Two-phase flow characterization for metal concentric tubes with capacitance measurement

The two-phase flow characteristics for the concentric tube would be different to a plain tube. But the existing research focused on the two-phase flow in circular tubes, without concentric structures. The purpose of this paper was to reveal the two-phase flow characteristics for R134a in both horizontal and vertical upward flow between two concentric tubes by capacitance sensors. The capacitive signals corresponding to different vapor qualities were measured with the mass fluxes from 18.37 kg m⁻² s⁻¹ to 64.31 kg m⁻² s⁻¹. The experimental data showed that the value of the time-averaged capacitive signal was mainly determined by the vapor quality. The measured capacitance was independent of the mass flux and electrode width, but greatly influenced by the flow direction. The result in horizontal flow was flatter when the fluid was closer to the single-phase and steeper at moderate vapor quality (from 0.1 to 0.9), while it was close to a straight line in vertical upward flow. Moreover, the capacitive signals had oscillation characteristics. By the probability density function, it was found that the amplitude features and oscillation levels were related to the flow patterns. The amplitude degree reflected the stability of the two-phase flow. The oscillation levels could be explained by the flow pattern transition. The corresponding relationship between flow patterns and oscillation characteristics was discovered, which could provide more information for the identification of flow patterns.