Heat release rate surrogate for ammonia–hydrogen premixed flames under various conditions

Abstract

Blending hydrogen has proven an efficient method to enhance the combustion stability of gaseous ammonia flames. Heat release rate (HRR), as an important parameter to indicate combustion process, is hard to be directly measured and highly dependent on the fuel components, equivalence ratios, and operation conditions. This paper presents a comprehensive study on developing a general HRR surrogate (HRRS) for ammonia–hydrogen premixed flames under various conditions. Firstly, reaction mechanisms for ammonia/hydrogen premixed combustion are evaluated under various conditions, by comparing the predicted laminar flame speeds with the experimental data collected from published literature. The reaction mechanism developed by Shrestha et al., (2021) performs the best under various conditions. Then, series of one-dimensional freely propagating premixed flames under various conditions are calculated using this reaction mechanism and analyzed to explore the effects of blending ratios, pre-heat temperatures, equivalence ratios, and pressures on the HRR reconstruction for ammonia–hydrogen premixed flames from the viewpoint of chemical kinetics, and also serve as a database for the identification of HRRS. It is found that equivalence ratio and blending ratio have significant effects on the HRR reconstruction, while the effects of pressure and initial temperature are relatively limited. Subsequently, the general HRRS, [NH₂]^{1.53} [OH]^{0.28}, is identified for ammonia–hydrogen premixed flames under various conditions using a grid-research optimization method. Finally, the general HRRS is further comprehensively validated on several 2D and 3D turbulent premixed flames of ammonia–hydrogen under various conditions. The comparisons with the two previous HRRSs also demonstrate that the present developed HRRS is superior and more stable temporally.