Capacitance sensor for measuring void fraction in small channels

The paper presents a method for the measurement of void fraction in two-phase flow of liquid and gas phases in small channels by means of capacitance sensor. It was assumed arbitrarily that the small channels are those of diameter less than 10 mm. The capacitance sensor investigated in this paper consisted of two electrodes mounted on the outer wall of a glass pipe, and an electronic circuit for signal detection. The capacitance of two electrodes depends on the mean electric permittivity of the medium between them. The mean permittivity is a function of permittivity of the channel material and the percentage of both phases of different dielectric constants: liquid and gas. In this paper, the void fraction in two-phase flow was determined from frequency deviation of a high-frequency oscillator, in which the capacitance sensor was connected to a resonant circuit with an inductance in parallel. The advantage of this method over other methods based on the direct capacitance measurement (for example, LCR bridge) is that the intermediate steps for the determination of void fraction from the capacitance changes are omitted, and the void fraction can be directly determined from the calibration curve. Changes in the capacitance of electrodes for various void fractions have been determined numerically from the distribution of electric field in this geometry.