CFD Simulation of R134a and R410A Two-Phase Flow in the Vertical Header of Microchannel Heat Exchanger

Transcription

1 Purdue Unversty Purdue e-pubs Internatonal Refrgeraton and Ar Condtonng Conference School of Mechancal Engneerng 2016 CFD Smulaton of R134a and R410A Two-Phase Flow n the Vertcal Header of Mcrochannel Heat Exchanger Yang Zou Unversty of Illnos at Urbana-Champagn, yangzou1@llnos.edu Pega Hrnjak pega@llnos.edu Follow ths and addtonal works at: Zou, Yang and Hrnjak, Pega, "CFD Smulaton of R134a and R410A Two-Phase Flow n the Vertcal Header of Mcrochannel Heat Exchanger" (2016). Internatonal Refrgeraton and Ar Condtonng Conference. Paper Ths document has been made avalable through Purdue e-pubs, a servce of the Purdue Unversty Lbrares. Please contact epubs@purdue.edu for addtonal nformaton. Complete proceedngs may be acqured n prnt and on CD-ROM drectly from the Ray W. Herrck Laboratores at Herrck/Events/orderlt.html

2 2367, Page 1 CFD Smulaton of R134a and R410A Two-Phase Flow n the Vertcal Header of Mcrochannel Heat Exchanger Yang ZOU 1, Pega HRNJAK* 1, 2 1 Unversty of Illnos at Urbana-Champagn, Department of Mechancal Scence and Engneerng, Urbana, IL, USA 2 Creatve Thermal Solutons Urbana, IL, USA Contact Informaton ( , yangzou1@llnos.edu, pega@llnos.edu) * Correspondng Author ABSTRACT Ths paper studes refrgerant maldstrbuton n the vertcal header of mcrochannel heat exchanger through both experment and CFD smulaton. In the experment, the two-phase R134a or R410A s crculated nto the transparent vertcal header through mult-parallel mcrochannel tubes n the bottom pass and exts through mult-parallel mcrochannel tubes n the top pass representng the flow n the heat pump mode of a reversble system. The expermental results are compared wth CFD smulaton. The Euleran-Euleran model n the commercal software Fluent s used to conduct smulaton. Qualtatve agreement between experment and CFD s obtaned. Both experment and CFD show that the dstrbuton s worse wth respect to nlet qualty due to the flow pattern n the header. Wth CFD smulaton, pressure drop and vod fracton nformaton n the vertcal header s obtaned. 1. INTRODUCTION Mcrochannel heat exchangers (MCHX) have come to the fronter of automotve, resdental, and commercal ar condtonng applcatons for ts advantages n compactness, hgher heat transfer, and possble charge reducton. However, refrgerant maldstrbuton n the header of MCHX creates unwanted superheated regon, where the heat transfer s much lower than the two-phase regon due to the lower heat transfer coeffcent of superheated vapor and less temperature dfference between refrgerant and ar, so t may reduce MCHX capacty by up to 30%, e.g. as n Byun and Km (2011) and Zou et al. (2014). Most studes on refrgerant maldstrbuton were nvestgated expermentally. Fe and Hrnjak (2002), Vst (2003), Bowers et al. (2006), and Jn (2007) studed the two-phase flow n the horzontal headers, whch usually appeared n the ndoor MCHX. Watanabe et al. (1995), Cho and Cho (2004), Lee (2009), Byun and Km (2011), and Zou and Hrnjak (2013a, 2013b, 2014a, 2014b, 2014c) nvestgated the refrgerant dstrbuton n the vertcal headers, whch were commonly used n the outdoor MCHX. Among these studes, some derved emprcal dstrbuton functons based on expermental results to smulate refrgerant dstrbuton. Vst (2003) appled the results of T-juncton studes to develop a qualty dstrbuton functon at the round tube juncton n the horzontal cylndrcal header. Jn (2006) proposed a dstrbuton functon n the horzontal header (upward flow n the mcrochannel tubes) by relatng the branch tube qualty wth the rato of vapor mass flux n the header mmedately upstream to total nlet vapor mass flux. Lee (2009) consdered the cylndrcal vertcal header as a seres of T-junctons, and predcted the lqud dstrbuton among flat tubes based on the studes of two-phase flow at T-juncton. Watanabe et al. (1995) defned the lqud take-off rato as the rato of lqud mass flow rate n the branch tube to lqud mass flow rate n the vertcal header mmedately upstream. In annular flow, the lqud take-off rato was constant. In froth or slug flow, the lqud take-off rato was a functon of vapor phase Reynolds number and lqud phase Weber number n the header mmedately upstream. Vapor was consdered as equally dstrbuted among the tubes based on the measurement. Byun

