Numerical studies of methane-air combustion in homogeneous and heterogeneous systems with detailed chemistry

The applications of Computational Fluid Dynamic (CFD) to simulations of two-dimensional homogeneous and heterogeneous methane-air combustion systems are reported in the present study. CFD has been established as a powerful research and design tool for fluid flow and heat transfer problems. When the code is coupled with proper models of chemical kinetics and transport properties, CFD not only can provide essential design information such as fuel efficiency and pollutant emission but also can be easily adjusted to explore influences of different burner configurations on pollutant formations.; In the first part of this thesis, a flamelet approach is proposed for facilitating rapid two-dimensional simulations of NO{dollar}rmsb{lcub}x{rcub}{dollar} emissions from laminar methane-air Bunsen flames. The objective is to develop a computationally efficient method for providing information on pollutant emission, such as NO{dollar}rmsb{lcub}x{rcub}{dollar} and CO. The numerical model is applied to studying NO formation in the secondary nonpremixed flame zone of fuel-rich methane Bunsen flames. By solving the steady-state flamelet equations with the detailed methane-air mechanism, a flamelet library is generated containing thermochemical information for a range of scalar dissipation rates. Modeling of NO formation is made by solving its conservation equation with chemical source term evaluated based on flamelet library using the extended Zeldovich mechanism and "NO reburning reactions." An optically-thin radiation heat transfer model is used to explore the potential effect of heat loss on thermal NO formation. With an established flamelet library, typical computing times are about five hours per calculation on a DEC workstation. The predicted mixing field, radial temperature profiles, and NO distributions compare favorably with recent experimental data obtained by Nguyen et al. (1996). The dependence of NO{dollar}rmsb{lcub}x{rcub}{dollar} emission on equivalence ratio is studied numerically and the predictions are found to agree reasonably well with the measurements by Muss (1997). The computed results show a decreasing trend of NO{dollar}rmsb{lcub}x{rcub}{dollar} emission with the equivalence ratio but an increasing trend in the CO emission index. By examining this trade-off between NO{dollar}rmsb{lcub}x{rcub}{dollar} and CO, an optimal initial equivalence ratio {dollar}rm phisb{lcub}i{rcub}{dollar} = 1.4 is found to yield the lowest combined emission.; Results of a numerical investigation into heterogeneous oxidation of methane-air mixture in a honeycomb catalytic reactor is reported in the second part of the thesis. An improved multistep surface reaction mechanism for methane oxidation on platinum is proposed so that surface ignition of lean methane-air mixtures is better modeled. The validation of the newly-proposed surface reaction mechanism is conducted by comparing predictions using this new mechanism with data from various experiments. Then the new surface chemistry mechanism is used with a two-dimensional flow code to model a methane-air catalytic reactor. The substrate surface temperatures are solved directly with a thermal boundary condition derived by balancing the energy fluxes at the gas-catalyst surface. The numerical results compare favorably with the measurements by Bond et al. (1996). A parametric study of pressure effects on the methane catalytic combustion shows that, despite the gas diffusivity decreases monotonically with pressure, the predicted methane conversion rate does not show the same trend. The predicted methane conversion rate first increases slightly, then it starts decreasing monotonically after the pressure reaches 2 atm.