Study on the mechanism for laminar burning velocity enhancement with ethane addition in ammonia premixed flames

Abstract

Ethane (C2) is the second largest component of natural gas, which exhibits superior combustion performance compared to methane (C1). The C2/NH₃ dual-fuel combustion strategy effectively mitigates the shortcomings of NH₃ flames. This study employed a spherical constant volume combustion chamber (CVCC) to measure the laminar burning velocity ($S_L$) of C2/NH₃ across various equivalence ($\varphi$) and blending ratios (${\chi }_b$). A chemical kinetics mechanism for C2/NH₃ is developed and optimized based on experimental data and existing models (GRI 3.0, SanDiego, CEU 1.1), achieving high fidelity in simulating $S_L$ for binary fuel flames. The concept of flame precursor waves is refined by analyzing the roles of concentration, temperature, and chemical waves in flame propagation. Unlike the absolute leading position of concentration waves, chemical and temperature waves closely couple, creating a positive feedback mechanism. An equation of sensitivity analysis is established to examine the effects of fluid dynamics, thermal diffusion, and Arrhenius effects on $S_L$. Thermal expansion coefficient ($\sigma$) and laminar flame thickness (${\delta }_l$) reflect fluid dynamic effects, with ${\delta }_l$ significantly influenced by $\varphi$. At low ${\chi }_b$, NH₃ dominates thermal diffusion, limiting $S_L$ and enhancing instability. The Arrhenius effect remains the primary factor, particularly significant at low ${\chi }_b$, while C2 somewhat weakens the effects. Near the stoichiometric ratio, fluid dynamic effects become more pronounced, but excessive fluid density gradients inhibit flame propagation under rich fuel conditions. Nevertheless, the favorable thermal diffusion properties of C2 enhance the thermal diffusion effect, maintaining $S_L$ at a higher level.