Why Non-Uniform Density Suppresses the Precessing Vortex Core
Why Non-Uniform Density Suppresses the Precessing Vortex Core
复制标题
为什么非均匀密度会抑制进动涡核
DOI：
10.1115/gt2013-95509
复制
发表时间：
2013
影响因子：
1.9
通讯作者：
C. Paschereit
中科院分区：
文献类型：
--
作者：
K. Oberleithner;S. Terhaar;Lothar Rukes;C. Paschereit
经历涡破裂的等温旋转射流已知易受自激流动振荡的影响。它们表现为旋进涡核和大尺度涡结构的同步增长。最近的理论研究将这些动力学与全球流体动力学不稳定模式的发生联系起来。这些全局模式也出现在反应流中，从而对混合特性和火焰动力学产生关键影响。然而，据观察，这些自激流振荡往往被抑制在反应流，而它们显然是存在于等温条件下。这项研究提供了强有力的证据表明，旋进涡核的抑制是由火焰造成的密度分层。通过考虑两种反应流配置揭示了这种机制：第一种配置代表了分离的蒸汽稀释天然气旋流稳定火焰具有强大的旋进涡核。第二个代表天然气旋流稳定火焰锚定在燃烧室入口附近，不表现出自激振荡。实验在通用燃烧室试验台上进行，并使用PIV和LDA捕获流动动态。相应的密度场近似从播种密度使用定量光片技术。实验结果进行了比较，来自流体动力学线性稳定性理论的整体不稳定性。理论推导的全球模式频率和测量的进动频率之间的良好协议提供了足够的证据来得出结论，自激振荡，确实是由全球流体动力学不稳定性。通过对有无密度层结的分析，明确地研究了密度场对全局不稳定性的影响。结果表明，不稳定性的显著变化是由内回流区的径向密度梯度引起的，而不是由平均速度场的变化引起的。本文的工作为分析实际燃烧构型的整体流体动力不稳定性提供了一个理论框架。它允许将火焰位置和由此产生的密度场与旋进涡核的出现相关联。Copyright © 2013 by ASME
Isothermal swirling jets undergoing vortex breakdown are known to be susceptible to self-excited flow oscillations. They manifest in a precessing vortex core and synchronized growth of large-scale vortical structures. Recent theoretical studies associate these dynamics with the onset of a global hydrodynamic instability mode. These global modes also emerge in reacting flows, thereby crucially affecting the mixing characteristics and the flame dynamics. It is, however, observed that these self-excited flow oscillations are often suppressed in the reacting flow, while they are clearly present at isothermal conditions. This study provides strong evidence that the suppression of the precessing vortex core is caused by density stratification created by the flame. This mechanism is revealed by considering two reacting flow configurations: The first configuration represents a detached steam-diluted natural gas swirl-stabilized flame featuring a strong precessing vortex core. The second represents a natural gas swirl-stabilized flame anchoring near the combustor inlet, which does not exhibit self-excited oscillations. Experiments are conducted in a generic combustor test rig and the flow dynamics are captured using PIV and LDA. The corresponding density fields are approximated from the seeding density using a quantitative light sheet technique. The experimental results are compared to the global instability properties derived from hydrodynamic linear stability theory. Excellent agreement between the theoretically derived global mode frequency and measured precession frequency provide sufficient evidence to conclude that the self-excited oscillations are, indeed, driven by a global hydrodynamic instability. The effect of the density field on the global instability is studied explicitly by performing the analysis with and without density stratification. It turns out that the significant change on instability is caused by the radial density gradients in the inner recirculation zone and not by the change of the mean velocity field. The present work provides a theoretical framework to analyze the global hydrodynamic instability of realistic combustion configurations. It allows relating the flame position and the resulting density field to the emergence of a precessing vortex core.Copyright © 2013 by ASME