3 2367, Page 2 and Km (2011) appled the approach of Watanabe et al. (1995). They related both vapor and lqud take-off rato wth vapor phase Reynolds number n the vertcal header mmedately upstream. Wth the wder range of test condtons and more fluds, Zou and Hrnjak (2013a, 2013b, 2015) found the nlet qualty and lqud phase Froude number were also mportant parameters to lqud take-off rato. Zou and Hrnjak (2015) generalzed R134a and R410A dstrbuton by relatng the lqud take-off wth the header nlet qualty as well as the vapor phase Reynolds number and lqud phase Froude number n the header mmedately upstream. In other studes, numercal methods were appled to study refrgerant dstrbuton. Moura (1995) numercally smulated ar-water dstrbuton n a two-pass MCHX wth vertcal headers based on the two-flud model. Only qualtatve agreement wth experment results was obtaned. Tompkns et al. (2002) dscretzed a header nto several control volumes and appled modfed separated flow model to smulate dstrbuton. Fe and Hrnjak (2004) conducted CFD smulaton of R134a flow n horzontal headers usng Euleran-Euleran model n the commercal software Fluent. Comparng wth experment data and vsualzaton mages, reasonable smulaton results were obtaned. Ablanque et al. (2010) presented a numercal model, usng the results of T-juncton studes to smulate the splttng flow phenomenon n the header. The accuracy of ths model strongly depended on the selecton of the T-juncton model. Stevanovc (2012) developed a computatonal mult-flud dynamc (CMFD) code based on numercal solvng of the mass and momentum balance equatons for the flow of each phase and the correspondng closure laws for the calculaton of nterface transfer of balanced parameters. Huang et al. (2014) proposed a co-smulaton approach by combnng CFD smulaton of the nlet vertcal header wth a ε-ntu based segmented heat exchanger model. The model s valdated aganst the expermental results. In ths study, t s attempted to mprove the understandng of twophase flow n the header through CFD smulaton based on expermental results. The upward flow n the ntermedate vertcal header of an outdoor reversble MCHX s smulated, mmckng the case when the outdoor reversble MCHX functons as evaporator n the heat pump mode. 2. EXPERIMENTAL METHOD The test loop was constructed to study R410A or R134a dstrbuton n the mcrochannel heat exchanger, as shown Fgure 1. The subcooled lqud refrgerant was pumped nto the nlet header. It was assumed that the sngle-phase subcooled lqud was dstrbuted evenly nto the mcrochannel tubes n the bottom pass, where the refrgerant was heated to desred qualty. The two-phase flud entered nto the test header and turned 90 o to flow upward n the bottom part. In the upper part of the header, due to maldstrbuton, dfferent amounts of lqud exted through the mcrochannel tubes n the top pass. In each ext tube, the refrgerant was heated agan to provde equal superheat at the ext. The sngle phase superheated vapor was then brought to the condenser. Through the recever and the subcooler, the subcooled lqud was returned to the pump. Fgure 1: System schematcs

