The Dynamics of the Flammable Zones Formed After the Release of a Fixed Mass of a Gaseous Fuel into Air

The initiation, growth and subsidence of flammable zones following the release of a fixed mass of a gaseous fuel into the over-layering atmosphere were investigated numerically. The fuel is assumed to be released at the base of an open to atmosphere, vertical, cylindrical container with negligible pressure difference. The transient equations of conservation of mass, momentum and energy that govern the convective and diffusive transport processes were solved using both a 3-D and a 2-D axis-symmetric CFD models. The results obtained from the two models were compared and led to the definition of a criterion that could be used to asses the limits of validity of the 2-D CFD model for the conditions considered.;Focus was made on the establishment of the concentration fields and subsequently the transient development of the flammable zones. These flammable zones change significantly in size, shape and location. Their evolution is a complex function of numerous variables such as the type of the fuel, initial fuel quantity, size of the container, and the atmospheric conditions. The numerical model permitted predicting the evolution of such flammable zones as well as the effects of the different influencing parameters. The characteristics and patterns of motion of the flammable zones when the fuel is lighter than air are discussed in detail and compared to when it is heavier than air.;A correlation was derived to estimate the propagation rate of the flammable zones for the cases simulated. This correlation was used as a basis to define a non-dimensional grouping to predict the propagation rate of the flammable zone for conditions within the range of the cases examined. The non-dimensional grouping was employed to compare the results for other different situations.;Some applications were considered in which, the buoyant flow of the dispersing gas was driven by heat transfer or double heat/mass transfer processes (rather than pure mass transfer as in the previous cases) in the same geometry and conditions. This is considered with respect to the following cases and situations: a) The dispersion processes of the LNG boil off vapour that is momentarily heavier than air due to its initial cryogenic temperatures (111K) b) The dispersion processes of lighter than air gases that are initially hotter than the surrounding atmosphere.;The resulting flow patterns and the dispersion rates in case (b) were compared to those obtained for the isothermal dispersion of fuel gases. It was observed that the spatial and temporal variations of the resulting density fields show some insignificant differences compared to the corresponding mass transfer dominated problems. However, the propagation rate of the hot gases that are lighter than air was found to be similar to that of equally buoyant gases under isothermal conditions.