Experimental And Computational Study On The Impingement Characteristics Of Diesel Spray

The diesel spray, impinged on the wall of combustion chamber, is almost inevitable because of the short distance between nozzle and combustion chamber for small bore diesel engine. The impingement has a significant impact on diffusion, evaporation and mixing process of fuel spray, further in affecting the combustion process and emissions of pollutants. So the structure and characteristics of impinging spray were got more and more attention by researchers.A lot of studies showed that, the impingement of spray had important influence on the distribution of spray. The purpose of this study is to find the distribution of spray by experiment and simulation research in modern diesel engine conditions. So, effects of impinging distance（12.5-32.5mm）, charge density（20-60 kg/m³）, ambient temperature（800-1100K）, injection pressure（180-260MPa）, nozzle hole diameter（0.1-0.18mm） and other parameters on the structure and characteristics of the impinging spray by using the planar laser induced exciplex fluorescence（PLIEF） technique and simulation under similar conditions of modern diesel engine was quantitative studied in a constant volume bomb in this dissertation.It is found that, a thin（about 1mm） and rich zone near the wall is formed into both liquid and vapor phase when the liquid spray impinges on the flat with exceeding 3 of the equivalence ratio of vapor phase which verifies the previous important conclusions. The range of rich zone of vapor phase near the wall decreases with the increasing of ambient density and injection pressure and with the decreasing of ambient temperature and orifice diameter. And the transient maximum equivalence ratio(Φ_max) of vapor spray appears near the impinging point or above in the free jet section depending on the liquid spray whether or not impinges on the wall. Φ_max is affected almost a little by ambient density and injection pressure, but decreases with the decreasing of ambient temperature and the orifice diameter.In the injection duration（1ms）, the impingement time of vapor-liquid spray is delayed even no impingement as the increase of impinging distance, the mass fraction of Φ>2 interval of vapor fuel decreases, the mass fraction of 1< Φ≤ 2 interval increases to a certain amount with little change, the mass fraction of 0< Φ≤ 1 interval increases gradually. The fuel distribution of numerical calculation has the same trend with the experimental results. The results of combustion simulation shows that: there is an optimal L/D（ratio of impingement distance and nozzle hole diameter） range based on trade-off of NO and soot emissions, in which the liquid spray do not imping the wall, the fuel quality of vapor spray is mainly distributed in the 1< Φ≤ 2 interval, and small in the Φ>2 and 0<Φ≤ 1 interval, without a large quality of NO and soot too. According to the calculation results, based on trade-off of NO and soot emissions, the empirical formula of optimal impingement distance can well predict the optimal impingement under different conditions.It is also found that, the transient liquid fuel mass fraction（quality ratio of liquid fuel and fuel） decreases when the liquid fuel impinges on the wall which leads to promote evaporation.