Research on Applying Capacitance Void Fraction Sensor to a Tapered Tube

Cryogenic fluid such as liquid hydrogen is used as the propellant for rockets. Cryogenic fluids are easy to boil and become gas-liquid two phase flow. Cavitation instability that occurs in cryogenic fluid has been a serious problem in pumps of rockets. However, this phenomenon is not fully understood. Therefore, revealing cavitation instability is an urgent matter. Previous research suggests that cavitation instability is likely to occur in conical diffusers. In this research, application of non-contact capacitive void fraction sensor to a tapered tube was attempted to measure the volume fluctuation of cavitation instability. The sensor was designed based on electric field analysis (EFA). EFA was conducted with two types of electrodes, and two fluid models. The result suggested that parallel type electrode was superior to spread out type electrode. However, low sensitivity of the sensor at the edge of electrode remained as a problem. To confirm the effect of the low sensitivity at the edge, the additional EFA was conducted. According to the results, the accuracy was the poorest when gas phase concentrated at the edge. Furthermore, flow experiment was performed to verify the accuracy of the void fraction sensor. Void fraction was measured in five flow patterns and the measurement was compared with void fraction calculated by using Smith’s equation and Thom’s equation. The result of the experiment corresponded with the void fraction from each equation within the margin of error.