A SEMIEMPIRICAL MODEL OF 2-PHASE FLOW OF REFRIGERANT-134A THROUGH SHORT TUBE ORIFICES

Measurements were conducted on Refrigerant-134a flowing through short tube orifices with length-to-diameter (L/D) ratios ranging from 5 to 2 0. Both two-phase and subcooled liquid flow conditions entering the sh ort tube were examined for upstream pressures ranging from 896 to 1448 kPa and for qualities as high as 10% and subcoolings as high as 13.9 degrees C. Data were analyzed as a function of the main operating vari ables and tube geometry. Semi-empirical models for both single- and tw o-phase flow at the inlet of the short tubes were developed to predict the mass flow of Refrigerant-134a through short tube orifices. Choked flow conditions for Refrigerant-134a were typically established when downstream pressures were reduced below the saturation pressure corres ponding to the inlet temperature. The flow rate strongly depended on t he upstream pressure and upstream subcooling/quality. The mass flow al so depended on cross-sectional area and short tube length. The mass fl ow model utilized a modified orifice equation that formulated the mass flow as a function of normalized operating variables and short tube g eometry. For a two-phase flow entering the short tube, the modified or ifice equation was corrected using a theoretically derived expression that related the liquid portion of the mass how under two-phase condit ions to a flow that would occur if the flow were a single-phase liquid . It was found that for sharp-edged short tubes with single- and two-p hase flow, approximately 95% of the measured data and model's predicti on were within +/-5% of each other.