Theoretical and Experimental Investigation of Cryogen Boil-Off for a Stationary Storage Vessel

Cryogen evaporation rate is of prime concern in various industrial as well as research settings. Thermal response of cryogens is quite important in their transportation (due to sloshing) and storage. A transient two-phase thermodynamic model, for a stationary liquid nitrogen cryocan, is developed for two cases, first to understand the boil-off rate (isobaric condition / vent open) and second to measure transient pressure evolution (closed condition / vent close) due to external heat in-leak. Experiments were performed for the validation of the model. A 25.5-liter cryocan of cylindrical geometry is considered with liquid nitrogen as a working fluid. Due to the high insulation of cryogenic vessels, boiling mechanism is governed by interfacial surface evaporation. In isobaric conditions of constant ullage pressure, thermal stratification in both liquid and vapour phases has been studied. Based on heat in-leak calculation and energy equation applied to the total fluid domain (considering temperature, pressure, and enthalpy of vaporization), the boil-off rate in cryogenic vessels is investigated. In closed conditions of the vent, due to the continuous evaporation process, the pressure inside the tank increases thus ullage pressure is determined experimentally from 1 bar to 2 bar due to self-pressurization.