Experimental investigation of the impact of temperature stratification on the occurrence of a global mode in swirling jets undergoing vortex breakdown

在现代燃气涡轮机燃烧室中，通常使用经历涡破裂的旋流射流。涡破裂伴随着涡核绕射流轴线的曲折运动。这种现象被称为旋进涡核或短PVC。对等温旋转射流中PVC的发生进行了广泛的研究。结果表明，PVC是一种全局性的不稳定模态，在整个流动区域内都是活跃的，而在非等温条件下，导致PVC阻尼的基本物理机制尚不清楚，这是等温旋转射流与非等温旋转射流（有燃烧或无燃烧）之间的知识差距。有必要了解这些机制，以开发一个有效的控制PVC在非等温流动。特别是在非等温旋流中，需要有效的控制。旋流燃烧室中PVC的控制可以用来影响大尺度流动结构对火焰动力学的影响。在化学反应流中，流体动力学不稳定性、热声不稳定性和几何边界条件以复杂的方式相互作用。因此，在本研究项目中，对简化系统进行了研究。这些系统是非等温的，但不发生化学反应。在这样做时，检查温度对PVC的具体影响。本文将详细讨论以下问题：控制参数雷诺数、旋流数和温度比对PVC的影响是什么？在非等温旋转射流中PVC的抑制是由稳定性的改变引起的吗？温度分布和PVC的抑制对流场有什么影响？澄清上述问题的基础是对无约束旋转射流非等温温度场和流场的广泛测量。稳态和瞬态测量的流量被用来显示的控制参数，即雷诺数，旋流数，和温度比的影响。通过适当的正交分解，PVC的空间结构提取的测量。流动的稳定性可以从PVC的时空演化中推导出来。这项工作承诺基本的新发现，考虑到非等温旋转射流及其稳定性。因此，PVC的发生和流动的温度分布之间的因果关系将被示出。

Swirling jets undergoing vortex breakdown are commonly used in modern gas turbine combustors. The vortex breakdown is accompanied by a meandering motion of the vortex core around the jet axis. This phenomenon is referred to as the precessing vortex core, or short PVC. Extensive research has been done on the occurrence of the PVC in isothermal swirling jets. It was demonstrated that the PVC is a global instability mode that is active in the entire flow domain.The knowledge gap between isothermal swirling jets and non-isothermal swirling jets with or without combustion is the fact that in the non-isothermal case it is so far unknown which fundamental physical mechanisms lead to the damping of the PVC. It is necessary to understand these mechanisms in order to develop an efficient control of the PVC in non-isothermal flows. Especially in non-isothermal swirling flows an efficient control is required. The control of the PVC in swirl combustors can be used to influence the bearing of large scale flow structures on the flame dynamics. In this way, the combustion efficiency can directly be influenced.In chemically reacting flows hydrodynamic instabilities, thermoacoustic instabilitites, and geometric boundary conditions interact in intricate ways. Therefore, simplified systems are examined in this research project. These systems are non-isothermal, but not chemically reacting. In doing so, the specific influence of the temperature on the PVC is examined. The following questions are going to be addressed in detail:What is the influence of the control parameters Reynolds number, swirl number and temperature ratio on the PVC?Is the suppression of the PVC in non-isothermal swirling jets caused by alterations of the stability properties?What are the impacts of the temperature distribution and the suppression of the PVC on the flow field?The bases for the clarification of the above questions are extensive measurements of non-isothermal temperature and flow fields of an unconfined swirling jet. Stationary and transient measurements of the flow are used to show the influence of the control parameters, namely Reynolds number, swirl number, and temperature ratio. By using proper orthogonal decomposition, the spatial structure of the PVC is extracted from the measurements. The stability properties of the flow can be derived from the spatio-temporal evolution of the PVC. This work promises fundamental new findings, considering the non-isothermal swirling jet and its stability properties. Thus, the causal relation between the occurrence of the PVC and the temperature distribution of the flow will be shown.