Three-dimensional Numerical Simulation Of Atomization And Combustion Characteristics Of Diesel Under Ultra-high Injection Pressures

In recent years,the development of economy and technology had made the automobile industry improve rapidly.But the problem of resource shortage and environmental pollution had become increasingly prominent The theme of saving resources and protecting the environment had gradually become the most important requirement in the development of diesel engines.Increasing injection pressure as an important measure to improve atomization and combustion performance of diesel had been gradually applied to new technologies of diesel engines.According to the development trend of products of international mainstream parts suppliers,the fuel injection system with injection pressure over 300MPa would appear soon.Scholars at home and abroad had gradually focused their research on the atomization process of ultra-high pressure fuel.Up to now,the internationally main viewpoint on ultra-high pressure was that the injection pressure exceeds 300MPa.Although some scholars had carried out many studies on the atomization characteristics of ultra-high pressure fuel spray,the content of studies were simpler and the injection pressure in these studies were basically less than 400MPa.Therefore,it was of great significance to explore the atomization characteristics and air entrainment characteristics of ultra-high pressure diesel spray.Firstly,the independence of the meshes designed in the calculation process was analyzed.Then the adaptability of the turbulence model and the atomization model was studied.The specific research contents are as follows:(1)Based on the experimental results of diesel spray penetration distance under 300MPa injection pressure,the effects of mesh generation method and minimum mesh size on the penetration distance were compared and analyzed.The optimal meshing method and the minimum mesh size of 2mmxl.5mm are determined.(2)Comparing the calculated and experimental values of the penetration distances under different turbulence models and atomization models respectively,the best RNGK-e turbulence model and KH-RT atomization model are determined.(3)Two model constants in KH-RT atomization model were calibrated by comparing the calculation results of penetration distance and SMD.The optimum model parameters Bi and CRT were determined to be 9 and 1.5 respectively.Secondly,the atomization process of diesel spray under different ambient conditions and injection pressure was studied from the aspects of macro-characteristic parameters,micro-characteristic parameters,gas-phase flow field status,distribution of material composition and air entrainment quality.The specific research contents are as follows:(1)Prediction of atomization characteristics and air entrainment characteristics of diesel spray under 300～700MPa injection pressures based on experimental conditions.(2)The effect of ambient pressure(3.5～5.5MPa)on the atomization characteristics and air entrainment characteristics of diesel spray was investigated when ambient temperature was room temperature and injection pressure was 300MPa.(3)When the ambient temperature was room temperature and ambient pressure was 4.5MPa,the atomization characteristics and air entrainment characteristics of diesel spray under 300～700MPa injection pressures were analyzed.(4)When the ambient pressure was 4.5MPa and the injection pressure was 300MPa,the effect of ambient temperature(800-1200K)on the atomization characteristics and evaporation characteristics of diesel spray was studied.(5)When the ambient pressure was 4～5MPa and the ambient temperature was 1000K,the effect of injection pressure(300-700MPa)on the atomization characteristics and evaporation characteristics of diesel spray was analyzed.It was found that the increase of spray pressure will increase the penetration distance and the total number of droplets under all environmental conditions.But the growth amplitude would gradually slow down.Finally,the effect of injection pressure(300～700MPa)on the combustion characteristics of diesel spray was studied by analyzing the distribution of gaseous temperature and pressure,flame temperature,heating rate,flame length,diesel and oxygen consumption,reaction rate,reaction heat release,CO2,Soot,NO emissions,and so on.It was found that the relative parameters of gas flow field and flame characteristics and reaction rate would increase with the increase of injection pressure,.But the growth amplitude would gradually slow down.