Design and calibration of capacitive sensors for measuring void fraction in vertical headers of microchannel heat exchangers

This paper presents new capacitive sensors built and calibrated to measure cross-sectional void fraction between tubes in vertical headers of microchannel heat exchangers (MCHEs). Eleven individual sensors are assembled into one test header to measure local capacitance independently in real-time. A calibration procedure based on visualization, pressure drop, signal patterns, and mass measurement (quick-closing valve technique, QCV) is proposed. After the calibration, most void fraction data predicted by all eleven sensors fall into ±10% deviations of the experimental results by QCV. The sensors are ready to be utilized to measure local void fraction along vertical inlet or intermediate headers of MCHEs in future studies.