NUMERICAL STUDIES ON THE DRAG COEFFICIENT OF A CONDENSING ELLIPSOIDAL BUBBLE

During a postulated pipe rupture accident in a nuclear reactor, the ensuing steam-air mixture is routed through the vent system to the suppression pool. Here, the steam condenses when it comes directly in contact with the sub-cooled pool of water. During this condensation process, several shapes of steam cavities are encountered based on the prevailing pool temperature and steam mass efflux into the pool. The bubble shape is known to distort from the spherical to spheroidal form at a relatively intermediate Eötvös and high Reynolds numbers. In this context, CFD numerical simulations have been carried out under the assumption of a quasi-steady state for an ellipsoidal bubble with an induced normal velocity on its surface representing the direct contact condensation of steam. The numerical simulations have been benchmarked for a spherical shape with induced blowing and suction on its surface. The estimated drag coefficients are in excellent agreement with analytical solutions available in the literature. Further simulations are underway to determine the parametric influence of non-sphericity, the magnitude of imposed mass flux on the ellipsoid surface, and the Reynolds number on drag forces experienced by the bubble. Further, this paper will also discuss the vorticity associated with the induced normal velocity and the contribution of the form and viscous drags to the total drag coefficient.