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

点击翻译按钮获取中文摘要

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.