This work describes an experimental investigation of the transient two-phase flow behavior of liquid nitrogen flowing through a pipeline during the chilldown process. The evaporation process and flow regime transitions were observed during chilldown. Initially pure vapor is observed in the visual test section. Then a moving film boiling front is observed to move through the test section. The length of the film boiling front appears to be very small compared with the length of the facility pipeline. After the film boiling front passes through the test section, a very high velocity stratified two-phase flow appears with a small liquid film thickness. As the pipeline is chilled, the film thickness grows. It appears that the high velocity two-phase flow suppresses nucleate boiling, and the dominant heat transfer mechanism is two-phase bulk turbulent convection. Evaporation occurs at the liquid/vapor interface. A high-speed video camera is used to observe the flow regime transitions during the transient chilldown process. Flow regime comparisons have been made with the Steiner [1], Kattan-Thome-Favrat [2], and Baker [3] flow regime maps.

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