Jacqueline O'Connor

Disturbance Field Characteristics of a Transversely Excited Burner

Transverse acoustic instabilities in premixed, swirl-stabilized flames are an important problem in low NOx combustors. Transverse excitation of swirling flames involves complex interactions between acoustic waves and fluid mechanic instabilities. This paper presents high-speed PIV characterization of the flow field characteristics of a swirling, annular jet under reacting and nonreacting conditions. These data show that the flame response to transverse acoustic excitation is a superposition of acoustic and vortical disturbances that fluctuate in both the longitudinal and transverse direction. In the nozzle near-field region, the disturbance field is a complex superposition of short wavelength and convecting vortical disturbances, as well as longer wavelength transverse and longitudinal acoustic disturbances. Very near the nozzle, distinct vortical structures are evident that are associated with the separating inner and outer annulus shear layers. Their relative phasing on the left and right side of the burner annulus changes by 180° under conditions where the burner centerline is nominally at a transverse acoustic velocity node and antinode. These suggest that the dominant excited instability mode of the annular jet changes from axisymmetric to helical as the structure of the acoustic mode shape changes. Farther downstream, these structures disappear rapidly and the disturbance field is dominated by the longer wavelength, transverse acoustic field.

Transverse to longitudinal acoustic coupling processes in annular combustion chambers

Combustion instability is a major issue facing lean, premixed combustion approaches in modern gas turbine applications. This paper specifically focuses on instabilities that excite transverse acoustic modes of the combustion chamber. Recent simulation and experimental studies have shown that much of the flame response during transverse instabilities is due to the longitudinal fluid motions induced by the fluctuating pressure field above a nozzle. In this study, we analyze the multi-dimensional acoustic field excited by transverse acoustic disturbances interacting with an annular side branch, emulating a fuel/air mixing nozzle. Key findings of this work show that the resultant velocity fields are critically dependent upon the structure of the transverse acoustic field and the nozzle impedance. Significantly, we also show that certain cases can be understood from relatively simple quasi one-dimensional considerations, but that other cases are intrinsically three-dimensional.

Dependence of the Bluff Body Wake Structure on Flame Temperature Ratio

This paper describes an experimental investigation of the wake and flame characteristics of a bluff body stabilized flame. Prior investigations have clearly shown that the wake structure is markedly different at “high” and “low” flame density ratios. This paper describes a systematic analysis of the dependence of the flow field characteristics and flame sheet dynamics upon flame density ratio, ρu/ρb, over the range 1.7< ρu/ρb <3.4. This paper shows that two fundamentally different flame/flow behaviors are observed – characterized here as Kelvin-Helmholtz and Von-Karman vortex street dominated - at high and low ρu/ρb values, respectively. These are interpreted here as a transition from a convectively to absolutely unstable flow. This transition manifests itself in several ways with decreasing ρu/ρb values, including (1) the spectrum of the flame motion and flow field transitions from a distributed to a narrowband peak at St~0.24, and (2) the correlation between the two flame front branches monotonically increases. Finally, the intermittent nature of the flow field is emphasized, with the relative fraction of the two different flow/flame behaviors monotonically varying with ρu/ρb.

Visualization of Shear Layer Dynamics in a Transversely Excited, Annular Premixing Nozzle

In this study, we investigate the response of a swirling annular jet flow and flame to transverse acoustic excitation. Characterizing this response is a necessary step towards understanding velocity-coupled transverse combustion instabilities in lean, premixed flames. These effects are investigated using smoke visualization, particle image velocimetry (PIV), and high-speed flame imaging. This study particularly focuses on the effects of excitation on unsteady vortex development in the shear layers, as well as the effects of high-amplitude acoustics on the time-averaged flow field. First, the time-averaged characteristics of the flow are discussed under both nominal and forced conditions. The shape of the flow changes significantly at high forcing amplitudes as a result of changes in the vortex breakdown structure. Next, the shear layer dynamics with and without acoustic forcing are considered. The shear layers are visualized using smoke visualization and PIV, and the result of vortex rollup on the flame is imaged using high-speed imaging of the flame. We hypothesize that the convectively unstable shear layers and absolutely unstable vortex breakdown bubble play different dynamical roles in controlling the flame response to excitation. The unsteady vortex breakdown bubble is primarily important through its impact on the time-averaged flow field upon which perturbations evolve. It really only changes character at high acoustic forcing amplitudes, resulting in significant variations to the time-averaged flame and flowfield. The shear layer rollup, responding at the frequency of acoustic forcing, creates large-scale wrinkles on the flame and is the main driver of flame response.

Impact of Flow Non-Axisymmetry on Swirling Flow Dynamics and Receptivity to Acoustics

In this study, we experimentally investigate both the intrinsic instability characteristics and forced response to transverse acoustic excitation of a non-reacting, swirling flow for application to combustion instability in annular gas turbine engines. The non-axisymmetry of the velocity field is quantified using an azimuthal mode decomposition of the time-averaged velocity field that shows that (1) the flow field is largely axisymmetric, (2) axisymmetry decreases with downstream distance, and (3) forcing does not significantly alter the time averaged shape of the flow field. The flow field is analyzed in a companion linear stability analysis that shows that the most unstable modes in the flow field are m=-1 and m=-2, which agrees with the experimental observations and shows that the intrinsic dynamics of this flow field are non-axisymmetric with respect to the jet axis. The linear stability analysis captures the spatial variation of mode strength for certain modes, particularly mode m=-1, but there are some deviations from the experimental results. Most notably, these deviations occur for mode m=0 at radii away from the jet axis. Experimental results of the forced response of the flow indicate that the intrinsic instability characteristics of the flow field have an impact on the forced-response dynamics. Response of the flow field to a velocity anti-node in a standing transverse acoustic field shows non-axisymmetric vortex rollup and the dominance of the m=-1 and m=-1 azimuthal modes in the fluctuating flow field. In the presence of a pressure anti-node, the m=0 mode of the fluctuating flow field is very strong at the jet exit, indicating an axisymmetric response, and ring vortex shedding is apparent in the flow measurements from high-speed Ply. However, further downstream, the strength of the axisymmetric mode decreases and the m=-1 and m=1 modes dominate, resulting in a tilting of the vortex ring as it convects downstream. Implications for flame response to transverse acoustic fields are discussed.

