Laminar Flamelets and the Bray, Moss, and Libby Model
This section is concerned with premixed turbulent combustion in circumstances in which the heat release reactions can be considered ‘fast’, a situation that is common in practical combustion systems. Several dimensionless parameters can help to identify the meaning of fast chemistry in this context. A Damköhler number, Da = τT/τc, compares a characteristic turbulent flow time scale with a laminar flame time. As explained in Chapter 1, when Da ≫ 1, combustion occurs in thin wrinkled interfaces separating unburned reactants from fully burned products. If also a Karlovitz number Ka = τL/τK is sufficiently small, where τK is the Kolmogorov time characterising the smallest eddies in a turbulent flow, these reacting interfaces will resemble unstretched laminar flames. It is then possible to divide a Reynolds-averaged Navier–Stokes (RANS) simulation or large-eddy simulation (LES) into two separate parts: a chemical kinetic analysis of a laminar flame and a subsequent fluid flow calculation. A third dimensionless quantity is, where τ = (Tb/Tu) − 1 is a heat release parameter and Tu and Tb are the temperatures in unburned reactants and fully burned combustion products, respectively. As explained in Chapter 1, gradient scalar transport expressions are found  to fail unless.