Experiments on the periodic instability of buoyant plumes and pool fires
Experiments on the periodic instability of buoyant plumes and pool fires
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DOI：
10.1016/0010-2180(93)90090-p
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发表时间：
1993-04
影响因子：
4.4
通讯作者：
B. Cetegen;T. A. Ahmed
中科院分区：
文献类型：
--
作者：
B. Cetegen;T. A. Ahmed
对浮力丙烷扩散火焰进行了实验研究，以确定引起这些火焰源附近周期性振荡的机制。这种现象，在文献中通常被称为“膨化”，表现为扩散火焰锋面在轴对称火源附近的准周期振荡，形成大规模燃烧的旋涡结构。实验诊断主要涉及在各种实验条件下的流动可视化、速度和压力测量。首先，用等温氦气或高温燃烧产物作为浮力流体，研究了来自直径0.10 m和0.3 m的浮力非燃烧羽流的行为。结果表明，氦羽状物在两个尺度上均表现出膨化现象，其膨化频率与火焰在D - 12尺度下的膨化频率相似。直径0.30 m燃烧器上的热燃烧产物羽流湍流较大，与后一种燃烧产物热气体羽流点火时产生的火焰相比，膨化程度较弱。其次，用不可燃气体稀释燃料，研究了热量释放的影响。研究发现，只要燃料流维持扩散火焰，膨化就会持续存在，尽管其强度随着稀释度的增加而减弱。第三，研究了内外扰动对火焰的影响。这些实验有力地表明，燃烧器表面附近的火焰锋面运动与大型燃烧结构的下游发展存在耦合。在实验基础上，提出了一种膨化机理。膨化机理包括：(1)悬浮羽流气体在静止环境中加速，在其表面上方一个燃烧器直径的范围内形成环形涡结构；(2)环形涡在向上对流时对燃烧器唇附近火焰表面的衰减影响；(3)悬浮气体在火焰包膜内的积累及其浮力加速度形成下一个涡结构。膨化频率随燃烧器直径的缩放与燃烧器表面上方一个直径高度内的环形涡的对流速度有关，如使用简单的运动学模型所示。
An experimental study of buoyant propane diffusion flames was undertaken to identify the mechanism responsible for the periodic oscillations near the source of these flames. This phenomenon, often referred to as “puffing” in the literature, exhibits itself as quasi-periodic oscillations of the diffusion flame front near the axisymmetric source of a fire with formation of large scale flaming vortical structures. Experimental diagnostics primarily involved flow visualization, velocity, and pressure measurements under a variety of experimental conditions. First, the behavior of buoyant, noncombusting plumes originating from 0.10-and 0.30-m-diameter sources was investigated with either isothermal helium or high temperature combustion products as the buoyant fluid. It was found that the helium plumes exhibited puffing at both scales with puffing frequencies similar to the flames scaling with D− 1 2. The plumes of hot combustion products on the 0.30-m-diameter burner were highly turbulent and puffing was very weak compared with the flames that were generated upon ignition of the latter hot gas plume of vitiated combustion products. Second, effects of heat release were studied by dilution of fuel with a noncombustible gas. It was found that puffing persisted as long as a diffusion flame was sustained by the fuel stream, although its intensity diminished with increasing dilution. Third, effects of disturbances both internal and external to the flame were studied. These experiments strongly suggest that there exists a coupling of the flame front motion near the burner surface with the downstream development of large-scale flaming structures. Based on the reported experiments, a puffing mechanism is suggested. The puffing mechanism involves (1) acceleration of buoyant plume gas in stagnant surroundings resulting in formation of a toroidal vortical structure within one burner diameter above its surface,(2) the decaying influence of the toroidal vortex on the flame surface near the burner lip as it convects upward, and (3) the accumulation of buoyant gas inside the flame envelope and its buoyant acceleration to form the next vortical structure. The scaling of puffing frequency with the burner diameter is connected to the convection speed of toroidal vortices within one diameter height above the burner surface as it was shown with the use of a simple kinematic model.