(47cr) Modelling of Time-Varying Dispersion from Ground-Level Liquid Pools or Vapour Area Sources

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 ground-level liquid pool or vapour area source. 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 a considerably improved model for linking between the pool and the cloud. It furthermore includes along-wind-diffusion effects (due to ambient turbulence) at the upwind and downwind edges of the cloud. This results in reduced concentrations while the cloud travels in the downwind direction which is particularly relevant for toxic releases. The new formulation presumes a number of ‘observers’ to be released at successive times from 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. 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 validated against experimental data including finite-duration carbon-dioxide vapour ground-level area releases (Kit Fox experiments) and time-varying LNG pools.

As indicated above the current paper considers the case of an unpressurised release from a ground-level liquid or vapour area source only. Previous separate papers have dealt with the cases of an elevated pressurised release without rainout and an elevated pressurised release with rainout.