Abstract
Swirl stabilized flows are the most widely deployed technology used to stabilize gas turbine combustion systems. However, there are some coherent structures that appear in these flows close to the nozzle whose occurrence and stability are still poorly understood during transition. The external recirculation zone and the Precessing Vortex Core to/from the Coanda effect are some of them. Thus, in this paper the transition of an Open Jet Flow-Medium Swirl flow pattern to/from a Coanda jet flow is studied using various geometries at a fixed Swirl number. Phase Locked Stereo Particle Image Velocimetry and High Speed Photography experiments were conducted to determine fundamental characteristics of the phenomenon. It was observed that the coherent structures in the field experience a complete annihilation during transition, with no dependency between the structures formed in each of the flow states. Moreover, transition occurs at a particular normalized step size whilst some acoustic shifts in the frequencies of the system were noticed, a phenomenon related to the strength of the vortical structures and vortices convection. It is concluded that a transient, precessing, Coanda Vortex Breakdown is formed, changing flow dynamics. The structure progresses to a less coherent Trapped Vortex between the two states. During the phenomenon there are different interactions between structures such as the Central Recirculation Zone, the High Momentum Flow Region and the Precessing Vortex Core that were also documented.
Highlights
Future flexible power plants will require new and improved systems for flexible usage of a variety of alternative fuels
Previous studies on swirling flows [2, 3] have identified the importance of such a region as a mechanism of stability to anchor flames and recirculate hot products, complementing the action of the Central Recirculation Zone (CRZ), a coherent structure that appears in the vicinity of the shearing flow and inside of the flame in Open Jet Flow-Medium Swirl (OJF-MS)
Results show that the flow can fall under 3 different conditions, A) Coanda stabilization with the appearance of a Coanda Vortex Breakdown; B) Transition, where a Trapped Vortex (TV) leads to the detachment of the flame and destruction of flow patterns; C) Open Jet stabilization with the formation of the CRZ and the appearance of the Precessing Vortex Core (PVC), External Recirculation Zone (ERZ) and High Momentum Flow Region (HMFR)
Summary
Future flexible power plants will require new and improved systems for flexible usage of a variety of alternative fuels. There are some structures whose interaction with the localised flow field during transition are still poorly understood. One of these structures is the External Recirculation Zone (ERZ) observed in Open Jet Flow-Medium Swirl (OJF-MS) flames with swirl numbers close to 1.0. Previous studies on swirling flows [2, 3] have identified the importance of such a region as a mechanism of stability to anchor flames and recirculate hot products, complementing the action of the Central Recirculation Zone (CRZ), a coherent structure that appears in the vicinity of the shearing flow and inside of the flame in OJF-MS. The OJF-MS features are similar to those of the Open Jet Flow-High Swirl, Fig. 1, but with less concave patterns due to longer and weaker CRZs product of increased axial velocities
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