This study first presents flow regimes and void fraction in horizontal and vertical tubes with R134a in the adiabatic condition and low mass flux range. Visualization results of horizontal flow patterns are compared to Wojtan-Ursenbacher-Thome flow regime map and have a good agreement. Both void fraction results for horizontal and vertical tubes are compared to some widely used correlations. Influences of tube orientation and mass flux on void fraction are discussed. At the same vapor quality condition, void fraction of horizontal tubes is larger than that of vertical tubes. Higher mass flux also results in larger void fraction compared that of lower mass flux.
A new capacitive sensor aimed to measure void fraction is designed and built. With this capacitive sensor, characterizing flow patterns appear in low mass flux range and measuring void fraction for horizontal and vertical tubes can be achieved. Signal characterization for each flow pattern is proposed. To apply this sensor to more applications in HVAC&R systems, electrodes with different axial lengths are built together to be compared. The comparison of signals from them show the signals are independent on axial length. Due to the non-uniformity of the electric field between the electrodes of the sensor, a calibration procedure of void fraction measurement must be finished before utilizations. An experiment based (quick closing valves (QCV)) calibration procedure is conducted. Two calibrated curves for measuring void fraction in horizontal and vertical tubes are proposed. With calibrated curves, sensors with similar configurations can be directly utilized to measure void fraction in further studies
