A review of oscillation mechanisms and the role of the precessing vortex core (PVC) in swirl combustion systems
A review of oscillation mechanisms and the role of the precessing vortex core (PVC) in swirl combustion systems
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DOI：
10.1016/j.pecs.2005.10.002
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发表时间：
2006
影响因子：
29.5
通讯作者：
N. Syred
中科院分区：
文献类型：
--
作者：
N. Syred
本文综述了旋进涡核（PVC）和其他不稳定性的发生，这发生在旋流燃烧系统，同时识别机制，它允许声学，燃烧和旋流动力学之间的耦合发生。最初，PVC在自由和受限的等温流动中的发生进行审查，通过描述其发生的斯特劳哈尔数和几何漩涡数。利用锁相粒子图像测速技术和激光多普勒测速技术对旋流排入开放环境时产生的三维流场进行了描述。这表明存在旋转和旋进的偏心涡流和相关的中心回流区（CRZ），延伸到一个燃烧器出口直径。在CRZ内和周围，靠近燃烧器口处存在轴向径向涡流，这一点已清楚地显示出来。典型地，发现一个大的主导PV，尽管可以存在许多较低幅度的谐波。这些现象的发生在很大程度上是旋流数和燃烧器几何形状的函数。在燃烧条件下的行为是更复杂的，PVC的发生和幅度也是燃料进入模式，当量比和水平的约束的强功能。轴向燃料进入，除了在特别弱的混合比，往往抑制涡核进动。在某些情况下，还发现了强的双PVC结构。预混或部分预混燃烧可以产生大的PVC，在结构上类似于等温发现：这是由于在旋流燃烧器出口处的火焰前缘的径向位置。如果防止火焰闪回到入口处，则与当量比约为0.7的等温条件相比，PVC的Strouhal数值由0.2激发。限制导致这个参数下降了三个因素非常弱的燃烧。对无约束旋流火焰的单独研究表明，即使涡核进动被抑制，产生的旋流火焰也是不稳定的，并且倾向于响应于流动中的微小扰动而摆动，最重要的是靠近燃烧器出口。另一种形式的不稳定性被证明是与射流进动，往往开始在非常低或零旋流数。射流旋进通常与特殊形状的喷嘴、大膨胀或海崖体相关，并且与PVC不同。Strouhal数被证明是至少一个数量级小于涡破裂后产生的PVC所产生的。旋流燃烧系统中的振荡和不稳定性进行了说明和分析，考虑几种情况下产生的稳定振荡旋流燃烧器/炉系统和两个PVC被抑制燃烧。第一种情况是在2 MW系统中产生的低频24 Hz振荡，其中由于与系统声学的相互作用，PVC频率被激发到等温情况的近六倍。相位锁定的速度和温度测量表明，火焰开始靠近燃烧器出口，周围的CRZ，但位于一个环的高速流动。在下游，火焰已经径向扩展通过高速区域，但是没有适当地占据整个炉。这使得火焰和旋流摆动，激发不稳定性。下一个系列的振荡发生在100千瓦的旋流燃烧器/炉系统，其中在100赫兹范围内的振荡被激发的流场类似于那些在脉动燃烧器中发现的流动周期性地停止在极限循环…
This paper reviews the occurrence of the precessing vortex core (PVC) and other instabilities, which occur in, swirl combustion systems whilst identifying mechanisms, which allows coupling between the acoustics, combustion and swirling flow dynamics to occur. Initially, the occurrence of the PVC in free and confined isothermal flows is reviewed by describing its occurrence in terms of a Strouhal number and geometric swirl number. Phase locked particle image velocimetry and laser doppler anemometry is then used to describe the three-dimensional flow fields, which are generated when swirling flow is discharged into an open environment. This shows the presence of a rotating and precessing off centred vortex and associated central recirculation zone (CRZ), extending up to one burner exit diameter. The presence of axial radial eddies close to the burner mouth, in and around the CRZ, is clearly shown. Typically one large dominant PV is found, although many harmonics can be present of lower amplitude. The occurrence of these phenomena is very much a function of swirl number and burner geometry. Under combustion conditions the behaviour is more complex, the PVC occurrence and amplitude are also strong functions of mode of fuel entry, equivalence ratio and level of confinement. Axial fuel entry, except at exceptionally weak mixture ratios, often suppresses the vortex core precession. A strong double PVC structure is also found under certain circumstances. Premixed or partially premixed combustion can produce large PVC, similar in structure to that found isothermally: this is attributed to the radial location of the flame front at the swirl burner exit. Provided the flame is prevented from flashing back to the inlets values of Strouhal number for the PVC were excited by ∼2 compared to the isothermal condition at equivalence ratios around 0.7. Confinement caused this parameter to drop by a factor of three for very weak combustion. Separate work on unconfined swirling flames shows that even when the vortex core precession is suppressed the resulting swirling flames are unstable and tend to wobble in response to minor perturbations in the flow, most importantly close to the burner exit. Another form of instability is shown to be associated with jet precession, often starting at very low or zero swirl numbers. Jet precession is normally associated with special shapes of nozzles, large expansions or bluff bodies and is a different phenomenon to the PVC. Strouhal numbers are shown to be at least an order of magnitude less than those generated by the PVC generated after vortex breakdown. Oscillations and instabilities in swirl combustion systems are illustrated and analysed by consideration of several cases of stable oscillations produced in swirl burner/furnace systems and two where the PVC is suppressed by combustion. The first cases is a low frequency 24Hz oscillation produced in a 2MW system whereby the PVC frequency is excited to nearly six times that for the isothermal case due to interaction with system acoustics. Phase locked velocity and temperature measurements show that the flame is initiated close to the burner exit, surrounding the CRZ, but is located inside a ring of higher velocity flow. Downstream the flame has expanded radially past the high velocity region, but does not properly occupy the whole furnace. This allows the flame and swirling flow to wobble, exciting instability. The next family of oscillations reviewed occur in a 100kW swirl burner/furnace systems whereby oscillations in the ∼40Hz range are excited with flow fields akin to those found in pulsating combustors where the flow is periodically stopped in the limit cycle of …