Characterization of capacitance sensor for the measurement of water droplet in gas

Abstract

The measurement of water droplet in gas may play an important role in air-condition, gas delivery under precise instrument environment or other applications. To fulfill the requirement of the detection of water droplet hold-up in gas-liquid two-phase flow, the characterization of capacitance sensor is studied. Two classicalcategories of capacitancesensors, which are capacitance sensors with concave electrode and helical electrode, areinvestigated comparatively by simulation and experiments. The simulation results show that concave capacitance appears better performances in the measurement of water droplet in gas. Droplet near the helical electrode (near the pipe wall) may cause considerably higher capacitance fluctuation than that locates centrally, which would result in high measure error or uncertainty in real applications. On the contract, the concave electrode has less capacitance fluctuation. The spanning angle of concave electrodes is the main factor that influences the sensor sensitivity distribution. To achieve the best angle of concave electrodes, the characterization of concave capacitance sensor is investigated theoretically. The sensitivity distribution is obtained by COMSOL Multiphysics and quantitativelyanalyzed. The sensitivity distribution of concave sensor has the shape of a saddle, high sensitivity in the edge of electrodes and negative sensitivity between the edges of different electrodes. The capacitancesare calculated while water droplet in different positions under different concave electrode angles. By interpolation calculation, the optimal concave capacitance sensor with homogeneous sensitivity distribution is obtained aselectrode spanning angle of 135° and verified by experiments. The test samplesmonitoring water droplet including small rubble bubble with water inside, small wet paper ball, etc., havebeen used to conduct the static experiment. The relationship between the capacitance variation and water droplet hold-up is obtained. The preliminary results show that the designed sensor is able to measure water droplet of 1ml.