Development of All Metal Multi-plate-type Capacitive Void Fraction Sensor for gas-liquid two-phase flow

Abstract

In the aerospace field, liquid hydrogen and oxygen which are “cryogenic fluid” are used as the propellant for rockets and hypersonic engines. These fluids become easily a gas-liquid two-phase flow in pipes and fluid machines. We have developed small capacitive void fraction sensors which can measure in real time, which is used to control the flow rate of cryogenic fuel and to understand typical two-phase flow phenomena such as the cavitation. The conventional capacitive sensors have a structure in which two electrodes sandwich a pipe made of dielectric material. However, the dielectric material has problems such as insufficient strength under high pressure condition and decreasing measurement accuracy due to changes in dielectric constant with temperature change. We developed a new capacitive sensor which consists of only metal parts to solve these problems. On the other hand, it may cause a decrease in measurement accuracy due to the structural limitation. This paper presents the results of the electric field analysis and the static fluid experiment using silicon oil and air to verify the accuracy of the sensor. The mean errors obtained by the analysis and the experiment are 3.47 % and 5.32 %, respectively, which almost satisfy the required specification. However, it was found that the accuracy is poor in some void fraction ranges, and the sensor needs to be improved.