Flow Dynamics in a Multiswirler Model Combustor Based on Large Eddy Simulation and Proper Orthogonal Decomposition Analysis

Abstract

Swirling flow is often employed in gas turbine combustion chambers for the sake of improving flame stability. Swirling flow induces not only recirculation zones but also large coherent structures, which show close relationship with flow dynamics and combustion instability. The flow dynamics including precessing vortex core (PVC) in simple swirlers is extensively studied, while the flow instability characteristics in a multiswirler combustor are not fully reported. In this paper, large eddy simulation (LES) of nonreacting turbulent swirling flow is conducted in a multiswirler burner, which comprises a pilot stage and a main stage. Flow dynamics in the multiswirler combustor are analyzed based on phase-averaged evolution of instantaneous flowfield. LES results are compared with particle image velocimetry (PIV) data in terms of mean and root mean square (RMS) velocities. Proper orthogonal decomposition (POD) is employed to identify the coherent structures in the multiswirling flow. Results show that LES results are in good agreement with particle image velocimetry (PIV) data. Main stage and pilot stage flow interact with each other generating highly turbulent swirling flow. PVC is successfully captured at the boundary of main recirculation zone (MRZ) in the pilot stage with a dominant frequency of 1915 Hz. The PVC leads to periodic azimuthal flow instability. POD analyses for the velocity fields show dominant high-frequency modes (modes 1 and 2) in the pilot stage. However, the dominant energetic flow is damped rapidly downstream of the pilot stage that it has little effect on the main stage flow.

Combustion dynamics analysis of a pressurized airblast swirl burner using proper orthogonal decomposition

“…The drag force acting on the coupled droplet momentum transfer is described by a modified Schiller-Naumann drag model in order to limit the behavior in the inertial regime as seen in equation (19). The Ranz-Marshall correlation was adopted for the overall droplet heat transfer coefficient which relates the Nusselt number to Prandtl and Reynold numbers for flow past spherical particles according to equation (20). The mass transfer that defines the evaporation process is then calculated based on whether the evaporation falls above or below the droplet boiling temperature as formulated in equation (21).…”

“…It is noteworthy that the frequency of the PVC in similar studied burners varies greatly between 400 and 2000 Hz and depends greatly on the inlet velocity and the velocity gradients in the inner swirling region. 21,19,20,22 Figure 10 depicts the instantaneous Lambda-2 criterion visualization of the vortex core region colored by axial velocity. The PVC can be identified as the dominant swirling vortical structure that originates at the base of the burner and spirals downstream in a helical fashion.…”

“…The precession frequency of the PVC in similar swirl-stabilized combustors has been shown to be highly sensitive to inlet air velocity and the gradient of rotational velocity and located in the inner shear layer of the vortex breakdown region. 19,20 While in most cases, the combustion phenomena have been shown to suppress the PVC, 21 it has been demonstrated that in some cases, the PVC frequency does not change between hot and cold flows. 22 The experimentally observed PVC frequency was compared to the numerically predicted value to assess the validity of cold flow simulations.…”