4 2367, Page 3 The lqud mass flow rate n each ext tube was obtaned by Equaton 1. m l, out, out, ( out, m 1 x ) where x f P, h ) (1) out, ( header out, The pressure n the header P header was estmated as the average of the measured heat exchange nlet and outlet pressures and the outlet enthalpy from the header (.e., nlet to each ext tube) h out, was calculated as n Equaton 2. h Q out, out, hsup, where h f ( Psup,,Tsup, ) mout, sup, (2) The lqud mass flow rate were generalzed and lqud fracton n Equaton 4. Unform dstrbuton was descrbed as LF = 0.2. An uncertanty propagaton analyss carred out n EES (2012). The uncertanty of lqud fracton s usually wthn 5%. LF n m l, out, m l, out, (3) A hgh speed camera, Phantom v4.2, was used for vsualzng the flow n the transparent header. The exposure tme of the camera was 80 μsec. The framng rate was at 2000 frames per second. The resoluton was 256x512 pxels. The transparent crcular header, made of the PVC tube, had fve nlet and fve ext mcrochannel tubes protruded nto the ½ depth of header s nner dameter. The geometres of the transparent header and alumnum mcrochannel tube are lsted n Table 1. The test condtons are shown n Table 2. The nlet mass flux G n, presented n Table 2, s defned by the smallest cross-secton area n the header where tube protruson s presented. Item Header geometry Inner dameter Header length Tube ptch Tube protruson Table 1: Vertcal header and mcrochannel tube geometres Data mm 170 mm for 5+5 header; 300mm for header 13 mm ½ depth and ¾ depth of nner dameter Mcrochannel geometry Shape Rectangular Number of ports 17 Length 0.54 mm Wdth 0.5 mm Hydraulc dameter 0.5 mm Table 2: Test condtons Item Data Saturaton temperature 5 o C for R410A; 10 o C for R134a Inlet qualty Inlet mass flow rate 2 6 g/s for 5+5 header Inlet mass flux kg/m 2 -s 3. CFD MODEL DESCRIPTION

5 2367, Page 4 The commercal software Fluent was used to model the two-phase flow n the ntermedate vertcal header. The model was based on the work of Fe and Hrnjak (2004) whch modeled the two-phase flow n the horzontal header wth downward round tubes. As suggested by Fe and Hrnjak (2004), due to the complexty of two-phase flow and rregular geometry, the 3-D doman of the round header was modeled n ths study, as shown n Fgure 2. The Hex/Wedge mesh as shown n Fgure 2 was generated, whose sze was kept small enough to have more than 2 meshes along the heght (tube mnor) of the mcrochannel tube. Fgure 2: System schematcs The steady smulaton was conducted wth ntegrated solver and mplct scheme. The Euleran method n the Euleran- Euleran multphase model was used, whch treated both vapor and lqud phases as contnuous phases. (Other methods n the Euleran-Euleran multphase model are the Volume of Flud method and the Mxture method.) Fe and Hrnjak (2004) showed that the Euleran method would gve the best results n smulatng the two-phase flow n the header, so t s chosen n ths study. The standard k-ε turbulence model was used for each phase because the turbulence transfer among the phases played a domnant role. The vapor flow was consdered as the prmary contnuous phase, whle the lqud droplets flow was consdered as the secondary phase. Fe and Hrnjak (2004) determned the unform droplet dameter based on Phase Doppler Partcle Anemometry measurement. In ths study, the droplet dameter was adjusted so that the smulated dstrbuton results agreed best wth the expermental dstrbuton results,.e. 25μm for R134a and R410A. Between the two phases, the drag force was modeled wth symmetrc drag coeffcent. The Phase Coupled SIMPLE (PC-SIMPLE) algorthm was used for the pressure-velocty couplng. The contnuty resdual hstory of Fluent n ths study was The stable convergence was observed after 2000 teratons. 4. RESULTS AND DISCUSSION Fgure 3 compares the smulated dstrbuton results wth the expermental results for R134a and R410A at m n = 6.25 g/s. The darkness of bar color represents dfferent branch tubes, the pale beng the lowest ext branch and the dark beng the hghest ext branch. For both fluds, CFD results show smlar trend as the experment. The best dstrbuton s at x n = 0.2. As qualty ncreases, the dstrbuton s worse because the bottom tubes receve less lqud than the top tubes. The man dfference between CFD and experment s the top tube. The lqud fracton of the top tube decreases wth respect to nlet qualty n experment whle the lqud fracton of the top tube s hgher wth respect to nlet qualty n CFD. However, for the other 4 tubes, the dstrbuton profles are very smlar between experment and CFD.

