Jacqueline O’Connor

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 aircraft 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 primary 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 capturing important physics in future modeling efforts.

Effect of Load on Close-Coupled Post-Injection Efficacy for Soot Reduction in an Optical Heavy-Duty Diesel Research Engine

The use of close-coupled post injections of fuel is an in-cylinder soot-reduction technique that has much promise for high efficiency, heavy-duty diesel engines. Close-coupled post injections, short injections of fuel that occur soon after the end of the main fuel injection, have been known to reduce engine-out soot at a wide range of engine operating conditions, including variations in injection timing, EGR level, load, boost, and speed. While many studies have investigated the performance of post injections, the details of the mechanism by which soot is reduced remains unclear. In this study, we have measured the efficacy of post injections over a range of load conditions, at constant speed, boost, and rail pressure, in a heavy-duty, optically-accessible research diesel engine. Here, the base load is varied by changing the main-injection duration. Measurements of engine-out soot indicate that not only does the efficacy of a post injection decrease at higher engine loads, but that the range of post-injection durations over which soot reduction is achievable is limited at higher loads. Optical measurements, including natural luminescence of soot and planar laser-induced incandescence of soot, provide information about the spatio-temporal development of in-cylinder soot through the cycle in cases with and without post injections. The optical results indicate that the post injection behaves similarly at different loads, but that its relative efficacy decreases due to the increase in soot resulting from longer main-injection durations.Copyright

Instability Mechanism in a Swirl Flow Combustor: Precession of Vortex Core and Influence of Density Gradient

Combustion instability is a serious problem limiting the operating envelope of present day gas turbine systems using a lean premixed combustion strategy. Gas turbine combustors employ swirl as a means for achieving fuel-air mixing as well as flame stabilization. However swirl flows are complex flows comprised of multiple shear layers as well as recirculation zones which makes them particularly susceptible to hydrodynamic instability. We perform a local stability analysis on a family of base flow model profiles characteristic of swirling flow that has undergone vortex breakdown as would be the case in a gas turbine combustor A temporal analysis at azimuthal wavenumbers m = 0 and m = 1 reveals the presence of two unstable modes. A companion spatio-temporal analysis shows that the region in base flow parameter space for constant density density flow, over which m = 1 mode with the lower oscillation frequency is absolutely unstable, is much larger that that for the corresponding m = 0 mode. This suggests that the dominant self-excited unstable behavior in a constant density flow is an asymmetric, m=1 mode. The presence of a density gradient within the inner shear layer of the flow profile causes the absolutely unstable region for the m = 1 to shrink which suggests a possible explanation for the suppression of the precessing vortex core in the presence of a flame.

Visualization of Shear Layer Dynamics in a Transversely Forced Flow and Flame

This study addresses the response of a swirling annular jet flow and flame to transverse acoustic excitation in order to better describe key velocity-coupled processes during transverse combustion instabilities in lean, premixed flames. In particular, visualization and velocimetry techniques provide information about the effects of acoustic excitation on unsteady vortex development in the shear layers. Without acoustic forcing, the shear layers roll up into small vortices, driven by the Kelvin–Helmholtz instability, that convect downstream and grow. In the presence of high-amplitude acoustic forcing, as would be present during a combustion instability, the acoustic oscillations drive shear layer vortices to undergo a strong rollup event. Smoke visualization provides visual evidence of the rollup, while particle image velocimetry measurements show the development and trajectory of this large structure. Finally, planar laser-induced fluorescence of OH shows how the large coherent structure causes flame wrin...

Disturbance-Field Decomposition in a Transversely Forced Swirl Flow and Flame

High-amplitude combustion instabilities are a destructive and pervasive problem in gas-turbine combustors. Although much research has focused on measuring the characteristics of these instabilities, there are still many remaining questions about the fluid-mechanic mechanisms that drive the flame oscillations. In particular, a variety of complex disturbance mechanisms arise during velocity-coupled instabilities excited by transverse acoustic modes. The resulting disturbance field has two components: the acoustic-velocity fluctuation from both the incident transverse acoustic field and the excited longitudinal field near the nozzle, and the vortical-velocity fluctuations arising from acoustic excitation of hydrodynamic instabilities in the flow. In this research, the relative contribution of these two components had been explored using proper orthogonal decomposition as a methodology for decomposing the velocity-disturbance field. Although proper orthogonal decomposition is successful at decomposing these t...

Combustor Dilution Hole Placement and Its Effect on the Turbine Inlet Flowfield

Dilution jets in a gas turbine combustor are used to oxidize remaining fuel from the main flame zone in the combustor and to homogenize the temperature field upstream of the turbine section through...

Flow dynamics in a variable-spacing, three bluff-body flowfield

This work explores the wake dynamics of systems with three bluff bodies with variable spacing. Studies of single-wake systems have shown that coherent wake vortices have a regular and predictable periodicity. A growing literature of dual-wake studies has shown that multi-wake systems are more stochastic than single-wake systems, and their dynamics are highly dependent on the spacing between the wakes. Here, we expand on this literature by investigating three-wake systems and find that the coherent dynamics of the wakes are highly intermittent. We use proper orthogonal decomposition to extract the most energetic modes of the three-wake system at six bluff-body spacings that span a range of dynamical “regimes.” After describing the time-dependent behavior of the interacting wakes in these regimes, we use a statistical approach to describe the relative phase between oscillations in each of the wakes, identifying regimes where oscillations are more or less random. Interestingly, the wake oscillations are less...

Comparison of Three Interacting V-Flames to a Single Bluff-Body Flame at Two Reynolds Numbers

The axial velocity profiles for three interacting bluff-body flames at Reynolds numbers of 4000 and 6000 are compared to a single-flame case at the same Reynolds numbers and equivalence ratio of φ=0.8. To facilitate a direct comparison between interacting and single-flame behaviors, an interacting flame case has been chosen to have a very similar time-averaged flow field as the single-flame configuration. While the inlet turbulence levels of the single-flame and interacting flame configurations are the same, the turbulence field development is different when multiple flames are present, leading to higher reactant Reynolds stresses along the c = 0.5 contour in the cross-stream direction when there is flame interaction. The principal directions are calculated and show that the flame surface normals tend to align with most compressive principal strain. Flame surface density is quantified for both flame configurations and indicates there is more flame wrinkling when flame interaction is present. The higher cross-stream Reynolds stresses cause increased flame wrinkling which results in faster flame brush growth when there is flame interaction. Flames are found to anisotropically redistribute turbulent kinetic energy, preferentially enhancing cross-stream fluctuations in the products. These results demonstrate that the multiple bluff-body flame behavior cannot be considered a superposition of the dynamics of the single bluff-body flame.