Flow regime based calibration of a capacitive void fraction sensor for small diameter tubes

The void fraction measurement technique used in this work is based on measuring the capacity between two electrodes. The electrodes are curved so they closely fit to the tube wall; due to this the measured capacitance is influenced by the spatial distribution of the phases and not only the void fraction. Hence, the measured capacitance does not linearly vary with the void fraction. In this work a method is proposed to account for the distribution of the phases and thus determine the void fraction based on capacitive measurements. The proposed method was applied to 270 measurement points. The tube diameter D is 8 mm, the mass flux ranges from 200 to 500 kg m−2s−1 and the vapour fraction ranges between 2.5% and 97.5%. Refrigerants R134a and R410A were used. The results were compared to the Steiner version of the Rouhani–Axelsson drift flux void fraction model. A very good agreement with this model was observed.

► calibration of a capacitive void fraction sensor for small diameter tubes.
► Due to curvature of the electrodes inhomogeneous electrical field.
► Capacitance also dependent on spatial distribution of phases.
► Different capacitance-void fraction curve for each flow regime is proposed.
► Good agreement with Rouhani–Axelsson drift flux void fraction model.