6 x n [-] x n [-] x n [-] x n [-] 2367, Page 5 n =6.25g/s n =6.25g/s n =6.25g/s n =6.25g/s Branch #5 Branch #4 Branch #3 Branch #2 Branch # Branch #5 Branch #4 Branch #3 Branch #2 Branch # Branch #5 Branch #4 Branch #3 Branch #2 Branch # Branch #5 Branch #4 Branch #3 Branch #2 Branch # % 20% 40% 60% 80% Lqud fracton [-] 0% 20% 40% 60% 80% Lqud fracton [-] 0% 20% 40% 60% 80% Lqud fracton [-] 0% 20% 40% 60% 80% Lqud fracton [-] (a) R134a (b) R410A Fgure 3: Comparson between experment and CFD dstrbuton profles of R134a and R410A Fgure 4 and Fgure 5 compare CFD lqud contours wth the experment flow vsualzaton. The churn and semannular regmes are dentfed from the vsualzaton for both R410A and R134a. At low nlet qualty n Fgure 4, t s observed n experment that the flow pattern s churn flow. Most of the header s occuped by lqud refrgerant wth bubbles, but at the top t s almost vapor only. Bubbles str the lqud though the mean velocty of lqud s upward. It s hard to dstngush the nterface of lqud and vapor. They are mxed almost homogeneously. As llustrated n Fgure 4(a), the lqud contour of CFD shows smlar churn flow pattern and vapor-only regon at the top of the header. Both experment vsualzaton and CFD velocty contour n Fgure 4(b) show that there s local vortex between the neghborng 2 mcrochannel tubes, whch helps to mx vapor and lqud unformly. Therefore, the opportunty of lqud supply to each branch tube s smlar, except for the top tube close to the vapor-only regon. The dstrbuton s good at low qualty. (a) Lqud volume fracton (compared wth experment vsualzaton)

7 2367, Page 6 (b) Velocty Fgure 4: CFD contours of R134a at m n=6.25g/s and x n=0.2 At hgh nlet qualty n Fgure 5, the flow pattern observed n experment s sem-annular flow. The sem-annular flow s lke annular flow, but due to the tubes protruson, the annulus s not complete. Most volume of the header s taken by vapor, but lqud s present n the form of lqud flm along the nner wall of the header. In the top extng regon, vapor wth lghter densty s much easer to turn and branches out, but lqud wth larger densty and hgher momentum tends to run through the header and bypassed the frst few mcrochannel tubes. As some flud branches out, the velocty n the header s reduced, and the lqud flm starts to separate from the wall at certan heght. The flow pattern becomes locally churn flow. Some lqud flows horzontally and leaves through the outlet mcrochannel tubes. Other lqud falls down through the gap between mcrochannel tube and round header, so that creates a large vortex n the header. At the top of the header, the momentum s further reduced due to the two-phase flud branchng out. It results n lqud cannot reach the top and the tubes there get very lttle f any lqud. Thus, the tubes n ths small local churn flow regon have hgher opportuntes to receve lqud resultng n bad dstrbuton. CFD lqud contour n Fgure 5(a) also llustrates hgh vod fracton n the header and the lqud s present as lqud flm. However, unlke the experment vsualzaton, the lqud flm flows all the way to the top header, turns and comes down from the other sde of the header, as also shown n the velocty contour n Fgure 5(b). It also creates a large vortex n the header, smlar to the experment. However, t results n the lqud exts through the top tube frst and then the bottom tubes, so the top tube has hghest lqud fracton and t causes dfferent dstrbuton profles from experment at hgh qualty as n Fgure 3. (a) Lqud volume fracton (compared wth experment vsualzaton)

8 2367, Page 7 (b) Velocty Fgure 5: CFD contours of R134a at m n=6.25g/s and x n=0.6 Wth the help of CFD, more nformaton regardng two-phase flow n the vertcal header such as pressure drop and vod fracton can be obtaned, whch may be dffcult to measure durng experment. Fgure 6 presents the locatons and notatons of pressure drop and vod fracton n the followng analyss. The pressure or vod fracton of each plane s the average of the cross-secton area. Fgure 7 shows the pressure drop of R134a and R410A n the top extng regon of the vertcal header. Zou and Hrnjak (2014b) measured the two-phase pressure drop of R134a n ths vertcal header, and the expermental results are compared wth CFD results n Fgure 7(a). The trend of pressure drop along the header s smlar between CFD and experment. Besdes, both experment and CFD show that at low nlet qualtes the pressure drop s postve whle at hgh nlet qualtes the pressure drop s negatve,.e. t s pressure gan nstead of pressure drop at hgh qualtes. Zou and Hrnjak (2014b) showed that such negatve overall pressure drop at hgh qualtes was because that the negatve momentum pressure drop (due to losng mass and flow deceleratng) was more domnant than the frcton and gravty pressure drops. Such negatve pressure drop n the top extng regon at hgh nlet qualtes s also llustrated n the pressure contour from CFD n Fgure 8. ΔP 4 ΔP 3 ΔP 2 ΔP 1 α 9 α 8 α 7 α 6 α 5 α 4 α 3 α 2 α 1 Fgure 6: Locatons of pressure drop and vod fracton n Fgure 7 and Fgure 9

