ANALYSIS OF THE THERMAL-DIFFUSION EFFECTS ON THE IGNITION OF HYDROGEN-AIR MIXTURES IN THE BOUNDARY-LAYER OF A HOT FLAT-PLATE

In this article the steady-state process leading to ignition of a comb ustible mixture of hydrogen, oxygen, and nitrogen by a hot flat plate in a boundary layer flow is studied, including the effect of thermal d iffusion. For sufficiently large values of the plate temperature, the ignition event corresponds to a typical chain-branching explosion with negligible heat release, in a first approximation. In the framework o f the reduced kinetic mechanism appropriate for this regime, the bound ary layer equations are solved by using the fact that the chain branch ing reaction H + O2 --> OH + O has a relatively large activation energ y. Assuming a three-layer structure, the governing equations reduce to a single integrodifferential equation for the concentration of atomic hydrogen. Close to the plate leading edge, the concentration of atomi c hydrogen around the plate surface is higher when thermal diffusion i s included, but, later on, this effect enhances the evacuation of atom ic hydrogen by the external convective/diffusive layer. Thus, in spite of the initial enrichment in molecular hydrogen by thermal diffusion, the final ignition distance is shown to be enlarged by a factor of ab out 2, under usual conditions. On the other hand, for low plate temper ature ignition, thermal diffusion produces the expected reduction in t he ignition distance.