Research On The Evaluation Of Diesel Spray Fuel/Air Mixting And The Fuel-rich Mixting Characteristic In Diversion-Type Combustion Chamber

The fuel/gas mixture of diesel spray is one of the most important factors which affects combustion and emission performance.At present,though many researches on diesel spray have been performed,there still exist some problems about experiment analysis method,breakup model and fuel/gas mixture need to be studied deeply.Therefore,in this work,the diesel spray characteristic and the fuel/gas mixture process were investigated with experiment and simulation.In addition,the relative results are applied to explore the mixture characteristic and the mechanism of the swirl-style combustion chamber under low excess air ratio conditions.A direct imaging processing code,and a schlieren imaging processing code,based on ‘two-peak' method and the method of stacking the subtraction images of each two adjacent images and the ‘isolated point denoising' process,were also developed respectively for the study of direct imaging and the schlieren imaging technique.It has been proven that the codes are high-efficiency and the adaptation is strong.Through the spray characteristic experiment,the effect of the four factors —background temperature,background gas density,orifice diameter and injection pressure—on the two evaluation parameters for fuel/gas mixture—the gas volume percentage and the average excess air ratio within the spray—was investigated.In addition,the sensitivity analysis of the four factors on the two evaluation parameters was also discussed.It was found that there exists an increase in the gas volume percentage with an increase in background temperature,background gas density and injection pressure or an decrease in orifice diameter.The sensitivity of background temperature on the gas volume percentage is the highest,and the second are orifice diameter and injection pressure.The last is backgroud gas density.There exists an increase in the average excess air ratio within the spray with an increase in background temperature,background gas density and injection pressure or an decrease in orifice diameter.The sensitivity of orifice diameter and injection pressure on the average excess air ratio within the spray is higher than that of background temperature and density.Through numerical analysis on spray characteristics,the effect of the different factors on the breakup time constant C2 in WAVE model was studied.It was found that the effect of orifice diameter,injection pressure and background gas density on the C2 are negligible.Background temperature has a strong effect on the C2.the C2 decreases with an increase in background temperature.The relationship between the C2 value and the background temperature seems to be of approximate inverse proportion.Therefore,a modified WAVE model with a variable C2 based on background temperature was proposed.In addition,the effect of background temperature on the relative velocity between droplets and gas was obtained.Furthermore,the reason that background temperature strongly affects the C2 was discussed.The simulation result based on the modified WAVE model shows that there exists a significant decrease in the liquid length with an increase in in-cylinder temperature.The rusult is more reasonable and the simulation accuracy is higher.A series of evaluation parameters for the fuel/gas mixture was proposed,including three points of view: the effect of the combustion chamber on spray and space utilization,the total characteristic of fuel/gas mixture within the spray,the distribution of the mixture gas concentration within the spray.With these evaluation parameters,the advantage and the mechanism of the swirl-style combustion chamber under low excess air ratio conditions were researched.It was found that the average velocity of rich mixture gas in the injeciton direction,which is the evaluation parameter about the effect of the combustion chamber on spray,decreases when the excess air ratio turns to be 1.5 from 2.0.However,for the omega combustion chamber,the decrease in the average velocity is disadvantageous,while for the swirl-style combustion chamber,that is advantageous.The swirl effect is enhanced in the swirl-style combustion when the excess air ratio decreases.Though the enhanced swirl effect hardly increased the value of the average excess air ratio within the spray,it restrains the increase of the equivalence ratio in the rich mixture gas mass fraction ranging from 2 to 4.In addition,it improves the combustible mixture gas mass fraction in the equivalence ratio range from 1 to 2.This is the main reason that the swirl-style combustion has an advantage in the fuel/gas mixture and combustion under low excess air ratio conditions.