Abstract: High speed digital camera is employed to acquire the typical flow patterns of liquid nitrogen flow boiling in vertical upward tubes with inner diameters of 1.931 mm, 1.042 mm and 0.531 mm. The flow patterns are bubbly flow, slug flow, churn flow and annular flow. Confined bubbles are observed in 1.042 mm and 0.531 mm tubes. Flow regime maps are depicted according to experimental results and the surface tension and diameter are considered to be the major factors affecting the flow pattern transitions. It is found that the slug/churn and churn/annular transition lines of the present experiment shift to lower superficial vapor velocity; while bubbly/slug transition lines to higher superficial vapor velocity compared to gas-water flow regime maps with the similar hydraulic diameters. The corresponding transition lines move to lower superficial velocity along with the reduction of tube inner diameter. The results indicate that no existing theoretical model can explain the flow regime maps in the present experiment.

Key words: Flow boiling, Flow pattern, High-speed photography, Liquid nitrogen, Micro/mini-channels