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The characteristics of bubble behavior have been in particular interest for decades due to its significant contribution to understanding the mechanism of heat transfer. In the present work, visualized experiment is conducted to study the bubble characteristics in subcooled flow boiling of a narrow rectangular channel under natural circulation. The experiments were performed at pressures of 0.2 MPa, with inlet subcooling ranging from 20 to 60 K and heat flux ranging from 100 to 300 kW/m 2 . A high-speed digital camera is used to capture the pictures of bubble behaviors. A sequence of image processing algorithms is used deal with the original bubble images to get relevant bubble parameters. We observe the whole process of a single sliding bubble lifetime and found most of bubbles slide along the heating surface after detaching the nucleation sites. Five typical sliding bubble growth paths are observed in the present experimental conditions. According to the analysis of the experimental data, it can be found that the liquid subcooling and wall superheat are the main factors that affect the bubble size during sliding in narrow rectangular channel under natural circulation condition. Due to the difference of driving force, the sliding velocity of bubble in forced circulation is always larger than that in natural circulation. At the same time, the bubble velocity changes significantly at different heat flux and shooting location.

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As a common heat transfer structure, the vertical round pipe is widely used in nuclear energy systems. The phase distribution characteristics of two-phase flow in a vertical round pipe is an important issue. For two-phase flow, combined with the local typical flow phenomenon, the analysis of the phase distribution characteristics such as the void fraction and the interfacial area concentration is helpful to research the two-fluid model and interfacial area transport equation. Based on the completed flow pattern experiment, an air-water two-phase flow characteristic experiment is carried out for the visualized experimental section of 10 mm ID vertical round pipe. In addition, the 5 × 7 total 35 measuring points of bubbly to slug flow transition region are selected, including 5 superficial liquid velocities (0.5, 1.0, 1.5, 2.0, 2.5 m/s) and 7 superficial gas velocities (0.08, 0.11, 0.13, 0.17, 0.21, 0.28, 0.35 m/s). High-speed camera is used to capture the flow patterns of bubbly to slug flow transition region. Four-sensor conductivity probe is used to measure the interfacial parameters such as void fraction and interfacial area concentration. Thus, the phase distribution characteristics of bubbly to slug flow transition region in the round pipe is analyzed. The obtained two-phase flow parameter data can provide data support for the development of two-fluid model and interfacial area transport equation.