Jacqueline O’Connor 1 e-mail: joconnor6@gatech.edu Tim Lieuwen School of Aerospace Engineering, Georgia Institute of Technology, 270 Ferst Drive, Atlanta, GA 30332-0150 Further Characterization of the Disturbance Field in a Transversely Excited Swirl-Stabilized Flame This paper describes an analysis of the unsteady flow field in swirl flames subjected to transverse acoustic waves. This work is motivated by transverse instabilities in annular gas turbine combustors, which are a continuing challenge for both power generation and air- craft applications. The unsteady flow field that disturbs the flame consists not only of the incident transverse acoustic wave, but also longitudinal acoustic fluctuations and vortical fluctuations associated with underlying hydrodynamic instabilities of the base flow. We show that the acoustic and vortical velocity fluctuations are of comparable magnitude. The superposition of these waves leads to strong interference patterns in the velocity field, a result of the significantly different wave propagation speeds and axial phase dependencies of these two disturbance sources. Vortical fluctuations originate from the convectively unstable shear layers and absolutely unstable swirling jet. We argue that the unsteady shear layer induced fluctuations are the most dynamically significant, as they are the pri- mary source of flame fluctuations. We also suggest that vortical structures associated with vortex breakdown play an important role in controlling the time-averaged features of the central flow and flame spreading angle, but do not play an important role in disturbing the flame at low disturbance amplitudes. This result has important implications not only for our understanding of the velocity disturbance field in the flame region, but also for captur- ing important physics in future modeling efforts. [DOI: 10.1115/1.4004186] Introduction Combustion dynamics, a coupling between resonant combustor acoustics and flame heat release fluctuations, has been an issue with propulsion and power generation technologies since the mid- dle of the twentieth century [1]. Initially explained by Rayleigh [2], this coupling can lead to high-cycle fatigue and engine dam- age, reduced operability windows, and increased emissions. For gas turbines, these instabilities have become more pronounced as engines have been optimized for low NO x emissions [3]. The main emissions abatement strategy, lean combustion, has lead to a rise in the severity of instabilities and the more frequent appear- ance of transverse instabilities in these engines. Transverse instabilities are a common instability mode in rockets [4–6], augmenters [7–9], and annular combustors [10,11], but have only recently become a significant issue for can-annular gas turbine systems [12]. These instabilities are characterized by acoustic pres- sure and velocity perturbations that oscillate normal to the direction of flow. Traditionally, longitudinal instabilities have been the domi- nant mode in can-annular engines and significant work has been done to understand the coupling mechanisms present in these insta- bilities [13–15]. More recently, work has been initiated to shed light on some of the flame response characteristics and coupling mecha- nisms for transversely forced flames [16–22]. Understanding the underlying mechanism for instability is a critical step in predicting the conditions under which instabilities do and do not appear. Two dominant coupling mechanisms in gas turbines are equivalence ratio [23–25] and velocity [26–29] cou- pling, where other mechanisms, such as pressure coupling [1], are believed to play a negligible role. In this work, we focus on veloc- ity coupled disturbances, by which we mean the flame response to acoustic and vortical velocity perturbations. Several studies have provided detailed characterizations of the way in which velocity disturbances lead to heat release oscilla- tions. First, acoustic velocity perturbations at the flame attachment point excite flame wrinkles that propagate the entire length of the flame at a speed approximately equal to that of the mean flow [30]. Vortical velocity disturbances originate at the nozzle (e.g., the rollup of the separating shear layer) and distort the flame as they propagate axially at the vortex convection speed [31]. Acous- tic disturbances also excite wrinkles as they propagate axially and/or transversely at the sound speed. Finally, several researchers have pointed to a swirl fluctuation mechanism that causes the flame angle to fluctuate with the swirl number [32–35]. Put to- gether, the flame is being simultaneously wrinkled by several sources, each with their own phase and convection speeds. As discussed in O’Connor et al. [22], the velocity disturbance field in a transversely forced flame is significantly more complex than in the longitudinal problem. The incident transverse acoustic perturbation disturbs the flame in an intrinsically nonaxisymmetric manner. In addition, the acoustic pressure fluctuation over the noz- zle leads to longitudinal acoustic fluctuation in the nozzle region, as shown in simulations by Staffelbach et al. [17], and in experiments by O’Connor et al. [22]. A similar phenomenon occurs in transverse rocket instabilities and is referred to as “injector coupling” [36–39]. Additionally, vortex roll-up at the base of the flame leads to further velocity disturbance sources. These disturbance mechanisms and their pathways are outlined in Fig. 1. Each of these processes can be notionally described with a transfer function, as is shown in Fig. 1. For example, the coupling between acoustic fluctuations at the nozzle and the resulting vortex rollup is characterized using an acoustic to hydrodynamic velocity transfer function. Additionally, the transverse to longitu- dinal acoustic coupling process provides an important connection back to the previous flame transfer function work performed for longitudinally excited flames [23,29,32,34,40]. 1 Corresponding author. Contributed by the Contributed by International Gas Turbine Institute (IGTI) for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received April 27, 2011; final manuscript received April 28, 2011; published online October 28, 2011. Editor: Dilip R. Ballal. Journal of Engineering for Gas Turbines and Power JANUARY 2012, Vol. 134 / 011501-1 Copyright V C 2012 by ASME Downloaded 05 Oct 2012 to 198.206.219.39. 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