A numerical study of laminar flame wall quenching

Laminar flame quenching at the cold wall of a combustion chamber has been studied, using a numerical model to describe the reactive flow. The model combines an unsteady treatment of the fluid mechanics and a detailed chemical kinetic reaction mechanism. Fuels considered included both methane and methanol. Catalytic reactions at the wall surface are not included in the kinetic model. The one-dimensional case of flame propagation perpendicular to the wall was studied. Two reference cases are described in detail for flame quenching at 10 atm pressure and a wall temperature of 300°K with stoichiometric mixtures of methane-air and methanol-air. In each case a conventional laminar flame propagates toward the wall, approaching to within a distance determined by the thermal flame thickness. Chemical kinetic factors, particularly differences between the temperature dependence of radical recombination reactions and conventional chain branching and chain propagation reactions, are shown to be responsible for quenching the flame near the wall. The flame stagnates, but fuel remaining near the wall diffuses out of the boundary region and is rapidly oxidized away from the wall. Subsequent model calculations demonstrate the effects of variations in pressure, fuel-air equivalence ratio, wall temperature, and type of fuel. Computer results from these methane and methanol flame quenching models indicate that the total unburned hydrocarbon content is considerably smaller than is commonly believed and that thermal wall quenching may not be the major source for hydrocarbon emissions from internal combustion engines at near-stoichiometric conditions.