EXTERNAL PUBLICATIONS

SUMMARY

The aim of this review is to determine whether current approaches and practice for modelling accidental atmospheric 'flashing' jet releases and subsequent dispersion - including possible rain-out -is consistent and up-to-date with information in the public domain. The study focuses on the two-phase aspects, with emphasis on flash atomisation and droplet dynamics downstream. Industrial processes and problems where similar thermo-fluid processes prevail are included within the remit. One essential outcome of the work is a proposal for an optimised and prudent strategy to develop quantitative methodologies for these problems in the short, medium and longer term. Several high-profile studies have been undertaken in this area over the last decade, so clear statements are required to explain where and why deficiencies still remain. The review considers models and validation data for the sub-processes of atomisation, atmospheric expansion to ambient pressure, two-phase dispersion, rainout, pool formation and re-evaporation. Most significantly, the source term for the downstream dispersion calculations are currently ill-defined due to lack of a justifiable, validated atomisation model, and no large-scale data-set for releases with significant rainout exists which links the initial post-expansion conditions with downstream rainout or airborne concentration. Only two models have been found which attempt to model the primary two-phase processes on a physical basis, the DNV 'Unified Dispersion Model' and its multi-compound generalisation by Exxon-Mobil; potential limitations of these models are discussed. Several correlations have been proposed which predict final rainout directly from initial release conditions, and these may be useful for engineering guidelines in the short term.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the author(s) alone and do not necessarily reflect HSE policy