Calibration of a capacitive void fraction sensor for small diameter tubes based on capacitive signal features

• Alternative calibration of a capacitive void fraction sensor for small diameter tubes.
• Capacitance also dependent on spatial distribution of phases.
• Existing method requires knowledge of vapour quality x and mass flux for each measurement.
• Proposed method allows calibration solely based on capacitance signal.
• Good agreement with Rouhani–Axelsson drift flux void fraction model and existing method.

In this paper, calibration of a capacitive void fraction sensor for small diameter tubes based on capacitive signal features is proposed. Existing calibration methods require the mass flux and vapour quality to be known, which poses serious issues for practical applications. In this work an alternative calibration technique is proposed, based on the statistical parameters of the measurement signal.The proposed method was applied to 270 measurement points. The inner tube diameter for all these points is 8 mm, the mass flux ranges from 200 to 500 kg/m²s and the vapour quality ranges between 2.5% and 97.5%. Refrigerants R134a and R410A were used. A good agreement was found, the results were compared to the Steiner version of the Rouhani–Axelsson drift flux void fraction model. The maximum average difference between the model and the predicted value was 1.3% with a standard deviation of 4%.