Large Eddy Simulation of the Low-Temperature Combustion Process of Diesel Spray Flames

Diesel spray flame is a complicated physics and chemistry process. Particularly at low-temperature combustion conditions, there are many unknown combustion phenomena. Therefore, in the present work, a numerical study was performed to investigate the combustion process of n-dodecane spray flame at different ambient temperatures by large eddy simulation(LES). First, for validation of the accuracy of the model, the simulated values were compared with the experimental data from the Engine Combustion Network, including the spray development process, ignition delay time, and flame lift-off length. Results showed that the LES-LEM can accurately capture diesel spray and combustion characteristics, which were close to the experimental data. The low-temperature combustion process of spray flame was then deeply analyzed. Results showed that increasing the initial temperature reduced the ignition delay time at first stage and overall ignition delay time, and the ignition delay time was prolonged at the second stage. Meanwhile, the peak of heat release rate will occur in the rich-mixture region with increasing initial temperature. Ketone peroxides and CH2O represented the multi-stage ignition process of the spray flame. Near the ignition delay time, the peak of mass fraction of OC12H23OOH gradually moved toward a rich-mixture position far from the stoichiometric mixture. In addition, we also studied the effects of initial temperature on the total heat release and the production of key intermediates, such as C2H2. The accumulated mass of the C2H2 in the soot precursors at 900K was larger than that under low-temperature conditions. This was because the ignition delay time and flame lift-off length were reduced at higher initial temperature, resulting in a relatively worse mixing of fuel and air and the formation of a more rich mixture. As a consequence, the soot formation became beneficial. © 2020, Editorial Board of Journal of Tianjin University(Science and Technology). All right reserved.