Effect of Channel Geometries on Flow Reversal in Microchannel Evaporators

Transcription

1 Purdue University Purdue e-pubs International Refrigeration and Air Conditioning Conference School of Mechanical Engineering 216 Effect of Channel Geoetries on Flow Reversal in Microchannel Evaporators Huize Li University of Illinois at Urbana-Chapaign, United States of Aerica, Pega Hrnjak University of Illinois at Urbana-Chapaign, United States of Aerica, Follow this and additional works at: Li, Huize and Hrnjak, Pega, "Effect of Channel Geoetries on Flow Reversal in Microchannel Evaporators" (216). International Refrigeration and Air Conditioning Conference. Paper This docuent has been ade available through Purdue e-pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional inforation. Coplete proceedings ay be acquired in print and on CD-ROM directly fro the Ray W. Herrick Laboratories at Herrick/Events/orderlit.htl

2 2471, Page 1 Effect of channel geoetries on flow reversal in icrochannel evaporators Huize LI 1, Pega HRNJAK 1,2 * 1 University of Illinois at Urbana-Chapaign, Mechanical Science and Engineering, Urbana, IL, USA huizeli2@illinois.edu, pega@illinois.edu 2 CTS, Creative Theral Solutions, Urbana, IL, USA * Corresponding Author ABSTRACT Flash Gas Bypass with venting of reversed vapor is ipleented in a R134a autootive air conditioning syste. The ass flow rate of reversed vapor is easured by a flow eter placed on the venting line. The effects of icrochannel geoetries including channel length and channel diaeter on the aount and frequency of flow reversal are investigated. A echanistic odel is developed to siulate the bubble dynaics inside of single icrochannel tube. This odel is validated against experiental data and used to explain the effect of channel length and diaeter on flow reversal. 1. INTRODUCTION Microchannel heat exchangers are widely used in air conditioning and refrigeration systes because of their copactness and enhanceent of heat transfer perforance. However icrochannel heat exchangers suffer fro the proble of refrigerant aldistribution, especially for evaporator. For condensers, refrigerant distribution has less effect on perforance, on the other hand, better separation of flow ight be desired instead of better ixing (Li and Hrnjak, 216). The none-unifor distribution of two phase refrigerant in evaporator can significantly deteriorate the evaporator perforance as well as syste efficiency. Boiling instability and flow reversal is one cause for refrigerant aldistribution. According to Tuo and Hrnjak (213a), flow reversal in icrochannel evaporators deteriorates refrigerant distribution, increase pressure drop, although ay increase local heat transfer coefficient. The focus of previous research on flow instability in icrochannels has been on heat sinks for sall scale cooling applications, such as electronic cooling. Brutin et al. (23) experientally investigated the unsteady boiling of n- Pentane in heated inichannels. They found that the unsteady boiling region was deterined by heat flux and ass velocity, and unsteady boiling created reverse flow and high aplitude fluctuation of pressure signal. Brutin and Tadrist (24) further studied the steady and unsteady boiling region deterined by heat flux and ass velocity. They found that under each heat flux they exained, there is a critical ass velocity (or Re nuber) that deliit steady and unsteady boiling. Under each heat flux, unsteady boiling only happens at low ass velocity region, which can also be interpreted as high exit quality region. They also exained both confined inlet and copliant inlet (with a buffer) and discovered that pressure drop oscillation has lower aplitude but higher frequency in copliant inlet case copared with confined inlet case. Wu and Cheng (23) studied water boiling in two silicon icrochannels with diaeters of and 82.8 icron. They believed a new type of oscillation with long period was discovered in which single phase flow and two-phase flow appear alternatively. The oscillation period in the large channel was 31 s, and it was 141 s in the saller