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

旋流射流是现代燃气轮机燃烧室中常用的一种旋流射流。涡旋破裂伴随着涡核绕喷流轴的曲折运动。这种现象被称为进动涡核，或简称PVC。人们已经对等温旋流中聚氯乙烯的产生进行了广泛的研究。研究表明，聚氯乙烯是一种在整个流区都活跃的整体失稳模式。在非等温情况下，目前尚不清楚是什么基本物理机制导致了聚氯乙烯的减振，这与有无燃烧的非等温旋转射流之间的认识差距。为了有效地控制聚氯乙烯在非等温流动中的流动，有必要了解这些机理。特别是在非等温旋流中，需要有效的控制。旋流燃烧室中聚氯乙烯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。在化学反应流动中，流体动力不稳定性、热声不稳定性和几何边界条件以错综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。这些体系是非等温的，但不会发生化学反应。在此过程中，考察了温度对聚氯乙烯的具体影响。本文将详细讨论以下问题：控制参数雷诺数、旋流数和温度比对聚氯乙烯有何影响？非等温旋流对聚氯乙烯的抑制是否是由于稳定性的改变？温度分布和对聚氯乙烯的抑制对流场有何影响？对无约束旋流的非等温温度和流场的广泛测量是澄清上述问题的基础。通过对流动的稳态和瞬变测量，揭示了控制参数雷诺数、旋流数和温度比的影响。通过适当的正交分解，从测量数据中提取了聚氯乙烯的空间结构。流动的稳定性可以从聚氯乙烯的时空演化中得到。考虑到非等温旋转射流及其稳定性特性，这项工作有望获得基本的新发现。因此，将显示PVC的发生与流动的温度分布之间的因果关系。

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.