Signal analysis and mass measurement based calibration of capacitive sensors for void fraction and flow regimes in horizontal and vertical round tubes using R134a

Flow patterns and void fraction are crucial descriptors of two-phase flows. This paper presents flow pattern identification and void fraction measurement with a non-intrusive method by a new capacitive sensor. In addition, this paper presents characterization of flow patterns with the capacitive sensor, correlated to visualization results obtained by a high-speed camera in horizontal and vertical round tubes (D=7mm) with R134a in developed adiabatic conditions. The sensor can characterize flow patterns observed within the test conditions. The sensor was calibrated based on mass measurements. After this calibration, the capacitive sensor can be directly used to measure void fraction in further studies. Three different axial lengths of electrodes in the sensor are built to explore the potential of application of this sensor in various HVAC&R systems. The comparison of signals from each of them shows that the signals are independent of axial length in the range explored.