Modeling terminal velocity and aspect ratio of a single bubble in distorted-particle and cap bubble regimes for pool scrubbing simulation

Abstract

Pool scrubbing is an essential filtration process that prevents the release of radioactive aerosols into the environment during severe accidents at nuclear reactors. During such events, a mixture of steam and/or various noncondensable gases containing radioactive aerosols is injected into the pool, where it forms bubbles. During aerosol particle transport, bubble dynamics play a notable role in determining the efficiency of particle removal within the bubbles. Clarifying these mechanisms is crucial for accurately modeling pool scrubbing processes. In practice, pool scrubbing codes based on the lumped parameter (LP) approach incorporate particle removal models; however, key hydrodynamic parameters—such as bubble rise (terminal) velocity and bubble aspect ratio—are modeled using simplified assumptions. The present study aimed to verify the validity of existing correlations for terminal velocity and bubble aspect ratio, with the long-term objective of proposing improved models. As part of this fundamental research, the behavior of a single bubble rising in a stagnant liquid, simulating a pool environment, was visualized through computational fluid dynamics (CFD) simulations employing an interface tracking/capturing method. The effective aspect ratio was calculated based on the bubble interfacial area and volume. Furthermore, correlations for single-bubble terminal velocity and aspect ratio were developed, taking into account the transition in bubble shape as the volume-equivalent diameter increased. The findings from the present study are expected to provide a foundation for the development and evaluation of future standalone pool scrubbing codes, including improved correlations for terminal velocity for bubble swarm, bubble aspect ratio, and other relevant hydrodynamic parameters.