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流的形成和发展进行了理论和实验研究。本研究的主要内容包括以下三个部分：(1)用激光片法可视化自由射流和射流扩散火焰中的湍流涡流。(2)从热流体动力学不稳定性的角度对紊流涡的形成和变形进行了数值计算。(3)湍流分析的图像处理测试。所得结果总结如下：(1)火焰内涡流的规模随运动粘度的增大而增大，垂直于射流轴向平面的涡流比轴向剪切层的涡流对夹带空气和燃料-空气混合的影响更大。在瞬态区，射流火焰的外层层流区形成热黏的烟灰层，烟灰层内部的燃油流弯曲，由于不稳定而沿流扩大，产生夹带涡。(2)粘性较低的射流核含有湍流涡流，在层流热地幔附近由于密度梯度和压力梯度的相互作用可能产生涡度，即斜压效应。这导致了界面层的不稳定性。当涡量增长速率大于黏度耗散速率时，火焰边界会发生变形和拉伸。(3)粒子图像测速可以确定湍流流场中瞬时速度矢量的空间分布，得到的速度矢量可以给出射流湍流的详细信息，如涡度、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.