A unified model for jet, heavy and passive dispersi on including droplet rainout and re-evaporation

The Unified Dispersion Model (UDM) models the dispersion following a ground-level or elevated two-phas e unpressurised or pressurised release. It allows for continuous, instantaneous, constant finite-duratio n, and general time-varying releases. It includes a unifie d model for jet, heavy and passive two-phase disper sion including possible droplet rainout, pool spreading and re-evaporation. It calculates the phase distrib ution and cloud temperature using either a non-equilibrium th ermodynamics model, a non-reactive equilibrium model, or an equilibrium model specific for HF (including effects of polymerisation). As part of the current work each of the modules in the UDM (passive, jet, and heavy dispersion; non- equilibrium, equilibrium and HF thermodynamics; steady, instantaneous or finite-duration releases; poo l spreading/evaporation on land or water) has been in vestigated. The fundamental underlying physics has been considered in conjunction with a literature review, comparison against wind-tunnel experiments, verifi cation of the numerical solution against analytical soluti ons (where possible) and sensitivity analyses. In a ddition the model has been compared against predictions by third-party models such as the HGSYSTEM dispersion model HEGADAS and the pool model GASP. As a result of this work the tuning present in the original UDM model has largely been eliminated, and model coefficients are obtained directly from established data in the literature which are largely based on wind-tunnel experiments. The latest UDM version implemented in the DNV software application PHAST 6.0 represents a significant revision and extension to all parts of the model. This paper includes an overview of the n ew model and a summary of the module verification and validation. The paper also includes a description of the valida tion of the overall UDM model against large-scale f ield experiments. The validation set of experiments incl udes both continuous and instantaneous releases. T hese experiments address complex phenomena such as aerosol discharges and HF thermodynamics, in addition to relatively simplistic releases. A subset of these experiments include those evaluated as part of the EEC programme SMEDIS ( Scientific Model Evaluation of Dense Gas D is persion Models).