This paper presents the transient analysis of fluid flow and heat transfer in a zero boil-off (ZBO) cryogenic storage tank of liquid hydrogen. The system includes a tank with cylindrical wall and oblate spheroidal top and bottom, a heat pipe located along the symmetric axis of the tank, and an active circulator. The heat pipe has a rounded evaporator section at its end. The active circulator is an assembly of a pump body, a suction tube for collecting fluid inside the tank, and a spray nozzle from which the fluid is discharged onto the evaporator section of the heat pipe, which is kept at a constant low temperature, where the heat is removed passively to the condenser section of the heat pipe located outside the tank and eventually to the ambient via a cryocooler. Whenever the maximum temperature inside the tank reaches the boiling point under the working pressure in the tank, the pump is activated to create a forced flow from the nozzle to cool off the heated fluid. After a preset period of time, the pump is shut down and standby until the maximum temperature reaches its threshold again and then the pump starts a new cycle. The transient simulations allow the visualization of flow field and temperature distribution, as well as the computation of maximum and mean temperatures of the fluid at various stages of the pump cycle.

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