Benjamin Emerson

Velocity and Flame Wrinkling Characteristics of a Transversely Forced, Bluff-Body Stabilized Flame, Part I: Experiments and Data Analysis

This article describes measurements of the response of bluff-body stabilized flames subjected to transverse acoustic waves. It is the first of a two-article series. The objective of this work was to extend prior studies of this nature to much higher Reynolds numbers and more severe environments that more closely mimic conditions encountered in applications. To this end, experiments were performed at flow velocities of 50 m/s and 100 m/s with inlet air temperatures ranging from 475–750 K. Two different modes of acoustic excitation were applied, corresponding to velocity and pressure nodes/antinodes along the combustor centerline. High-speed imaging and phase-locked particle image velocimetry (PIV) were used to characterize the spatio-temporal flame front and velocity field response. The data show that the disturbance field and the flame front response amplitude exhibit a nonmonotonic spatial distribution with interference patterns. The phase of the flame front response at the forcing frequency varies nearly linearly with downstream distance, and corresponds to a phase speed that is slightly less than the mean flow velocity. Significantly, these results show that the key features of the flames magnitude and phase characteristics are quite similar to those observed in much lower flow velocities.

Helical Flow Disturbances in a Multinozzle Combustor

This paper describes an experimental investigation of a transversely forced, swirl stabilized combustor. Its objective is to compare the unsteady flow structures in single and triple nozzle combustors and determine how well a single nozzle configuration emulates the characteristics of a multi-nozzle one. The experiment consists of a series of velocity field measurements captured on planes normal to the jet axis. As expected, there are differences between the single and triple-nozzle flow fields, but the differences are not large in the regions upstream of the jet merging zone. Direct comparisons of the time averaged flow fields reveal a higher degree of non-axisymmetry for the flowfields of nozzles in a multi-nozzle configuration. Azimuthal decompositions of the velocity fields show that the transverse acoustic forcing has an important influence on the dynamics, but that the single and multi-nozzle configurations have similar forced response dynamics near the dump plane. Specifically, the axial dependence of the amplitude in the highest energy axisymmetric and helical flow structures is quite similar in the two configurations. This result suggests that the hydrodynamic influence of one swirling jet on the other is minimal and, as such, that jet-jet interactions in this configuration do not have a significant influence on the unsteady flow structures.Copyright

Velocity and Flame Wrinkling Characteristics of a Transversely Forced, Bluff-Body Stabilized Flame, Part II: Flame Response Modeling and Comparison with Measurements

This article analyzes the response of bluff-body stabilized flames to transverse acoustic waves. Data were obtained for bluff-body flames at flow velocities of 50 m/s and 100 m/s with inlet air temperatures ranging from 475–750 K. Two different modes of acoustic excitation were applied, corresponding to velocity and pressure nodes/antinodes along the combustor centerline. High-speed imaging and phase-locked particle image velocimetry (PIV) were used to characterize the spatio-temporal flame front and velocity field response. The key objective of the study is to compare measurements of the fluctuating velocity and flame wrinkling using the G-equation, e.g., to compare how the ensemble averaged unsteady flame wrinkling gain/phase predicted by solving the G-equation using the measured velocity field as inputs compares to the measured values. These results show good qualitative agreement between the comparisons and measurements, and quite good quantitative accuracy in many of the cases. These comparisons also enable insight into the features controlling the unsteady flame wrinkling; for example, it enables insight into the relative contributions of acoustic and vortical disturbances on the flame wrinkling characteristics, whose different propagation velocities lead to interference patterns and oscillatory flame wrinkle amplitude characteristics.

Simultaneous High Speed (5 kHz) Fuel-PLIE, OH-PLIF and Stereo PIV Imaging of Pressurized Swirl-Stabilized Flames using Liquid Fuels

This paper describes implementation of simultaneous, high speed (5 kHz) stereo PIV, OH and fuel-PLIF in a pressurized (up to 5.2 atm), liquid fueled, swirl stabilized flame, representative of a gas turbine combustor. The experiments were performed to characterize the flowfield, qualitative heat release and fuel spray distributions, and flame dynamics. Acquiring high speed OH-PLIF in pressurized, liquid fuel systems is difficult due to the fuel’s absorption and emission spectra strongly overlapping that of the OH fluorescence spectrum. To overcome the fuel emission polluting the OH signal, the OH and fuel fluorescence signals were partially separated by using two cameras with differing spectral filters and data acquisition timing, as the emission from OH and fuel differ both in spectral width and time. The first camera captured only fuel-PLIF, while the second captured fuelPLIF and OH-PLIF. The fuel-PLIF images were used to compute two intensity thresholds, separating each image into regions of no fuel, fuel only and an intermediate region. In the region of no fuel, OH was detected in the second camera. In the intermediate region there was a mix of fuel and OH. Instantaneous and time-averaged results are discussed showing the flow field, flame position and dynamics, and spray distribution from the fuel signal for two different multi-component liquid fuels (Jet-A and C-5), at two inlet temperatures of 450 and 570 K, and three pressure of 2.1, 3.5 and 5.2 bar. The flame shape in some cases is described as M-shaped, existing both inside and outside of the annular swirling jet produced by the nozzle, while in other cases no reaction is apparent on the inside. The spray penetration and distribution, and flame position are sensitive to the various conditions, while the flow field topology is qualitatively insensitive. Furthermore, elevated pressure as expected sharpens all spatial gradients in the data.

