Laminar flame speed is an intrinsic characteristic of premixed combustible mixtures.
[http://www.clarke-energy.com/2013/laminar-flame-speed/ Laminar Flame Speed] It is the speed at which an un-stretched
laminar
Laminar means "flat". Laminar may refer to:
Terms in science and engineering:
* Laminar electronics or organic electronics, a branch of material sciences dealing with electrically conductive polymers and small molecules
* Laminar armour or "band ...
flame will propagate through a quiescent mixture of unburned reactants. Laminar flame speed is given the symbol ''s''
L. According to the thermal flame theory of
Mallard
The mallard () or wild duck (''Anas platyrhynchos'') is a dabbling duck that breeds throughout the temperate and subtropical Americas, Eurasia, and North Africa, and has been introduced to New Zealand, Australia, Peru, Brazil, Uruguay, Argen ...
and
Le Chatelier, the un-stretched laminar flame speed is dependent on only three properties of a chemical mixture: the
thermal diffusivity
In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the rate of transfer of heat of a material from the hot end to the cold end. It has the SI ...
of the mixture, the
reaction rate
The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit ...
of the mixture and the temperature through the flame zone:
is
thermal diffusivity
In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the rate of transfer of heat of a material from the hot end to the cold end. It has the SI ...
,
is reaction rate,
and the temperature subscript u is for unburned, b is for burned and i is for ignition temperature.
Laminar flame speed is a property of the mixture (fuel structure, stoichiometry) and thermodynamic conditions upon mixture ignition (pressure, temperature). Turbulent flame speed is a function of the aforementioned parameters, but also heavily depends on the flow field. As flow velocity increases and
turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
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 fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid be ...
in the flame zone. As a result, the flame front of a turbulent flame will propagate at a speed that is not only a function of the mixture's chemical and transport properties but also properties of the flow and turbulence.
See also
*
Flame speed The flame speed is the measured rate of expansion of the flame front in a combustion reaction. Whereas ''flame velocity'' is generally used for a fuel, a related term is explosive velocity, which is the same relationship measured for an explosive. ...
*
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in wh ...
References
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Combustion