tube. Wu and Cheng (24) continued their study about boiling instability in parallel icrochannels. They first adjusted the water inlet pressure, then gradually increased heat flux. They discovered liquid/two-phase alternating flow at heat flux fro W/c 2, then continuous two-phase flow at heat flux of 18.8 W/c 2, and lastly liquid/two-phase/vapor alternating flow at heat flux of 22.6 W/c 2. They found that liquid/two-phase/vapor alternating flow created the largest pressure oscillation, while continuous two-phase generated the least pressure oscillation. Wu and Mudawar (24) studied boiling instability of water in parallel icrochannels under high heat fluxes. They were focusing on two types of dynaic instability: 16 th International Refrigeration and Air Conditioning Conference at Purdue, July 11-14, 216

3 2471, Page 2 pressure drop oscillation and parallel channel instability. They found that pressure drop oscillation can be greatly suppressed by a throttling valve placed before icrochannels. Parallel channel instability, which is uch ilder than pressure oscillation, can also be alleviated by ore throttling. They also discovered that under the sae ass flux, higher heat flux will help to stabilize the boiling process because the syste becoes stiffer with increasing pressure drop and a saller portion of two-phase flow. Hestroni et al., (25) proposed detailed stages in one cycle of periodic boiling process. Soon after nucleation, a bubble grows very rapidly to the size coparable to the diaeter of the channel (step a); then the bubble expands bidirectionally (step b); after the downstrea edge of the bubble reaches channel outlet, the bubble begins to vent (step c); few liquid droplets are left on the wall after bubble depletion (step d); along with bubble depletion, pressure inside of the channel decreases and fresh liquid starts to ove in (step e); finally, a new cycle starts (step f). Through experients, they also found that pressure oscillation caused by boiling instability increases with increasing quality. Hetroni et al., (26) tried to describe boiling instability by diensionless groups. They picked Nusselt nuber (Nu), Eotvos nuber (Eo) and Boiling nuber (Bo). They correlated initial fil thickness and Nu/Eo with Bo and believed Bo (q/h fg) is the bond between oentu and energy equations. Chen and Gariella (26) visualized FC-77 flow in parallel silicon icrochannels. They found that under high heat fluxes (>427 kw/ 2 ), flow reversal occurs near the inlet of the icrochannels, causing oscillation of pressure drop. They also concluded that flow reversal, which creates changes of ass flow rate, causes the alternation of flow regie near the outlet of the icrochannel. Fro their results, it can be seen that higher heat flux leads to greater chance of reverse flow, but this coparison is ade under the sae ass flux and changing outlet quality (under sae ass flux, higher heat flux will create higher exit quality). Harirchian and Gariela (28) visualized FC-77 flow in parallel silicon icrochannels with different geoetries. They located a throttling valve before the heat sink to suppress instability, but flow reversal was still observed at their highest heat flux exained. Huh et al. (27) studied boiling instability in one icrochannel. They discovered that there is a phase shift between pressure drop oscillation and ass flow oscillation. A sudden increase of ass flow rate occurred at the peak of pressure drop. They also found that an increase of ass flow rate lead to saller oscillation aplitude and shorter period. Under the sae ass flux, increasing heat flux (equivalent to higher exit quality) created higher oscillation aplitude and longer period. Zhang et al. (29) investigated excursive instability in parallel icrochannels. They found that increasing operation pressure, channel diaeter, and channel length and adding an inlet restrictor could alleviate instability. The heat transfer and fluid flow conditions in heat sinks for electronic cooling are significantly different fro those in air conditioning systes. In heat sinks, the incoing flow is