Analysis of high-pressure hydrogen, methane, and heptane laminar diffusion flames: Thermal diffusion factor modeling

Direct numerical simulations are conducted for one-dimensional laminar diffusion flames over a large range of pressures (1⩽P0⩽200 atm)(1⩽P0⩽200 atm) employing a detailed multicomponent transport model applicable to dense fluids. Reaction kinetics mechanisms including pressure dependencies and prior validations at both low and high pressures were selected and include a detailed 24-step, 12-species hydrogen mechanism (H2/O2 and H2/air), and reduced mechanisms for methane (CH4/air: 11 steps, 15 species) and heptane (C7H16/air: 13 steps, 17 species), all including thermal NOx chemistry. The governing equations are the fully compressible Navier–Stokes equations, coupled with the Peng–Robinson real fluid equation of state. A generalized multicomponent diffusion model derived from nonequilibrium thermodynamics and fluctuation theory is employed and includes both heat and mass transport in the presence of concentration, temperature, and pressure gradients (i.e., Dufour and Soret diffusion). Previously tested high-pressure mixture property models are employed for the viscosity, heat capacity, thermal conductivity, and mass diffusivities. Five models for high-pressure thermal diffusion coefficients related to Soret and Dufour cross-diffusion are first compared with experimental data over a wide range of pressures. Laminar flame simulations are then conducted for each of the four flames over a large range of pressures for all thermal diffusion coefficient models and results are compared with purely Fickian and Fourier diffusion simulations. The results reveal a considerable range in the influence of cross-diffusion predicted by the various models; however, the most plausible models show significant cross-diffusion effects, including reductions in the peak flame temperatures and minor species concentrations for all flames. These effects increase with pressure for both H2 flames and for the C7H16 flames indicating the elevated importance of proper cross-diffusion modeling at large pressures. Cross-diffusion effects, while not negligible, were observed to be less significant in the CH4 flames and to decrease with pressure. Deficiencies in the existing thermal diffusion coefficient models are discussed and future research directions suggested.