Acting mechanism of low ambient temperature on wall-impinging diesel spray ignition at an extensive range

Abstract

The impinging spray ignition has been widely studied, but the mechanism is unclear, especially at low temperatures. Thus, the 3-D simulation was conducted at ambient temperatures of 740–1200 K and wall temperature of 393 K. The results reveal that the fuel–air mixing and ignition processes of an impinging jet are approximately the same as that of a free jet at 1000–1200 K. However, as the temperature decreases, the attached fuel mass increases linearly, and the adhesion ratio is only 4.5% at 1000 K but as high as 32.6% at 780 K. The wall cooling causes the evaporation rate to drop sharply, thus the fuel mass fraction suitable for autoignition decreases. With the decrease of ambient temperature, the maximum temperature after cool flame decreases linearly, and the transition time from cool to hot flames is greatly extended. Compared to free jet, the heat release of impinging jet decreases significantly, and the ignition threshold increases by 40 K. The competition between chemical reaction and fuel diffusion rate divides the impinging ignition into four different modes: (1) The flame initially occurs at the center and then spreads to the edge. (2) The hot flame at the edge spreads to the center. (3) The hot flame spreads to the center but the central region misfires. (4) Only cool flame is observed at the edge. At low temperatures, most of the fuel film remains. At 900 K, 34.4% of the fuel film participates in combustion, but at 780 K it is reduced to 6.6%.