OR-94-02-2 — Two-Phase Flow of HFC-134a and CFC-12 Through Short-Tube Orifices

Short-tube orifices have been used as an expansion device on automotive and residential air conditioners in the United States. The design for optimum performance of these systems requires predicting correct flow characteristics through short tubes for a given set of operating conditions. Insufficient data are available on how any of the new replacement refrigerants will perform in short-tube orifices relative to conventional refrigerants. Compares experimental mass flow results with HFC-134a and CFC-12 for a variety of inlet conditions and short-tube geometries. Five sharp-edged short-tube orifices with 7 less than L/D less than 20 and 1.10mm (0.0435 in.) less than D less than 1.72mm (0.0676 in.) were tested with HFC-134a and CFC-12. Both two-phase and subcooled liquid flow conditions entering the short tube were examined for condensing temperatures ranging from 35.4 deg C (96 deg F) to 53.8 deg C (129 deg F), for subcooling as high as 13.9 deg C (25 deg F), and for qualities as high as 10% at the inlet of the short tube. The effects of downstream pressure were also investigated by lowering the downstream pressure from the upstream saturation pressure down to 310 kPa (45 psia). For HFC-134a, two types of measurements were made during this study – mass flow tests and pressure distribution inside the orifice. For CFC-12, only mass flow tests were performed. The results with CFC-12 were compared with those of HFC-134a with the same upstream temperature conditions. A comparison of these two refrigerants was made as a function of the main operating variables (upstream pressure, subcooling, and downstream pressure) and short-tube geometry. Semi-empirical models for both HFC-134a and CFC-12 were developed for the prediction of mass flow rate through short tubes.

KEYWORDS: replacing, R134a, R12, refrigerants, tubes, orifices, expansion valves, motor cars, domestic, heat pumps, unit air conditioners, designing, optimisation, performance, calculating, fluid flow, properties, comparing, experiment, geometry, subcooling, condensation temperature, pressure, measuring, pressure difference