Investigation on Refrigerant Distribution in Evaporator Manifolds
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To provide essential design information of microchannel evaporators, an experimental study was conducted on the effects of geometry, operating conditions and fluid properties on the distribution of refrigerant and pressure drop in horizontal heat exchanger manifolds. An experimental facility with a visualization section for mimicking a real heat exchanger manifold geometry was developed. Under realistic operating conditions, measurements of refrigerant distribution were conducted by measuring mass flow rates and vapor quality of all branch tube groups (individual adjacent heat exchanger tubes were grouped in groups) for a total of 60 test cases. The flow direction within the heat exchangers is vertically upward. Stratified flow is observed for the end inlet case of the dividing manifold due to the gravitational effect. The liquid level increases along the dividing manifold because the liquid is traveling farther than the vapor due to inertia difference. Near the end of the manifold, the liquid level is almost constant. For the side inlet case, it is observed that the incoming refrigerant impinges on the inner side wall of the manifold, and is divided symmetrically near the inlet, and the interface between the vapor and the liquid has a V-shaped form near the inlet. Based on the measurements, it is observed that for the end inlet case, the profile of the branch tube inlet vapor quality is of a "stepwise" shape. There exist two almost constant value regions, one of about 100% vapor quality near the inlet and the remainder of about 12% vapor quality with a very short transition region. For the end inlet case, as the manifold inlet mass flow rate increases, the number of branch tube groups having almost 100% tube inlet vapor quality increases also because the vapor-liquid interface is moving farther towards the end of the manifold due to the increased momentum. However, for the side inlet case, there is no such region having 100% branch tube inlet vapor quality. For the side inlet case, the profile of the branch tube inlet vapor quality is symmetric. Near the inlet, the branch tube inlet vapor quality is about 60 ~ 70%, and near the end of the manifold, the branch tube inlet vapor quality is about 20%. In between two regions, the branch tube inlet vapor quality decreases monotonously along the manifold. The flow distribution is strongly affected by the manifold inlet location and/or manifold inlet geometry and manifold inlet vapor mass flux. Correlations are proposed using the T-junction concept in a modified form from Watanabe et al.'s method (1995). For R-410A and R-134a tests with both inlet cases, 90% of measured vapor inlet quality data and 90% of measured liquid fraction of taken off data are within predicted values ± 0.1. To investigate the effect of refrigerant maldistribution on the performance of the tested heat exchangers, heat exchanger simulations were conducted. Based on the heat exchanger simulation results using test results for the refrigerant distribution, for the side inlet case, the capacity degradation based on the uniform distribution at the tested inlet manifold mass flow rates (at 30, 45 and 60 g/s) is 5 ~ 8%. For the end inlet case, as the inlet manifold mass flow rate increases, the capacity degradation based on the uniform distribution ranges from 4% to 15% as a function of the manifold inlet mass flow rate. Therefore, the side inlet is preferred for a wide range of mass flow rates compared to the end inlet.