A Study of the Thermo Fluid Dynamic Instability Focussing on the Formation and Dissipation of Turbulence in Diffusion Flames

对射流扩散火焰中湍涡的形成和发展进行了理论和实验研究。本研究的主要内容如下：(1)用激光光片方法显示自由射流和射流扩散火焰中的湍流旋涡。(2)从热流动力不稳定性的角度预测湍流涡旋的形成和变形的数值计算。(3)图像处理对湍流的分析试验。研究结果表明：(1)火焰中的涡流尺度随运动粘性的增大而增大，垂直于射流轴线平面内的涡流对气流卷吸和油气混合的影响大于轴向剪切层的影响。在瞬变区，喷流火焰的外部层流区形成了一个热粘性的烟灰层，在其内部，由于不稳定，燃料流被弯曲并沿气流被放大，产生了卷吸空气的涡流。(2)较低粘性的喷流核含有湍流涡流，由于密度和压力梯度的相互作用，可能在层流热地幔附近产生涡度，即所谓的斜压效应。这导致了界面层的不稳定性。当涡量增长速率大于粘性耗散速率时，火焰边界发生变形和拉伸。(3)粒子图像测速技术有助于确定湍流流场中瞬时速度矢量的空间分布，所获得的速度矢量提供了射流中湍流的详细信息，如涡量分布、剪切应变率和湍流尺度等。

Formation and development of turbulent eddies in jet diffusion flames were investigated theoretically and experimentally. The main points of the present study consist of three parts as follows: (1) Visualization of the turbulent eddies in free jets and jet diffusion flames by a laser-light sheet method. (2) Numerical calculations for predicting formation and deformation of turbulent eddies from the standpoint of the thermo fluid dynamic instability. (3) Tests of the picture processing to analyze the turbulent flows. The obtained results are summarized as follows.(1) The scale of eddies in the flame increases with the kinematic viscosity, and the eddies in the plane perpendicular to the jet axis have greater influence on air entrainment and fuel-air mixing than those of the shear layer in the axial direction. In the transient region, a hot and viscous soot layer are formed in the outer laminar region of the jet flame, in the inside of which a fuel flow is bent and is enlarged along the stream due to instability to produce the air-entrainment eddy.(2) A less viscous jet core contains turbulent eddies which may produce vorticity near the laminar hot mantle due to interaction between density and pressure gradients, so called the baroclinic effect. This results in instability of the interfacial layer. Deformation and stretching of the flame boundary take place once the vorticity growth rate becomes stronger than that of dissipation due to viscosity.(3) Particle image velocimetry is useful to determine the spatial distribution of instantaneous velocity vectors in the turbulent flowfield, and the obtained velocity vectors give the detailed information on turbulence in jet flows, such as profiles of vorticity, shear strain rate, and scale of turbulence.