Implementing and assessing the importance of multicomponent transport properties using direct numerical simulations of premixed, turbulent flames

摘要

Implementing multicomponent diffusion models in numerical combustion studies is computationally expensive due to the challenges involved in computing diffusion coefficients. As a result, mixture-averaged diffusion treatments are used to avoid these costs. However, to the authors' knowledge, the accuracy and appropriateness of the mixture-averaged diffusion models has not been verified for three-dimensional turbulent premixed flames. This study will evaluate the role of multicomponent mass diffusion in premixed, high-Karlovitz hydrogen flames, neglecting secondary Soret and Dufour effects. Direct numerical simulation (DNS) of these flames is performed by implementing the Stefan-Maxwell equations in NGA. A semi-implicit algorithm decreases computational expense of inverting the full multicomponent ordinary diffusion array while maintaining simulation accuracy and fidelity. The algorithm is unconditionally stable provided the components of the mixture-averaged and multicomponent diffusion coefficient matrices are of order one over the number of species or smaller, and performance scales approximately with the number of species squared. One-dimensional simulations of premixed hydrogen flames are performed and compared with matching cases in Cantera to verify this method. Premixed two-dimensional, unstable and three-dimensional, turbulent hydrogen flames are simulated and compared with previous mixture-averaged DNS results. Simulation conditions are carefully selected to match previously published results and ensure valid comparison. A priori analysis shows similar relative angles between the species diffusion flux vectors and the species gradient vectors between mixture-averaged and multi-component results. Further, a posteriori analysis demonstrates negligible differences in conditional means of the fuel mass fraction and its diffusion source term against temperature for mixture-average and multicomponent transport, respectively, between the two cases.