喷雾碰壁燃烧数值模拟研究

Numerical Simulation Study of Spray Wall Impingement Combustion

燃油喷雾碰壁是小型高压直喷式柴油机中普遍存在的现象.燃油碰壁会影响缸内燃烧过程,进而显著地影响柴油机的动力学、经济性和排放性.为了更好地认识燃油喷雾碰壁燃烧现象,该研究采用数值模拟的方法对该过程进行计算,探究其特殊的燃烧特性.研究结果表明:在碰壁喷雾的两阶段燃烧过程中,喷雾碰壁促进喷雾径向发展半径和卷吸高度的增加,促进近壁面油气混合,在近壁面形成有利于低温点火的条件.低温燃烧反应在混合气较为稀薄的近壁面区域开始,随后向碰壁喷雾中心浓混合气区域发展.随着低温氧化燃烧持续放热,碰壁喷雾雾束中心区域温度逐渐升高,同时积累大量甲醛.由于喷雾碰壁会导致碰壁雾束中心形成较浓混合气,并且低温燃烧放热量较少,导致部分碳氧化物无法完全燃烧,增加了碳烟的生成量.另外随着高温燃烧的进行,温度持续升高,碰壁喷雾卷吸更多环境气体,进而产生大量氮氧化物.

Fuel spray wall impingement is a common phenomenon in small high⁃pressure direct injection diesel engines.Fuel spray wall impingement influences the in⁃cylinder combustion process,and significantly impacts the engine’s dynamics,fuel economy,and emissions.To better understand the combustion characteristics of fuel spray wall impingement,the numerical simulation was applied to calculate the process and explore this process.The results show that,during the 2⁃stage combustion process of spray wall impingement,the impinge⁃ment promotes the radial development radius and the vortex height of the spray,enhances oil⁃gas mixing near the wall,and forms favorable conditions for low⁃temperature ignition near the wall.Low⁃temperature combus⁃tion reactions start in the near⁃wall region,where the mixture is relatively dilute,and then develop into the dense mixed gas area in the center of the impinging spray.As low⁃temperature oxidation combustion continues to release heat,the maximum temperature in the center of the impinging spray will gradually increase,and a large amount of CH2O will accumulate.Meanwhile,the impinging spray can cause the formation of a more con⁃centrated mixture in the center of the impinging spray,and low⁃temperature combustion would release less heat,resulting in the incomplete combustion of some carbon,and increasing the amount of soot generated.Additionally,as high⁃temperature combustion proceeds,the temperature will continue rising,and the impinging spray will draw more oxygen,generating a large amount of NO_(x)through oxidation reactions.