湍流预混燃烧火焰结构的分形维数特征研究

Study on the Flame Structure of Turbulent Premixed Combustion of Engines Using Fractal Method

发动机湍流预混燃烧的火焰结构具有自相似性,将分形技术这一新的非线性方法技术引入对湍流预混火焰结构的研究中,基于自行设计的以光学发动机和高速摄像机为核心的试验系统,得到了湍流预混燃烧的火焰结构图像。利用修正数盒法的分形图像处理方法,得到了描述湍流预混燃烧火焰的分形维数特征。在此基础上,对于发动机转速、空燃比、点火提前角和燃料性质等参数对火焰分形维数的影响规律进行了探索性的研究。研究结果表明,湍流预混燃烧火焰的分形维数在燃烧期内先随曲轴转角增加而增加,在达到最大值后,随曲轴转角增加而减小,证明在燃烧中期,火焰锋面具有最大的扭曲度;发动机转速的增加、点火角的提前、混合气的加浓均会使燃烧火焰的分形维数均有不同程度的增加。因此表明上述因素会使燃烧进行得更为剧烈,火焰结构的扭曲程度加强,火焰传播速度加剧。

Due to the existence of self-similarity of engine turbulent premixed flame, the fractal method is applied in this paper to study the flame structure of turbulent premixed combustion in IC engines. Using an optical engine and a high-speed digital camera, the images of flames of premixed combustion are obtained. With an image processing method called ＂modified boxing method＂, the fractal dimension of the flame structure is picked up from the combustion images, and the effects of some engine operation parameters on the fractal dimension of flame structure, such as engine speed, ignition timing, air-fuel ratio and fuel property, are studied in details. The results show that during the combustion process, the fractal dimen- sion shows an increase at early crank angle and a decrease in late crank angle, and this indicates the maxi- mum distortion of flame front in the middle of the combustion. While increasing the engine speed, advan- cing the ignition timing and enriching the mixture, the fractal dimension all increase dramatically, and this suggests that all these parameters will promote the distortion of flame structure and flame propagation.