"Bump燃烧室"内新概念稀扩散燃烧混合气形成机理的研究

基于自行研制的实验装置,用片状激光诱导荧光法(PLIF)对普通商用柴油喷雾的撞壁混合过程进行了实验研究,并用CFD数值分析软件对其进行了模拟计算,二者结果基本吻合．平板和实际燃烧室的实验及计算结果均表明,撞壁射流在遇到限流沿(Bump)后会剥离壁面,形成二次空间射流,扩大撞壁射流与空气的空间混合体积及混合速率,出现与周围空气迅速混合的“闪混”现象,减少壁面燃油堆积量．计算结果还表明,Bump的存在改变了缸内气流运动的流场结构,Bump附近旋向相反的“双涡结构”极大地增强了二次空间射流对周围空气的卷吸,促进了燃油与空气的混合,是Bump燃烧室内稀混合气形成及稀扩散燃烧的关键所在．

A Study on the Mechanism of New Conceptual Mixture Formation of Lean Diffusion Combustion in the Bump Combustion Chamber of a Diesel Engine

Based on a self-made optical experimental facility and a CFD numerical analysis code, experimental investigations and numerical simulations of the mixing process of an impinging diesel fuel spray were carried out. The computed results were in good agreement with the experimental ones obtained by planar laser-induced fluorescence (PLIF) method by comparison. Both measured and computed results with a plane or an actual combustion chamber wall indicated that the wall-impingement jet would stripped off the wall and formed a secondary space jet when it met with the bump on the wall. This resulted in the rises of space mixing volume and mixing rate of the wall-impingement jet with the surrounding air, and a "flash mixing" phenomenon appeared with rapid mixing of diesel fuel and air. And the amount of fuel accumulated on the wall was reduced. The computational results also showed that the bump on the chamber wall changed the flow field structure of in-cylinder gas motion, and that the counter-flow "two-vortex-structure" near the BUMP significantly enhanced entrainment of the secondary jet to the surrounding air and thus promoted the mixing of fuel with air. And it was the key to the formation of lean mixture and lean diffusion combustion in the bump combustion chamber.