Capacitance sensor is a non-intrusive devices used for the void fraction measurement in two-phase flows. Capacitance sensor is formed by two electrodes placed at the outside walls of a dielectric pipe, through which a two-phase medium flows. The capacitance of the sensor depends on the geometric arrangement of electrodes and distribution of the electrical field, which depends on the electrical permittivity of the medium around the electrodes and in the pipe. For a two-phase gas–liquid​ flow, the effective permittivity of dielectric between the electrodes varies due to the variation in the percentage of gas and liquid in the pipe. In this paper, the variation of sensor capacitance were determined numerically for the annular-flow pattern. The capacitance was calculated by the integration of electric field distribution around two symmetrical concave electrodes in the form of cylinder fragments using the Gauss law. This shape of electrodes was chosen because the sensitivity of this type of sensor is the highest, and is the most often used in practice. The results of numerical simulations were compared with experimental results. The capacitance changes due to void fraction variation were determined experimentally by the method of frequency deviation. It was noticed that regardless of complex geometry of the electric field between two concave electrodes, the electric field lines within the pipe walls do not deviate much from the parallel distribution. From the simulations for various arc lengths of the electrodes resulted that the changes of relative capacitance of the sensor are almost the same for all arc lengths, although the absolute value of capacitance of the sensor depends on this arc length of the electrodes. The numerical results on electric field distribution for annular flow also indicated that the model of two parallel capacitances, representing the liquid and gas phases, usually assumed in various papers as the most consistent with experimental results, can be used for the representation of two separate phases in two-phase flows.

电容传感器是一种非侵入式设备，用于测量两相流中的空隙率。电容传感器由放置在介电管外壁的两个电极形成，两相介质流过该两个电极。传感器的电容取决于电极的几何布置和电场分布，电场的分布取决于电极周围和管道中介质的介电常数。对于两相气液流动，由于管道中气体和液体百分比的变化，电极之间的有效介电常数会发生变化。在本文中，通过数值确定了环形流模式下传感器电容的变化。使用高斯定律，通过将两个对称的凹电极周围呈圆柱体碎片形式的电场分布积分来计算电容。选择这种形状的电极是因为这种类型的传感器的灵敏度最高，并且在实践中最常用。将数值模拟的结果与实验结果进行了比较。通过频率偏差的方法，实验确定了由于空隙率变化而引起的电容变化。注意到，不管两个凹电极之间的电场的复杂几何形状如何，管壁内的电场线与平行分布的偏差都不大。从电极的各种电弧长度的模拟得出的结果是，尽管传感器的电容的绝对值取决于电极的该电弧长度，但是传感器的相对电容的变化对于所有电弧长度几乎都是相同的。环形流动电场分布的数值结果还表明，代表液相和气相的两个并联电容模型通常在各种论文中被认为与实验结果最为吻合，可以用来表示两个独立的相在两相流中。