The Flickering Candle: Transition to a Global Oscillation and Turbulence in a Thermal Plume
The Flickering Candle: Transition to a Global Oscillation and Turbulence in a Thermal Plume
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DOI：
10.1007/978-94-011-5118-4_2
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发表时间：
1998
期刊：
影响因子：
--
通讯作者：
T. Maxworthy
中科院分区：
文献类型：
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作者：
T. Maxworthy
已经对相对较低的燃料流量和使用各种燃烧器类型的丙烷扩散火焰的特性进行了许多实验。使用光学方法观察火焰及其上方的羽流。我们观察到从稳定火焰（其上方的羽流在低流速下表现出螺旋不稳定性）到整个火焰和羽流下部在较高流速下在明确频率下表现出轴对称不稳定性的转变。如果燃烧器由直的、直径恒定的管制成，并且燃料垂直喷射，则振荡开始时的频率和流量与之前的测量结果一致。对于水平喷射燃料的燃烧器，火焰长度非常小，因此它只能被视为其上方不稳定羽流的热源。在这些情况下，振荡频率很稳定，并且不受中等强度的外部声源引起的扰动的影响。火焰/羽流振荡幅度随距燃烧器尖端距离的测量显示出指数依赖性。这些结果以及对各种外部修改效果的观察，例如环形逆流对稳定火焰的不稳定效应，随后首先产生轴对称不稳定，随后产生低频螺旋振荡；强压力扰动等与以下观点一致：向轴对称状态（火焰闪烁时）的转变是由本研究中使用的燃烧器尖端处或附近的绝对不稳定流动的有限区域强制产生的全局激发振荡。这种解释中固有的信息反馈可以通过在火焰尖端上方放置障碍物来增强，在这种情况下，引起闪烁所需的流速会降低。此外，通过控制出口条件来增加穿过燃烧器周围外壳的流速，可以稳定闪烁的火焰。基于先前对表现出全局振荡的类似类型的流动的理论和实验研究，预计会得到这样的结果。最后，羽流的上游通过一系列复杂的涡旋相互作用和扭曲进一步转变为湍流。
A number of experiments have been performed on the properties of propane diffusion flames at relatively low fuel flow rates and using a variety of burner types. Optical methods were used to observe the flame and plume above it. We have observed the transition from a steady flame, with the plume above it exhibiting a helical instability at low flow rates, to an axisymmetric instability at a well defined frequency, of the whole flame and the lower part of the plume, at higher flow rates. In the case of burners made of straight, constant-diameter tubing, with the fuel injected vertically, the frequency and flow rate at onset of oscillation agreed with previous measurements. For burners in which fuel is injected horizontally the flame length was very small, so that it could be considered merely as a heat source for the unstable plume above it. Under these circumstances the frequency of oscillation was robust and unaffected by perturbations caused by a moderately-strong, external sound source. Measurements of the amplitude of flame/plume oscillation with distance from the burner tip showed an exponential dependence. These results and the observation of the effects of various external modifications, e.g. the destabilising effect, on a steady flame, of an annular counterflow with the subsequent generation of, first, an axisymmetric instability followed by a low-frequency, helical oscillations; a strong pressure perturbation, etc., are consistent with the view that the transition to the axisymmetric state, at which the flame flickers, is one to a globally-excited oscillation forced by a finite region of absolutely unstable flow at or near the tip of the burners used in this study. The information feed-back inherent in this explanation could be enhanced by placing an obstacle above the flame tip, in which case the flow rate required to induce flickering was decreased. Also, increasing the flow velocity through the enclosure surrounding the burner, by manipulation of the exit conditions, stabilised the flickering flame. Such a result would be expected based on prior theoretical and experimental studies of similar types of flow exhibiting global oscillations. Finally, the upper reaches of the plume underwent a further transition to turbulence through complex sequence of vortex interactions and distortions.