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流涡的形成和发展进行了理论和实验研究。本文的主要研究内容包括以下三个方面：（1）自由射流和射流扩散火焰中湍流涡的激光片光显示。(2)从热流体动力学不稳定性的观点预测湍流涡旋的形成和变形的数值计算。(3)对湍流流动进行图像处理分析的试验。所得结果总结如下。(1)随着运动粘度的增大，火焰中旋涡的尺度增大，垂直于射流轴线的平面内的旋涡对掺混的影响大于轴向剪切层的旋涡。在瞬态区，在射流火焰的外层层流区中形成热的和粘性的烟灰层，在该烟灰层的内部，燃料流由于不稳定性而被弯曲并沿流沿着扩大，从而产生空气卷吸涡流。(2)低粘性喷流核心包含湍流涡旋，由于密度梯度和压力梯度之间的相互作用，即所谓的斜压效应，湍流涡旋可能在层流热地幔附近产生涡度。这导致界面层的不稳定。当涡量增长率大于粘性耗散率时，火焰边界发生变形和拉伸。(3)粒子图像测速技术可用于确定湍流流场中瞬时速度矢量的空间分布，所获得的速度矢量给出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。

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.