1、AN-04-1 -2 Flow Distribution Issues in Parallel Flow Heat Exchangers Pega S. Hrnjak, Ph.D. Member ASHRAE ABSTRACT This paper discusses issues of single- and two-phase flow distribution in manifolds of heat exchangers:flow in the tube after the expansion valve, flow regimes in the manifold, and press
2、ure drop in the heat exchanger channels. A few typical options for reasonable distribution are shown. INTRODUCTION Heat exchangers with parallel flow are attracting signifi- cant attention lately. Most new designs are being made in plate or microchannel forms. Figure 1 presents two typical plate hea
3、t exchangers: (a) for liquids and (b) for air. Those mentioned first are typically made in a single-pass version, while the second have generally more than four passes. Heat exchangers with microchannels (hydraulic diameter less than 1 mm) are experiencing even more rapid design changes than the com
4、monly used round tube plate fin designs. Those that use channels in the range of 0.5 to 1 .O mm are rela- tively common in mostly mobile applications due to compact- ness and performance. One example of a microchannel heat exchanger for carbon dioxide is shown in the Figure 2. Figure 3 presents two
5、heat exchangers of the same type as shown in Figure 2. One is open to display round microchannel ports. The header shown in Figure 3 has an unusually small diameter to be able to serve high operating pressures with a relatively thin wall. The header representative for those work- ing fluids that ope
6、rate under pressures less than 4 MPa is shown in Figure 4. Figure 5 presents a different type of header for a multipass serpentine arrangement. Interest in microchannel heat exchangers (MCHE) is strong also due to possible charge reduction that could be important for some fluids (such as hydrocarbon
7、s, ammonia, and even CO, to some extent) and some applications (in densely populated areas). With the reduction of internal diam- eter, there is an increased need for a larger number of parallel channels to control pressure drop at an acceptable level and for realistic capacities. Low pressure drop
8、is needed to maintain acceptable values of coefficients of performance (COP) for the entire system. Since the number of parallel channels typically goes over several hundred, conventional techniques of two- phase refrigerant distribution techniques are not feasible. Upcoming sections will describe s
9、ome issues of single- and two-phase distribution in the headers of parallel flow heat Figure I Plate heat exchangers: (a) liquidirefrigerant on the left and (b) plate evaporator on the right. P.S. Hrnjak is research professor in the Department of Mechanical and Industrial Engineering and Co-director
10、 at the Air Conditioning and Refrigeration Center, University of Illinois at Urbana-Champaign, Urbana, Ill. 02004 ASHRAE. 301 Figure 2 Aluminum microchannel heat exchanger for Figure 3 Headers of microchannel heat exchanger for a higher pressures (up to 15 MPa) used in high-pressure transcritical CO
11、, system. Open transcritical CO2 systems. header shows round microchannel ports nominally 0.8 mm. Figure 4 A typical cylindrical header for microchannel heat exchangers that operates at lowerpressures. Figure 5 Earlyprototype of a short header with radial tube inlet for multipass serpentine heat exc
12、hangers. exchangers: the effect of orientation of the inlet and outlet in the headers and two ways to improve the distribution of two- phase flow. These two distributor approaches try to cope with different inertial forces of vapor and liquid by either (1) homogenizing the flow or (2) separating it
13、and distributing each phase separately into the heat exchanger passage. These approaches are technically acceptable for a limited number of parallel channels (or circuits). When the number of channels is typically greater than 30, these distributors are less reason- able and the situation in the hea
14、der is more complex. Some elements of the two-phase flow in the header with associated distribution are described in the upcoming section. through each path. Nevertheless, it will be shown in the following paragraphs that even that task is demanding and cannot be realized perfectly. The pressure dro
15、p situation in the microchannel heat exchanger shown in Figure 2 is presented in Figure 6. The expectation that placing the inlet and the exit on opposite sides will provide a uniform distribution is not completely correct. Assuming that the microchannels and the heat exchanger are manufactured well
16、 (channels have equal hydraulic diam- eters and all of them are open), the problem arises from the variable flow rate through the header. That variable flow rate causes a varying pressure drop along the header, which affects distribution. The other element affecting pressure drop along OME ELEMENTS
17、IMPRoVING DISTRIBUTIoN Single Phase the inlet and outlet header is the difference in gas density due to pressure drop in the channels. Figure 6 presents the Distribution of single-phase fluid among parallel chan- nels is typically realized by ensuring equal total pressure drops elements for a model
18、of a real heat exchanger (shown in Figure 2) that was experimentally examined in detail with nitrogen 302 ASHRAE Transactions: Symposia gas and operating with R-744 and well modeled. Details can be found in Yin et al. (2002). Symbols are maintained as in the paper and are cited without detailed expl
19、anation. They indi- cate geometry and pressure loss coefficients. Model results for distribution of pressures in the headers and flow rates through the channels are shown in Figure 7- for the case of inlet and outlet location at the opposite sides in Figure 7a and for the case of inlet and outlet lo
