Application of liquid nitrogen to cooling is widely used in such fields as cooling of the high temperature superconducting devices, cryosurgery and so on. In order to have a full understanding of the flow and heat transfer characteristics of liquid nitrogen in micro-tube, high-speed digital photography was employed to acquire the typical flow boiling patterns of liquid nitrogen in micro-tubes in the experiments. The main flow patterns were bubbly flow, slug flow, churn flow and annular flow. And the confined bubbly flow and mist flow were also observed. These flow patterns were characterized on the flow regime maps. And the surface tension force and the size of the tube diameter were found to be the major factors affecting the flow pattern transitions.

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