Predictions of Line Chilldown Boiling Regime Transitions by a Coupled CFD-Subgrid Boiling Model Validated against 1G LN2 Experiments

Abstract

Before filling a propellant tank on the ground or in Space, the transfer line between the donor and receiver tanks must be cooled down, preferably while sacrificing the least amount of the cryogenic fluid. The cryogenic line chill-down process involves transitions between different flow boiling regimes, namely film boiling, transition film boiling, and nucleate boiling, which are complex and gravity-dependent. Attempts to capture these boiling phenomena and to predict the transitions between them in a Computational Fluid Dynamics (CFD) framework are new and challenging. The present work addresses this challenge by following an Eulerian approach in which a homogeneous two-phase mixture model is used together with the Lee phase change formulation to capture the chilldown film boiling regime in the framework of the ANSYS Fluent® CFD code. The chilldown nucleate boiling regime is predicted by a mechanistic subgrid model that accounts for the nucleation, growth, departure diameter, and shedding frequency of the bubbles. The subgrid model is encoded and coupled to the CFD model via a user-defined function for the wall-fluid heat flux calculations. The coupled CFD-Subgrid model is validated against published experimental data for the chill-down of a heated stainless-steel pipe in 1g using liquid nitrogen (LN2). The CFD predictions of the wall temperature evolution, rewetting temperature, and transition between film and nucleate boiling agree well with experimental measurements for LN2 flow in a vertical pipe. Physical insights derived from the CFD simulations and validation are described, and the strengths and weaknesses of the modeling approach are presented and discussed. Recommendations for future improvements of the CFD model are also provided.