Direct numerical simulation of nonpremixed flame–wall interactions

The objective of the present study is to use detailed numerical modeling to obtain basic information on the interaction of nonpremixed flames with cold wall surfaces. The questions of turbulent fuel–air–temperature mixing, flame extinction, and wall-surface heat transfer are studied using direct numerical simulation (DNS). The DNS configuration corresponds to an ethylene–air diffusion flame stabilized in the near-wall region of a chemically inert solid surface. Simulations are performed with adiabatic or isothermal wall boundary conditions and with different turbulence intensities. The simulations feature flame extinction events resulting from excessive wall cooling and convective heat transfer rates up to 90 kW/m2. The structure of the simulated wall flames is studied in terms of a classical mass-mixing variable, the fuel–air based mixture fraction, and a less familiar heat loss variable, the excess enthalpy variable, introduced to provide a measure of nonadiabatic behavior due to wall cooling. In addition to the flame structure, extinction events are also studied in detail and a modified flame extinction criterion that combines the concepts of mixture fraction and excess enthalpy is proposed and then tested against the DNS data.