Multiple optical diagnostics on effects of fuel properties on spray flames under oxygen-enriched conditions

Abstract Spray flames development in diesel engines is typically diffusion combustion process controlled by mixing rate, which may cause considerable NOx and soot emissions. Breakthroughs in the field of oxygen preparation technology make it possible to apply oxygen-enriched combustion of spray flames, which has the potential to improve the thermal efficiency of engines and reduce pollutant emissions. In this paper, multiple optical diagnostics were applied to investigate spray flames at oxygen-enriched conditions. The experiments were conducted in an optical constant volume vessel, simulating the in-cylinder conditions of diesel engines. High-speed imaging, two-color pyrometry, and OH* chemiluminescence were used to investigate flame structure, soot flame temperature and KL factor, and flame lift-off length, respectively. The main fuel properties were separated by contrasting different fuels, including diesel (D100), 70% diesel + 30% n-heptane (D70H30), 70% diesel + 30% iso-octane (D70O30), 70% diesel + 30% n-butanol (D70B30), all of which were mixed by volume. The oxygen concentration of the ambient gas was in the range of 21%-70%. Experimental results show that the oxygen concentration has significant influence on the shape, structure, temperature, and species distributions of spray flame, while the fuel properties including cetane number, latent heat of vaporization and oxygen content, have more effects on flame ignition and soot formation under relatively lower oxygen concentration. With the increase of oxygen concentration, the flame lift-off length (FLoL) experiences a fast decrease firstly and then goes down slightly until it remains basically unchanged. The overall trend of FLoL under high oxygen concentrations shows that the FLoL of D100 is the shortest while the FLoL of D70B30 is the longest, and FLoL values of D70H30 and D70O30 are almost under the same level. With the increase of oxygen concentration, the effects of fuel properties on the ignition and development of the spray flame are weakened, the natural luminosity of spray flame increases, the length and width of spray flame reduce in size. At higher oxygen concentrations, the differences in KL factors and soot flame area among all test fuels become smaller. Under the same concentration, pixel averaged flame temperature of D100 is slightly higher than that of the other three fuels, while D70B30 is the lowest. The temperature differences among four fuels are no more than 100 K.