thn 27, nd decree with louver ngle when louver ngle i greter thn 27.

9 251, Pge ().8.7 (b) j_cor.2.18 _cor j _exp _exp Fig. 7: Comprion o experimentl dt nd correltion or the j nd ctor Figure 7 how the comprion o experimentl dt nd the prediction by Chng nd Wng correltion (1994) or the j nd ctor: experiment nd the correltion re within 1% or j.ctor nd 2% or ctor. The devition i t very low level nd lightly incree with the louver ngle. Tht indicte tht thi unique, one ided HX i very good repreenttion o opertion o the in o thi kin in the rel, complete het exchnger, or tht eect o lightly reduced in eiciency i in the noie. 5.2 The perormnce in roting Eect o louver ngle on ir-ide preure drop () RH=7%, Fp=18pi, Lp=1.4mm (b) RH=8%, Fp=18pi, Lp=1.4mm Air preure drop [p] time [min] Air preure drop [p] 2 15 Figure 8: Eect o reltive humidity on the ir-ide preure drop or het exchnger with louver ngle 15, 27 nd 39 or the in pitch 18pi during the irt ive roting period, ce velocity: 3.5m/ Eect o reltive humidity o inlet ir (here 7% nd 8%) on ir ide preure drop i hown in Fig. 8. or the multi-louvered in with dierent louver ngle, 15, 27 nd 39, or the in pitch18 pi during the irt ive roting period. A expected, the preure drop t the beginning o ech cycle incree in irt our cycle due to retention o the wter ter derot tht w in quntity uicient to be retined, not drined. Thi i due to greter wter retention cpbility on the urce o the in compred to reltively mll quntity o rot between in becue low rot denity. The urce could retin wter rom multiple roting period. The wter retined block the low nd thu incree the preure drop cro the het exchnger. Even more, the wter ct pritic therml m becue it chnge phe in ech rot/derot cycle. It tke our to ive cycle to ccumulte wter to it limit nd the drin will trt. Tht will reult in unchnged vlue o preure drop t the beginning o ech rerot cycle. Thi phenomenon w viully conirmed by photogrph tht were tken during the tet nd will be explined lter in the pper. More on thi iue cn be ound in Zhng nd Hrnjk (21) which how tht in ome ce it i needed even to eight cycle to come to tedy tte condition

10 251, Pge 9 Figure 9: Viul obervtion o wter retention ter the irt nd econd reroting period. Figure 9 on the let depict wter retention on the urce o the in ter the irt deroting period. Thi explin the incree in the preure drop t the beginning the irt reroting period. The picture o the in ter the econd derot (on the right) how tht there i more wter on the urce o the in thn in the irt deroting period. Thi i due to the dditionl wter rom the rot tht w melted during the econd derot, but h not been removed. Thi explin the incree in the preure drop t the beginning o the econd reroting period when compred with the preure drop t the beginning o the irt reroting period () Initil orting cycle (d) The third reroting cycle 2 2 Air preure drop [p] Air preure drop [p] Air preure drop [p] (b) The irt reroting cycle (c) The econd reroting cycle Air preure drop [p] Air preure drop [p] (e) The ourth reroting cycle Figure 1 Air-ide preure drop dt or ech cycle o the het exchnger with the in pitch 18pi or three louver ngle: 15, 27 nd 39 under the reltive humidity 7% Figure 1 how detil o the preure drop progreion in Figure 8() or het exchnger with 18 pi nd louver ngle: 15, 27 nd 39, by ligning ll cycle to trt rom the zero time. Figure 1() compre the preure drop during the initil roting period. At the beginning o the rot ccumultion period, het exchnger with lrger louver ngle hve higher preure drop. A time progree to ner the point where deroting i necery, the in with 39 louver ngle h nerly the me preure drop the 27 ce. The het exchnger with in with louver ngle o 15 h the lowet preure drop, probbly becue duct directed low i r le ected by rot bridging

