Evaluating The Performance Of Refrigerant Flow Distributors

CFD was applied to evaluate the performance of both existing and improved designs for refrigerant distributors. In general, it is better to utilize a spherical base as compared with other shapes and to locate the orifice close to distributor base. These changes tend to improve the robustness of the distributor in terms of providing even flow and phase distribution in different outlet branches when the orifice and/or distributor are not oriented optimally. In addition, experiments were performed that tend to validate the general trends associated with the CFD results. Introduction Two-phase refrigerant is generally distributed to individual circuits within the evaporator of a vapor compression air conditioner or refrigerator. Often times, an orifice is integrated into a distributor housing to provide a low cost method for expansion and refrigerant distribution. Ideally, the mass flow rates and qualities of the refrigerant exiting the different branches of the distributor should be equal in order to obtain the best performance for the evaporator and the system as a whole. However, this is generally not the case. Very little has been published regarding the analysis of refrigerant flow distributors. Generally, refrigerant flow distributors are designed using a trial-and-error process. Nakayama (2000) did experimental investigations involving a refrigerant distributor and proposed a new distributor that was claimed to have better performance with respect to flow distribution. The improved design utilized a capillary mixing space rather than an orifice. This design resulted in much more even flow rates for the individual branches within the refrigerant distributor. In a companion paper, Li et. al (2002) demonstrated that commercially available CFD tools can be used to analyze the phase distribution and separation phenomena in refrigerant distributors. The current paper describes results of the application of CFD to evaluate the performance of both existing and improved designs for refrigerant distributors. In addition, experiments were performed to validate the general trends associated with the CFD simulations. Distributor Geometries and Performance Criteria Table 1 and Figures 1 – 5 describe geometries that were considered in this study. The figures are drawn to scale and show the internal volumes where refrigerant flows. Each of the distributors has four branches and an orifice that is located at the centerline of the inlet to the distributor. The Type 1 and Type 3 geometries are commercially available, while the other 3 incorporate design modifications. In the Type 1 geometry, the base of the distributor is convex with respect to the flow and comes to a point. This design was conceived so as to provide a single point of contact for separation of the flow. The Type 3 design uses a cone-shaped base that is concave with respect to the flow and was conceived so as to provide a mixing chamber for distribution of refrigerant. Type 2 and 4 are the same as Type 3, except the cone is replaced with flat and spherical surfaces, respectively. The Type 5 design is a modification of Type 4 where the orifice has been moved closer to the distributor base and the depth of the chamber has been reduced. For all of the distributors, the center of the base is on the centerline of the distributor along with the center of the orifice. Results are presented in terms of uneven flow and quality distribution performance indices. For a given branch i, the uneven flow and quality indices are m m mi i m & & & − = , ε (1) x x xi i x − = , ε (2) where εm,i and εx,i are the indices for uneven flow and quality, i m& and i x are individual branch flow rates and qualities, and m& and x are average mass flow rates and qualities for all four branches. For a given distributor, average performance indices for uneven flow and quality are defined as ( ) m m m i i m & & & ∑ = − = 4