On real gas and molecular transport effects in high pressure mixing and combustion

Linear stability analyses and direct numerical simulations (DNS) are conducted to study real gas and cross-diffusion (i.e. Soret and Dufour diffusion) effects in high pressure mixing layers and one-dimensional laminar diffusion flames. The formulation includes the fully-compressible form of the governing equations with generalized multicomponent cross diffusion in the presence of temperature, pressure, and concentration gradients (derived from non-equilibrium thermodynamics and fluctuation theory). Real gas effects are included by means of a cubic Peng-Robinson equation of state. Realistic transport models are employed for the viscosity, heat capacity, thermal conductivity, binary mass diffusion coefficients, and thermal diffusion factors. Five different models for high pressure thermal diffusion factors are assessed in comparisons with experimental data and in their impacts on high pressure flame simulations. While no model was able to predict all of the experimental data, the models typically bound the data and will therefore provide bounds on the potential importance of Soret and Dufour diffusion in high pressure flame simulations. The real gas and ideal gas models are first compared as they affect the linear inviscid stability of parallel shear flow base flow profiles provided by similarity analysis. The ideal gas model typically results in over predictions in the mixing layer growth rates, and substantial errors in predictions of the fluid densities, compressibilities, heat capacities, and sonic speeds. The impact of Soret and Dufour cross-diffusion on high pressure combustion is then addressed through simulations of laminar diffusion flames described by increasingly complex kinetics models. A parametric study of several simple one-step reactions is first presented based on a single thermal diffusion factor model allowing for a thorough analysis of all terms in the heat and mass flux vectors. Cross-diffusion effects increased as the reacting species molecular weight ratios increased with flame temperature departures > 100K in comparison to the standard Fickian and Fourier transport model. All five thermal diffusion factor models (as well as purely Fourier/Fickian diffusion) are then tested in simulations of H2 = Air (47-step, 12 species), H2 - O 2 (37-step, 8 species), CH4 - Air (11-step, 15 species), and C7 H16 - Air (13-step, 17 species) laminar diffusion flames (including NOx chemistry). Soret and Dufour effects generally resulted in significant peak flame temperature reductions for all flames. Pollutant species concentrations were also substantially altered by cross-diffusion. A parametric study based on the initial ambient pressure was also conducted which showed a general increase in the cross-diffusion effects with pressure for all the flames except CH4 - Air. For both the H 2 and C7H16 flames the temperature departures from the Fourier/Fickian model are > 100 K at the large pressures encountered in many practical combustion devices. The results highlight the importance of cross-diffusion and the continuing need for further research in high pressure combustion modeling.