The necessity of using detailed kinetics in models for premixed combustion within porous media

Abstract

Models for premixed combustion within porous inert media (PIM) are complicated by the highly nonlinear radiative exchange terms in the energy equation for the solid matrix in addition to the stiffness of the set of gas phase equations. Therefore, prior researchers have simulated the gas-phase reactions using single-step chemistry. In the present work, predictions are made using both single-step and multistep kinetics mechanisms. It is concluded that it is essential to use multistep kinetics if accurate predictions of the temperature distributions, energy release rates, and total energy release are sought. Obviously, this is also true if predictions of the composition profiles and emissions are sought. Single-step kinetics is shown to be adequate for predicting all the flame characteristics except the emissions for the very lean conditions under which equilibrium favors the more complete combustion process dictated by global chemistry. The first predictions of NO and CO emissions from PIM burners are presented and compared with experimental data. The model predicts the CO emissions very accurately and predicts the NO trend correctly but overpredicts the NO emissions for φ > 0.8. The present multistep PIM burner model does not accurately reproduce the data for the burning speed and NO emissions for nondilute mixtures. These discrepancies can be only partially attributed to experimental uncertainties and/or imprecise knowledge of the properties of the solid matrix. Thus, it is concluded that important aspects of the physical processes within PIM combustors are not well simulated at present.