Capacitance sensor design for refrigerant two-phase flow characterization

A capacitance sensor configuration has been designed to characterize horizontal two-phase flow of reftigerants, based on the difference in electrical permittivity of the vapour and liquid phase. A concave electrode design with active guarding was chosen. Finite element simulations were made for optimizing the design. The thickness of the dielectric layer separating the two-phase flow from the electrodes is an important design parameter and was investigated in detail. The effect of the different flow regimes on the electrical field distribution and total capacitance was simulated for several inscribed electrode angles. An electronic circuit was built to accurately measure the capacitance. Experimental test results with air-water flow in transparent 9mm tubes are presented. Experimental results with refrigerants are necessary for further development of objective and accurate flow regime transition and two-phase flow pattern maps during evaporation or condensation of refrigerants.