Maybe you, chemical people could answer my question. Imagine a fuel injector surrounded by a coaxial air stream. The fuel and air flow are parallel, coaxial and axisymmetric. Both fluids obey ideal gas equation. Let's consider steady and compressible flow, i.e, variations of [tex]\rho[/tex] are important. At low Mach numbers, the flow velocity is small enough to allow both reactants to be mixed by means of molecular diffusion and convection, resulting in a diffusion flame that can be seen experimentally. I think chemical reaction takes place in the mixing region of both substances. The thickness of this mixing layer I think it will depend on how important is the diffusion mechanism in the transport phenomena. But my question is about high Mach numbers, in particular M>>1 (supersonic). What would happen?. Will there be enough time for molecular diffusion?. If not, it would be possible to the reaction to take place?. Moreover, what do you think it will happen if fuel stream is subsonic and air is supersonic? Any help is welcomed.