11 251, Pge 1 the louver (ee Fig. 5.). The opertionl (rerigerting) time, which i conequence o the derot criterion o ive time incree compred to dry preure drop, i longet or the in with 39 louver ngle. Figure 1(b) how the preure drop in the irt reroting period. The 39 louver ngle pecimen till h the highet preure drop t the beginning o the irt reroting period, but the 27 pecimen degrde more quickly. () At the beginning o the irt reroting period Wter retention t plexigl -in interce Wter retention in mll mount t plexigl-in interce Virtully no wter retention oberved ter irt rerot. θ=15 θ=27 θ=39 (b) At the beginning o the econd reroting period Wter retention i higher thn in Wter i bridging the in in irt rerot everl re. Smll mount o wter retention i ner the triling edge θ=15 θ=27 θ=39 (c) At the begining o the third reroting period. Wter retention i prevlent t the triling edge. Wter i bridging the in in everl re More wter i retined on the in thn ter econd re rot, but it i till mll mount. θ=15 θ=27 θ=39 (d) At the beginning o the orth reroting period Wter retention i primrily t the Wter retention i primrily triling edge. Fin bridging occur. t the triling edge. Wter retention i in greter mount thn previou reroting period θ=15 θ=27 θ=39 Figure 11: Het exchnger with in pitch 18pi nd louver ngle 15, 27 nd 39 t beginning o rerot Figure 11 how the wter retention on the urce o the in ter ech derot or het exchnger with 18 pi nd louver ngle, 15, 27 nd 39. Figure 11() how tht ll het exchnger retin wter, but in moderte mount

12 251, Pge 11 t the beginning o the irt reroting period. Het exchnger with 39 h virtully no wter retention oberved on the in urce. Het exchnger with 27 hd wter t the plexigl-in interce in mller mount thn het exchnger with 15. Tht vlley repreent loction o the tube where condente or wter ter derot tend to ty. Thi itution could be explined by eier retention/bridging in the mller louver ngled het exchnger. There re two primry pth o wter removl in HX with verticlly oriented tube nd they were clerly viible in the experiment illutrted in Figure 12. The irt pth i through the louver, where the wter low downwrd, long the louver edge, then move to the upper prt o the lower below, nd gin downwrd ollowing the louver nd on, due to the orce o grvity. Overll movement i down. The m lux o wter incree towrd the bottom. The other pth i in the irlow direction towrd the triling edge o the in due to the her orce rom the ir. Thi direction generlly doe not reult in wter removl unle blow-o (crry-over) occur or pecil proviion re mde. Wter Droplet Pth 1 Microchnnel Tube Pth 2 Lrget ditnce between louver nd in Fin Het exchnger with =39 Louver Smller ditnce between louver nd in Fin Louver Het exchnger with =27 Smllet ditnce between louver nd in Fin Louver Figure 12: Illutrtion o wter movement nd removl mode. Het exchnger with =15 Figure 13: Drwing illutrting the impct o the louver ngle on the chnnel re tht i vilble or wter to low. Needle to y wter removl i very importnt ince the lrger mount o wter retined on the in bridge the louver turning louver directed into duct directed low, block the irlow nd once het exchnger i turned to rerigertion mode, the wter reeze nd ty on the urce o the in. The ice plug cn lo bridge the louver, which lo cue the low to chnge rom louver directed low to duct directed low. In ddition, Thi tke wy the het trner enhncement provided by the louver. When non-ti blower re ued, it will drticlly decree the ir m low rte. Alo, the ice ct dditionl therml reitnce to het trner thu cuing decree in cpcity. Needle to y it tke energy to turn the wter retined into the ice nd to melt it in derot. It w een tht once droplet i creted it ty or everl cycle ometime even not melting in derot, until inlly grow o big tht it i moved by