Numerical simulation of superheated jets using an Eulerian method

Abstract

[EN] Flash boiling is the rapid phase change of a pressurised fluid that emerges in ambient conditions below its vapour pressure. Flashing can occur either inside or outside the nozzle depending on the local pressure and geometry and the bubble formation leads to interfacial interactions that eventually influence the emerging spray. Lagrangian methods which exist in literature to simulate the flash atomisation and inter-phase heat transfer employ many simplifying assumptions. Typically, sub-models used for the break-up, collisions and evaporation introduce an extensive empiricism that might result in unrealistic predictions for cases like flashing. In this study, a fully Eulerian approach is selected employing the Σ − Y model proposed by Vallet and Borghi. The model tracks liquid structures of any shape and computes the spray characteristics comprising a modified version for the transport equation of the surface density. The main goal of this study is to investigate the performance of this model in flash boiling liquids using the Homogeneous Relaxation Model (HRM) developed by Downar-Zapolski, a model capable of capturing the heat transfer under sudden depressurisation conditions accounting for the non-equilibrium vapour generation. The model in this present study considers that the instantaneous quality would relax to the equilibrium value over a given timescale which is calculated using the flow field values. A segregated approach linking the HRM and Σ − Y is implemented in a compressible formulation in an attempt to quantify the effects of flash boiling in the spray dynamics. The developed model is naturally implemented in RANS in a dedicated solver HRMSonicELSAFoam. Results from simulations of two-phase jets of different subcooled fluids through sharp-edged orifices show that the proposed approach can accurately simulate the primary atomisation and give reliable predictions for the droplet sizes and distribution. Strong effects of the flashing and turbulent mixing on the jet are demonstrated. The model is tested for turbulent flows within small nozzles and was developed within the open source code OpenFOAM.