In the present study, ammonia/air premixed combustion in marine engine relevant conditions has been numerically investigated. Following the potential of ammonia as an alternative carbon-free fuel for engine applications, this work aims to expand the fundamental understanding regarding the ammonia combustion at high pressure and humidity conditions of marine engines. The effects of the mixture equivalence ratio, compression ratio, and particularly the relative humidity on the essential characteristics of ammonia combustion, including laminar flame speed, heat release, and specifically NO emission and the regarding reaction pathways, were extensively investigated at high pressure and temperature relevant to marine engine conditions. The engine’s relevant conditions were simulated by using an isentropic compression step while increasing the temperature and pressure of the mixture to the operating conditions. Then, a one-dimensional flame solver was used to simulate the combustion step. The results show that the laminar flame speed could be decreased by up to 17.5% by increasing the air humidity at every compression ratio, which can lead to stability problems during combustion. However, NO production was mitigated by humid air in fuel-lean conditions. The maximum exhaust NO was decreased by 12% by increasing the relative humidity from 0 to 100%. Nevertheless, the NO reaction pathways did not change with respect to the relative humidity of the mixture, meaning that H2O did not add or change any specific pathway. Combustion efficiency also did not change with the relative humidity.

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