The Effects of Cracking Ratio on Ammonia/Air Non-Premixed Flames under High-Pressure Conditions Using Large Eddy Simulations

Abstract

Ammonia is a promising carbon-free fuel. However, one of the main challenges for ammonia combustion is the high level of (Formula presented.) emissions. In this study, simulations were conducted for ammonia/air laminar counterflow flames and turbulent non-premixed jet flames in the KAUST high-pressure combustion duct (HPCD) at a pressure of 5 bar, with two ammonia cracking ratios of 14% and 28%. The influence of ammonia cracking ratio on the flame structure and (Formula presented.) formation mechanism were examined. The laminar counterflow flame results showed that (Formula presented.) is one of the most critical species related to (Formula presented.) formation and (Formula presented.) is mainly generated through the path of (Formula presented.). For the turbulent flames, the flamelet/progress variable (FPV) approach was employed in the context of large eddy simulations (LES) for high-fidelity simulations. The simulation results were compared with the measured data with promising agreements, which proves the accuracy of the FPV method for the present flames. It was shown that with increasing cracking ratio, not only the flame reactivity is enhanced, but also the generation of (Formula presented.) is increased. The correlation between (Formula presented.) and (Formula presented.) is weaker when compared to that between (Formula presented.) and radicals such as (Formula presented.), (Formula presented.) and (Formula presented.) in the entire flame. Through the distribution of (Formula presented.) source terms, it was found that the (Formula presented.) source term has a higher absolute value in the upstream region and the absolute value rapidly decreases with increasing streamwise distance. The total (Formula presented.) source term is positive in the fuel-lean zone and shows negative values in the fuel-rich zone.