Impinged Diesel Spray Combustion Evaluation for Indirect Air-Fuel Mixing Processes and Its Comparison with Non-Vaporing Impinging Spray Under Diesel Engine Conditions 2019-01-0267

Under low-temperature combustion for the high fuel efficiency and low emissions achievement, the fuel impingement often occurs in diesel engines with direct injection especially for a short distance between the injector and piston head/cylinder wall. Spray impingement plays an important role in the mixing-controlled combustion phase since it affects the air-fuel mixing rate through the disrupted event by the impingement. However, the degree of air entrainment into the spray is hard to be directly evaluated. Since the high spray expansion rate could allow more opportunity for fuel to mix with air, in this study, the expansion rate of impinged flame is quantified and compared with the spray expansion rate under non-vaporizing conditions. The experiments were conducted in a constant volume combustion chamber with an ambient density of 22.8 kg/m³ and the injection pressure of 150 MPa. The ambient temperature was set to 900 K and 1000 K to study the effect of ambient temperature on the air-fuel mixing process for combustion experiments. Under the non-reacting experiments, the chamber was filled with nitrogen at an ambient temperature of 423 K. The injection pressure was 150 MPa and ambient density was 22.8 kg/m³. The patterns of liquid spray expansion and flame propagation on the impinging plate were visualized by a high-speed camera from bottom view by using Mie scattering and natural luminosity, respectively. An in-house MATLAB code was used to post-process the images. The local flame front and expansion rate were compared with the expansion distance and expansion rate under non-vaporizing conditions, respectively. The low ambient temperature has been found to slow the flame expansion rate at the early stage of combustion after the impingement. In addition, local temporal heat flux at three different locations on the impinging plate was obtained to quantify heat loss by flame impingement. The local heat flux of impinging point is governed by the flame temperature due to the direct flame impingement while the local heat flux near to the impinging point is governed by the flame expansion rate and flame temperature.