Combustion is a complex physical-chemical process of converting original products into combustion products during exothermal reactions, which is accompanied by an intense release of heat. Chemical energy stored in the components of original mixture can be also released in the form of hear radiation and light. The lightened region is referred to as flame.
Consider combustion process in a gas jet. In real conditions, such a jet is turbulent. The gas jet inflammation causes formation of a diffusion torch of axisymmetric fusiform shape. Chemical combustion reactions take place in a thin surface layer of the torch. In the first approximation this layer can be considered as a surface, where concentration of fuel and oxidant are equal to zero, and the diffusion fluxes of fuel and oxidant to this surface are in stoichiometric relation. Diffusion combustion front has zero propagation velocity; therefore it cannot stay by itself on the jet flowing upwards.
Flame stabilization occurs in the lowest part of the torch, where combustion process is different. A turbulent layer of gas mixing with air appears at the initial non-burning part of the jet surface. In this layer, the gas concentration reduces slowly in the radial direction, and the oxidant concentration increases. A homogeneous mixture of fuel and oxidant with almost stoichiometric composition appears in the middle part of the mixing layer. After inflammation of such a mixture, the flame can propagate in the mixing layer even towards the flow, if the combustion velocity exceeds the local flow velocity. But since the jet velocity increases towards the exit cross section, at a certain height the jet velocity becomes equal to the combustion velocity, and the flame stabilizes at the jet surface at this height.
It is impossible to calculate velocity of the turbulent jet combustion precisely, however, there are ways to estimate it.