Refrigerant flow regime detection with a capacitance void fraction sensor

Flow regime prediction in air-conditioning units is of great importance for designing the inlet distributor and the pass layout of the evaporators/ condensers. An important parameter of the two-phase flow inside the tubes is the void fraction. Void fractions and flow regimes are often related in flow pattern maps. In order to refine and expand these maps, more experiments have to be performed. At the Ghent University's Applied Thermodynamics and Heat Transfer research group a precise void fraction sensor was developed based on the dielectric constant (capacitance) difference between the gas and fluid phase. Measures were taken to improve the accuracy and reliability of the measurements. An electronic component was developed with a fast response that measures picofarad differences. The sensor is, due to its inherently simple operating principle, both robust and accurate. The sensor was first tested for air-water flow. An analyses of all different flow regimes of the Baker map was made. At the University of Pretoria's Thermoflow Research Group the sensor was mounted in a testrig for refrigerant two-phase flow (R22) to further analyze the different flow regimes. The void fraction signals analysis is used as a way to characterize the flow regimes. In addition, flow patterns were verified using high-speed digital videography.