Flow Mal-Distribution In Micro-channel Evaporator (2002)

The present study experimentally investigated flow mal-distribution and phase separation in micro-channel tubes. The experimental apparatus consisted of a test section with micro-channel tubes, a refrigerant pump, a mass flow meter, a pre-heater, 15 gas-liquid separators, 5 sets of liquid and gas flow meters, and a plate heat exchanger. The test section made of a header pipe and 15 micro-channel tubes. Each micro-channel tube brazed to the header pipe had 8 rectangular ports with the hydraulic diameter of 1.24mm. Flow mal-distribution for 15 micro-channel tubes got improved by changing the flow direction to the inlet header from parallel to in-line flow condition. Mass flow rate fraction showed the maximum at channel number 1 under both parallel and in-line flow conditions. The inlet quality didn’t affect much on the flow distribution under the parallel and in-line flow conditions. Phase separations under the in-line flow condition differed less than those under the parallel flow condition. The difference of phase separation ratios for 15 micro-channel tubes decreased as the inlet quality increased. INTRODUCTION The fin-tube heat exchangers were widely applied for the residential air-conditioner. With the increase in the use of micro-channel heat exchangers for industrial and commercial applications because of their small size, weight and refrigerant charge, Bae and Han[1] applied micro-channel heat exchanger for the residential airconditioner. Residential air-conditioners with micro-channel condenser have been used in the market. Microchannel evaporator, however, is not to be utilized for residential application. There are two major reasons for its limitation. The first one is the condensate problem inside the louvered fin. It prohibits air from passing through the heat exchanger, and then severely deteriorates thermal performance. The second one is the flow mal-distribution problem in micro-channel tubes. Due to the flow mal-distribution and phase separation in micro-channel tubes, part of the evaporator is filled with superheated vapor and then thermal performance is decreased very much. Even though flow mal-distribution and phase separation problems are important, a few studies have been done for simple T type or Y type junctions by using air-water mixture[2,3] or steam-water mixture[4-6]. To author’s knowledge, very few study by using refrigerants has been reported so far. Author’s previous study[7] reported the flow distribution and phase separation in T-junction by using R22. Two-phase flow distribution and phase separation depend on lots of geometrical and dynamic parameters. Geometrical parameters are header diameter, header orientation, micro-channel tube diameter, tube pitch, tube length, etc. Dynamic parameters are inlet quality, mass flux, thermal boundary condition, etc. The study on the flow distribution and phase separation contains the complexity of test including selection of geometric and dynamic parameters, pressure control of the system, setting of system balance, proper selection of gas-liquid separator, two-phase flow rate measurement, etc. Asoh et al.[8] and Kariyasaki et al.[9] studied the flow distribution through the multi-pass branch tube with different direction of branch tube. Watanabe et al.[10] and Rong et al.[11] reported the flow distribution in the header of serpentine and stacked plate heat exchanger, respectively. Rong et al.[11] specifically investigated the effects of the header orientation on the flow distribution. Recently visualization in the inlet header of plate type evaporator was investigated by Fei et al.[12]. They reported effect of flow rate and quality on the flow distribution for different flow regimes. Oh and Lee[13] showed the simulation results on the effect of inlet angle, shape and location of refrigerant flow and height of protruding flat tube in the inlet header of parallel flow heat exchanger for single phase flow. Beaver et al.[14] presented experimental results on the effect of flow mal-distribution in micro-channel evaporators using the CO2 for heat pump application. Even though the flow mal-distribution and phase separation problems in the evaporator are very serious comparing with the condenser application, they were not studied systematically by considering geometry and dynamic parameters for optimal design of the microchannel evaporator as mentioned before. Thus, they have to be systematically investigated. Some studies on the flow mal-distribution and phase separation for refrigerant system are going on in a couple of research labs. As one of them, the present study investigated the influence of the inlet flow direction and inlet quality on the flow maldistribution and phase separation in the manifold of the micro-channel evaporator. EXPERIMENTAL APPARATUS AND PROCEDURE Figure 1 shows the schematic diagram of the experimental apparatus for the present study. The experimental apparatus consists of a test section of micro-channel evaporator, a refrigerant pump, a mass flow meter, a preheater, 15 gas-liquid separators, 5 sets of liquid and gas volumetric flow meters, and a plate heat exchanger. The test section consisted of an aluminum header pipe with outer diameter of 22mm and inner diameter of 19.4mm and 15 micro-channel tubes. The details and photographs of the test section are shown in Figure 2. Each microchannel tube had 8 rectangular ports with a hydraulic diameter of 1.24mm, and was brazed to the header. Each micro-channel tube was connected to the copper tube with outer diameter of 6.35mm through the outlet header. The outlet header has separators in it for not mixing two-phase flow in each micro-channel. Then, each tubes were connected to the 15 gas-liquid separators respectively. The volumetric flow rates for the liquid and vapor were measured by rotameters.