A Comparison of the Transfer Functions and Flow Fields of Flames With Increasing Swirl Number

Abstract

The frequency response of premixed swirled flames is investigated by comparing their Transfer Function (FTF) between velocity and heat release rate fluctuations. The equivalence ratio and flow velocity are kept constant and four different swirling injectors are tested with increasing swirl numbers. The first injector features a vanishing low swirl number S = 0.20 and produces a flame anchored by the recirculating flow in the wake of a central bluff body. The three other swirling injectors produce highly swirled flows (S > 0.6) leading to a much larger internal recirculation region, which size increases with the swirl level. When operating the burner at S = 0.20, the FTF gain curve smoothly increases to reach a maximum and then smoothly decreases towards zero. For the highly swirled flames (S > 0.6), the FTF gain curve shows a succession of valleys and peaks attributed to interferences between axial and azimuthal velocity fluctuations at the injector outlet. The FTF phase-lag curves from the vanishing low and highly swirled flames are the same at low frequencies despite their large differences in flame length and flame aspect ratio. Deviations between the FTF phase lag curves of the different swirled flames start above the frequency corresponding to the first valley in the FTF gain of the highly swirled flames. Phase averaged images of the axial flow fields and of the flame chemiluminescence are used to interpret these features. At forcing frequencies corresponding to peak FTF gain values, the cold flow response of all flames investigated is dominated by large coherent vortical structures shed from the injector lip. At forcing frequencies corresponding to a valley in the FTF gain curve of the highly swirled flames, the formation of large coherent structures is strongly hindered in the cold flow response. These observations contrast with previous interpretations of the mechanisms associated to the low FTF response of swirled flames. It is finally found that for flames stabilized with a large swirl number, heat release rate fluctuations result both from large flame luminosity oscillations and large flame volume oscillations. For conditions leading to a small FTF gain value, both the flame luminosity and flame volume fluctuations are suppressed confirming the absence of strong perturbations within the flow at these frequencies. The experiments made in this work reveal a purely hydrodynamic mechanism at the origin of the low response of swirling flames at certain specific frequencies.

Combining analytical models and LES data to determine the transfer function from swirled premixed flames

“…It allows for an easier interpretation of LES results than in real scale burners. A wide selection of diagnostics has been employed by Gatti et al [47,48] , including velocity measurements with a hot wire, pressure measurements using microphones, LDV and PIV of the flow at the injection exit and in the flame residence area. FDF data is available for forcing amplitudes ranging from 10 to 70% RMS of the bulk velocity at the hot wire location.…”

Flame Transfer Functions and Dynamics of a Closely Confined Premixed Bluff Body Stabilised Flame With Swirl

“…The potential utility of this approach, in reducing the significant complexity associated with the reacting flow to a simple response function, has resulted in considerable effort in understanding the behaviour, scaling and generality of such functions [6][7][8][9][10][11].…”

“…In particular a number of recent studies have focused on understanding the presence of multiple time scales in the response, due to vorticity oscillations generated both at the injector lip and further upstream from the swirler geometry [7][8][9], or from nonuniform mixtures and therefore equivalence ratio oscillations [6].…”

“…Previous studies have focused on the effect of the location of an axial swirler [7,12,13], the difference between axial and tangential inlet swirler [10], the influence of swirl number [9] or the geometry of the injector [14]. A common feature of all of these studies is the effective interference which can be introduced under certain conditions, resulting from the interaction between velocity oscillations generated at the combustor inlet and the upstream convective vorticity oscillations generated at the swirler.…”

Influence of the Laminar Burning Velocity on the Transfer Functions of Premixed Methane and Propane Swirl Flames

“…However, these two studies did not analyze the effects of parameters such as S L on the FVR phenomenon. More recently, Gatti et al [16,17] investigated the influence of swirl intensity and central bluff body on the FTF. They observed that the strength of the AGV defines the response of the flame.…”