A numerical study of thermal and chemical effects in interactions of n-heptane flames with a single surface

Abstract

The thermal and chemical effects of a one-dimensional, premixed flame quenching against a single surface are studied numerically. Fuels considered include n-heptane and molar-based mixtures of 95/5 and 70/30 percent n-heptane and hydrogen, respectively. A reduced gas-phase kinetic mechanism for n-heptane is employed. Wall boundary conditions investigated include both an adiabatic and an isothermal wall with temperatures ranging from 298 to 1200 K. The effects of equivalence ratio variations between 0.7 and 3 are investigated. The computations with n-heptane and n-heptane/hydrogen mixtures show that for wall temperatures greater than 400 K heat release rates have a higher value for the wall-interacting flame than for the freely propagating flame. It is also seen that the peak wall heat flux increases with increasing wall temperatures up to 1000 K. Chemical pathway analysis reveals the importance of radical recombination reactions at the surface to the heat release profiles of this study. The effect of H, O, and OH radical recombination near the inert wall is observed to lower the heat release spike on a 750 K isothermal boundary. The concentrations of intermediate hydrocarbons in the near-wall region are studied and related to unburned hydrocarbon formation in an engine cylinder. It is shown that a simple one-step global reaction rate expression for n-heptane fuel conversion cannot reproduce the flame–wall trends observed with the reduced n-heptane mechanism.