20、cation on the same side in Figure 7b. This is the case of single-phase exper- iments with nitrogen flowing through at a given flow rate. There is greater friction pressure drop than deceleration pres- sure gain. Please note the difference in the pressure drop gradi- ent along the inlet and the outle
21、t header. The reason for this a) inlet and exit on opposite sides Figure 6 Figure 7 -320 . b) inlet and exit on the same sides 320 difference is the lower absolute pressure in the outlet header and, consequently, lower fluid density, which increased veloc- ity. Comparing two pictures, one could see
22、that even though pressure drops in headers are not much different, the mass flow maldistribution is significantly greater in the case of same side location. To provide more uniform distribution, some solutions are based on the branching principle, as shown in Figure 8. That approach is reasonable wh
23、en space and cost allow. The concept is based on equalization of pressure drop. Neverthe- less that approach does not ensure good distribution for the case of two-phase flow because it does not ensure equal divi- sion of the flow at each branch. Even small imperfection in orientation may have signif
24、icant effect on distribution. Two Phase In the case of distribution of two-phase mixture in the headers (manifolds), the situation is more complex due to differences in the thermophysical properties of each phase, density, viscosity, and surface tension in particular. These affect the intensity of t
25、he inertial, gravitational, shear, and capillary forces on each ofthe phases. Consequently, the inter- action of these forces when changing direction in the header determines the vapor fraction in each channel of the heat exchanger. A ypical approach used in the industry for distributing two-phase f
26、low to evaporator circuits is presented in Figure 9. It is based on the principle of single-phase flow, generating ;I -I- yp“ 1 I/ /d f 4 kDet / kloss,ehd cexp rout Let I / Exit header I 1 I I Exit tube I I I I Multichannel heat exchanger in single-phase flow. Figure 8 Branching header: Distribution
27、 ofpressures along the headers und trend in massflow rates per channel mr in the microchannel heut exchanger shown in Figures 2 and 6 for two inlet and exit orientations. ASHRAE Transactions: Symposia 303 View A J+, jet with front pooling; J+rp, jet with rear pooling; J, jet; MF, misty. The lines ar
28、e given for two diameters of inlet pipes: 6.4 and 9.5 mm. Flow of each phase was measured independently with emphasis on liquid flow rate. Standard deviation from the average value of liquid flow rates through each pipe STD was used as one of the measures of uniformity. The best distribu- tion regio
29、n indicated with the lowest value of the standard deviation (STD 0.05) is presented by a small circle in the chart. Next good distribution regimes are shown in the solid ellipse and then dotted ellipse. SUMMARY AND CONCLUSIONS This paper presented three types of heat exchangers with parallel flow ch
30、annels in which performance could be signif- icantly affected (up to 20% reduction of system COP Beaver et al. 20001) by imperfect distribution. Some major distribution situations and approaches are presented for single-phase (opposite location of inlet and outlet) and two-phase flow (distribution i
31、n homogenous zone and separation of the phases with separate distribution). For a horizontal header and downward flow in branches, five flow regimes are identified and a new two-phase flow map is presented. Zones of good distribution for the case of a hori- zontal downward flow header are indicated.
32、 REFERENCES Beaver, A., P.S. Hmjak, J. Yin, and C.W. Bullard. 2000. Effects of distribution in headers of micro-channel evap- orators on transcritical COz heat pump performance. O 0.05 0.1 0.15 0.2 025 0.3 035 0.4 0.45 Vyror qudity Figure 12 Flow regimes in horizontal header with indication of areas
33、 with diflerent levels of distribution (low value of standard deviation STD represents good distribution). Mass fluxes are based on inner diameter of inlet pipe. ASH RAE Transactions: Symposia 305 Proceedings of the ASME Advanced Energy Systems Division, Orlando, FL, AES-Vol. 40, pp. 55-64. Yin, J.M
34、., C.W. Bullard, and P.S. Hrnjak. 2002. Single phase pressure drop measurements in microchannel heat exchanger. Heat Transfer Engineering 23(4): 3-1 2. BIBLIOGRAPHY Bernoux, P., P. Mercier, and M. Lebouche. 2001. Two phase flow distribution in a compact heat exchanger. Proc. of the Third Internation
35、al Conference on Compact Heat Exchangers, Davos. Fei, P., D. Cantrak, and P. Hrnjak. 2002. Refrigerant distribu- tion in the inlet header of plate evaporators. SAE paper 2002-01 -0948, World Congress 2002. Philpott, M.L., M.A. Shannon, N.R. Miller, C.W. Bullard, D.J. Beebe, A.M. Jacobi, P.S. Hrnjak,
36、 T. Saif, N. Aluni, H. Sehitoglu, A. Rockett, and J. Economy. 1999. Inte- grated mesoscopic cooler circuits (IMCCS). Interna- tional Mechanical Engineering Congress and Exposition, Nashville, TN, Nov. 15-20. Rong, X.Y., M. Kawaji, and J.G. Burgers. 1996. Gas-liquid and flow rate distributions in sin
37、gle end tank evaporator plates. SAE Technical Paper Series, pp. 133-141. Tae, S-J., and K. Cho. 2002. Two-phase flow split and pres- sure drop of R-22 in branch tubes. 9th International Refrigeration and Air Conditioning Conference at Pur- due, July 15-1 9, West Lafayette, Ind. Tompkins, D.M., T. Yo
38、o, P. Hrnjak, T. Newell, and K. Cho. 2002. Flow distribution and pressure drop in microchan- ne1 manifolds. 9th International Refrigeration and Air Conditioning Conference at Purdue, July 15- 19, West Lafayette, Ind. Vist, S., and J. Petersen. 2002. Two-phase flow distribution in compact heat exchanger manifolds. Compact Heat Exchangers Symposium, Grenoble. 306 ASHRAE Transactions: Symposia