norally subcooled and the exit quality is very low, while in icrochannel evaporators of A/C systes, the fluid at the inlet is either two-phase or saturated liquid and the outlet is norally superheated. In addition, the heat flux in electronic cooling applications is uch higher than that in air conditioning applications. Last but not least, water is the ost coonly used fluid in the reviewed studies, and its therophysical properties (vapor density, heat of vaporization, etc.) are significantly different with those of hydrofluorocarbons (R134a, R41A, etc.). To the best of the authors knowledge, the flow reversal in real systes was first observed at CTS (Creative Theral Solutions) in 26 and later published by Bowers et al (212). The only studies of flow reversal and boiling instability in realistic air conditioning systes in open literature were carried out by Tuo and Hrnjak. They (213b) invented a new syste configuration to vent the reversed vapor in the evaporator. They found that the reverse vapor accounted for 2-8% of the total supplied liquid into the evaporator. By venting the reverse vapor, 5 % of capacity and 3 % of COP iproveent could be achieved copared with the flash gas reoval (FGB) AC syste baseline. The Flash Gas Bypass syste was first used for systes with icrochannel evaporators by Beaver, Hrnjak (2), later Elbel and Hrnjak (24). The focus of this research is to experientally investigate boiling instability and flow reversal in icrochannel evaporators under conditions and geoetries which are relevant to air conditioning applications. Meanwhile, a echanistic odel of bubble dynaics in icrochannels will be developed and validated using experiental results. This odel will be used to explain the effect of different geoetries on flow reversal. 2. EXPERIMENTAL SETUP AND TEST CONDITIONS The scheatic drawing of the facility is shown in Figure 1Error! Reference source not found.. The variable speed copressor and the icrochannel condenser are coponents that are used in a ajor brand vehicle. Three different icrochannel evaporators were exained in the syste with detailed geoetry suarized in Table 1. Two heat exchangers have identical channel length but with different channel diaeter, and another two share the sae 16 th International Refrigeration and Air Conditioning Conference at Purdue, July 11-14, 216

4 2471, Page 3 icrochannel tube cross-section but with different length. The cross-sections of two icrochannel tubes are shown in Figure 2. In order to easure the flow rate of reversed vapor, revised flash gas bypass (FGBR) configuration invented by Tuo and Hrnjak (213b) is eployed (as shown in Figure 3), which is a odification of Flash Gas Bypass (FGB) configuration. In FGB ode, a separation vessel is placed before the evaporator, ensuring that only liquid is supplied to the evaporator and vapor is bypassed to the inlet of a copressor. Based on FGB configuration, two venting ports are added near both ends of the inlet header in FGBR ode, directing the reversed vapor to the top of the separation vessel. The dynaic ass flow rate of reversed vapor is easured by a ass flow eter, which is placed on the venting line d h Figure 1. Scheatic drawing of the test facility Figure 2. Cross-section of two types of MC exained Table 1. Key geoetries and operating conditions of three different heat exchangers (HX) Hydraulic diaeter () Nuber of tubes Channels /tube Length () Refrigerant-side area /channel ( 2 ) Heat flux (kw/ 2 ) Superheat ( ) HX HX HX Figure 3. Evaporator setup in FGBR ode 16 th International Refrigeration and Air Conditioning Conference at Purdue, July 11-14, 216