ir. Figure 13 illutrte the eect tht the louver ngle h on the low o wter through the louver. When the louver ngle i lrge, the ditnce nd correponding re between the louver nd the in i lrger thn when the louver ngle i mll, when other louver geometricl prmeter re held contnt. Thu, it i le likely or wter to get cught up or bridge the louver. Thi i importnt ince hving wter low rom upper in to lower in nd thu, wter low down the het exchnger more reely or het exchnger with 39 thn the other two het exchnger. Thu the wter lowed to lower portion o the het exchnger. Thi i importnt ince thi leve the top portion o the het exchnger with mll mount o wter retention. Figure 1(c) conirm tht obervtion: the het exchnger with 39 i ble to run or much longer operting time (up to 5%) thn the other two het exchnger nd h imilr preure drop 15.. There i lo noticeble incree in the preure drop t the beginning o the roting which indicte tht ll o the het exchnger hve retined more wter thn in the previou period. Het exchnger with 27 h the wort preure drop proile (the mot enitive to rot) by the time the deroting criterion i met. Photogrph preenting wter retention t the beginning o the econd reroting period re in Figure 11(b). It i gin hown tht het exchnger with 39 h the mllet mount o wter retined in the re where the picture re tken. The reon or thi behvior w decribed bove. All three het exchnger hve greter mount o

13 251, Pge 12 wter retention in the viewble re thn in the previou rerot cycle. It i lo importnt to note tht when bridging occur, it typiclly hppen t or ner the triling edge due to the ir low puhing greter mount o wter towrd the exit. The preure drop in the third rerot re hown in Figure 1(d). The trend re the me in the previou cycle. Thi i gin due to dditionl wter being retined on the urce o the het exchnger. The het exchnger with the mllet louver ngle gin h the mllet preure drop t the beginning o the reroting period, but it degrde much more quickly when compred to het exchnger with 39. Het exchnger with 27 nd 39 hve nerly identicl preure drop t the beginning o the third reroting period. Photogrph ter the third derot re hown in Figure 11(c). Agin it pper tht wter dringe i better in in with lrger louver ngle. Wter ccumulte in lrger quntitie ner the triling edge o the het exchnger. The preure drop in the ourth reroting period re plotted in Figure 1(e). Sitution i the me in previou cycle, jut expnded. Photogrph t the beginning o the ourth reroting period nd re hown in Figure 11(d). It pper tht the re where the picture were tken i nerly turted -wter bridged the in ner the triling edge or het exchnger with louver ngel 15 nd 27. Thu, comprion o the preure drop chrcteritic under the cyclicl roting condition, ter everl reroting cycle, the het exchnger with the lrget louver ngle hd the mot deirble preure drop chrcteritic due to the bet dringe Eect o louver ngle on overll het trner coeicient () RH=7%, Fp=18pi, Lp=1.4mm (b) RH=8%, Fp=18pi, Lp=1.4mm U [W/K -1 ] 8 U [W K -1 ] time [min] Figure 14: Eect o humidity on overll het trner coeicient or het exchnger with louver ngle 15, 27 nd 39 or the in pitch 18pi het exchnger during the irt ive roting period The overll het trner coeicient chrcteritic or the multi-louvered in het exchnger with dierent louvered in geometrie re preented in Fig. 14 or two ir humiditie nd ive cycle. Figure 15 how the detil o the proce in Figure 14. Figure 15() how tht the het exchnger with 27 h the highet het trner coeicient during the beginning period o the irt (nd lter every) roting cycle. The overll het trner coeicient with the mllet louver ngle (15) h the lowet overll het trner coeicient throughout the tet. It i not urprie when looking in the inormtion in the Figure 5, nd lo Fig. 6(b). Neverthele the louver ngle o 27 h lrger