Research On The Characteristic And Combustion Of The Spray Impingement Near The Wall By Means Of Optical Diagnostics

Advanced combustion technology is the key to improve the thermal efficiency and reduce emissions in the internal combustion engines.The process of combustion in limited space is complex and turbulent,which determines the wall will affect the process of flame development and pollutant evolution.The effect of the wall will decrease the comprehensive performance of internal combustion engine.In this paper,methods of laser induced incandescence(LII),laser induced fluorescence(LIF)and two-color high-speed imaging were the diagnostic methods to detect the mixture near the wall,structure of the combustion flame and the formation of PAH and soot.Firstly,PAHs and soot formation were studied for a blend of n-heptane and toluene in rich partially-premixed flames and non-premixed flames that were diluted with inert gas in order to control flame temperature and added wall to change the flame structure.Laser-induced fluorescence(LIF),laser-induced incandescence(LII)and two-color pyrometry were used to study the effects of flame temperature on PAHs and soot evolution.It was found that the evolution of PAHs and soot depends strongly on flame temperature.Small PAHs(A2&A3)were formed at 800-1200 K,larger PAHs(A4)were formed at 1200-1400 K,and the soot formation began at 1600-1800K.In the rich mixture,the temperature increased promote soot formation instead of the oxidation.The fluorescence signal of PAH increased first and then weakened when the wall surface was raised,and the signal intensity was the highest at 20 mm and 25 mm.The temperature at different heights has the opposite effect on the distribution of PAH:when the wall is low,PAH increases as the wall temperature increases,and when the wall is higher,PAH decreases as the wall temperature increases.Then,The characteristic of the spray was investigate in the constant volume bomb.It found that,the wall-impingement effect decrease the spray penetration and increase the cone angle by comparing the free spray.The distance of between the wall and spray tip is most important factor to change the characteristic of the spray.The effects of different wall-temperatures on spray-impingement have been investigated in a high-temperature high-pressure constant volume combustion vessel.A cooling equipment was used to adjust the temperature difference between the wall and the ambient gas.The results shows that the liquid phases of spray do not reach the wall except the condition of low wall-temperature and high injection pressure.The liquid penetration develops and then becomes constant after 1ms from the start of injection.With the increase of injection pressures(600-1600 bar),the liquid concentration of spray decreases,however the liquid penetration decreases insignificant.The wall temperature and the injection pressure have little influence on liquid interpenetration process.For the vapor phase of spray,the high concentration regions(equivalence ratio(?)>1)mainly distribute in the area of 10 mm away the impact point on the wall.The decreasing wall temperature deteriorates the mixing process of fuel and ambient gas,but the effect is weakened with the increase of injection pressure.At last,The qualitative mixture concentration distributions and the flame characteristics of wall-impinging diesel fuel spray at different wall temperatures and ambient pressures were investigated in a high-temperature high-pressure constant volume combustion vessel.as the wall temperature increases,the vapor-rich field extends to the region close to the wall and the area of high vapor fluorescence intensity increases.However,the wall temperature only has a little influence on the liquid phase of the spray.The flame luminosity intensity increases and the ignition delay decreases as the wall temperature increases.Cases at higher wall temperature show more soot emissions,higher soot formation and oxidation rate.In the condition of Pa=4 MPa,the fuel has evaporated completely before impinging on the wall.The flame area and height at different wall temperatures are nearly the same.The ignition position is observed near the impingement point.However,in the condition of P_a=2MPa,there are some liquid phase of the fuel impinging on the wall.The flame area and height increase as the wall temperature increases.The ignition position is always observed in the wall jet region.The distance between ignition position and the axis of the spray decreases as the wall temperature increases.