Refrigerant Distribution Effects on the Performance of Microchannel Evaporators

Transcription

1 Purdue Universit Purdue e-pubs International Refrigeration and Air Conditioning Conference School of Mechanical Engineering 2012 Refrigerant Distribution Effects on the Performance of Microchannel Evaporators Chad D. Boers Helena Mai Stefan Elbel Predrag S. Hrnjak llo this and additional orks at: Boers, Chad D.; Mai, Helena; Elbel, Stefan; and Hrnjak, Predrag S., "Refrigerant Distribution Effects on the Performance of Microchannel Evaporators" (2012). International Refrigeration and Air Conditioning Conference. Paper This document has been made available through Purdue e-pubs, a service of the Purdue Universit Libraries. Please contact epubs@purdue.edu for additional information. Complete proceedings ma be acquired in print and on CD-RM directl from the Ra W. Herrick Laboratories at Herrick/Events/orderlit.html

2 2173, Page 1 Refrigerant Distribution Effects on the Performance of Microchannel Evaporators Chad D. BWERS 1*, Helena MAI 1, Stefan ELBEL 1,2, Pega HRNJAK 1,2 1 Creative Thermal Solutions 2209 N Willo Rd, Urbana, 61802, USA chad.boers@creativethermalsolutions.com Universit of Illinois 1206 West Green St, Urbana, 61801,USA * Corresponding Author ABSTRACT The performance of air to refrigerant heat exchangers can be affected greatl b both refrigerant and air flo distribution. Maldistribution of either fluid stream can lead to reduced heat transfer effectiveness of the heat exchanger and thus loer sstem efficienc and capacit. Microchannel evaporators are especiall susceptible to refrigerant maldistribution as the tpicall have man parallel channels fed b a common header or manifold. This paper outlines general trends in refrigerant flo distribution in such evaporators taking into account various heat exchanger orientations. In addition, links beteen heat exchanger performance and refrigerant flo distribution characteristics in microchannel evaporators are made using both analtical and experimental methods. These results sho that unification of the liquid refrigerant flo distribution in the evaporator can lead to quantifiable improvements in evaporator performance. 1. INTRDUCTIN Maldistribution in manifolds providing fluid to parallel flo heat exchangers is a complicated problem that hile greatl studied, is still little understood. Man have noted the importance that flo regime and development pla in this area, but a complete understanding of its characteristics and effects is lacking. Keller (1949), in one of the earlier studies of this ver problem, endeavored to improve the distribution in a heat exchanger for compressed air b examining the placement of the exit from the discharge manifold, ith respect to the iet. In recent ears, the stud of such manifolds suppling to-phase flo to heat exchangers has become more important as microchannel technolog and small diameter tubes become more popular. Yoo et al. (2002) performed experimental ork in the distribution of an air ater mixture to 15 parallel microchannels through a manifold ith a rectangular cross section. The studied the effect of iet length, manifold orientation, flo orientation, mass flo, and iet qualit. Most of their results shoed severe maldistribution ith ater floing into the first and last channels, but not in the middle channels. Vist and Pettersen (2003 & 2004) conducted distribution experiments using both R134a and C2 in a horizontal manifold feeding 10 parallel heat exchanger tubes using counterfloing ater jackets as a heat source. The flo as oriented both in the upard and donard direction ith liquid in the upard flo tending to distribute most heavil in the last tubes and in the first tubes in the case of donard flo. Vist and Pettersen also examined the effect that entrance length had on distribution and found that a shorter entrance length led to enhanced distribution. International Refrigeration and Air Conditioning Conference at Purdue, Jul 16-19, 2012