decree in perormnce thn the louver ngle o 39, both in ech cycle nd in ech conecutive cycle. Another importnt chrcteritic i igniicnt drop in het trner coeicient in the roting cycle (while doing the job): orm initil vlue round 9 Wm-2-1 It drop to round Wm-2-1 beore derot. The irt reroting period w initited immeditely ter the irt deroting period w terminted (2min). Once the rot lyer hd melted completely to wter, rerigernt w once gin llowed to low through the het exchnger nd thu the cooling period trted. The overll het trner coeicient during the irt reroting period or thee three het exchnger were determined nd re hown in Figure 15(b). All three het exchnger how light decree in the overll het trner coeicient t the beginning o the roting period in ech conecutive cycle until reching the tedy vlue. It pper tht it i due to light incree in wter retined on the in urce. The wter ct n dditionl therml reitnce to het trner nd my ect the ir low through the in/louver tructure. Neverthele, the impct o wter retention on the het trner coeicient pper to be very mll. Comprion o three louver ngle how the me order: the het exchnger with 27 h the highet het trner coeicient (but not or much) during the beginning period o the irt reroting cycle but it h lrger

14 (d) The third reroting cycle 251, Pge 13 decree in perormnce thn the louver ngle o 15 nd 39. The het exchnger with the mllet louver ngle (15) h the lowet het trner coeicient throughout the tet. U [W K-1] 8 () Inicil roting cycle U [W K-1] 8 U [W K -1 ] (b)the irt reroting cycle U [W K-1] (e) The ourth reroting cycle U [W K -1 ] (c)the econd reroting cycle Figure 15 Air-ide overll het trner coeicient dt or ech cycle o the het exchnger with the in pitch 18pi or three louver ngle: 15, 27 nd 39 under the reltive humidity 8% Figure 15(c) how the itution in the econd rerot. The mjor dierence i development o the 39o louver the bet perormer, due to: the highet het trner coeicient, the let reduced het trner coeicient in roting, nd the longet opertion in roting. Agin, during the third rerot, there i light, miniml decree in the overll het trner coeicient when compred to the previou reroting period (Figure 15(d)). Thi i due to dditionl wter being retined by the het exchnger. The trend pper tht the het exchnger with lrger louver ngle hve higher overll het trner coeicient throughout. The itution ter the ourth derot i hown in Figure 15(e). The me trend the third reroting period re oberved. Agin, the overll het trner coeicient i higher or lrger louver ngle. The tedy opertion eem to hve been reched. Thu, comprion o the perormnce chrcteritic (preure drop nd overll het trner coeicient) under the cyclicl roting condition indicte tht the het exchnger with louver ngle o the in 39 h the bet overll chrcteritic (or 18 pi nd 3.5 m/ ce velocity). It i becue the perormnce in the ully repetitive condition (ter initil 3-5 reroting cycle) i wht mtter or ctul opertion, even 27 ngle h the highet overll het trner coeicient during the irt ew cycle, or the 15 het exchnger h the lowet preure drop. 5. SUMMARY AND CONCLUSIONS Thi pper preent the reult o n experimentl invetigtion o the perormnce o erpentine-louvered-in, microchnnel het exchnger in periodic roting. The ocu i on the eect o louver ngle: 15, 27 nd 39 or the three in pitche (12, 15 nd 18pi) on preure drop nd overll het trner coeicient. When erching or good deign typiclly higher het trner i ccompnied with higher preure drop nd the election o good combintion i unction o the ppliction. In the ce o opertion in roting deigner could lo k: would the good perorming geometry in dry condition till be good in roting. Reult preented in the pper indicte importnt trend in tht repect: the combintion tht w not good in dry condition (39 o nd 18pi in Fig. 4) turned to be very good in roting (Fig. 1 nd 15). Even more importnt i the ct tht the dierence how more nd more prominent in lter conecutive roting cycle. Thee opertion (numerou conecutive roting reroting cycle) re the mot typicl or rel operting itution nd re typiclly neglected in tudie o