5 2471, Page 4 3. RESULTS AND DISCUSSION 3.1 Model Developent and Validation The details of the odel can be found in Li and Hrnjak (216). The velocity and acceleration of each liquid slug is deterined by the pressure in the bubbles located upstrea and downstrea: dvi P i1, d A Pi, u A Ffric, l, i l, ig l, i (3.1) dt At the sae tie, the velocity difference of two neighboring liquid slugs is deterined by the ass and energy balance of the bubble which is sandwiched in between. d v, i Ll, i Ll, i1 hlv q D( Lv, i ) (3.2) dt 2 2 dv, i dlv, i A (3.3) dt v dt The increase of the bubble length is caused by the velocity difference on both ends and the decrease in length of the liquid slug located upstrea and downstrea. In the end, the velocity difference of two neighboring liquid slugs is forulated (equation 3.5) as a function of heat flux, densities, heat of vaporization, lengths of liquid and vapor slugs and channel diaeter. dl v, i Ll, i L v, i Ll, i1 Lv, i Vi 1 Vi q D( ) / l Ahlv q D( ) / l Ahlv (3.4) dt V i1 Ll, i Ll, i 1,, 1 (, )(1 v Ll i Ll i v q D Lv i ) 4 q( Lv, i )(1 ) 2 2 l 2 2 l Vi (3.5) Ah Dh v lv v lv Vapor slug control volue Ll,i+1/2 Vi+1 Pi,d Lv,i Pi,u Ll,i/2 Vi D Liquid slug control volue Lv,i/2 Pi,u Flow direction Ll,i Lv,i-1/2 Vi Pi-1,d Figure 4. Control volues for liquid and vapor slugs D Figure 5. Coparison between flow visualization and siulation results 16 th International Refrigeration and Air Conditioning Conference at Purdue, July 11-14, 216

6 2471, Page 5 Knowing the pressures at the channel inlet and outlet (boundary conditions) and the velocity difference of any two neighboring liquid slugs, the pressure at each vapor slug (pressure will be different at the leading and trailing edges of the bubble, if vapor pressure drop is considered) and the velocity of each liquid slug can be calculated iteratively. The siulation results are copared with visualization results (Tuo and Hrnjak 214) at the sae geoetry and siilar operating conditions as shown in Table 2. Pressure drop in the odel was odified to be 3 kpa to better atch with the flow visualization results. Table 2. Experiental conditions which are used as odel inputs Ites Unit Value Evaporation pressure kpa 325 Heat flux kw/ 2 8 Pressure drop kpa 1.3 The top row in Figure 5 shows the coparison of bubble oveent in experient and siulation. The locations of three selected liquid slugs are highlighted using green, red and blue lines. The top left figure consists of 9 snapshots taken fro the high speed video of the diabatic in-tube flow. The top right figure is generated using the nuerical values of the locations of bubbles and liquid slugs at different points in tie. It can be seen that the odel is able to capture the experiental phenoenon of forward flow-reverse flow-forward flow. The botto row in Figure 5 shows the coparison of bubble velocity in experient and siulation. The velocities in the experient are calculated using the traveling distance of a certain liquid slug divided by the corresponding tie interval. The botto right figure is generated using the nuerical values of the velocities of bubbles and liquid slugs at different points in tie. The odel is able to predict that the three liquid slugs begin to reverse around 2s, and the velocity of the top liquid slug (green line) increases back to a positive value around 5s followed by the next two liquid slugs. The bubble velocities of all three slugs see to be the sae before 15s in the experient. This is ost likely because the ajority of the heat input is used to superheat the new coing liquid refrigerant, but this factor is not considered in the odel. 3.2 Effect of Channel Diaeter In this study, HX1 and HX2 are chosen since they have the sae channel length (19 ) but different channel diaeter (1.4 and.5 ). Siilar operation conditions are iposed for both heat exchangers (around 7.8 kw/ 2 heat flux and 7.5 superheat). As shown in Figure 6(a), the ass flow rate of vented vapor (for whole heat exchanger) has a very obvious periodic oscillation in HX1, which is in phase with the oscillation of inlet and outlet pressures. Through Fast Fourier Transforation (FFT) of the teporal data, the doinant frequency is found to be.73 Hz. In HX2 (shown in Figure 6(b)), the frequency of oscillation increases to 1.5 Hz, but the aplitude significantly reduces. The ratio of vented vapor flow rate to the total ass flow rate is uch higher in HX2 (14.8%) than in HX1 (6.6%). Bubble dynaics in these two channels are siulated under the sae operation conditions as in the experients. The siulation and