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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.

This paper presents the design and building of capacitive sensors for measuring void fraction in headers of microchannel heat exchangers with R134a. The capacitive sensors utilize different electrical properties (dielectric constant) between the liquid and vapor phases to measure the void fraction of two-phase flow. The test section (a header) is made of 3D printed partitions with a total length of 185.8 mm and an inner diameter of 15.8 mm (D). It has eleven microchannel-tube protrusions with a depth of half inner diameter. All microchannel tubes are capable of being connected to real aluminum tube-like real heat exchangers in the future study. Eleven capacitive sensors locate between protrusions. Each sensor has two concave-plate electrodes with the axial length of the half inner diameter (D/2). With these eleven sensors, void fractions along the header can be measured spontaneously. Preliminary validation of the sensors is also presented in this paper.

© 2020 The Society of Powder Technology Japan Hierarchical micro/nano-structured FePO4 (FP) particles were environmental-friendly and controllable synthesized through synergistic intensification of ultrasound and impinging streams. Nano-scale FP seeds were firstly synthesized via ultrasound irradiation intensified impinging stream reaction (UISR), where the enhanced turbulent micromixing in the impinging streams benefits the nucleation process while the application of ultrasound to the impinging streams further intensifies such micromixing. The ultrasound frequency and power applied were 20 kHz and 600 W. The hierarchical micro/nano-structured FP particles are obtained when taking sufficient reaction time and stirring FP nano-seeds in a continuous stirring tank reactor (CSTR). Effects of the sonochemical pre-treatment, mean residence time and rotation speed on the physical-chemical properties of FP were investigated systematically. The characterization revealed that under the optimum operating condition, the FP particles presented near-spherical hierarchical micro/nano-structure, consisting of nanoscale primary grains. The mechano-chemical effects on the synthesized FP particles during different synthesis process were also studied.

This paper presents flow regimes and void fraction in horizontal and vertical round tubes ID 7 mm with R134a in the adiabatic conditions and low mass flux (40-150 kg/m2s for horizontal tubes and 65-115 kg/m2s for vertical tubes) captured by a high-speed camera. Horizontal flow patterns are compared to Wojtan-Ursenbacher-Thome flow regime map and some modifications are proposed. Void fraction results for both 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.

This paper presents void fraction and flow regimes determined by three methods: visualization (high speed camera), mass measurement (quick-closing valves) and a newly developed capacitance sensor. In a way, this is a calibration process for a capacitance sensor. It is shown that new sensor can characterize flow patterns in low mass flux range and measure void fraction for horizontal and vertical tubes. A calibration procedure of void fraction measurement is based on a mass measurement (quick-closing valves). Two calibration curves for measuring void fraction in horizontal and vertical tubes are developed. With calibration curves, sensors with similar configurations can be directly utilized to measure void fraction in further studies.