15 251, Pge 14 roting. In other word: CONCLUSIONS FROM DRY TEST AND EVEN FIRST TWO THREE REFROSTING CYCLES DO NOT HOLD IN REAL OPERATION WHERE MULTIPLE CONSECUTIVE FROSTING OCCURS. It i ound tht the preure drop t the beginning o the irt roting cycle (clen opertion) or ll in pitche incree with louver ngle, but the eect o louver ngle on the preure drop decree or louver ngle lrger thn 27. For the in with 18 pi the preure drop t the beginning o the irt roting cycle or the louver ngle 27 i bout 21% higher thn 15 nd or louver ngle 27 i bout 8% lower thn 39. Compring the reult rom the tet run on the pecimen with three louver ngle, 15, 27 nd 39, or the three in pitche, 12 pi, 15 pi, nd 18 pi, (gin I do not ee two other in pitche: 12 nd 15) the overll het exchnger coeicient, t the beginning o the irt roting cycle, incree with louver ngle or mll louver ngle (louver ngle le thn 27 in thi ce), but or the greter louver ngle it eect on the overll het exchnger coeicient vrie with in pitch. For Fp=12 pi, the overll het exchnger coeicient t the beginning o the irt roting cycle incree with louver ngle. For Fp=18pi, the overll het exchnger coeicient t the beginning o the irt roting cycle incree with louver ngle when louver ngle i le thn 27, nd decree with louver ngle when louver ngle i greter thn 27. The overll het trner coeicient t the beginning o the irt roting cycle or louver ngle 27 with in pitch 18 pi i bout.7% higher thn 39 nd bout 8 % higher thn 15. Severl (here typiclly our) repetitive derot cycle were needed to rech turtion in wter retention. Ech het exchnger hd n incree in the preure drop t the beginning o irt our conecutive reroting period due to exce wter being retined. For the in pitch 18 pi, ter everl reroting cycle, the het exchnger with the lrget louver ngle hd the highet overll het trner coeicient throughout nd hd the mot deirble preure drop due to the bet dringe. The photo how tht the het exchnger with the lrget louver ngle hd the let mount o wter in the viible region. Thu, the geometry with lrger louver ngle w ble to drin the wter down the het exchnger more eiciently. A wter drin downwrd, it ill re tht re more uceptible or the wter to be retined in, but it lo goe to re tht drin more eily. When lrge quntitie o wter re preent, it i eier or the wter to be removed o o the het exchnger by the irlow puhing the wter o o the triling edge. Thu, the concluion rom thee tet were tht the het exchnger with the lrget louver ngle hd the mot deirble behvior ter everl reroting period with repect to both the overll het trner coeicient, preure drop, nd wter movement. Thi concluion i very importnt becue it how hortcoming o the choice bed on the only one roting period, typicl or mny tudie in thi ield. The election o the high ngle louver would not hve been jutiibly mde bed on the reult o the irt cycle. A re (m 2 ) C P peciic het (J kg 1 K 1 ) D hydrulic dimeter (m) D vg verge devition (%) D men men devition (%) dp F (-) F F F d h p F preure drop (kp) cro-low correction ctor in depth (mm) in height (mm) in pitch (pi) in pcing (on center) (mm) riction ctor (-) G ir-low m vel. (kg m -2-1 ) h convective het trner coeicient (W m -2 C -1 ) NOMENCLATURE h g ltent het o blimtion (or wter vpor) (J kg -1 ) j Colburn ctor (-) k therml conduct. (Wm -1 C -1 ) L p louver pitch (mm) m m low rte (kg 1 ) Nu Nuelt number (-) Pr Prndtl number (-) het trner rte (W) q Re Reynold number bed on hydrulic dimeter (-) Re ir ide Reynold number Lp bed on louver pitch (-) T temperture ( C or K) T L Tube length (m) T mjor tube mjor (mm) T minor tube minor (mm) T tube pcing (on center) (mm) time (min) t U overll het trner coeicient (W K 1 m 2 ) UA overll conductnce (W K 1 ) x the dt element (-) T lm log-men temperture dierence ( C) thickne (m) reltive humidity (%) overll urce eiciency (-) roted-in eiciency (-) vicoity o luid (P-) louver ngle denity (kg m -3 ) ir velocity between in (m 1 )

