Simulation of Coyote series trials-Part II: A computational approach to ignition and combustion of flammable vapor clouds

Accidental releases of flammable gases may lead to major fires with extensive effects on the surroundings, mainly due to the intense thermal load emissions. In this paper, a computational approach based on fluid dynamics techniques was attempted aiming at the estimation of resulting thermal radiation emissions and overpressure in large scale cloud fires. In particular, the work dealt with the simulation of Coyote series trials, which conducted in 1981 by Lawrence Livermore National Laboratory (LLNL) and involved the release, dispersion, ignition and combustion of unconfined natural gas clouds in the open-air. In the computations, the CFD code CFX 5.7 was utilized which, in addition to the standard three-dimensional Navier-Stokes equations, incorporates the k-e model for turbulence modeling, the Eddy Dissipation model for combustion and P1 model for radiation transport modeling. Computational thermal radiation histories were compared with experimental data from totally four trials showing a reasonably good agreement for several locations in the field. Discrepancies were laid on overestimation of the thermal load receipted at a certain location, nevertheless within a factor-of-two of the observed values. Moreover, positive peak overpressures were sufficiently low to indicate that the combustion of the cloud yielded a flash fire rather than an explosion.