ICONE11-36303 NUMERICAL INVESTIGATION ON THE BEHAVIOR OF A DROPLET AND VAPOR FILM DUE TO SHOCK WAVE

Abstract

The droplet behavior in coolant is one of the key phenomena that govern the dynamics and energetics of molten fuel-coolant interactions (MFCI) during a postulated severe accident in a nuclear reactor. The violent interaction between droplet and coolant may lead to a steam explosion, which may contribute early failure of reactor vessel or containment. Phenomenology of steam explosion is very complex, involving intense thermal-hydrodynamics. With the high temperature melt droplet falling into the volatile liquid coolant, the droplet surface experiences intensive film boiling. The formation of vapor film on the droplet surface mitigates the deformation of the droplet because of the distinct properties. In one of hypothetical mechanisms of steam explosion, for example, it is believed that the collapse of the vapor film, triggered by a shock wave, enables liquid-liquid contact and potential for coolant to penetrate into the melt droplet. This phenomenon may cause excessive steam production and droplet fragmentation. Interest in modelling of MFCI has motivated experimental and analytical investigation on steam explosion. But steam explosion is so fast and complex phenomenon, it is very difficult (if not impossible) to capture every detail of the process. The recent development of computational fluid dynamics may improve significantly the understanding of the mechanism of the steam explosion during MFCI. In our previous works, an advanced front-capturing algorithm, called the level set method, was successfully developed for the simulation of two-phase (bubble or droplet) and three-phase (droplet with vapor film in coolant) flows. The interfaces were implicitly captured by smooth level set functions. This method leaves only one set of conservation equations to be solved for all the multi phases. A high order solver, named cubic-interpolated pseudo-particle (CIP) method, is used to solve conservation equations. In the present work, a shock wave is introduced to the multiphase system. For the purpose of comparison, the revolutions of both single bubble and single droplet due to shock wave are simulated, as well as behavior of a droplet covered by a vapor film under shock wave. Due to the complexity of the problem, only hydrodynamic process under isothermal conditions is studied. Under isothermal condition, the vapor film is introduced initially. A two-dimensional model was chosen. The shock wave is introduced at the bottom as the case in most single droplet steam explosion experiment. The other boundaries are set to Neumann (open) conditions. Through the simulation, further understanding on the mechanism of melt droplet evolution due to shock wave is obtained. Due to the high viscosity and density of droplet, the effect of shock wave on droplet deformation is less significant than that on bubble deformation. However, for the droplet with vapor film in a water pool, existence of the vapor film mitigates the droplet deformation properties due to shock wave. The collapse of vapor film and the penetration of water into the droplet are observed under certain conditions.