Turbulence, scalar transport, and reaction rates in flame-wall interaction

Abstract

Turbulent premixed flames are studied in a Couette channel flow using direct numerical simulation (DNS). The combustion is simulated by a single-step reaction with heat release, and the flow is fully developed turbulence at a Reynolds number of 504 based on channel half-width The flow configuration consists of a V-shaped flame held in place by a flame holder. One flame is far from the walls and remains adiabatic, while the other flame interacts with a wall and is nonadiabatic. The flame alters the turbulent velocity profile. Near-wall viscous sublayer scaling is found provided both the pressure gradient and the local, average wall shear stress are used. However, in the log layer, the standard scaling does not collapse the velocity profiles, indicating the importance of additional transport mechanisms caused by the flame. The turbulent length scale is reduced in the flame brush region. In the adiabatic flame, the reduction is to approximately the laminar flame thickness. In the nonadiabatic wall flame, the reduction is not as dramatic due to wall heat loss causing weaker reaction rates. The probability density function (PDF) of the scalar progress variable has a classical bi-modal shape in the adiabatic flame. Thus, in the adiabatic flame, the DNS results compare well with the standard Bray-Moss-Libby (BML) expressions for the turbulent scalar flux and flame surface density. However, in the nonadiabatic flame, which interacts with the wall, the scalar PDF is less bi-modal. In this case, the DNS results and the BML expression for the flame surface density does not correctly capture the heat loss effects of the wall. Modifications to the flame surface area are suggested and compare well with the DNS results.