Extending the mixture fraction concept to address under-ventilated fires

A common technique in computational fluid dynamics (CFD) modeling of fire is to assume single step, infinitely fast combustion, in which case the transport equations of all gas species can be combined into one for a single conserved scalar called the mixture fraction. While this approach is adequate for many engineering applications, for fire scenarios that require predictions of CO formation or flame extinction, this approach is inadequate. This paper describes a method of extending the mixture fraction concept to address two-step chemistry. The two-step chemistry allows for flame extinction and the prediction of CO formation and destruction. The mixture fraction is decomposed into components representing the states of the two-step chemistry. The new model is demonstrated with two test cases: a slot burner using direct numerical simulation (DNS) and a reduced scale enclosure using large eddy simulation (LES). Results of the new model are compared with experimental data and simulations using single-step chemistry.