9 2367, Page 8 (a) R134a (b) R410A Fgure 7: Pressure drop of R134a and R410A n the vertcal header ΔP=-160Pa ΔP=300Pa Fgure 8: CFD pressure contours of R134a at m n=6.25g/s and x n=0.6 Fgure 9 presents the vod fracton of R134a and R410A n the vertcal header from CFD. Even though the flow vsualzaton s taken durng experment, t s very dffcult to quantfy vod fracton because of the complex flow patterns. Ths nformaton s added wth the help of CFD. It s shown n Fgure 9 that even at low nlet qualty, at least 80-90% volume of the vertcal header s occuped by the vapor. Thus, t may be very dffcult to mx vapor and lqud unformly n the header. To acheve good dstrbuton, ventng some vapor out of the header may be a more effectve soluton, as presented n Tuo and Hrnjak (2011) wth the method called flash gas bypass.

10 2367, Page 9 (a) R134a (b) R410A Fgure 9: Vod fracton of R134a and R410A n the vertcal header 5. CONCLUSIONS Ths study nvestgates the two-phase flow of R134a and R410A n the vertcal header of mcrochannel heat exchanger. The CFD smulaton s carred out n the commercal software Fluent and the smulaton results are compared wth the expermental results. The dstrbuton profles from CFD are very smlar to those from experment except for the top tube. Both CFD and experment show that as nlet qualty ncreases more lqud exts through the top tubes, and the dstrbuton becomes worse. Ths s due to the flow pattern n the header. At low nlet qualtes, the flow pattern s churn, and the mxng of vapor and lqud s unform. However, the flow pattern n the header s sem-annular at hgh nlet qualtes. The hgh speed lqud flm would bypass the bottom ext tubes and flow to a hgher locaton, then the lqud s only avalable for a few tubes at the top. These flow patterns are observed from both CFD and experment. Besdes, the CFD smulaton shows that the pressure drop n the top extng regon (top half part) of the header s postve at low qualty but negatve at hgh nlet qualty. The negatve pressure drop n the header may seem counterntutve, but ths s because that the flow decelerates as the two-phase flud branches out and the negatve momentum pressure s domnant at hgh qualtes. Based on the vod fracton from CFD smulaton results, there s at least 80-90% vapor n the header. It mght be very dffcult to mx vapor and lqud unformly, especally at hgh qualtes. Probably some other method (e.g. flash gas bypass method) should be appled to vent out some vapor for mprovng refrgerant dstrbuton. NOMENCLATURE G Mass flux (kg/m 2 -s 1 ) Subscrpts Enthalpy (kj/kg) Branch number LF Lqud fracton (-) n At the smallest m Mass flow rate (g/s) area n the mddle n Number of the outlet tubes (-) of the header P Pressure (kpa) l Lqud Q Power of the heaters (kw) out Out of the header T Temperature (K) sup Superheated x Qualty (-) sub Subcooled α Vod fracton (-) v vapor REFERENCES Ablanque, N. Olet, C., Rgola, J. Prez-Segarra, C., & Olva, A. (2010). Two-phase flow dstrbuton n multple tubes. Int. J. Thermal Sc., 49,