3 2173, Page 2 Zhang et al. (2003) expanded on the ork done b Yoo et al. (2002) b studing the distribution of R134a in a horizontall oriented manifold feeding 15 parallel microchannels ith donard flo. Results shoed that the liquid flo tpicall as maldistributed to the first fe channels. In their examination of to-phase flo distribution in parallel flo heat exchangers Webb and Chung (2005) studied the effect of vapor qualit, header area, and protrusion effects in a horizontal header feeding 20 microchannels tubes ith donard flo. In all cases, the liquid tended to maldistribute toard the first tubes. Boers et al. (2006) studied the effect of entrance length, microchannel protrusion, mass flo rate, and qualit on distribution of to-phase R134a in a horizontal manifold feeding 15 parallel microchannels ith donard flo. As ith others, the found that the liquid flo tended toard the first fe channels in this orientation. The to entrance lengths studied shoed ver different results. In the case of the longer entrance length (267 mm) increases in iet qualit, mass flo, and protrusion depth all helped to improve the distribution of the liquid phase. In the case of the shorter iet, these effects ere much smaller since the distribution as alread more uniform than in the longer entrance case. Also of note is that through flo visualization, Boers et al. sa that protrusion of the microchannels helped to promote mixing of the to phases, hich has been seen here and elsehere to result in better distribution. In their ork on to-phase flo distribution, Kim and Sin (2006) studied the effects of mass flo rate, iet qualit, microchannel protrusion, flo orientation (upard and donard), and outlet direction on an air and ater mixture in a horizontal header feeding 30 microchannels. In the comparison of upard flo and donard flo it as seen that the flo tended to maldistributed liquid toard the rear and toard the front, respectivel. The mass flo rate and qualit ere varied from 16 g/s to 45 g/s and 0.2 to 0.6, respectivel. Increases in both ere seen to improve distribution. Microchannel protrusion as varied from none to ½ of the diameter of the manifold. r the donard flo configuration, the protrusion significantl improved the liquid phase distribution. In the case of upard flo, no significant effect of protrusion as seen. The did also note that the protrusion of the microchannels significantl changed the flo pattern observed in the header. In their stud of distribution of to-phase R134a in a horizontal manifold, Kim et al. (2006 & 2007) looked at the effect of flo direction (upard and donard), microchannel protrusion, mass flo rate, and iet qualit. Again seeing to distinct trends hen the flo direction as changed. In upard flo, most of the liquid floed through the channels at the end of the manifold; hile in donard flo, most of the liquid floed through the channels at the front of the manifold. Kim et al. varied the iet mass flo rate from 16 g/s to 90 g/s and sa that in donard flo, increased mass flo tended to improve distribution, this trend as exactl reversed in the case of upard flo. Loer iet qualities shoed more uniform distribution results. Poggi et al. (2007) experimentall studied the distribution of to-phase HFE 7100 in a transparent 16 mm manifold feeding 8 parallel multi-port microchannels. The studied both horizontal and vertical orientation, finding that header orientation had a significant effect on distribution. Horizontal headers ith vertical donard flo through the microchannels tended to see most of the liquid flo going into the first channels. When the header as oriented verticall, ith the flo going donard in the header, the last channels received most of the liquid flo. Kim and Han (2008) expanded on the ork done b Kim and Sin (2006) in horizontal headers feeding 10 parallel microchannels ith a to-phase mixture of air and ater. The studied the effect of microchannel protrusion on distribution and found that in the donard flo configuration increasing protrusion forced more ater to the back of the header hich tpicall lacked ater flo. In the upard flo configuration, increasing the depth of protrusion resulted in more liquid into the front part of the header, hich tpicall lacked ater flo. 2. EFFECT F EVAPRATR RIENTATIN F REFRIGERANT FLW DISTRIBUTIN In order to understand the effects of heat exchanger orientation on refrigerant flo distribution, a microchannel evaporator designed for an air-conditioning sstem of approximatel 10 kw using R410A as examined in three separate orientations. The orientations examined are shon in Figure 1. To orientations are ith the microchannel tubes oriented verticall and the headers horizontall. In one (Horizontal Header Verticall Donard Flo), the refrigerant iet header as located on the top ith the bulk refrigerant flo don through the microchannels. In the other (Horizontal Header Verticall Upard Flo), the iet manifold as located at the bottom of the coil International Refrigeration and Air Conditioning Conference at Purdue, Jul 16-19, 2012

