Two-phase jet releases, droplet dispersion and rainout I. Overview and model validation

Many accidents involve two-phase releases of hazardous chemicals into the atmosphere. This paper describes the results of the fourth phase of a Joint Industry Project (JIP) on liquid jets and two-phase droplet dispersion. The objective of Phase IV of the JIP was to generate experimental rainout data for non-flashing experiments, and to develop recommendations for the best methodology to predict rainout [total rainout mass and its spatial distribution (‘distributed’ rainout)].Phase IV of the JIP first included rainout experiments by the UK Health and Safety Laboratory (HSL) for sub-cooled releases of water and xylene with a range of orifice sizes and stagnation pressures. See the companion paper II for further details. Secondly model validation was carried out by DNV Software for these experiments using different correlations for the initial droplet size (Sauter Mean Diameter, SMD), i.e. the CCPS SMD correlation and the Phase III JIP SMD correlation. The validation includes flow rates, droplet size, distributed rainout and cloud temperature drop. Subsequently validation was considered for a wider range of experiments from the literature (sub-cooled and superheated releases) for both SMD and total rainout. Adopted rainout methods comprised both methods including explicit modelling of the droplets (using an extended version of Phast dispersion model UDM), as well as more simple methods based on rainout correlations without droplet modelling. Recommendations are made for the most accurate droplet size and rainout modelling. A modified CCPS UDM droplet size correlation has been shown to agree best against experimental rainout data.

► Flow rate, droplet size and distributed rainout validation using consequence modelling package Phast.
► Validation includes extensive range of experiments involving sub-cooled and superheated liquid jets.
► Added set of rainout experiments for water and xylene sub-cooled releases.
►  Explicit modelling of droplets with modified CCPS droplet-size criterion produces best rainout predictions.
► Recommendations for most accurate droplet size and rainout modelling implemented in Phast.