Mass measurement based calibration of a capacitive sensor to measure void fraction for R134a in smooth tubes

This paper presents a new capacitive sensor built and used to measure void fraction of both horizontal and vertical upward flow in the low mass flux range in circular tubes with an inner diameter of 7 mm. Three sensors with different axial lengths (D, 2D/3, and D/2) are built and evaluated to examine the possibility of utilizing shorter sensors in applications with space limitations. Results show all three sensors have the capability to measure void fraction. Due to the nonlinear relation between capacitive signals and void fraction, a calibration procedure based on mass measurement (quick-closing valve technique, QCV) is proposed. After the calibration procedure, most void fraction data measured by the sensor falls into the ±15% deviations of the experimental results by QCV for horizontal flow and ±10% for vertical upward flow. Sensors with the same design can be utilized directly to measure void fraction for the similar test conditions in the future studies.