4 2173, Page 3 ith refrigerant floing up through the microchannels. r both cases, the refrigerant exit of the coil as located on the opposite side from the iet. The other orientation as ith the microchannel tubes horizontal and headers vertical. In the first (Horizontal Tubes Verticall Upard Flo), the refrigerant entered the iet header from the bottom. The exit from the coil to the suction of the compressor as located at the top of the coil. The refrigerant flo distribution in all cases as analzed using infrared thermograph. In addition to analzing the refrigerant flo distribution in the coil through infrared thermograph, the aluminum iet header as removed and replaced ith a transparent header to visualize the refrigerant flo regimes in the header itself for to of the orientations. This visualization provides a link beteen the flo regimes in the header and the distribution of the refrigerant to the man parallel channels of the coil. The presented results ere obtained using split tpe air conditioning sstems ith variable speed compressor using R410A as the orking fluid tested at AHRI Standard 210/240 Test Condition A. Figure 1: Microchannel evaporator coil orientations 2.1 Verticall riented Microchannel Tubes Due to condensate drainage issues in horizontal tube orientation, orienting the microchannel arra verticall in evaporators is an attractive option. To cases of horizontal header orientation ere investigated. The first as ith the refrigerant iet located on the bottom left (from the air iet perspective) of the coil and exiting from the top right. An infrared image tpical of the coil operating in this orientation is shon in Figure 2. What is apparent from this image is that approximatel one third of the coil nearest the iet receives ver little liquid refrigerant. This results in the flo in that portion of the coil becoming superheated ver quickl and the entire upper left corner of the heat exchanger having little to no temperature difference available to accomplish heat transfer. International Refrigeration and Air Conditioning Conference at Purdue, Jul 16-19, 2012

5 2173, Page 4 Visualization in the iet header shoed a flo regime that as indicative of the flo distribution seen in the infrared image shon in Figure 2. Specificall, the to-phase mixture enters the manifold and separation begins almost immediatel. This separation leads to a stratified flo regime in the first third of the header. This stratification, vapor on the top and liquid on the bottom, is hat causes the first third of the heat exchanger to receive ver little liquid refrigerant. At approximatel one-third of the distance don the iet header, the flo undergoes a phenomenon ver similar to a hdraulic jump. From this point on, the cross section of the manifold is filled ith liquid, up to the height of the microchannel iets. An illustration of the flo regime over the entire length of the header is shon in Figure 3. UT IN Figure 2: Tpical refrigerant flo distribution in a horizontal iet header ith verticall upard flo through microchannels IN Figure 3: Tpical flo in a horizontal iet header ith verticall upard flo through microchannels Another interesting flo phenomena noticed in this orientation is the flo of vapor refrigerant coming from the microchannels against the bulk flo direction. A sequence of high speed images, Figure 4, provides an example of this behavior in the second microchannel from the left. In images 1 through 3, o one or to small bubbles are observed near the iet of the second microchannel. In image 5, a larger vapor jet beings to discharge from the microchannel tube against the bulk flo direction. This jet increases in size as the frames progress from image 5 International Refrigeration and Air Conditioning Conference at Purdue, Jul 16-19, 2012

6 2173, Page to image 8. In images 9 and 10, the vapor discharge from the microchannel ceases and the injected vapor rises through the liquid. This behavior as observed to occur in a random fashion in all microchannels located in the portion of the iet header here the liquid level as as high as the iet of the microchannel. It is postulated that this backflo of vapor is caused b the sudden expansion of the refrigerant in the microchannel tube. While the example presented here is for a horizontal header ith the flo verticall upard through the microchannels, similar behavior has been observed in all orientations and even ith other orking fluids Figure 4: Reverse flo in horizontal iet header ith verticall upard flo through microchannels The second case investigated ith horizontal headers as ith the flo entering through the iet manifold located on the top of the heat exchanger. An infrared image of the refrigerant flo distribution tpical of this orientation is shon in Figure 5. In this case, it appears as though the flo is preferentiall fed to the microchannels closest to the iet. While the portions of the coil that are fed ith liquid refrigerant appear to have relativel uniform distribution, nearl one third of the heat exchanger appears to receive little if an liquid refrigerant. As ith the previous orientation, this results in that portion of the coil becoming superheated ver quickl and having little to no temperature difference available to accomplish heat transfer. IN UT Figure 5: Tpical refrigerant flo distribution in a horizontal iet header ith verticall donard flo through microchannels International Refrigeration and Air Conditioning Conference at Purdue, Jul 16-19, 2012

