# Laminar flame speed

Laminar flame speed is a property of a combustible mixture.[1] It is the speed at which an un-stretched laminar flame will propagate through a quiescent mixture of unburned reactants. Laminar flame speed is given the symbol sL. According to the thermal flame theory of Mallard and Le Chatelier, the un-stretched laminar flame speed is dependent on only three properties of a chemical mixture: the thermal diffusivity of the mixture, the reaction rate of the mixture and the temperature through the flame zone:

$s_\mathrm{L}^{\circ} = \sqrt{\alpha \dot{\omega} \left( \dfrac{T_\mathrm{b} - T_\mathrm{i}}{T_\mathrm{i} - T_\mathrm{u}} \right)}$

$\alpha$ is thermal diffusivity,

$\dot{\omega}$ is reaction rate,

and the temperature subscript u is for unburned, b is for burned and i is for ignition temperature.

While the laminar flame speed is a property of the mixture alone, the same is not true for turbulent flame speed - or turbulent burning velocity as it is more correctly called. As flow velocity increases and turbulence is introduced, a flame will begin to wrinkle, then corrugate and eventually the flame front will be broken and transport properties will be enhanced by turbulent eddies in the flame zone. As a result, the flame front of a turbulent flame will propagate at a velocity that is not only a function of the mixture's chemical properties but also properties of the flow and turbulence.