(47h) Numerical Simulation of Cryogenic Boiling

The main source of hazard associated with cryogen is the loss of primary containment which can lead to vaporization, two-phase spreading and vapor dispersion. Numerous studies have been conducted in modeling vapor dispersion, but much less attention has been given to phenomena such as vaporization and two-phase spreading despite the fact that those are critical to the accurate prediction of the consequences of spill. Vaporization and two-phase spreading models are known as source term model since results of these models are used to perform the vapor dispersion calculations. As a result, LNG vapor cloud dispersion models are highly sensitive to the accuracy of the vapor generation rate estimated from the source term model. The vaporization rate modeling includes boiling and evaporation. During the initial spillage, the surface temperature of the substrate, such as concrete or insulated dyke material, is much higher than the boiling point of the cryogen thus boiling takes place mainly due to the heat conduction from substrate. When the temperature of the substrate falls close or equal to the boiling point, conduction heat transfer becomes insignificant in comparison to convective heat transfer from air to liquid thus the vapor generation is mostly due to the evaporation. In this study, our focus is to model the boiling heat transfer due to conduction for an accurate prediction of the vaporization rate that will subsequently be combined with evaporation model and two-phase spread model. A CFD based approach is considered rather than integral modeling approach for future unification of vaporization, two-phase spreading and vapor dispersion models.

The boiling of a fluid have 3 different regimes, i.e., film boiling, transition boiling and nucleate boiling. Each mode of boiling occurs over a range of excess temperatures (surface temperature exceeding the saturation temperature). Nucleate boiling is characterized by the formation of bubbles from randomly distributed nucleation sites over the surface whereas the film boiling is characterized by the formation of a continuous blanket of vapor film on the heating surface with bubbles leaving the surface at a regular intervals. Transition mode is much less understood but it connects nucleate and film regimes. Liu et al. proposed a volume-of-fluid (VOF) method with a sub-model of phase change to simulate cryogenic pool boiling using the commercial CFD software Fluent. A user defined interphase mass transfer model considering both homogenous and heterogeneous boiling was proposed. Three different boiling regimes were identified, and the boundaries of each regime were well predicted. It was also found that the simulation can reasonably predict the experiments in a film boiling regimes. However, the heat flux for nucleate boiling was under-predicted by Liu’s model. In this work, the predictions in the regime of nucleate boiling will be improved by seeking a better sub-model to describe the homogenous model, which is supposed to be critical to predict nucleate boiling.