Two-phase flow modelling is strongly dependent on flow patterns. For the purpose of objective flow pattern identification, a capacitance sensor was developed for horizontal two-phase flow in small diameter tubes. Finite element simulations were made during design to study the effect of vapour distribution, wall thickness and electrode angle. A test rig was constructed and a series of experiments was done with horizontal air–water flow in a 9 mm tube. The sensor test results are presented in time, amplitude and frequency domain. Flow regime characterization with the capacitance measurements is clearly possible.

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A capacitive void fraction sensor was developed to study the objectivity in flow pattern mapping of horizontal refrigerant two-phase flow in macroscale tubes. Sensor signals were gathered with R410A and R134a in a smooth tube with an inner diameter of 8mm at a saturation temperature of 15°C in the mass velocity range of 200 to 500 kg/m²s and vapour quality range from 0 to 1 in steps of 0.025. A visual classification based on high speed camera images is made for comparison reasons. A statistical analysis of the sensor signals shows that the average, the variance and a high frequency contribution parameter are suitable for flow regime classification into slug flow, intermittent flow and annular flow by using the fuzzy c-means clustering algorithm. This soft clustering algorithm predicts the slug/intermittent flow transition very well compared to our visual observations. The intermittent/annular flow transition is found at slightly higher vapour qualities for R410A compared to the prediction of [Barbieri et al., 2008, Flow patterns in convective boiling of refrigerant R134a in smooth tubes of several diameters, 5th European Thermal-Sciences Conference, The Netherlands]. An excellent agreement was obtained with R134a. This intermittent/annular flow transition is very gradual. A probability approach can therefore better describe such a transition. The membership grades of the cluster algorithm can be interpreted as flow regime probabilities. Probabilistic flow pattern maps are presented for R410A and R134a in an 8 mm I.D. tube.

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