Coupled interactions of a helical precessing vortex core and the central recirculation bubble in a swirl flame at elevated power density

Abstract

The PRECCINSTA GTMC was studied at elevated pressure and power density with 6 kHz stereoscopic particle image velocimetry (SPIV), OH* chemiluminescence (CL), and 100 kHz dynamic pressure measurements. This technically premixed, swirl stabilized flame exhibited self-excited thermoacoustic oscillations with limit-cycle behavior. A helical precessing vortex core (PVC) was detected within the inner shear layer, between the central recirculation bubble (CRB) and the reactant jets. The PVC was found to be the delineating flow feature for combustion dynamics even at elevated pressure. Sparse dynamic mode decomposition (DMD) of the velocity fields deconvolved the dynamics into a thermoacoustic and PVC mode. The precession of the PVC was at a non-harmonic frequency to the thermoacoustic oscillations, and at least twice that of findings at atmospheric conditions. Nevertheless, the continuous and persistent structure of the PVC allows it promote unsteady heat release to sustain the thermoacoustic cycle. The three dimensional structure of the reactant jets, central recirculation bubble, and PVC was reconstructed by double phase conditioning the reconstructed velocity field. The surface of the CRB was observed to transition between asymmetric and symmetric states depending on the thermoacoustic phase. Analysis of the swirling strength values on the CRB surface indicates the interaction strength between the hydrodynamic structures of the PVC and CRB. When this coupling is large, the heat release determined by the mean OH*-CL intensity is maximum. These findings indicate a critical role of the PVC and CRB interaction on combustion in unstable swirl flames at conditions closer to those found in a modern gas turbine engine.

A combined oscillation cycle involving self-excited thermo-acoustic and hydrodynamic instability mechanisms

“…Among the different phenomena studied in these works are the role of mass flow rate and equivalence ratio oscillations 9,48 , the dynamics of coherent vortex structures, i.e. the PVC [49][50][51][52] and symmetric vortex shedding 50 , as well as the transition between bi-stable flame shapes [53][54][55][56][57] . It is important to note that each of these studies only focused on one specific feedback mechanism at a time.…”

Effect of hydrogen enrichment on the dynamics of a lean technically premixed elevated pressure flame

“…A PVC accompanied by a single frequency-locked HV has been observed in previous studies of the PRECCINSTA burner at atmospheric [15,[24][25] and elevated pressures [22][23]. The PVC distorts the inner recirculation zone flow [18,[23][24][25], and the helical vortices affect flame stabilization by providing stagnation points and rolling up the flame. The HV also modifies the heat release by increasing flame surface area [18,[23][24][25].…”

“…This study aims to examine the effects of H2 enrichment of NG in a gas turbine model combustor with preheated air and at elevated pressure combined using detailed optical diagnostics. The burner used is based on the PRECCINSTA design, which has been studied widely experimentally [18][19] and computationally [20][21] mostly at atmospheric pressure, but also at elevated pressure [22][23]. The fuel is mixed with preheated air in the swirler.…”

“…The PVC distorts the inner recirculation zone flow [18,[23][24][25], and the helical vortices affect flame stabilization by providing stagnation points and rolling up the flame. The HV also modifies the heat release by increasing flame surface area [18,[23][24][25]. In this paper we will refer to the PVC and associated HV as the PVC structure.…”

Thermo-acoustic Instabilities in the PRECCINSTA Combustor Investigated Using a Compressible LES-pdf Approach

“…A significant number of experimental investigations have since been carried out attempting to identify the underlying mechanisms involved in the flame oscillation. Among the different phenomena studied in these works are the dynamics of (8) (Steinberg et al 2010;Caux-Brisebois et al 2014;Oberleithner et al 2015;An et al 2016;Zhang et al 2019), the interaction between velocity and equivalence ratio fluctuations (Stöhr et al 2017) and the transition between bistable flame shapes (Yin et al 2017;Stöhr et al 2018). Most computational studies on the other hand, have focused on validating LES combustion models based on the reference case involving 'stable' operating conditions with a global equivalence ratio of glob = 0.83 and thermal power of P th = 30 kW.…”