As a common heat exchange structure, vertical pipe is widely used in neaclear reactors and other systems. Flow pattern and flow regime map are the basis of thermal-hydraulic calculation. In this paper, the experiment of air-water two-phase flow pattern was carried out based on the visual test section of 10 mm vertical pipe. Three impedance meters were used to measure the void fraction at three axial positions (L/D = 66, 148, and 229) along the vertical pipe. Compared with the flow pattern criterion proposed by Mishima and Ishii, six superficial liquid velocities are selected for design conditions, which are 0.5 m/s, 1.0 m/s, 1.5 m/s, 2.0 m/s, 2.5 m/s and 3.0 m/s, respectively. And the superficial gas velocities are ranging from 0 to 30 m/s. Considering the inlet and export effects of pipe flow, the void fraction parameters from L/D = 148 (#2 impedance meter) were applied for analysis. Based on the experimental data and random forest algorithm, the flow pattern was identified for each condition, and the flow regime map of 10 mm vertical pipe was established. It is concluded that the flow pattern transition point of air-water flow in the 10 mm vertical pipe was different from that of the Mishima-Ishii’s model, and the inner diameter of the pipe will affect the flow pattern formation and flow pattern transition.

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