THE HOMOGENEOUS EQUILIBRIUM APPROXIMATION IN MODELS OF AEROSOL CLOUD DISPERSION

The dispersion of heavier-than-air aerosol clouds is of great relevanc e to the analysis and assessment of hazards in the chemical and petroc hemical industries. There are many simple mathematical models for heav y gas dispersion but, in most cases, two-phase dynamics are either ign ored or treated in a very simple way, without any convincing scientifi c justification. In fact almost all of those models which do treat two -phase phenomena rely on the ''homogeneous equilibrium'' assumption, w hether they be for jet dispersion, for gravity dominated dispersion, o r for any other aspect of a two-phase release. Here we test the homoge neous 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 des cription for some envisaged release situations, and guidance is given on where the homogeneous equilibrium model is not likely to be adequat e. The homogeneous equilibrium model seems to be adequate for evaluati ng the dispersion of heavier-than-air clouds containing droplets small er than 100 mum.