Experimental investigation of fuel spray and combustion with wall impingement under premixed conditions: A comparative analysis of flat wall and 2-D piston cavity

In small-bore diesel engines, spray impingement phenomena often occur. To investigate the fuel mixing and combustion characteristics during spray-wall impingement, experimental studies were conducted using Laser Absorption Scattering (LAS), OH∗ chemiluminescence, and two-color pyrometry for two impingement modes under the premixed combustion conditions (flat-wall and 2-D piston cavity). The results indicate that vapor-phase concentrations near the walls of both impingement modes are relatively high. The 2-D piston cavity impingement exhibits richer droplets fuel near the wall, with higher vapor-phase concentrations and lower evaporation rates. In the late stage, the evaporation rate of 2-D piston cavity impingement exhibits a smoother variation, implying increasing difficulty in spray evaporation in later stages. Intense reaction zones of both impingements are near the wall surface. The combustion process of flat-wall impingement spray flames is relatively gentle, with shorter ignition delays and unburned regions upstream of the spray. The initial rise rate of OH∗ intensity in 2-D piston cavity impingement is faster, with higher and sharper peaks. Soot from both impingement modes mainly accumulates near the wall region, with flat-wall impingement exhibiting more low-soot combustion reaction areas. The flame temperature of 2-D piston cavity impingement is higher, with longer duration of high-temperature flames and more richer soot concentration. The distribution of soot in 2-D piston cavity impingement is more dispersed, not only concentrating near the impingement point. Downstream of the spray, along the fuel flow path the distribution of fuel gas phase, OH∗ intensity, and KL values exhibits a double-peak distribution resembling the letter “M" with each peak corresponding to each other. This suggests that the concentration distribution of the spray directly influences the combustion process. Finally, we created schematic diagrams of spray and flame development under premixed conditions for flat wall and 2-D piston cavity impingement. These help researchers and engineers better understand fuel mixing and combustion characteristics, offering insights for designing and optimizing small engine piston chambers.