in local propagation speed of a laminar flame arise due to what is called flame stretch. Flame stretch can happen due to the straining by outer flow velocity field or the curvature of flame; the difference in the propagation speed from the corresponding laminar speed is a function of these effects and may be written as:

where is the unit normal to the flame surface (pointing towards the burnt gas side), is the flow velocity field evaluated at the flame surface and is the surface divergence