Microgravity cryogenic boiling heat transfer with application to ZBO and pipe chilldown

A Zero Boll-Off (ZBO) pressure control scheme based on the latent heat transport concept has been developed for reduced gravity condition. The key elements of this scheme involve the removal of vapor by the jet flow from the tank wall (shear-off), entrainment of bubbles by the jet flow and condensation of entrained bubbles by a cryocooler. In microgravity, we observed that vapor volumes were fragmented into individual bubbles which slid on the heater surface at 25 to 90% of the jet velocity. For the two-phase flow and heat transfer study, we have focused on the liquid nitrogen convective film boiling during the tube chilldown process. The initial stage is associated with a hot wall which causes the two-phase flow to take the form of a vapor core with scattered small liquid chunks. When the wall becomes slightly cooler, the inverted annular flow is seen. When the wall temperature decreases further, the two-phase flows are experiencing a transition from the film boiling (inverted annular flow) to the stratified flow. Finally when the wall is chilled down enough, a stable stratified two-phase flow was observed.