Expansion and evolution of heavy gas and particulate clouds

Abstract

Physical and mathematical models have been applied to stages of two phase releases following the failure of pressurised vessels. The initial expansion stage has been modelled using an experimental apparatus involving the measurement of pressure versus time and of Freon-11 aerosol particle sizes and velocities. An analysis of the experimental data combined with an appreciation of the thermodynamics has allowed estimations of the time dependent processes. The analytical description of the critical pressure decrease on the opening of the vessel is given. The later evolution stage has been mathematically modelled by assuming that the two phases have separate velocities. The cloud motion characteristics have been shown to depend upon the degree of hydrodynamic interaction between the particles via the gaseous phase. For small particle fractional volumes, this interaction is small and each particle behaves as a single particle corresponding to a “filtration” regime. However, for larger concentrations the air between the particles becomes entrained due to the particle motion and the cloud velocity exceeds that of the single particle. In this “entrainment” regime large scale vortex motion occurs. The cloud is transformed either into two cylindrical vortices for planar symmetry or into a toroidal vortex for axial symmetry. The regime of the cloud motion defines the sedimentation characteristics of the particles.