16 251, Pge 2 Subcript initil vlue by correltion cor d dp exp derot dew-point by experiment r in l n in ree-low rot inlet ltent het nozzle out r tot w outlet rerigernt (or rerigernt ide) enible het totl riction wll moit ir (or ir ide) REFERENCES Blekjin, G., Ktz, D., Het trner rom uperheted vpor to horizontl tube, A.I.Ch.E. J., 4 (1) 1958, pp Achichi A, Cowell T. A., Het trner nd low riction chrcteritic o lt tube nd louvered plte in urce. Exp. Therml Fluid Sci. Vol. 1; p Bowmn, R.A., Mueller, A.C., Ngle, W.M., 194. Men temperture dierence in deign. Trn. ASME 62, Crlon, D. M., Hrnjk, P. S., nd Bullrd, C. W., 21. Depoition, ditribution, nd eect o rot on multi-row het exchnger perormnce. Deprtment o Mechnicl nd Indutril Engineering. Urbn, Univerity o Illinoi, ACRC TR-183. Kim J, Groll E.A., 23. Perormnce comprion o unitry plit ytem uing microchnnel nd in-tube out-door coil. ASHRAE Trnction, 19(2), p Kondepudi S. N, O Nel D. L., The Eect o rot growth on extended urce het exchnger perormnce - Review. ASHRAE Trnction 93(2), Kondepudi S. N, O Nel D. L., The Eect o rot growth on the perormnce o louvered inned tube het exchnger. Interntionl Journl o Rerigertion. 12, P Mchielen, C.H.M., Kerchbumer, H.G., Inluence o rot ormtion nd deroting on the perormnce o ir cooler: tndrd nd dimenionle coeicient or the ytem deigner. Int. J. Rerigertion 12, Webb, R.& P. Truger, Flow tructure in the louvered in het exchnger geometry. Exp. Therm. Fluid Sci, 4; P Snkr Pdhmnbhn, Lorenzo Cremchi, Dniel Fiher, John Knight., 28. Comprion o rot nd derot perormnce between microchnnel coil nd in-nd-tube coil or het pump ytem. Proceeding o the 28 Interntionl Rerigertion nd Air Conditioning Conerence t Purdue, Vol. I, p Shh, R.K., Therml entry length olution or circulr tube nd prllel plte. In: Proc. 3rd Ntionl het m trner conerence, Indin Int. Technology, Bomby, vol. 1, HTM Ky W. M. nd A. L. London. (1984). Compct Het Exchnger,3rd Ed. New York: McGrw-Hill Book Compny. Yn, W. M., H. Y. Li nd Y. L. Ty, 25. Thermoluid chrcteritic o roted inned-tube het exchnger, Int. J. Het M Trner, 48, P Xi, Y., Jcobi, A.M., 24. An exct olution to tedy het conduction in two-dimenionl lb on one-dimenionl in: ppliction to roted het exchnger. Int. J. Het M Trn. 47, Xi, Y., Jcobi, A.M., 25. Air-ide dt interprettion nd perormnce nlyi or het exchnger with imultneou het nd m trner: wet nd roted urce. Int. J. Het M Trn. 48, Xi Y, Y. Zhong, P. S. Hrnjk, nd A. M. Jcobi, 26. Frot, derot, nd rerot nd it impct on the ir-ide therml-hydrulic perormnce o louvered-in, lt tube het exchnger. Inerntionl Journl o Rerigertion, 29:7, Y.J. Chng, C.C. Wng, A generlized het trner correltion or louver in geometry, Int. J. o Het nd M Trner 4 (3), Zhng, P. nd P. S. Hrnjk, 29. Air-ide perormnce evlution o three type o het exchnger in dry, wet nd periodic roting condition. Interntionl Journl o Rerigertion, Vol. 32, p Zhng, P. nd P. S. Hrnjk, 21. Eect o ome geometric prmeter on perormnce o PF2 het exchnger in periodic roting, Interntionl Journl o Rerigertion,Vol. 33, p Zhng, P. nd P. S. Hrnjk, 21. Air-ide perormnce o prllel-low prllel-in (PF2) het exchnger in equentil roting, Interntionl Journl o Rerigertion, Vol. 33, p ACKNOWLEDGEMENT We re grteul or inncil nd technicl upport rom Modine Mnucturing Compny nd or technicl upport rom the Air Conditioning nd Rerigertion Center (ACRC) t the Univerity o Illinoi.