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...design information the minichannel evaporators. Flow visualization in the horizontal manifold showed that the flow patterns of the two-phase refrigerant before and after the liquid-vapor transition are in stratified flow for the end-inlet location and in bubbly and stratified flows for the side-inlet location. Test results showed that the normalized standard deviation of the liquid mass flow rate, which indicates the degree of maldistribution, changes from 0.088 to 0.263 with manifold inlet location changes, from 0.034 to 0.141 with manifold inlet mass flow rate changes, from 0.037 to 0.082 with tube number changes, and from 0.027 to 0.055 with tube pitch changes. The normalized standard deviation of the liquid mass flow rates showed that the liquid refrigerant flow distribution is strongly affected by the manifold inlet location and the manifold inlet mass flow rate but is primarily independent of tube pitch. The side-inlet location showed a better liquid refrigerant flow distribution than the end-inlet location by more effectively mixing of the liquid and the vapor refrigerant from the inlet. Therefore, the side-inlet location is preferred to the end-inlet location.
INTRODUCTION
The two basic demands for air-conditioning and refrigeration systems are improving the efficiency and reducing the size of the equipment. Compact heat exchangers, especially aluminumbrazed heat exchangers, meet these demands very well. As manufacturing technology in aluminum-brazed compact heat exchangers advances, the hydraulic diameter of the tube channel has been reduced to a size of less than 1 mm. The reduced cross-sectional area of each tube requires many parallel tubes to keep the pressure drop across the heat exchangers within a reasonable range. Having many parallel tubes leads to refrigerant maldistribution issues when the minichannel heat exchanger is used as an evaporator. Refrigerant flow maldistribution is defined as a nonuniform distribution of the liquid flow rates. Since refrigerant maldistribution causes an overall deterioration of the heat exchanger performance, it is essential to understand the optimal design parameters and operating conditions.
Literature Review
Many researchers experimentally and numerically investigated this issue of refrigerant maldistribution in the evaporator for conventional and minichannel heat exchangers, as summarized in Table 1. Several researchers investigated two-phase distribution in manifolds with branch tubes (Horiki and Osakabe 1999; Kariyasaki et al. 1995; Rong et al. 1995; Zietlow et al. 2002; Tompkins et al. 2002; Lee and Lee 2005; Webb and Chung 2005; Watanabe et al. 1995a, 1995b; Asoh et al. 1991; Vist and Pettersen 2004; Fei et al. 2002; Sa et al. 2003; Cho et al. 2002; Vist 2004), most of which were conducted with air/water or low vapor pressure fluorocarbons. Only a few works were conducted with high-pressure working fluids such as R-22 and [CO.sub.2] (Sa et al. 2003; Cho et al. 2002; Vist 2004). It has been reported that gravity is an important force affecting the two-phase distribution in horizontal manifolds with vertical branch tubes. In a manifold with upward branch tubes, vapor flows predominantly through the branch tubes near the inlet, while liquid flows through the branch tubes near the far end of the manifold. Although many authors reported experimental measurements for the two-phase refrigerant distribution in the manifold, there are few works published regarding experimental measurements under realistic operating conditions, with realistic geometry and with R-410A as a working fluid. The objective of the current study is to provide essential design information through an experimental study of the effects of the geometry of the heat exchangers and operating conditions on the distribution of refrigerant R-410A in heat exchanger manifolds.
Table 1. Summary of Previous...
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