Computation 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. One of the important features of porous burners is their presumed low levels of nitrogen oxides. In the present work, the computed NO_x is compared with similar conventional premixed burners and measured nitrogen oxides in porous burners. In order to accurately compute the nitrogen oxides levels in porous burners, both prompt and thermal NO_x 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 ensure that the influence coefficients are always positive to reflect the real effect of neighboring nodes on a typical central node. Finite-difference equations are solved iteratively for velocity components, pressure correction, gas enthalpy, species mass fractions and solid matrix temperature. The grid used to solve the solid energy equation is extended inside the zero-porosity solid annular wall of the burner porous disk. This was found useful for computing the solid wall temperature with high accuracy. A two-dimensional, discrete-ordinate, model 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 studied numerically under equivalence ratio ranging from 0.5 to 0.8. Twenty-one species are included, involving 55 chemical reactions. The computed solid wall temperature profiles are compared with experimental data of similar porous burners. The obtained agreement is fairly good. The present numerical results show that as the equivalent ratio decreases, the reaction zone moves downstream. Moreover, as the flame speed increases, the NO_x mole fraction increases. Some reacting species, such as H₂O, CO₂ and H₂ increase steadily inside the reaction zone; they stay appreciable in the combustion products. However, unstable products, such as HO₂, H₂O₂ and CH₃, first increase in the preheating region of the reaction zone; they are then consumed in the remaining part of the reaction zone. The numerical results show that most of the formed NO_x is composed of nitric oxide. The velocity and temperature profiles were accurately predicted using a grid of 80×80 while the nitrogen oxides were computed accurately utilizing a finer grid of 160×160.