Frequency Locking and Vortex Dynamics of an Acoustically Excited Bluff Body Stabilized Flame

This paper presents measurements of the forced response of a bluff body stabilized flame. The nonreacting, unforced flow exhibits intrinsic oscillations associated with an unstable, global wake mode. This same global mode persists in the reacting flow at low density ratios, but disappears at high flame density ratios where the flow is dominated by the convectively unstable shear layers. The flow responds quite differently to forcing in these two situations, exhibiting a roughly linear input-output character in the convectively unstable regime, but exhibiting nonlinear behavior, such as frequency locking, during global mode oscillations. In this work, a reacting bluff body wake is subjected to harmonic, longitudinal, acoustic forcing, providing a symmetric disturbance. This experiment is conducted at several density ratios as well as different spacing between the forcing frequency and global mode frequency. As the spacing between these frequencies is narrowed, the wake response is drawn away from the global mode frequency and approaches (and eventually locks-into) the forcing frequency. This observation seems to be linked to the spatial distribution of vorticity fluctuations, as well as the symmetry of the vortex shedding. For example, for large spacing between the forcing and global mode frequencies, there is significant response at the forcing frequency in the shear layers, but little response is observed along the flow centerline (which itself exhibits strong oscillations at the global mode frequency). This seems to be due to the symmetry of vortical structures that are shed at the forcing frequency. For small spacing between these frequencies, however, the vortices shed symmetrically at the forcing frequency, but quickly stagger into an asymmetric configuration as they convect downstream. For such cases, we observe a strong response of the wake at the forcing frequency. The axial distance required for such staggering to occur is a function of the spacing between the forcing frequency and natural frequency, the flame density ratio, and the intensity of the forcing.

Response of an Annular Burner Nozzle to Transverse Acoustic Excitation

Transverse combustion instabilities are increasingly problematic in land based and aerogas turbine engines. A common problem in rockets and augmenters, these transverse modes have a circumferential and/or radial structure and are often associated with high frequency oscillations. Depending on its location in the combustor, a flame experiences different parts of a standing circumferential acoustic wave, or in the case of a spinning waveform, a flame sees different parts of the wave structure over time. The current study investigates the response of an annular, swirling nozzle to two limits of the standing wave transverse excitation structure, associated with a pressure node and antinode at the nozzle centerline. The resulting disturbance field is significantly different for these two cases. This paper presents results showing how the disturbance field is composed of long wavelength, multidimensional acoustic disturbances and shorter wavelength convecting vortical disturbances. The flow vorticity originates in the separating boundary layers of the inner and outer annulus and rolls up into larger structures inside and outside of the annular jet. These structures, in turn, merge downstream in a staggered fashion into a single, larger vortex that convects downstream in the annular jet centerline. This paper quantifies the relative strengths and phasing of these disturbances, and discusses the key features of the disturbance field exciting the flame.

Effects of Fuel Molecular Weight on Emissions in a Jet Flame and a Model Gas Turbine Combustor

The objective of this study is to understand the effects of fuel volatility on soot emissions. This effect is investigated in two experimental configurations: a jet flame and a model gas turbine combustor. The jet flame provides information about the effects of fuel on the spatial development of aromatics and soot in an axisymmetric, co-flow, laminar flame. The data from the model gas turbine combustor illustrate the effect of fuel volatility on net soot production under conditions similar to an actual engine at cruise. Two fuels with different boiling points are investigated: n-heptane/n-dodecane mixture and n-hexadecane/ n-dodecane mixture. The jet flames are nonpremixed and rich premixed flames in order to have fuel conditions similar to those in the primary zone of an aircraft engine combustor. The results from the jet flames indicate that the peak soot volume fraction produced in the n-hexadecane fuel is slightly higher as compared to the n-heptane fuel for both nonpremixed and premixed flames. Comparison of aromatics and soot volume fraction in nonpremixed and premixed flames shows significant differences in the spatial development of aromatics and soot along the downstream direction. The results from the model combustor indicate that, within experiment uncertainty, the net soot production is similar in both n-heptane and n-hexadecane fuel mixtures. Finally, we draw conclusions about important processes for soot formation in gas turbine combustor and what can be learned from laboratory-scale flames. [DOI: 10.1115/1.4037928]

IMPACT OF PVC DYNAMICS ON SHEAR LAYER RESPONSE IN A SWIRLING JET

Combustion instability, or the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a non-reacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m=0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m=1 and m=-1 modal content. By comparing the relative magnitude of the m=0 and m=-1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental. results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.

Combustion Instabilities in Lean Premixed Systems

Combustion instabilities are one of the most costly and technically challenging issues in lean, premixed combustion systems. While combustion instabilities, or thermacoustic oscillations more generally, have been noted in a variety of applications for several centuries, they are particularly problematic in lean, premixed combustion systems. Combustion instability is characterized by undesirably high acoustic and heat release rate oscillations inside a combustor chamber. In this chapter, we discuss the fundamentals of thermoacoustic feedback cycles, as well as the different coupling mechanisms by which combustor system acoustics create a feedback loop with flame heat release rate oscillations. Finally, an overview of combustion control strategies, particularly those employed in lean combustion systems, is discussed with references to future design and research directions.