Multiple Parameter Mixture Fraction With Two-Step Combustion Chemistry for Large Eddy Simulation.
Multiple Parameter Mixture Fraction With Two-Step
Combustion Chemistry for Large Eddy Simulation.
Floyd, J. E.; McGrattan, K. B.
Interflam 2007. (Interflam '07). International
Interflam Conference, 11th Proceedings. Volume 2.
September 3-5, 2007, London, England, 907-918 pp, 2007.
simulation; mixture fraction; combustion chemistry;
formulation; combustion models; laminar flames;
diffusion flames; experiments; carbon monoxide; fire
models; equations; validation; burners; extinction
A common approach for treating combustion in practical
fire models is to use the mixture fraction, a conserved
scalar to which all gas species can be related.
Typically, infinitely fast chemistry is assumed, in
which case the technique works well for fires scenarios
in which there is an adequate supply of oxygen. A
somewhat more complex approach is to create flamelet
libraries that map temperature and mixture fraction to
species mass fractions. This has been shown to work well
in small scale simulations and is widely used in the
combustion community. However, for simulations of fires
in large structures, the inability to resolve flame
temperatures and scalar dissipation rates, regardless of
the turbulence model used, make detailed flamelet models
impractical. Therefore, we seek a methodology that
allows us to describe incomplete combustion and flame
extinction at large scale while staying within the basic
framework of the mixture fraction. In the proposed new
framework, the mixture fraction retains its classic
definition as the mass fraction of gas that originates
as fuel. However, with a single value of the mixture
fraction it is not possible to account for products of
incomplete combustion, or even the mixing of unburned
fuel and oxygen. Instead, we need to decompose the
mixture fraction into constitutive parts that represent
the products of the different reactions. The number of
components depends on the complexity of the phenomena.
For example, to account for local flame extinction and
also the production/destruction of CO, we need to
decompose the mixture fraction into three components.
This paper will document the new mixture
fraction approach and test it against three sets of
experimental data of varying scale: a slot burner, a
hood experiment, and a compartment fire experiment. All
three sets of experiments involve relatively clean
burning fuels because the emphasis is on CO, not soot,