Bethany Huelskamp

Bio: Bethany Huelskamp is an academic researcher. The author has contributed to research in topics: Extinction (optical mineralogy) & Combustion. The author has an hindex of 4, co-authored 12 publications receiving 97 citations.

Improved Correlation for Blowout of Bluff Body Stabilized Flames

Abstract: The lean extinction limit is one measure of the stability of combustion systems. Over the past 60 years there have been many papers on the extinction of bluff body flame holders. Early in the work many experiments were conducted over a range of flame holders, pressures, temperatures, and fuels. The authors typically tried to derive empirical correlations for the lean limit as a function of global conditions that seemed to have arbitrary exponents. These authors typically concluded that the extinction seemed to be some function of Damkohler number. More recently, since the advent of high speed diagnostics and computers, several observations about the extinction process have been made. General conclusions are that the extinction process is a wake phenomenon where the flame is highly strained and dominated by large vortices. In this paper a new correlation for lean extinction is derived using a linear least squares fit and over 800 pieces of historical and current experimental data. Fits of various dimensionless parameters are made, but the best fit is that of a Damkohler number with ignition delay as the chemical time scale, verifying many previous conclusions. Finally, it is concluded that flame holder size, and not shape, is the driving parameter representing geometry.

Characterization of Bluff-Body-Flame Vortex Shedding Using Proper Orthogonal Decomposition

Abstract: Flame stabilization has been of interest for many decades. Bluff-body flame stabilization has been incorporated in gas turbine engines as a means of secondary combustion in high-speed flows. The current work is focused on understanding vortex shedding and its contribution to both blow off and flame stability. Two modes of shedding, Kelvin-Helmoltz and Von-Karman, have been observed to play a major role in the stability and blow off of these bluff-body flames. Typically researchers have observed these modes visually but have been unable to quantify the effective contribution under various flow conditions. The present work is focused on the implementation of Proper Orthogonal Decomposition (POD) as a means of characterizing the energy and nature of these shedding modes as flames transition to acoustic instabilities and blow off. POD provides a new method of assessing the shedding mode and complements the pure visualization and vorticity calculations performed to date. POD is implemented on high-speed images of bluff-body flames at multiple equivalence ratios in an experimental test section. During this equivalence-ratio scan, the flame transitions to an acoustic instability. By incorporation of POD, the symmetric and asymmetric energy contributions through instability and blow off can be described.

Influence of Fuel Characteristics in a Correlation to Predict Lean Blowout of Bluff-Body Stabilized Flames

Abstract: This study employed experimental data collected at the Air Force Research Laboratory (AFRL) as well as data from a review of past literature to develop a correlation for predicting lean blowout through the use of a least-squares curve-fit method. Combining data from the literature with data from AFRL allowed significant variations within the dataset with regard to velocity, flameholder diameter and shape, pressure, temperature, and fuel. Gaseous, single-component fuels as well as multi-component jet fuels were included in the study. The study reports new jet fuel blowout data. The laminar flame speed and ignition delay time calculated using detailed chemical kinetics mechanisms were used in the correlations to determine the chemical timescales relevant to lean blowout.The correlation presented here indicates that the lean blowout of bluff-body stabilized flames is dependent on the Damkohler number, with fuel variation being a significant factor. The ratio of the flameholder diameter to the lip velocity was found to influence the lean blowout. This ratio represents the fluid-mechanic timescale in the Damkohler number. Pressure, temperature, and the hydrogen-to-carbon ratio of the fuel affect the reactivity of the mixture, contributing to the chemical timescale in the Damkohler number. For a limited dataset, the ignition delay time is an adequate representation of the chemical timescale.© 2015 ASME

Application of the Cross Wavelet Transform and Wavelet Coherence to OH-PLIF in Bluff-Body Stabilized Flames

Abstract: A number of high-speed experimental techniques provide the opportunity to collect spatio-temporal information about the evolution of the flowfield and reactions in turbulent flames. This is particularly important for understanding static and dynamic stability in combustion devices that require bluff bodies to anchor the flame. In such cases, KelvinHelmholtz and Von Karman shedding modes have been known to affect flame stabilization and blow-off, leading to an interest in studying the coherent structure dynamics with high data bandwidth and advanced data mining techniques. One of the primary goals of this work is to employ the continuous wavelet transform (CWT) to characterize and quantify the nature of shedding in bluff body stabilized flames. Planar laser-induced fluorescence (PLIF) experimental data have been collected under different flow rates and equivalence ratios for a V-gutter flame holder, and coherent structures are analyzed using single- and multi-point (cross) wavelet transforms. This enables evaluation not only of the dominant modes within the flow, but also the time-dependence of these modes as a function of location within the flame. These methods are used to evaluate the potential for utilizing advanced numerical tools for extracting information about coherent structure dynamics in complex reacting flows.