experiental results are suarized in Table 3. Although siulation results are not quantitatively siilar with experiental results, but they shows the sae trends with the experients: There is ore flow reversal being generated in the channel with.5 diaeter in ters of relative agnitude (vented vapor flow rate divided by the total ass flow rate supplied to the evaporator) with a higher frequency. Table 3. Key flow reversal related easureents and siulation results in HX1 and HX2 Ites Mass flux in channels (kg/ 2 ) Ratio (a) Experient Siulation Experient Siulation Experient Siulation HX %.9% HX % 5.3% (a): Ratio is defined as: vented vapor flow rate divided by the total ass flow rate supplied to the evaporator ( rev tot ) 16 th International Refrigeration and Air Conditioning Conference at Purdue, July 11-14, 216

7 2471, Page 6 Mass flow rate of vented vapor [g/s] Tie [s] Aplitude spectru of venting flow rate [-] Hz Pressure [kpa] Mass flow rate of vented vapor [g/s] Pressure [kpa] Tie [s] Tie [s] Tie [s] Pin Pin Pout Pout Aplitude spectru of Pin and Pout [-] (a) Aplitude spectru of venting flow rate [-] Aplitude spectru of Pin and Pout [-] (b).6 Pin Pout.5.8 Hz Hz zoo in Pin Pout Hz zoo in Figure 6. Teporal variations and frequency analyses of easured variables in HX 1 and HX 2 Refer to equation 3.5, under the sae heat flux, saller channel diaeter will lead to ore rapid growth of a bubble. There are larger differences of the velocities of the liquid slugs which are located upstrea and downstrea of the bubble. Frictional resistance created by liquid or vapor slugs is proportional to velocity squared. Due to a ore drastic increase of liquid slug velocity in saller channels, the frictional resistance is ore concentrated downstrea in the tube. Due to higher frictional resistance downstrea, flow in saller channels tends to decelerate quicker and the pressure peak will shift ore downstrea fro the inlet. Figure 7 and 8 show the siulated flow regie and the corresponding pressure distribution inside of channels with 1.4 and.5 diaeter, respectively. One periodic cycle is divided into 1 steps to deonstrate the evolution of flow regie and pressure distribution. For the large tube (1.4 ), the highest pressure (indicated by the darkest red) occurs at 78% of the cycle tie and is located very close to the tube inlet, creating sall aount of flow reversal, which is confired by the flow regie shown on the left side of Figure 7. In the figures of flow regie, the black and white areas indicate liquid and vapor. Dash lines were used to illustrate the locations of the chosen liquid slugs in tie. In the channel with saller diaeter (.5), highest pressure occurs at 67% of the cycle tie and is observed near 2 % of the tube length after the inlet, creating a negative pressure gradient in the first 2 % of the tube. Due to the negative pressure gradient which occurs earlier and covers ore areas, there is ore reverse flow in the channel with.5 diaeter, and this phenoenon is also deonstrated by the flow regie shown on the left side of Figure th International Refrigeration and Air Conditioning Conference at Purdue, July 11-14, 216

8 2471, Page 7 Flow regie Cycle: % 1% 2% 3% 4% 5% 6% 7% 8% 9% 1% Pressure distribution % 1% 2% 3% 4% 5% 6% 7% 8% 9% 1% Outlet: kPa forward flow Inlet: 315kPa Figure 7. Flow regie and pressure distribution of R134a inside of a icrochannel with 1.4 diaeter and 19 length at 1 different points in tie within one periodic cycle Flow regie Pressure distribution Cycle: % 1% 2% 3% 4% 5% 6% 7% 8% 9% 1% % 1% 2% 3% 4% 5% 6% 7% 8% 9% 1% Outlet: 361.7kPa forward flow Inlet: 363kPa Figure 8. Flow regie and pressure distribution of R134a inside of a icrochannel with.5 diaeter and 19 length at 1 different points in tie within one periodic cycle 3.3 Effect of Channel Length In this study, HX2 and HX3 were chosen since they have the sae channel diaeter (.5 ) but different channel length (19 and 51 ). Siilar operation conditions are iposed for both heat exchangers (around 6.8 kw/ 2 heat flux and 6.5 superheat). By coparing Figure 6(b) and Figure 9, it can be seen that