Capacitive Void Fraction Measurements and Probabilistic Flow Pattern Mapping of Horizontal Refrigerant Flow

A capacitive void fraction sensor was developed to study the dynamic behavior of refrigerant two-phase flows in horizontal macro-scale smooth tubes of evaporators used in air-conditioning and heat pump applications. A first prototype was successfully tested with air-water flow. The dielectric constant of liquid HFC refrigerants however is much smaller than the one of water, which results in much smaller capacitances to be measured. Therefore an improved version of the capacitance transducer was developed to make dynamic measurements of these HFC fluids possible. The sensor signal can be used for flow pattern detection and void fraction measurements of high pressure HFCs. A macro-scale test facility for two-phase flow and heat transfer studies of high pressure HFCs was designed and constructed. The adiabatic test section consists of a sight glass with high speed camera and the capacitive void fraction sensor. Two datasets of sensor signals were gathered using R410A and R134a respectively (G = 200–500kg/m²s and x from 0 to 1 in steps of 0.025 at T_sat = 15°C). To find more objectivity in flow pattern mapping, a signal analysis was performed. The average, the variance and a high frequency parameter were found suitable for flow regime classification into slug flow, intermittent flow and annular flow by using the fuzzy c-means clustering algorithm. A probabilistic flow pattern map is presented for the HFC data. These maps clearly quantify the width of the transition zones and can be applied for probabilistic heat transfer and/or pressure drop modeling.