Direct combustion of liquid ammonia sprays offers a promising carbon-free energy conversion pathway, but the fundamental droplet–droplet interactions governing phase change, ignition, combustion, and pollutant formation remain poorly understood. In this study, we present interface-resolved unit-cell simulations coupling non-ideal vapor–liquid equilibrium, detailed gas-phase kinetics, and multi-component transport to elucidate the autoignition and combustion dynamics of monodisperse ammonia droplet clouds. Results reveal a non-monotonic, U-shaped dependence of the critical autoignition temperature and ignition delay on inter-droplet spacing. An optimal cloud density emerges where physical confinement effectively pools heat and radicals to overcome severe evaporative cooling, substantially lowering the ignition threshold. Originating in extremely fuel-lean regions, the pre-ignition kernel undergoes a topological transition: in dense clouds, the kernel propagates outward and accumulates heat and radicals near the symmetry boundary before ignition, whereas in sparse clouds, it reverses direction prior to reaching the boundary. Crucially, the threshold for this kernel detachment aligns with the optimal cloud density for fastest ignition. Chemical explosive mode analysis demonstrates a universal two-stage autoignition sequence: initial radical runaway phase driven by , , and accumulation, followed by thermal runaway dominated by the exothermic reaction. After ignition, rapid surface water uptake suppresses evaporative cooling and inevitably drives the droplet to boiling. Finally, a fundamental emission trade-off is identified, demonstrating that single-stage spray combustion cannot simultaneously eliminate nitrogen oxides ( ), , and unburned . As the global equivalence ratio ( ϕ g ) increases, emissions plummet at the expense of elevated slip, while exhibits a distinct bell-shaped peak. Even at ϕ g = 1 , significant unburned persists due to early flame extinction, creating a reducing environment that favors reduction. Consequently, operating the primary spray under moderately rich conditions ( ϕ g > 0 . 8 ) to intrinsically suppress , followed by secondary oxidation for slip, emerges as an optimal staged-combustion strategy. Novelty and significance statement This study establishes previously unrecognized links between micro-scale droplet interactions and the macroscopic autoignition, combustion, and emission dynamics of ammonia clouds. The primary novelty lies in uncovering a U-shaped dependence of autoignition thresholds and delay times on inter-droplet spacing. This behavior arises from a newly identified topological transition of the pre-ignition kernel, which originates in extremely fuel-lean regions. At an optimal cloud density, physical confinement effectively pools heat and radicals, overcoming evaporative cooling and lowering the ignition threshold. Furthermore, this work provides fundamental proof that single-stage combustion cannot simultaneously eliminate , , and unburned . Notably, even under global stoichiometric conditions, early flame extinction causes substantial slip with negligible emissions. The significance of this research is twofold: it provides a theoretical foundation for the autoignition and combustion dynamics of ammonia sprays, and it physically mandates the necessity of staged-combustion strategies, offering guidelines for designing low-emission ammonia combustors.
Li et al. (Tue,) studied this question.