An experimental study of two-phase flow of HFC-134a through short tube orifices

With the phase out of conventional CFC and HCFC refrigerants, air conditioning and heat pump systems will have to be redesigned to accommodate new replacement refrigerants. An important component in these systems is the expansion device. Short tube orifices are a common expansion device used on residential sized air conditioners and heat pumps in the United States. The design for optimum performance of these systems requires predicting correct flow characteristics through short tube orifices for a given set of operating conditions. Very little data are available on how these new replacement refrigerants will perform in short tube orifices relative to conventional refrigerants. This paper presents the results of an experimental study to evaluate the flow characteristics of HFC-134a flowing through short tube orifices in heat pump systems. Three types of measurements were made during this study: mass flow tests, pressure distribution inside the orifice, and flow visualization. Measured data were analyzed and discussed as a function of operating variables and short tube geometry. In general, the variables (upstream pressure, subcooling, and quality, and orifice diameter and length) affecting two-phase flow through orifices with HFC-134a were the same as those found with HCFC- 22. For operating saturation temperatures upstream of the orifices of 45.4°C, the mass flow produced with HFC-134a varied from 16 to 23 percent less than that for HCFC-22 under choked flow conditions. Choked flow conditions for HFC-134a were typically established when downstream pressures were reduced below the saturation pressure corresponding to the inlet temperature. The flow rate was strongly dependent on the upstream pressure and upstream subcooling/quality, but it was insensitive to downstream flow conditions. As the inlet quality increased from 0 percent to 5 percent, a sharp drop in flow was measured due to a large increase in void fraction at the entrance of the short tube. Pressure distribution data indicated the presence of a vena contracta and metastable flow near the orifice entrance.