Modelling of Time-Varying Dispersion for Releases Including Potential Rainout

Many accidents involve releases of hazardous chemicals into the atmosphere. Many commonly used atmospheric dispersion models are limited to continuous or instantaneous releases only, and cannot accurately simulate time-varying releases. The current paper discusses a new enhanced dispersion formulation accounting for time-varying effects resulting from a time-varying release (e.g. resulting from depressurisation in a vessel or pipe) and/or from a time-varying pool (following rainout or from a spill). This new formulation is implemented in the Unified Dispersion Model (UDM), and is planned to be included in a future version of Phast.

The new model includes effects of along-wind diffusion, which are particularly relevant in the far-field for toxic releases, and ignoring these effects leads to significant over-prediction of concentrations in the far-field.

The new formulation presumes a number of ‘observers’ to be released at successive times from the point of discharge or the upwind edge of the pool.  The UDM first carries out ‘pseudo steady-state’ calculations  for each observer, where the ‘release data’ correspond to the time at which the observer is released, and where the observer ‘picks’ up vapour while travelling over the evaporating pool. Subsequently effects of along-wind diffusion are included by means of Gaussian integration over the downwind distance.

First the above new UDM formulation has been verified against the time-varying HGSYSTEM model HEGADAS-T.  For the case of an elevated time-varying pressurised release without rainout, the UDM results were consistent with the HEGADAS-T results. For the case of dispersion directly from a ground-level pool, the UDM appears to provide more accurate results in the near-field because of more accurate observer calculations while observers are moving over the pool.

Subsequently the UDM has been further tested for more general cases including elevated releases with rainout.