Simulation of the air/fuel mixing of an HSDI diesel engine. Part 1: a new dense spray vapour coupling submodel

摘要

A numerical study has been performed to investigate the soot emission from a high-speed single-cylinder direct injection diesel engine. The computational conditions were set to be the same as the test conditions in the experiments where measurements had been performed at two running speeds with two injector protrusions. It was shown that the KTVA CFD code can predict the experimental trend, where at a low-speed running condition a higher smoke reading is reached when increasing the injector protrusion into the piston chamber and, conversely, a lower smoke reading was recorded for the same change in injector protrusion at a high running speed condition. Although computational predictions yielded the same trend as the experimental results, the magnitudes of the smoke emissions were an order of magnitude over those predicted. Evidence of inappropriate air/fuel mixing of the model was seen via rates of heat release analyses, especially in the high-speed conditions. Therefore, efforts to reduce this discrepancy by way of improvements to the KTVA submodels were made. In particular, modifications to the breakup and evaporation models have been made in order better to represent the mixing of the high-speed liquid jets. A gaseous sphere per liquid droplet model was employed to improve the current KTVA model by improving the mass coupling effects, which effectively delays the addition of spray source terms to the gas phase equations. Results of the modified models showed improvements in the vapour dispersion of the atomizing liquid jet, thus affecting the mixing rates and predicted smoke emissions. Further improvements on the momentum coupling will be presented in Part 2 of this work.