7 2173, Page 6 Flo visualization results in the iet manifold shoed a trend that is the reverse of that seen in a horizontal header feeding refrigerant up through the microchannels. Namel, that flo separation does not appear to begin directl at the iet. The flo enters the header as a relativel homogenous mixture and does not begin to separate significantl until approximatel one third of the a through the header. At this point, the flo begins to stratif, ith the denser liquid on the bottom. This liquid then feeds the tubes closest to the iet preferentiall, until most of the liquid is gone, approximatel to-thirds of the distance through the header. The diagram in Figure 6 illustrates the flo regimes observed over the length of the header, hile the high speed images in Figure 7 sho flo visualization at various locations. IN Figure 6: Tpical flo in a horizontal iet header ith verticall donard flo through microchannels Halfa through Header Immediatel after Iet Separated little liquid Separating Homogenous To-Thirds through Header Figure 7: Flo regimes at several locations in iet header for a horizontal iet header ith verticall donard flo through microchannels 2.2 Horizontall riented Microchannel Tubes Currentl, man outdoor units of split tpe sstems using microchannel heat exchangers orient the coil ith the headers vertical and microchannel tubes horizontal. This orientation can be beneficial from a cost perspective as the aspect ratio in such units is often large, meaning that a vertical header ould be much shorter than a horizontal one. As the header is one of the more expensive components of the heat exchanger, reducing the length of the headers is desirable. While operation of microchannel heat exchangers in such orientation as an evaporator is not currentl preferred due to condensate drainage issues, several attempts have been made to optimize fin geometr to mitigate this particular problem. If the air-side challenges can be addressed, understanding of the refrigerant flo distribution in such orientations ill become important. In order to investigate these phenomena, the microchannel evaporator as oriented ith the headers vertical and the tubes horizontal and to cases ere investigated. The first as ith the refrigerant iet located at the bottom of the iet header and the exit at the top of the outlet header. The visualization of the refrigerant flo distribution as seen in the infrared image is shon in Figure 8. From the infrared image, it appears as though ver top portion of the coil receives little if an liquid flo. While the middle portion of the coil appears to receive the most liquid refrigerant, the bottom appears to have slightl less liquid refrigerant floing through the microchannels. Even though there is significant flo maldistribution still present ithin the coil a qualitative comparison of the heat exchanger in this orientation to those described above ould suggest that the heat transfer area of the heat exchanger is better used. International Refrigeration and Air Conditioning Conference at Purdue, Jul 16-19, 2012

8 2173, Page 7 UT IN Figure 8: Tpical refrigerant flo distribution in a verticall upard flo in the iet header 3. REFRIGERANT FLW DISTRIBUTIN EFFECTS N PERFRMANCE Boers et al. (2010) outlined a statistical methodolog for quantifing both the refrigerant distribution and the effective use of heat transfer area using infrared thermograph. The parameter as developed to rate heat exchanger distribution on a scale from zero to one in evaporator mode; zero being the highest degree of liquid maldistribution and one being uniform distribution. This method also has the advantage of being both non-invasive and lo cost. The folloing are examples that sho the link beteen an improvement in refrigerant flo distribution and an increase in heat exchanger performance using this refrigerant distribution parameter as a measure of refrigerant flo distribution. As a means of relating the refrigerant flo distribution in the heat exchanger to its performance, the distribution as rated using the above method and performance metrics ere evaluated for the horizontal header ith vertical don flo and the vertical header orientations at AHRI Standard 210/240 test condition A. r these tests, the volumetric airflo rate on both condenser and evaporator ere maintained hile also maintaining subcooling. Using the results obtained ith the horizontal header, to comparison points ith the vertical header ere investigated, matching refrigerant flo rate and matching superheat at the exit of the evaporator. Evaporator performance results as ell as the distribution rating parameters are shon in Table 1. The difference in orientation resulted in an increase of the distribution rating parameter from 0.72 in the case of the horizontal header to 0.86 for the vertical header in both cases. This quantitativel confirms hat as mentioned previousl, that the distribution in the vertical header orientation allos for more of the heat exchanger to be used for heat transfer. There are a fe other indicators that point toard the evaporator having improved performance from more uniform refrigerant flo distribution. In the case here the flo rate of the vertical header orientation as matched to the International Refrigeration and Air Conditioning Conference at Purdue, Jul 16-19, 2012

