The homogeneous equilibrium approximation in heavy gas dispersion models

The dispersion of heavier-than-air aerosol clouds is of great relevance to the analysis and assessment of hazards in the chemical and petrochemical industries. There are many simple mathematical models for heavy gas dispersion but, in most cases, two-phase dynamics are either ignored or treated in a very simple way, without any convincing scientific justification. In fact almost all of those models which do treat two-phase phenomena rely on the homogeneous equilibrium assumption, whether they be for jet dispersion, for gravity dominated dispersion, or for any other aspect of a two-phase release. Here the authors test the homogeneous equilibrium model against a more sophisticated aerosol approach, in the context of two-phase ammonia clouds released in dry and moist air. It is shown that the simpler model does indeed provide a good description for some envisaged release situations, and guidance is given on where the homogeneous equilibrium model is not likely to be adequate. The homogeneous equilibrium model seems to be adequate for evaluating the dispersion of heavier-than-air clouds containing droplets smaller than 100 micrometers. In this paper the dispersion model DRIFT is compared with the aerosol model AERCLOUD