Detonation peninsula of different stoichiometric ammonia/hydrogen/air mixtures under engine-relevant conditions

This study numerically investigates the detonation development of carbon-free fuels, namely ammonia and hydrogen (NH3 and H 2 ), using one-dimensional (1D) simulations under the end-gas autoignitive con-ditions relevant to internal combustion (IC) engines. Five stoichiometric NH3/H2/air mixtures with differ-ent NH3/H2 blending ratios are studied. A 1D hot spot with varied lengths and temperature gradients is used to induce different ignition modes. The detonation peninsulas are quantitatively identified by two non-dimensional parameters, namely the resonance parameter , xi, and the reactivity parameter , epsilon. In-creasing the H 2 blending ratio up to 80% results in a unique horn-shaped detonation peninsula, i.e., the magnitude of the upper and lower xi limits, xi u,l, near the leftmost boundaries of the detonation peninsula of the rich-H 2 mixtures becomes larger by an order of magnitude as compared to those of the lean-H 2 mixtures. Such behavior is attributed primarily to the large heat diffusion of hydrogen, leading to rapid heat dissipation of the hot spot and the significantly decreased transient xi over time, thus promoting detonation development. The analysis reveals that the characterization of detonation propensity in the rich-H 2 mixtures needs to account for the fast heat diffusion of the initial hot spot, in which the initial magnitude of xi is not representative of its detonability. As such, a correction factor, beta, weighted by the ignition Damkohler number, is proposed to resolve the discrepancy of the xi u,l limits between different NH3/H2/air mixtures. With this correction, the transient magnitudes of xi, xi t, prior to the main ignition are well predicted such that a unified shape of the detonation peninsula for different NH3/H2/air mixture compositions is achieved.(c) 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.