Evaluating the Effect of Obstruction Separation Distance on Flammable Gas Dispersion Using FDS

When Computational Fluid Dynamics (CFD) models are used for field siting studies on gas-handling facilities, the separation distance between structures is an important consideration. Previous studies have evaluated the CFD model Fire Dynamics Simulator (FDS) against experimental data for flammable gas release and dispersion in obstructed environments. These studies found that FDS gave results with an acceptable accuracy when compared to the experimental data. However, in these studies only a limited set of configurations of obstructions in the flow path were evaluated, as these experiments typically included only a single configuration. Additionally, the studies used previous versions of FDS, which has since seen significant upgrades to the species transport submodel in the latest release.

The current study consist of two elements; (1) evaluate the latest version of FDS against full-scale gas release test data, and (2) determine the effects of different obstruction configurations on gas dispersion, as predicted by FDS. Results from the FDS model can be highly sensitive to the resolution of the numerical grid. Therefore, a subset of simulations were performed with different cell sizes to determine the coarsest grid that produced accurate results.

Full-scale test data was chosen where gas was released over land, rather than water, and included multiple large obstructions, comparable to buildings, in the dispersion path. This particular data set had been modeled in a previous version of FDS, and results were determined to be acceptable. Current modeling results established that the latest version of FDS continues to adequately model the gas dispersion around downwind obstructions and the grid resolution necessary for accurate prediction. Building on this result, a systematic study of different obstruction separation distances using the current version of FDS was performed. As the separation distance was decreased, the channeling effects, eddy formations, and the resulting flammable gas concentrations were evaluated. Both liquefied natural gas (LNG) and liquefied petroleum gas (LPG) releases were considered.