9 2173, Page 8 horizontal, the exit superheat is 5 K higher and the capacit is marginall higher. While in the case here the superheat as matched, the flo rate and consequentl the cooling capacit are both increased dramaticall. Table 1: Performance comparison beteen to orientations at AHRI condition A Header rientation Horizontal Vertical (Matched Florate) Vertical (Matched Superheat) Evap. utlet Superheat (K) Evap. utlet Pressure (kpa) Refrigerant Flo Rate (g/s) Cooling Capacit (kw) Distribution Parameter IR Image While the increased capacit ould point toard sstem level improvements; in both cases, the evaporator exit pressure is significantl loer than ith the horizontal header. This decrease in suction pressure ould likel offset hatever gains ma have been achieved in increasing the cooling capacit. The loering of evaporation pressure as likel caused b retained condensate on the evaporator increasing the heat transfer resistance. This postulation is supported b the air-side pressure drop results. In the case of the horizontal header, air-side pressure drop over the evaporator as approximatel 56 Pa. In the case of the vertical headers, the air-side pressure drop almost doubled to 105 Pa. In order to confirm that the offset in performance improvement as caused b condensate retention, the same comparison as made at a modified AHRI test condition A. The o modification to the test condition as to reduce the de point of the indoor air to ensure that it as belo the refrigerant evaporation temperature. This ensured that no condensate as formed/retained on the coil and alloed for a comparison ithout the added effect of poor condensate drainage. The results of these tests are shon in Table 2. Again, the distribution rating parameter is higher for both vertical header cases, increasing from 0.72 to 0.82 and r the matched refrigerant flo rate case, the exit superheat as increased from 5 K to 8.6 K and the cooling capacit as increased b more than 3%. The notable change from the test performed ith a latent contribution to the cooling load as the increase of the evaporation pressure b more than 50 kpa. Ver similar trends are seen in the case here the exit superheat as matched. The cooling capacit as increased b more than 5% and the evaporation pressure as increased b close to 40 kpa. These results provide even clearer indications that improvements in the heat exchanger performance caused b more uniform refrigerant flo distribution ere offset some b poor condensate removal. International Refrigeration and Air Conditioning Conference at Purdue, Jul 16-19, 2012