CONVECTIVE AND ABSOLUTE INSTABILITIES IN REACTING BLUFF BODY WAKES

This paper describes the variation of bluff body wake structure with flame density ratio. It is known that the bluff body flow structure at “high” and “low” flame density ratios is fundamentally different, being dominated by the convectively unstable shear layers or absolutely unstable Von Karman vortex street, respectively. This paper characterizes the aforementioned transition and shows that the bifurcation in flow behavior does not occur abruptly at some ρu/ρb value. Rather, there exists a range of transitional density ratios at which the flow exists intermittently in both flow states, abruptly shifting back and forth between the two. The fraction of time that the flow spends in either state is a monotonic function of ρu/ρb. This behavior is to be contrasted with lower Reynolds number, laminar flow problems where the convective/absolute instability transition occurs at a well defined value of bifurcation parameter. With this distinction in mind, however, this paper also shows that local parallel stability analyses developed for laminar base wake flows can capture many of the observed flow dependencies. These results have important implications on the dynamics of high Reynolds number, vitiated flows, where typical parameter values fall into the highly intermittent flow regime characterized in this study. This suggests that such flows exhibit two co-existing dynamical states, intermittently jumping between the two.

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.

Measurements and Analysis of Bluff-Body Flame Response to Transverse Excitation

This paper describes the interaction of bluff body stabilized flames with 450 Hz transverse acoustic standing waves at flow velocities up to 100 m/s. Two different modes of acoustic excitation were applied, corresponding to pressure and velocity nodes along the bluff body centerline. Time resolved measurements of both the flame front and velocity field were obtained. These measurements of the spatio/temporal distribution of the flame front were compared to level set equation prediction using the measured velocity field as an input, or vice-versa. These studies show that the measured flame response characteristics are qualitatively captured in almost all cases, with quantitative differences varying from values that are quite low to a factor of two. A key implication of this work is that the important features of the unsteady flame dynamics at high velocity, vitiated flow conditions are understood, but further work is needed for quantitative prediction.

Dynamics of a Transversely Forced, Bluff Body Stabilized Flame

This paper describes an experimental and modeling analysis of the response of bluff body stabilized flames to transverse acoustic waves. In these experiments, high speed video of the flame was processed to characterize the flame front response to transverse acoustic excitation at various flow conditions. Unsteady velocity field measurements were obtained with PIV to obtain the corresponding time averaged and fluctuating disturbance field. Data were taken at flow conditions with inlet temperatures between 480K and 755K, and flow velocities of 50 m/s and 100 m/s. Two different modes of acoustic excitation were applied at 450 Hz, corresponding to velocity and pressure nodes/antinodes along the combustor center. The flame front response at first increases almost linearly with downstream distance, exhibits oscillatory behavior, and then decays. The phase of the flame front response at the forcing frequency decays with downstream distance nearly linearly, and corresponds to a convective velocity of disturbances that is slightly less than the mean flow velocity. These results are similar in many respects to prior observations from longitudinally forced flames. These experimental results are compared with predictions from a flame front tracking G-equation. This equation shows that the key processes controlling the response are 1) the anchoring of the flame at the bluff body, 2) the excitation of flame-front wrinkles by the oscillating acoustic and vortical velocity, and 3) flame propagation normal to itself at the local flame speed. The validation studies using this model show qualitatively good comparison of the velocity field data and the flame response data measured experimentally. These experiments also show that flame wrinkling is induced by both acoustic and vortical disturbances, whose different propagation velocities leads to interference and oscillatory flame wrinkle amplitude characteristics.

Spatio-temporal linear stability analysis of stratified planar wakes: Velocity and density asymmetry effects

This paper explores the hydrodynamic stability of bluff body wakes with non-uniform mean density, asymmetric mean density, and velocity profiles. This work is motivated by experiments [S. Tuttle et al., “Lean blow off behavior of asymmetrically-fueled bluff body-stabilized flames,” Combust. Flame 160, 1677 (2013)], which investigated reacting wakes with equivalence ratio stratification and, hence, asymmetry in the base flow density profiles. They showed that highly stratified cases exhibited strong, narrowband oscillations, suggestive of global hydrodynamic instability. In this paper, we present a local hydrodynamic stability analysis for non-uniform density wakes that includes base flow asymmetry. The results show that increasing the degree of base density asymmetry generally has a destabilizing effect and that increasing base velocity asymmetry tends to be stabilizing. Furthermore, we show that increasing base density asymmetry slightly decreases the absolute frequency and that increasing the base veloc...

Helical Flow Disturbances in a Multi-Nozzle Combustor

This paper describes an experimental investigation of a transversely forced, swirl stabilized combustor. Its objective is to compare the unsteady flow structures in single and triple nozzle combustors and determine how well a single nozzle configuration emulates the characteristics of a multi-nozzle one. The experiment consists of a series of velocity field measurements captured on planes normal to the jet axis. As expected, there are differences between the single and triple-nozzle flow fields, but the differences are not large in the regions upstream of the jet merging zone. Direct comparisons of the time averaged flow fields reveal a higher degree of non-axisymmetry for the flowfields of nozzles in a multi-nozzle configuration. Azimuthal decompositions of the velocity fields show that the transverse acoustic forcing has an important influence on the dynamics, but that the single and multi-nozzle configurations have similar forced response dynamics near the dump plane. Specifically, the axial dependence of the amplitude in the highest energy axisymmetric and helical flow structures is quite similar in the two configurations. This result suggests that the hydrodynamic influence of one swirling jet on the other is minimal and, as such, that jet-jet interactions in this configuration do not have a significant influence on the unsteady flow structures.Copyright