Optical Diagnosis Of Spray Flame And Wall-impinging Combustion Of Different Volatile Fuels

Increasingly stringent emissions regulations and "two-carbon" goals require constant innovation in internal combustion engine technology.Finding suitable alternative fuels,improving the thermal efficiency of internal combustion engines,and reducing carbon emissions are important development directions for future engines.Many carbon-neutral fuels,such as alcohol-based fuels,are much more volatile than traditional diesel fuels.Therefore,exploring the development characteristics of spray flames and near-wall flames of different volatile fuels has important theoretical significance and application value for the optimization of fuel characteristics,the development of internal combustion engine combustion theory and efficient and clean combustion technology.In this paper,the flame structure,flame development characteristics,combustion characteristics,intermediate product formation characteristics and soot emission characteristics of three fuels(n-hexane,PRF26 and D53)are test by optical diagnostic techniques such as high-speed self-luminous imaging,RGB two-color method,and self-luminous spectroscopy in a high temperature and high pressure constant volume combustion bomb.The results can be seen as following.Firstly,in the case of a spray flame,in the early stage of n-hexane combustion,a blue premixed flame can be found,and there is a pale yellow flame in the later stage.As the boiling point of the fuel increases,the area of ??the blue flame in the early stage gradually decreases,and the area of ??the yellow flame in the later stage gradually increases,and the SINL of flame gradually increased.As the boiling point of the fuel increases,the distance between the ignition position and the nozzle increases,the ignition delay period is shorter,and the flame area is larger.The results of the self-luminescence spectra of the three fuels show that the main product in the flame is soot radiation.Hence,the RGB two-color method is used to qualitatively analyze the soot emission of the flame and it can be seen that the soot generation in the flame of the high-boiling fuel is higher however the flame temperature is low.Secondly,in the event of flame wall-impingement,the near-wall flame of the three fuels has higher flame SINL,more soot emission,and lower flame temperature compared with the spray flame.The flame brightness distribution near the wall surface of n-hexane is more uniform and the intensity is lower.As the volatility decreases,the flame brightness area is closer to the wall surface area,and the SINL increases.The wall temperature will affect the flame near the wall that the lower wall temperature will lead to prolonged fuel evaporation and mixing time,which will increase the ignition delay time.When the wall temperature is low,the self-luminous brightness distribution of the flame is more dispersed,and the SINL of the flame lower,soot emissions are reduced.The spray impingement angle also affects the flame near the wall.The intensity of the flame tip vortex and the flame height increase with the decrease of the spray impingement angle.The momentum component of the spray perpendicular to the wall actually affects the ignition process.Smaller spray impingement angles should be used to improve mixing and combustion.Therefore,impingement angle optimization,ie,flexible injectors,can be employed to improve combustion.Finally,through the research in this paper,it is found that the ignition position of the highly volatile fuel is shorter from the nozzle,which prolongs the ignition delay period and increases the flame area;the flame brightness and temperature distribution near the wall of the higher volatile fuel are more uniform.With or without wall,the high volatile fuels had lower KL factors and higher flame temperatures,indicating a potential for lower soot emissions and higher NOx emissions from the engine.