Laminar flame speed
Encyclopedia
Laminar flame speed is a property of a combustible mixture. 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:
is thermal diffusivity,
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.
Thermal diffusivity
In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It has the SI unit of m²/s...
of the mixture, the reaction rate
Reaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
of the mixture and the temperature through the flame zone:
is thermal diffusivity, 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
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
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.