Void fraction measurements of refrigerant flow in small diameter tubes using a capacitive sensor

Abstract

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.

The void fraction is one of the key parameters in two phase flows. Quite a few methods have been proposed to measure this void fraction. Yet many of these methods are quite complex, intrusive and/or are not applicable to all possible void fraction ranges. Capacitance measurements have already been shown to characterize the two phase flow behaviour of refrigerants. Due to the curvature of the electrodes for circular tubes, the measured capacitance does not vary linearly with the void fraction. In this work a method is proposed to calibrate a capacitive sensor and thus determine the void fraction based on capacitive measurements. The proposed method was applied to 270 measurement points. The tube diameter D for all these points is 8mm, the mass flux ranges from 200 to 500 kg/m²s and the vapour fraction ranges between 2.5% and 97.5%. Refrigerants R134a and R410A were used. The results for these data points were compared to the Rouhani-Axelsson drift flux void fraction model. A very good agreement with this model was observed.