the oscillation aplitude of vented vapor flow is higher in HX3 than in HX2, but the frequency is lower in HX3 than in HX2. Larger oscillation is due to the fact that longer tube contains ore refrigerant ass and ore vapor will be vented within one periodic cycle. At the sae tie, ore inertia associated with ore refrigerant ass will slow down the periodic process, which contributes to slower frequency. Although the ass flux in HX3 is 1.5 ties higher than that in HX2, the ratio of vented vapor flow rate to the total ass flow rate supplied to the evaporator in HX3 (1.1%) is only slightly lower than that in HX2 (14.6%), Bubble dynaics in these two channels are siulated under the sae operation conditions as in the experients. The siulation and experiental results are suarized in Table 4. Although siulation results are not quantitatively siilar with experiental results, but they shows the sae trends with the experients: There is slightly ore flow reversal being generated in the channel with 19 length in ters of relative agnitude (vented vapor flow rate divided by the total ass flow rate supplied to the evaporator) with a slightly higher frequency. For the channel with 51 length, the siulated flow regie and pressure distribution at 1 selected ties within one cycle are presented in Figure 1. The location of the highest pressure is also close to 2% of the tube length after the inlet and it occurs at 68.6% of the cycle tie, which is siilar with the case in 19 tube. Despite the large difference of ass flux in both channels, the relative agnitude of vented vapor flow rate in both channels differs not significantly (shown by both experients and siulation), and this can be attributed to the siilar pressure distribution along two channels. 16 th International Refrigeration and Air Conditioning Conference at Purdue, July 11-14, 216

9 2471, Page 8 Mass flow rate of vented vapor [g/s] Pressure [kpa] Tie [s] Tie [s] Pin Pout Aplitude spectru of ventling flow rate [-] Aplitude spectru of Pin and Pout [-] zoo in zoo in Hz Hz Pin Pout Figure 9. Teporal variations and frequency analyses of easured variables in HX 3 Flow regie Pressure distribution Cycle: % 1% 2% 3% 4% 5% 6% 7% 8% 9% 1% % 1% 2% 3% 4% 5% 6% 7% 8% 9% 1% Outlet: 384kPa forward flow Inlet: 4kPa Figure 1. Flow regie and pressure distribution of R134a inside of a icrochannel with.5 diaeter and 51 length at 1 different ties within one periodic cycle Table 4. Key flow reversal related easureents and siulation results in HX2 and HX3 Ites Mass flux in channels (kg/ 2 ) Ratio Experient Siulation Experient Siulation Experient Siulation HX % 5.3% HX % 4.9% CONCLUSION It has been found experientally that under the sae heat flux (based on the refrigerant side area), icrochannel evaporators with sae length but saller diaeter induce ore reverse flow ( rev tot ) at a higher frequency, and icrochannel evaporators with sae diaeter but longer length produce slightly less reverse flow ( rev tot ) 16 th International Refrigeration and Air Conditioning Conference at Purdue, July 11-14, 216

10 2471, Page 9 at a slightly slower frequency. The effect of diaeter on flow reversal (relative ass flow rate ratio and frequency) is ore significant than the effect of channel length. A newly developed echanistic odel which can siulate bubble dynaics in single icrochannel explains the experiental results: under the sae heat flux and saller diaeter, the vapor slug grows ore rapidly (according to equation 3.5), locating ore flow resistance to the downstrea part of a channel. The higher downstrea resistance will decelerate the incoing flow quicker and generate negative pressure gradient in larger upstrea areas, which leads to ore flow reversal at higher frequency. For icrochannel evaporators with sae diaeter but different length, the pressure distribution along channel length is rather siilar. Slightly ore flow reversal ( rev tot ) at a higher frequency in shorter heat exchangers is attributed to less incoing ass flux. NOMENCLATURE A area ( 2 ) D diaeter (D) F force (N) g acceleration of gravity ( 2 /s) h enthalpy (kj/kg) L