A DNS study of heat release rate surrogates with unity and non-unity exponents for ammonia/air premixed flames

Ammonia is a carbon-free fuel that can potentially be used to reduce the greenhouse effect. However, the characteristics of ammonia combustion are not well understood, which presents a major challenge for the development of improved combustion technologies. Heat release rate (HRR) is a critical quantity of combustion, but it is impossible to be measured directly and quantitatively in experimental studies. In the present work, HRR surrogates with unity and non-unity exponents of ammonia/air combustion were developed based on laminar premixed flame results, and then were validated in turbulent flames using three-dimensional direct numerical simulations (DNS). Both lean and rich conditions were considered, and the simulations were carried out under various temperature and pressure conditions. It was found that the elementary reaction NH2+NO = N2+H2O acts as an important contributor to HRR, and the product of NH2 and NO concentrations, i.e. [NH2][NO], is shown to be a promising HRR surrogate. Other HRR surrogates such as [NH3][OH], [NH2][H] and [NH2][O] were also considered, and their performance was evaluated. In order to further improve the accuracy of prediction, the HRR surrogates with non-unity exponents were proposed, and the optimal exponents were identified, resulting in the following optimal HRR surrogates: [NH2]0.86[NO]0.97 and [NH2]0.77[O]0.65. The proposed HRR surrogates were also evaluated in turbulent ammonia/air premixed flames. It was shown that the HRR surrogates with non-unity exponents generally outperform those with unity exponents. These results are helpful for characterizing ammonia/air combustion, and can be used in practical combustion applications. (c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.