Numerical Study On The Mechanisms Of The Phenomena Involved In Vapor Explosions

Vapor explosion is one of the consequences of fuel-coolant interactions in a severe accident of a nuclear reactor. In nuclear plants, there is then a risk of release of radioactive fission products into the environment. So vapor explosion is an important topic to investigate in nuclear safety. Vapor explosion is a compositive multiphase phenomenon including four stages: coarse premixing, triggering, propagation and expansion. The mechanisms involved in the vapor explosion process have not been understood, which prevents predicting the explosion precisely.The mechanisms of the phenomena involved in vapor explosions are studied with numerical simulation. Vapor explosion is a very complex dynamic progress with heat and mass transfer, which challenges the robustness and applicability of the numerical algorithms. In this study Volume of Fluid (VOF) method is used to track interface and is extended to complex domain, phase change and compressible flow. A computational fluid dynamics code for multiphase flow is developed for this purpose. A hot particle moving in water with film boiling is simulated, and the drag force on the particle is investigated. Based on the simulation results, a new drag model including pressure drag and friction drag is established with the help of theoretical analysis. In this model, the pressure drag force is affected by the surface tension and the shape of the liquid-vapor interface in the vicinity of the particle. The model is validated with experimental data. In addition, different heat transfer correlations are evaluated with the simulation results.In simulation of thermal fragmentation of a melt drop under a pressure pulse, it is found that Richtmyer-Meshkov instability determines the growth of the initial disturbances on the coolant-vapor interface. As the vapor film collapses, the coolant will contact the drop at a series of local points. The local generation of high-pressure vapor at the surface will cause a small crater. A spike appears between two craters. The collapse of the vapor film above the spike could not suppress the formation of spike, but will bifurcate the spike. It is also indicated that the diffraction of the pressure wave is important to trigger a thermal fragmentation.It is shown in simulation that the drop deformation and surface instability are the main mechanisms in hydrodynamic fragmentation. After passing of shock wave, flow separation does not occur in the early time and the pressure is nearly recovered at the back stagnation point. The drop is squeezed by these pressures and pulled out at the equator. In a liquid-liquid system, the density ratio is much larger than that in a gas-liquid system, so the drop deformation is especially important, and affects the fragmentation time evidently.