Numerical analysis of flashing pipe flow using a population balance approach

The phenomena of initially subcooled liquid flashing into vapour due to depressurization frequently occur in the nature and technology. They are very complex systems, and the fluid dynamics is affected by the interplay of many different sub phenomena, including bubble nucleation, evaporation, condensation, coalescence, and breakup. Research on the flashing flows has received much attention, but CFD modelling and simulation of such scenarios is still challenging, because knowledge is often insufficient for a precise mathematical description of the physical problems. Attempts of numerical analysis having been made before are all based on the assumption of mono-disperse bubbles by prescribing either the size or number density or using a mixture model, which deviates largely from the physical picture. In the present work, a CFD-PBM coupled approach is extended for the investigation of bubble dynamics in a flashing pipe flow. The comparison with experimental data demonstrates that the model is effective in capturing the physics of vapour bubbles' generation and growth as well as their spatial motion and distribution during the decompression. Although further polishing of the bubble coalescence and breakup as well as interphase momentum and turbulence transfer models is necessary, the agreement between measured and simulated cross-section averaged flow parameters such as void fraction, liquid temperature and bubble size distribution is satisfying. The wide range of bubble size changing confirms the necessity of using a poly-disperse approach instead of mono-disperse assumptions.