11 2367, Page 10 Bowers, C.D., Hrnjak, P.S., & Newell, T.A. (2006). Two-phase refrgerant dstrbuton n a mcro-channel manfold. Proc. 11th Int. Refrgeraton Ar Condtonng Conf. at Purdue, West Lafayette, IN, USA, R161. Byun, H.W., & Km, N.H. (2011). Refrgerant dstrbuton n a parallel flow heat exchanger havng vertcal headers and heated horzontal tubes. Exp. Thermal Flud Sc., 35, Cho, H., & Cho, K. (2004). Mass flow rate dstrbuton and phase separaton of R-22 n mult-mcrochannel tubes under adabatc condton. Mcroscale Thermophyscal Eng. 8(2), Fe, P., & Hrnjak, P.S. (2004). Adabatc developng two-phase refrgerant flow n manfolds of heat exchangers. Techncal Report TR-225, Ar Condtonng and Refrgeraton Center, Unv. Illnos at Urbana-Champagn, Urbana, IL, USA. Huang, L., Lee, M.S., Saleh, K., Aute, V., & Radermacher, R. (2014). A computatonal flud dynamcs and effectveness-ntu based co-smulaton approach for flow mal-dstrbuton analyss n mcrochannel heat exchanger headers. App. Thermal Eng., 65, Jn, D.H. (2006). Investgaton on refrgerant dstrbuton n evaporator manfolds. Ph.D. Thess, Unversty of Maryland, College Park, MD, USA. Lee, J.K. (2009). Two-phase flow behavor nsde a header connected to multple parallel channels. Exp. Thermal Flud Sc., 33, Moura, F.L. (1990). Experment and numercal study on the two-phase flow dstrbuton between two passes of a heat exchanger. ASME Fluds Eng. Dvson, 99, Stevanovc, V., Cucuz, S., Carl-Messner, W., Maslovarc, B., & Prca, S. (2012). A numercal nvestgaton of the refrgerant maldstrbuton from a header from a header towards parallel channels n an evaporator of automotve ar condtonng system. Int. J. Refrg., 55, Thompkns, D.M., Newell, T.A.,& Hrnjak, P.S. (2002). Sngle phase, two-phase modelng; X-Ray vsualzaton for a mcrochannel manfold dstrbuton system. Techncal Report TR-206, Ar Condtonng and Refrgeraton Center, Unv. Illnos at Urbana-Champagn, Urbana, IL, USA. Tuo, H., & Hrnjak, P.S. (2011). Flash gas bypass n moble ar condtonng system wth R134a. Int. J. Refrg., 35, Vst, S. (2003). Two-phase flow dstrbuton n heat exchanger manfolds. Ph.D. Thess, Norwegan Unversty of Scence and Technology, Trondhem, Norway. Watanabe, M., Katsuta, M., & Nagata, K. (1995). General characterstcs of two-phase flow dstrbuton n a multpass tube. Heat Transfer-Japanese Res., 24, Zou, Y., & Hrnjak, P.S. (2013a). Experment and vsualzaton on R134a upward flow n the vertcal header of mcrochannel heat exchanger and ts effect on dstrbuton. Int. J. Heat and Mass Trans., 62, Zou, Y., & Hrnjak, P.S. (2013b). Refrgerant dstrbuton n the vertcal header of the mcrochannel heat exchanger measurement and vsualzaton of R410A. Int. J. Refrg., 36, Zou, Y., & Hrnjak, P.S. (2014a). Effect of ol on R134a dstrbuton n the mcrochannel heat exchanger wth vertcal header. Int. J. Refrg., 40, Zou, Y., & Hrnjak, P.S. (2014b).Two-phase pressure drop n the vertcal header of mcrochannel heat exchanger. Int. J. Refrg., 44, Zou, Y., & Hrnjak, P.s. (2014c). Effects of flud propertes on two-phase flow n the vertcal header and refrgerant dstrbuton among parallel tubes n the mcrochannel heat exchanger. App. Thermal Eng., 70, Zou, Y., Tuo, H., & Hrnjak, P.S. (2014). Modelng refrgerant maldstrbuton n mcrochannel heat exchanger wth vertcal headers based on expermentally developed dstrbuton results. App. Thermal Eng., 64, Zou, Y., & Hrnjak, P.S. (2015). Comparson and generalzaton of R410A and R134a dstrbuton n the mcrochannel heat exchanger wth the vertcal header. Sc. and Tech. for the Bult Envronment, 21, ACKNOWLEDGEMENT Ths paper s part of the Ph.D. work of the frst author at Unversty of Illnos Urbana-Champagn. The authors would lke to thank the support from the sponsors of Ar Condtonng and Refrgeraton Center at the Unversty of Illnos Urbana-Champagn.