涡流运动降低柴油机混合气浓度及碳烟排放的数值分析

为了揭示涡流运动对柴油机混合气形成及碳烟排放的影响规律，采用经过实验验证的喷雾及湍流模型，用CFD数值分析软件对某车用柴油机燃烧室内不同涡流条件下柴油喷雾的混合气浓度、速度矢量场、燃油液滴空间分布及油束特性进行了模拟计算．模拟结果表明，当涡流比从0到5．0依次增大时，过喷孔轴线的铅垂面内喷雾浓度场局部浓区燃空当量比逐渐降低，而过喷孔轴线且与铅垂面垂直的平面内喷雾浓度场局部浓区的燃空当量比则先降后升，而不是逐渐下降．只有合理选择剖切平面，即选择过喷孔轴线且与铅垂面垂直的平面，才能正确评价涡流运动对燃烧室内混合气浓度分布及碳烟形成区域分布的影响规律．组织燃烧室内气流运动，须兼顾与气缸轴线垂直的水平面内的涡流运动和过气缸轴线的铅垂面内的湍流或滚流运动．涡流比太大，铅垂面内的湍流或滚流太弱，会削弱喷雾射流对燃烧室底部空气的卷吸，降低燃烧室底部的空气利用率．随涡流比增大，射流顺涡流方向的弯曲度增大，不同喷孔的油束会发生相互干涉，在靠近气缸中心的区域内形成局部浓混合气，不利于降低碳烟排放．对具体的燃烧室结构和喷油系统，合理匹配涡流运动十分必要．

Numerical Analysis of Reduction in Mixture Concentration and Soot Emissions in a Diesel Engine by Swirl

In order to reveal the effects of swirl on mixture formation and soot emissions in a diesel engine, numerical simulations of mixture concentration, velocity vector field, fuel droplet spatial distribution and spray characteristics of a diesel spray under different swirl ratios in the combustion chamber of an automotive diesel engine were performed, using a CFD three-dimensional numerical analysis code and experimentally-validated models of spray and turbulence. The simulated results indicate that, with the increase of swirl ratios from 0 to 5.0, the fuel-air equivalence ratios of the local richest area decrease gradually in the vertical plane which passes through an injector hole axis,while they first decrease and then increase in the plane which passes through the same injector hole axis and is perpendicular to the vertical one. It is essential to choose a correct section. The plane which passes through the injector hole axis and is perpendicular to the vertical one is selected for the purpose of exactly assessing the effects of swirl on mixture concentration and soot formation region distribution. Attention nust be paid to both the swirl in the horizontal plane which is perpendicular to the cylinder axis and the turbulence or tumble in the vertical plane which passes through the cylinder axis when air flow in the combustion chamber is organized. If the swirl ratio is too high, and the turbulence or tumble in the vertical plane is too weak, the entrainment of the spray jet to the air at the bottom of the combustion chamber will be impaired, and the air utilization ratio will be reduced. With the swirl ratio increased, the jet camber along the swirl direction becomes heavy, jets of different injector holes interfere with each other, and a local richest mixture region appears in the vicinity of the cylinder center. This is not beneficial to reduction in soot emissions. So it is imperative that swirl should be reasonably matched with a specific combustion chamber geometry and fuel injection system.