Deconvolution of reacting-flow dynamics using proper orthogonal and dynamic mode decompositions.

Abstract: Analytical and computational studies of reacting flows are extremely challenging due in part to nonlinearities of the underlying system of equations and long-range coupling mediated by heat and pressure fluctuations. However, many dynamical features of the flow can be inferred through low-order models if the flow constituents (e.g., eddies or vortices) and their symmetries, as well as the interactions among constituents, are established. Modal decompositions of high-frequency, high-resolution imaging, such as measurements of species-concentration fields through planar laser-induced florescence and of velocity fields through particle-image velocimetry, are the first step in the process. A methodology is introduced for deducing the flow constituents and their dynamics following modal decomposition. Proper orthogonal (POD) and dynamic mode (DMD) decompositions of two classes of problems are performed and their strengths compared. The first problem involves a cellular state generated in a flat circular flame front through symmetry breaking. The state contains two rings of cells that rotate clockwise at different rates. Both POD and DMD can be used to deconvolve the state into the two rings. In POD the contribution of each mode to the flow is quantified using the energy. Each DMD mode can be associated with an energy as well as a unique complex growth rate. Dynamic modes with the same spatial symmetry but different growth rates are found to be combined into a single POD mode. Thus, a flow can be approximated by a smaller number of POD modes. On the other hand, DMD provides a more detailed resolution of the dynamics. Two classes of reacting flows behind symmetric bluff bodies are also analyzed. In the first, symmetric pairs of vortices are released periodically from the two ends of the bluff body. The second flow contains von Karman vortices also, with a vortex being shed from one end of the bluff body followed by a second shedding from the opposite end. The way in which DMD can be used to deconvolve the second flow into symmetric and von Karman vortices is demonstrated. The analyses performed illustrate two distinct advantages of DMD: (1) Unlike proper orthogonal modes, each dynamic mode is associated with a unique complex growth rate. By comparing DMD spectra from multiple nominally identical experiments, it is possible to identify "reproducible" modes in a flow. We also find that although most high-energy modes are reproducible, some are not common between experimental realizations; in the examples considered, energy fails to differentiate between reproducible and nonreproducible modes. Consequently, it may not be possible to differentiate reproducible and nonreproducible modes in POD. (2) Time-dependent coefficients of dynamic modes are complex. Even in noisy experimental data, the dynamics of the phase of these coefficients (but not their magnitude) are highly regular. The phase represents the angular position of a rotating ring of cells and quantifies the downstream displacement of vortices in reacting flows. Thus, it is suggested that the dynamical characterizations of complex flows are best made through the phase dynamics of reproducible DMD modes.

Dynamics of robust structures in turbulent swirling reacting flows

Abstract: High-speed synchronized stereo particle-imaging velocimetry and OH planar laser-induced fluorescence (PIV/OH-PLIF) measurements are performed on multiple $R{-}\unicode[STIX]{x1D703}$ planes downstream of a high-Reynolds-number swirling jet. Dynamic-mode decomposition (DMD) – a frequency-resolved data-reduction technique – is used to identify and characterize recurrent flow structures. Illustrative results are presented in a swirling flow field for two cases – the nominal flow dynamics and where self-excited combustion driven oscillations provide strong axisymmetric narrowband forcing of the flow. The robust constituent of the nominal reacting swirl flow corresponds to a helical shear-layer disturbance at a Strouhal number ( $St$ ) of ${\sim}0.30$ , $St=fD/U_{0}$ , where $f$ , $D$ and $U_{0}$ denote the precessing vortex core (PVC) frequency ( ${\sim}800~\text{Hz}$ ), the swirler exit diameter (19 mm) and the bulk velocity at the swirler exit ( $50~\text{m}~\text{s}^{-1}$ ) respectively. Planar projections of the PVC reveal a pair of oscillating skew-symmetric regions of velocity, vorticity and OH-PLIF intensity that rotate in the same direction as the mean tangential flow. During combustion instabilities, the large-amplitude acoustics-induced axisymmetric forcing of the flow results in a fundamentally different flow response dominated by a nearly axisymmetric disturbance and almost complete suppression of the large-scale helical shear-layer disturbances dominating the nominal flow. In addition, reverse axial flows around the centreline are significantly reduced. Time traces of the robust constituent show reverse axial flows around the centreline and negative axial vorticity along the inner swirling shear layer when the planar velocity is in the same direction as the mean tangential flow. For both stable and unstable combustion, recurrent flow structures decay rapidly downstream of the air swirler, as revealed by the decreasing amplitude of the velocity, axial vorticity and OH-PLIF intensity.