Direct Numerical Simulation of combustion near a carbonaceous surface in a quiescent flow

In this work, unsteady gasification and oxidation of carbonaceous materials with gas-phase combustion under oxy-combustion conditions is studied. An original analytical solution is derived to validate the numerical species boundary condition for the heterogeneous surface reactions with frozen gas-phase. Direct Numerical Simulations accounting for multi-species diffusion and detailed chemistry with both species and energy boundary conditions taking into account the Stefan   flux were subsequently performed to study combustion near a carbonaceous wall. Transition from gasification to solid carbon oxidation and gas-phase ignition is evidenced. The competition between homogeneous and heterogeneous reactions is discussed based on the analysis of the time-dependent surface and gas-phase species consumption and production rates. Transient histories of surface species as well as spatial profiles of gas-phase species, heat release and temperature are presented for two identified structures of gas-phase CO/O2CO/O2 reaction zone. In order to quantify the error induced by the Hirschfelder & Curtiss approximation for the multi-species diffusion and to evaluate the importance of the Dufour and Soret terms, a library dedicated to the computation of complex diffusion phenomena has been used to rebuild a priori, species and Soret fluxes for mass diffusion as well as Fourier, species and Dufour   diffusion heat fluxes. Eventually, characteristic times of gas and surface reactions and those corresponding to the switch between the two structures of the gas-phase reaction zone are obtained over a wide range of initial O2/CO2O2/CO2 concentrations.