10 2173, Page 9 Table 2: Performance comparison beteen to orientations at modified AHRI condition A Header rientation Horizontal Vertical (Matched Florate) Vertical (Matched Superheat) Evap. utlet Superheat (K) Evap. utlet Pressure (kpa) Refrigerant Flo Rate (g/s) Cooling Capacit (kw) Distribution Parameter IR Image 4. CNCLUSINS In order to gain better understanding of the effect that orientation has on the refrigerant flo distribution, experimental investigations of a microchannel evaporator using R410A ere conducted in three different orientations. The results shoed that hen the refrigerant iet header is located at the bottom of the coil ith the refrigerant flo up through the microchannels, the first portion of the heat exchangers receives little, if an liquid refrigerant. In a sense, this causes that portion of the heat exchanger to be almost useless in the heat transfer process. Conversel, hen the horizontal header is located at the top of the coil feeding the microchannels ith refrigerant in the donard direction, the first part of the heat exchanger is preferentiall fed ith liquid refrigerant. Hoever, the result is ver similar, that nearl one-third of the heat exchangers is not used effectivel. In addition, interesting phenomena in the iet header, such as reverse flo and hdraulics jumps ere observed. Liquid refrigerant distribution as improved hen the heat exchanger as rotated 90 such that the header as oriented verticall ith flo entering at the bottom. Performance of the horizontal iet header at the top case and the vertical iet header ith an entrance at the bottom case ere compared using conventional sstem level performance metrics as ell as a novel technique for quantifing refrigerant distribution in microchannel heat exchangers. Results obtained at AHRI condition A appeared to be mixed due to the negative contribution of poor condensate drainage in the vertical header case. r this reason, a modified condition A test ith no humidit as evaluated and shoed that improvements in refrigerant flo distribution offer quantifiable improvements in coil performance. REFERENCES AHRI, 2008, AHRI Standard 210/240 Performance Rating of Unitar Air-Conditioning & Air-Source Heat Pump Equipment Boers, C.D., Hrnjak, P.S., Neell, T.A., 2006, To-Phase Refrigerant Distribution in a Microchannel Manifold, International Refrigeration and Air Conditioning Conference at Purdue, Paper # R161 International Refrigeration and Air Conditioning Conference at Purdue, Jul 16-19, 2012

11 2173, Page 10 Boers, C.D., Wujek, S.S., Hrnjak, P.S., 2010, Quantification of Refrigerant Distribution and Effectiveness in Microchannel Heat Exchangers Using Infrared Thermograph, International Refrigeration and Air Conditioning Conference at Purdue, Paper # 2117 Keller, J.D., 1949, The Manifold Problem, Journal of Applied Mechanics, March 1949, p Kim, N.H., Park, T.K., Ham, J.H., 2006, To-Phase Separation of R134a in a Header of a Parallel Flo Heat Exchanger, International Refrigeration and Air Conditioning Conference at Purdue, Paper # R045 Kim, N.H., Ham, J.H., Kim, D.Y., 2007, To-Phase Refrigeration Distribution in a Parallel Flo Heat Exchanger, International Congress of Refrigeration 2007, Paper # ICR07-B1-457 Kim, N.H., Han, S.P., 2008, Distribution of Air-Water Annular Flo in a Header of a Parallel Flo Heat Exchanger, International Journal of Heat and Mass Transfer, 51, p Kim, N.H., Sin, T.R., 2006, To-Phase Flo Distribution of Air-Water Annular Flo in a Parallel Flo Heat Exchanger, International Journal of Multiphase Flo, 32, p Poggi, F., Macchi-Tejeda, H., Marechal, A., Leducq, D., Bontemps, A., 2007, International Congress of Refrigeration 2007, Paper # ICR07-B1-647 Vist, S., Pettersen, J., 2003, To-Phase C2 Distribution in a Compact Heat Exchanger Manifold, 2nd International Conferenceon Heat Transfer, Fluid Mechanics, and Thermodnamics, Paper # VS2 Vist, S., Pettersen, J., 2004, To-Phase Flo Distribution in Compact Heat Exchanger Manifolds, Experimental Thermal and Fluid Science, 28, p Webb, R.L., Chung, K., 2005, To-Phase Flo Distribution to Tubes of Parallel Flo Air-Cooled Heat Exchangers, Heat Transfer Engineering, 26, p.3-18 Yoo, T., Hrnjak, P.S., Neell, T.A., 2002, An Experimental Investigation of To-Phase Flo Distribution in Microchannel Manifolds, Technical Report TR-207, Air Conditioning and Refrigeration Center, Univ. Illinois at Urbana-Champaign Zhang, Q.M., Hrnjak, P.S., Neell, T.A., 2003, An Experimental Investigation of R134a Flo Distribution in Horizontal Microchannel Manifolds, Technical Report TR-223, Air Conditioning and Refrigeration Center, Univ. Illinois at Urbana-Champaign International Refrigeration and Air Conditioning Conference at Purdue, Jul 16-19, 2012 Poered b TCPDF (.tcpdf.org)