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Experimental investigation of local displacement speeds of wrinkled unsteady flames.

机译：起皱的不稳定火焰局部位移速度的实验研究。

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Local flame displacement speeds were measured along the flame front of laminar unsteady premixed flames wrinkled by laminar toroidal vortices. The displacement speed is argued to be the most important and sensitive parameter that must be simulated correctly in numerical simulation of turbulent flames. An axisymetric flame wrinkle is created in order to measure all components of the normal velocity vector and stretch; particle image velocimetry (PIV) and high-speed shadowgraph cinematography yield the difference between the interface velocity and the reactant gas velocity—the displacement speed. This repeatable flame-vortex interaction problem provides a useful test data to assess direct numerical simulation models and flame stretch theory.; Lean and rich methane-air, and lean propane-air flames were investigated to assess stable and unstable preferential diffusion effects on local displacement speeds. The strength of the laminar toroidal vortices was varied from 1.4 to 10 times the unstretched laminar flame burning velocities, S L. Results showed that the local displacement speeds follow the trends predicted by the laminar steady-state theory, but not the magnitudes. Large variations in local displacement speed measurements were found, ranging from –6 to 10 times SL. The negative values occur at the two locations that the steady-state theory predicts low values. Planar laser induced fluorescence of the OH radical was used successfully to locate the flame front boundary in the velocity fields obtained from the PIV images.; Microgravity studies at NASA Lewis 2.2 second drop tower showed that when the stabilizing influence of buoyancy is removed, the wrinkling amplitude of the premixed flames caused by toroidal vortices increases by as much as a factor of three. The degree of wrinkling for preferential diffusion unstable flames is larger than those showed by stable ones. Vorticity fields obtained from the PIV images (at one-g) indicate that baroclinic torques due to buoyancy create “flame-generated vorticity” which induces a velocity that suppresses flame wrinkles. Scaling concepts indicate that the ratio of Baroclinic torques to the Rayleigh-Taylor stabilizing force scales inversely with Froude number. In the present experiment, the velocity induced by Baroclinic torques is equal to that induced by the Rayleigh-Taylor forces. Thus, baroclinic torques in this experiment are important but not dominant.
机译：沿着由层状环形涡旋起皱的层状非稳定预混火焰的火焰前沿测量局部火焰位移速度。位移速度被认为是在湍流火焰的数值模拟中必须正确模拟的最重要和最敏感的参数。为了测量法向速度矢量和拉伸的所有分量，会产生一个轴心火焰皱纹。粒子图像测速技术（PIV）和高速阴影摄影技术可产生界面速度和反应气体速度之间的差异，即位移速度。这个可重复的火焰-涡旋相互作用问题提供了有用的测试数据，以评估直接数值模拟模型和火焰拉伸理论。研究了稀薄的浓甲烷空气，稀薄的丙烷空气火焰，以评估稳定和不稳定的优先扩散对局部位移速度的影响。层状环形涡流的强度是未拉伸层状火焰燃烧速度 S L 的1.4到10倍。结果表明，局部位移速度遵循层流稳态理论预测的趋势，而不是幅度。发现局部位移速度测量值存在很大差异，范围是 S L 的–6到10倍。负值出现在稳态理论预测的低值的两个位置。平面激光诱导的OH自由基荧光已成功用于在从PIV图像获得的速度场中定位火焰前边界。在NASA Lewis 2.2下降塔的微重力研究表明，消除浮力的稳定影响后，由环形涡流引起的预混火焰的起皱幅度增加了三倍。优先扩散不稳定火焰的起皱程度大于稳定火焰表现出的起皱程度。从PIV图像获得的涡度场（为1 g）表明，由于浮力而产生的斜压产生了“火焰产生的涡度”，从而产生了抑制火焰褶皱的速度。标度概念表明，斜压转矩与瑞利泰勒稳定力之比与弗洛德数成反比。在本实验中，斜压扭矩引起的速度等于瑞利-泰勒力引起的速度。因此，该实验中的斜压扭矩很重要，但不是主要的。

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