Numerical prediction of nitrogen oxides in radiant porous burner flows

The present paper is concerned with the numerical computation of flow, heat transfer and chemical reactions in porous burners. The porous solid matrix acts as a host for redistributing the thermal energy transferred to it from the hot reacting gases. Inside the porous matrix, heat is transferred down stream by conduction and radiation. This thermal energy is then transferred to the incoming cold fuel/air mixture to initiate the chemical reaction processes and thus stabilize the flame. One of the important features of porous burners is its presumed low levels of NO concentration. In the present work, the computed NOx is compared with experimental data and open premixed flames. In order to accurately compute the nitric oxide levels in porous burners, both prompt and thermal NOx mechanisms are included. In the present work, the porous burner species mass fraction source terms are computed from an 'extended' reaction mechanism, controlled by chemical kinetics of elementary reactions. The porous burner has mingled zones of porous/nonporous reacting flow, i.e., the porosity is not uniform over the entire domain. Finite-volume equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to insure that the influence coefficients are always positive. Finite-difference equations are solved, iteratively, for velocity components, pressure correction, gas enthalpy, species mass fractions and solid matrix temperature. A non-uniform (80×80) computational grid is used. The grid used to solve the solid energy equation is extended inside the solid annular wall of the porous burner, to improve its modeling. A discrete-ordinate model with S4 quadrature is used for the computation of thermal radiation emitted from the solid matrix The porous burner uses a premixed CH4-air mixture, while its radiating characteristics are required to be studied numerically under equivalence ratios 0.6 and 0.5. Twenty-five species are included, involving 75 elementary chemical reactions. The computed solid wall temperature profiles are compared with experimental data for similar porous burners. The obtained agreement is fairly good. Some reacting species, such as H2O, CO2, H2, NO and N2O increase steadily inside the reaction zone. However, unstable products, such as HO2, H2O2 and CH3, increase in the preheating zone to be depleted afterward.