The response of stratified swirling flames to acoustic forcing: Experiments and comparison to model

Abstract

The gradient of local equivalence ratio in reacting mixtures significantly affects the flame structure and their corresponding response to acoustic velocity perturbations. We study the effect of acoustic velocity fluctuations on flames created by two co-annular, swirling streams with different equivalence ratios to simulate the effects of pilot-mains split. The flames are stabilized both by a bluff body and by swirl. The flame responses were measured via chemiluminescence as a function of frequency, in the linear perturbation range. A linearized version of the G-equation model is employed to describe the flame dynamics, combined with effects of axial and azimuthal velocity perturbations downstream of the swirlers. The model accounts for the phase shift between the main acoustic and swirler vortical perturbations, which propagate at different speeds. The very different flame structures generated by different fuel splits lead to different flame responses. Models based on time delay of vortical disturbances are able to capture the behaviour reasonably well for the case of outer fuel enrichment, but offer limited agreement for the case of the inner enriched flame, particularly under higher mean equivalence ratios.

Effect of Modulation of the Inlet Velocity and Equivalence Ratio Gradients on the Stabilization of Stratified Axisymmetric Bluff-Body Flames

“…The placement of the lean and ultralean flame disposition illustrated in Figure 3 clearly indicates that the limiting reacting configuration chosen as a reference for the present study indeed represents a weaker flame topology, closer to LBO topology, in terms of anchoring position and stability, when compared to previous robust flame configurations used for modulation studies [21]. Comparisons between measurements and simulations of detailed velocity and temperature fields reported in [3] have suggested that the variations in the transition from lean to ultralean flame behavior were followed adequately by the simulation methodology and this lends some initial credibility for extending the basic approach to investigate the forced flames configurations.…”

“…The distortion seen between the premixer inlet velocity and the afterbody exit Φ signal at 50 Hz can be considered the result of the interaction of the perturbed in-cavity flow with the fuel injection leading to a phase shift of 180 ∘ and an amplitude deviation of more than 100%. The effect of the premixer section on the afterbody exit Φ modulation varies with frequency forcing, as demonstrated below in the heat release profiles; this combination yields modulations of both the velocity and mixture gradients at inlet to the main reaction zone, with variable frequency content and amplitude in a fashion that is different from previously reported studies [11,21]. At the lower frequency of 50 Hz the overshoots in heat release response are significant and exhibit a phase difference of around 180…”

Extracting flame describing functions in the presence of self-excited thermoacoustic oscillations

“…1 ), which has been used before to study the forced response of stratified flames [18] and the interaction between forcing and self-excitation in premixed flames [13,19,20] .…”

“…An extensive discussion of the shape of the short tube transfer function is available in Ref. [20] . There is clearly a significant difference between the transfer functions obtained with self-excitation (long tube, LT) and without self-excitation (short tube, ST), for the low and high power case.…”

“…Such dips in gain creating a node and change in phase are usually associated with the interference of two time scales, here most likely between the acoustic and swirler transfer function [19,20] . The gain in the self-excited long tube case varies significantly from that in the short tube, with different values at low frequencies, and a significant decrease past the location of the resonant frequency.…”