The formation and dispersion of flammable zones within vertical cylindrical enclosures following the release of a fixed mass of gaseous fuels into air

The initiation, growth and subsidence of flammable atmospheres following the release of a fixed mass of a gaseous fuel into the overlaying air within a vertical cylindrical container was investigated numerically. An axisymmetric two-dimensional mathematical model based on the transient equations of conservation of mass, momentum and energy was developed and solved numerically to describe the transport processes occurring as a result of the combined effects of molecular diffusion and buoyancy. Such transport processes result in the establishment of concentration fields and subsequently, to the development of flammable mixture zones that represent serious fire and explosion hazards. These flammable mixture zones change significantly in size and location with time and their evolution is a complex function of numerous variables that include the type of fuel involved, initial fuel quantity, size of the container, conditions and composition of the overlaying atmosphere. The numerical model developed in this work permitted to predict the evolution of such flammable zones as well as the effects of the different influencing parameters on this evolution.;The differences between the patterns of flammable zones resulting from molecular diffusion-driven and buoyancy-controlled mixing processes have been put in evidence. Also, it was found that binary mixtures made of two different types of fuel resulted in flammable zones whose pattern and extent were closely related to the composition of the binary mixture considered. In the same line, it was shown that the impact of adding diluents in the system on the development of flammable zones was different in situations of homogeneous or stratified fuel-air mixtures. A parametric study to assess the effects of key controlling dimensionless group numbers was performed in relation to the time of first appearance of flammable mixtures at the exit of the cylindrical container and the time when the system became ultimately safe. Finally, guidelines for minimizing risks of fire and damageable emissions have been provided through this investigation.