Hydrodynamics During the Transient Evolution of Open Jet Flows from/to Wall Attached Jets

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 FlowMedium 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.

Identification and dynamics of coherent structures in a Coanda swirling jet flow

“…It is speculated that the higher frequency in the CoJF is a consequence of its stagnation to the bottom surface, or alternatively, because another coherent structure with a higher frequency has appeared in the field, the outer PVC. This behaviour has also been previously reported in the work of Valera-Medina et al [27].…”

“…This vortical structure keeps the jet attached to the bottom wall by means of the Coanda effect. It has previously been identified as a Coanda Vortex Breakdown (CoVB), which is a product consequence of the pressure, recirculation and swirling motion of the flow [27]. The centre of the vortex has a pressure minimum, keeping the jet attached to the wall [23,33].…”

“…5. Next to the outer PVC, a weaker second vortex is appearing in the inner shear layer, which is similar to the inner PVC of the OJF, thus creating a CoVB pair [27].…”

“…Therefore, this research is based on continuation of previous work [27] to decipher the underlying coherent structures that are present in a CoJF. The fundamental idea of the research is based on first having a particular geometry at which the system would present both flow patterns under the same inlet flow conditions, hence to be able to identify and compare the coherent structures present in each of the flow patterns.…”