length () ass (kg) ρ density (kg/ 3 ) P pressure (kpa) q heat flux (kw/ 2 ) t tie (s) V velocity (/s) Subscript evap evaporation d downstrea i index l liquid rev reversed vapor tot total supplied liquid u upstrea v vapor REFERENCES Brutin, D., Topin, F., & Tadrist, L. (23). Experiental study of unsteady convective boiling in heated inichannels. International Journal of Heat and Mass Transfer, 46(16), doi:1.116/s17-931(3)93- Brutin, D., & Tadrist, L. (24). Pressure drop and heat transfer analysis of flow boiling in a inichannel: Influence of the inlet condition on two-phase flow stability. International Journal of Heat and Mass Transfer, 47(1-11), doi:1.116/j.ijheatasstransfer Wu, H. Y., & Cheng, P. (23). Visualization and easureents of periodic boiling in silicon icrochannels. International Journal of Heat and Mass Transfer, 46(14), doi:1.116/s17-931(3)39-5 Wu, H. Y., & Cheng, P. (24). Boiling instability in parallel silicon icrochannels at different heat flux. International Journal of Heat and Mass Transfer, 47(17-18), doi:1.116/j.ijheatasstransfer Hestroni, G., Mosyak, A., Pogrebnyak, E., & Segal, Z. (25). Explosive boiling of water in parallel icrochannels. International Journal of Multiphase Flow, 31(4), doi:1.116/j.ijultiphaseflow th International Refrigeration and Air Conditioning Conference at Purdue, July 11-14, 216

11 2471, Page 1 Hestroni, G., Mosyak, A., Pogrebnyak, E., & Segal, Z. (26). Periodic boiling in parallel icro-channels at low vapor quality. International Journal of Multiphase Flow, 32(1-11), doi:1.116/j.ijultiphaseflow Chen, T., & Gariella, S. V. (26). Measureents and high-speed visualizations of flow boiling of a dielectric fluid in a silicon icrochannel heat sink. International Journal of Multiphase Flow, 32(8), doi:1.116/j.ijultiphaseflow Harirchian, T., & Gariella, S. V. (28). Microchannel size effects on local flow boiling heat transfer to a dielectric fluid. International Journal of Heat and Mass Transfer, 51(15-16), doi:1.116/j.ijheatasstransfer Huh, C., Ki, J., & Ki, M. H. (27). Flow pattern transition instability during flow boiling in a single icrochannel. International Journal of Heat and Mass Transfer, 5(5-6), doi:1.116/j.ijheatasstransfer Zhang, T., Tong, T., Chang, J. Y., Peles, Y., Prasher, R., Jensen, M. K., Phelan, P. (29). Ledinegg instability in icrochannels. International Journal of Heat and Mass Transfer, 52(25-26), doi:1.116/j.ijheatasstransfer Tuo, H., & Hrnjak, P. (213a). Periodical reverse flow and boiling fluctuations in a icrochannel evaporator of an air-conditioning syste. International Journal of Refrigeration, 36(4), doi:1.116/j.ijrefrig Tuo, H., & Hrnjak, P. (213b). New approach to iprove perforance by venting periodic reverse vapor flow in icrochannel evaporator. International Journal of Refrigeration, 36(8), doi:1.116/j.ijrefrig Tuo, H., & Hrnjak, P. (214). Visualization and easureent of periodic reverse flow and boiling fluctuations in a icrochannel evaporator of an air-conditioning syste. International Journal of Heat and Mass Transfer, 71, doi:1.116/j.ijheatasstransfer Li, H. and Hrnjak, P., "Visualization and Analysis of Periodic Reverse Flow in an Autoobile Microchannel Evaporator," SAE Technical Paper , 216, doi:1.4271/ Beaver, A. C., J. M Yin, C. W. Bullard, and P. S. Hrnjak, An Experiental Investigation of Transcritical Carbon Dioxide Systes for Residential Air Conditioning, ACRC CR18, July Beaver, A., P. Hrnjak, J. Yin, and C. Bullard, "Effects of Distribution in Headers of Microchannel Evaporators on Transcritical CO2 Heat Pup Perforance," Proceedings of the ASME Advanced Energy Systes Division, 2, Orlando, FL, pp Elbel, S. and P. Hrnjak, 24, "Flash Gas Bypass for Iproving the Perforance of Transcritical R744 Systes that Use Microchannel Evaporators," International Journal of Refrigeration, 27:7, , 12 pp. ACKNOWLEDGEMENT The authors thankfully acknowledge the support provided by the Air Conditioning and Refrigeration Center at the University of Illinois at Urbana-Chapaign. 16 th International Refrigeration and Air Conditioning Conference at Purdue, July 11-14, 216