Optical Study Of Diesel Spray Combustion Characteristics Under Wall Influence

The phenomenon of spray impingement in direct injection engine can be divided into liquid-wall impinging and vapor-wall impinging.As everyone knows,the liquid-wall impinging will cause combustion deterioration,and the amount of unburned hydrocarbon and soot increases.In the engine design,the liquid-wall impinging will be avoided as much as possible,but the vapor-wall impingement phenomenon can not be avoided,and the influence rule of spray,combustion and soot formation process is more complex.In this paper,based on the constant volume combustion chamber test platform,a multifunctional spray wall impingement device is designed and manufactured.A variety of advanced optical testing techniques are used to study the spray wall impingement process and the soot generation characteristics in the flame under different fuel injection pressure,environmental conditions and wall impinging conditions,and compare with the free spray combustion process.The main research contents and innovative research conclusions are as follows:(1)A multi-function,multi parameter controllable spray wall impingement device is designed and developed on the high pressure and high pressure constant volume combustion chamber test bench.It realizes the study of vapor-wall impinging combustion and soot characteristics under high temperature and high pressure environment.Based on schlieren method,the quantitative measurement of the parameters such as the vapor spray development of free spray and impinging spray,the vapor spray penetration,the spray rebound height and the ignition delay under the condition of high temperature and high pressure combustion were obtained.Based on the combined optical testing technology of OH* chemical fluorescence method and the diffused back-illumination extinction imaging method,the flame lifted-off length,liquid length and soot mass were obtained synchronously.(2)Under the same conditions,a comparative study of free spray and impinging wall spray shows that the vapor spray penetration is basically the same in front of the wall.And the larger the temperature gradient near the wall,the higher the ignition delay and the flame lifted-off length than the free spray,especially when the ignition position is on the wall,the heat transfer loss is greater,and the ignition delay is postponed.The corresponding flame lifted-off length is longer.(3)The effects of fuel injection pressure,environmental conditions and wall conditions on the development and combustion characteristics of free spray and impinging wall spray were investigated.The results show that,the ignition delay and flame lifted-off length of spray impinging wall and free spray are basically the same under different environmental conditions.They all shorten with the increase of ambient temperature,ambient density and ambient oxygen concentration.With the increase of injection pressure,the ignition delay decreases,but the flame lifted-off length is longer.When the ambient temperature is low,the ignition time is earlier when the distance is short.When the ambient temperature is high,the ignition time is earlier when the distance is long.With the increase of wall temperature,the ignition delay is shortened and the cycle fluctuation is smaller.The spray rebound height increases with the increase of ambient temperature and injection pressure after impinging,while the environmental density,oxygen concentration,wall distance and wall temperature have little effect on the spray rebound height.(4)The effects of fuel injection pressure,ambient conditions and wall conditions on the soot characteristics of free spray and spray impinging wall are studied.Because of the longer flame lifted-off length and the heat transfer between the flame and the wall,and the soot quality of spray impinging is smaller than that of free spray.The concentration of soot increases with the increase of ambient temperature,with the increase of ambient pressure,and decreases with the increase of ambient pressure.The change of ambient oxygen concentration is related to the distance of the wall.When the impinging distance is larger,the oxygen concentration is less affected.When the impinging distance is small,the oxidation rate of soot is higher and the quality of soot decreases significantly.With the decrease of impinging distance,the gas mixture is uneven and the soot mass increases;With the increase of wall temperature,the lifted-off length of flame shortens and the soot mass increases.