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 [1] to fail unless.