Flamelet LES of solid fuel/ammonia co-combustion with differential diffusion

Differential diffusion is expected to be important in a pulverized coal/ammonia co-combustion system due to the co-existence of heavy species in volatiles and light species in the ammonia stream. In this work, the multi-stream flamelet model for piloted pulverized coal/ammonia co-combustion is further extended to incorporate differential diffusion in the framework of large-eddy simulation (LES). The flamelet solutions are obtained by solving the flamelet equations with differential diffusion, and tabulated as a function of four mixture fractions, reaction progress variable and total enthalpy. The governing equation for the total mixture fraction is derived based on element conservation, and the effects of differential diffusion are explicitly considered by an additional term, i.e., the differential diffusion term. The governing equations for the other manifold coordinates with differential diffusion are formulated in a similar way. The differential diffusion terms are closed by relating them to the manifold coordinates based on the 1D flamelet assumption. The suitability and performance of the proposed flamelet model with differential diffusion are evaluated by comparing with the detailed chemistry solutions for a laminar pulverized coal/ammonia counterflow flame and the flamelet model based on the unity Lewis number assumption through an a priori analysis. A fully-coupled flamelet/LES is applied to a laboratory-scale pulverized coal/ammonia CRIEPI burner, and the simulation results are compared with the available experimental data. The comparisons show that the flamelet model with differential diffusion can accurately predict the thermo-chemical quantities, and outperforms the flamelet model based on the unity Lewis number assumption, particularly for the prediction of temperature and NO${}_x$ species mass fractions.