capacitive sensor, void fraction, headers, microchannel heat exchangers, R134a
This paper presents the calibration procedures of the capacitive sensors for measuring void fraction in headers with R134a. The sensors utilize the difference of dielectric constants between the liquid and vapor phases of the two-phase mixture. The 3D printed header has eleven microchannel-tube protrusions. Eleven capacitive sensors are located between protrusions. All sensors are made to be identical as much as possible in the manual fabrication procedures. Because the electric field within each sensor is not perfectly uniform, sensors need to be calibrated before measuring void fraction. Another transparent 3D printed header with the same inner geometry is installed at the same location of the facility for visualization and pressure drop measurement purposes. By controlling valves, the flow is directed to either header. The calibration is based on three methods: mass measurement (quick-closing valves, QCV), visualization (with a high-speed camera), and capacitive signals obtained from eleven individual sensors. After the calibration procedure, all sensors are ready to measure the void fraction in vertical headers.