Evolution mechanism of n-dodecane spray ignition characteristics with variable impingement distance under high-altitude environment

Fuel spray inevitably impinges on the combustion chamber when the aviation piston engine operates in a high-altitude and low-temperature environment, which affects the mixture formation and ignition. To elucidate the impact of wall involvement on the ignition characteristics of spray, a series of optical experiments were conducted for free sprays and impingement sprays, with ambient temperatures ranging from 660 ∼ 800 K and impingement sprays with distances ranging from 20.56 ∼ 50.21 mm. Moreover, the distribution patterns of spray flow fields, temperatures and equivalence ratios were analyzed. The study results reveal that the spray-wall interaction (SWI) significantly modifies n-dodecane spray ignition characteristics, with experimentally observed reductions of 10.8 % in minimum ignition temperature (from 740 K under free spray conditions to 660 K at 20 mm impingement distance) and 67 % in ignition delay time (IDT) (from 4.5 ms to 1.5 ms at 740 K ambient temperature conditions). Both the minimum ignition temperature and IDT exhibit progressive decreases with reduced impingement distance. This enhancement originates from the wall jet vortex dynamics, where the induced negative tangential velocity promotes enhanced entrainment of high-temperature ambient gas. This accelerated thermal feedback to low-temperature reactions (LTR) elevates local mixture reactivity, achieving critical high-temperature reaction (HTR) thresholds faster than free spray conditions. Reduced impingement distances can increase the negative tangential velocity by up to 7.9 m/s at the same moment, and form a fuel-lean zones (Φ = 0.5 ∼ 1.0), with the temperature rise occurring 0.7 ms earlier, thereby optimizing LTR-to-HTR transition kinetics.