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Stephen Daniel Hammack

Stephen Daniel Hammack
USAF Air Force Research Laboratory · Aerospace Systems Directorate

About

43
Publications
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801
Citations
Introduction

Publications

Publications (43)
Article
In this study, we demonstrate successful development of a predictive model that detects both the fuel-air equivalence ratio (ϕ) and local pressure prior to plasma formation via machine-learning from the laser-induced plasma spectra; the resulting model enables measurement of a wide range of fuel concentrations and pressures. The process of model ac...
Conference Paper
Liquid atomization plays an important role in automobiles, propulsion, power generation, and chemical industries. A liquid injection scheme capable of rapidly generating fine droplets for faster evaporation is sometimes preferred. Injection of fuel containing dissolved gas can be one of the promising injection schemes. With an optimized injection c...
Article
The dynamics of using a pulse detonation wave to ignite a supersonic cavity flameholder was investigated. Fueling conditions ranging from lean to rich were investigated. Simultaneous 40-kHz schlieren imaging, OH planar laser-induced fluorescence, and chemiluminescence imaging were used to elucidate the fluid dynamics and combustion physics througho...
Article
The effects of inlet distortion and shockwave interaction on fuel-air distribution and ignition in a cavity-based flameholder with supersonic flow were investigated using nanosecond-gated laser-induced breakdown spectroscopy (n-LIBS). Experiments were conducted in a Mach-3 freestream direct-connect-type wind tunnel with direct cavity fueling. Ethyl...
Article
Full-text available
Combustion characteristics of a mesoscale burner array have been studied using several diagnostic and analysis techniques. The array was specifically configured to enhance overall combustion stability, particularly under lean operating conditions, by promoting flame to flame interactions between neighboring elements. The 4 × 4 burner array demonstr...
Article
Full-text available
Simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) utilizing R-branch transitions in the C-X (0,0) band were performed at a 10-kHz repetition-rate in a turbulent premixed flame. The CH lines at 310.690 nm (from the R-branch of the C-X band) used here have greater efficiency than A-X and B-X transitions, which...
Article
Full-text available
Combustion stability is an important consideration for many energy systems due to its impact on performance and efficiency. Flame dynamics that govern combustion stability are often complex and difficult to resolve, particularly from experimental data. However, recent advances in post-processing techniques, such as dynamic mode decomposition (DMD),...
Article
Interactions between turbulent structures (eddies) and premixed flame fronts were characterized with simultaneous CH2O-/OH-PLIF and stereo-PIV at high framing rate (20 kHz). The high temporal resolution of these measurements permitted the tracking and subsequent characterization of the evolution of turbulence-flame interactions. Common events were...
Article
Planar laser-induced fluorescence (PLIF) of hydroxyl (OH) and formaldehyde (CH2O) radicals was performed alongside stereo particle image velocimetry (PIV) at a 20 kHz repetition rate in a highly turbulent Bunsen flame. A dual-pulse burst-mode laser generated envelopes of 532 nm pulse pairs for PIV as well as a pair of 355 nm pulses, the first of wh...
Article
This communication describes and demonstrates an approach to making simultaneous multi-scalar measurements with reduced equipment requirements. Specifically, this letter describes and demonstrates the ability to simultaneously acquire high-quality planar laser-induced fluorescence (PLIF) images of CH2O and either CH, OH, or a combination of CH and...
Article
Planar laser-induced fluorescence (PLIF) of hydroxyl (OH) and formaldehyde (CH2O) radicals was performed alongside stereo particle image velocimetry (PIV) at a 20 kHz repetition rate in a highly turbulent Bunsen flame. A dual-pulse burst-mode laser generated envelopes of 532 nm pulse pairs for PIV as well as a pair of 355 nm pulses, the first of wh...
Article
Full-text available
This study demonstrates advancement in a low-pulse energy methylidyne (CH) planar laser-induced fluorescence (PLIF) method that facilitates its application alongside flows seeded for particle image velocimetry (PIV) or other particle scattering based methods, as well as in high scattering environments. The C-X (0,0) R-branch excitation and filtered...
Article
Visualizing the structural features of turbulent premixed flames is critical for the validation of fundamental combustion theories. This work compares two methods for visualizing reaction layers of flames. The first is planar laser-induced fluorescence (PLIF) of CH radicals. The second involves the product of simultaneously acquired PLIF images of...
Article
This paper presents high-fidelity flame structure measurements of premixed methane–air Bunsen flames subjected to extreme levels of turbulence. Specifically, 28 cases were studied with longitudinal integral length scales (Lx) as large as 43 mm, turbulence levels (u′/SL) as high as 246, and turbulent Reynolds (ReT,0) and Karlovitz (KaT) numbers up t...
Article
Recent results have experimentally demonstrated the feasibility of obtaining 3D (three-dimensional) combustion measurements using tomographic VLIF (volumetric laser-induced fluorescence), specifically of the CH radical, representing the flame surface. To elucidate the fundamental capabilities and limitations of the VLIF technique, this work reports...
Article
Full-text available
Turbulent flame dynamics in an ethylene-fueled model scramjet is experimentally investigated in Mach 4.5 freestream flows with a total temperature of 2600 K using a pulsed arc-heated hypersonic wind-tunnel facility at the University of Notre Dame. Gaseous ethylene fuel is injected into the combustor through a supersonic nozzle, and is autoignited b...
Article
Full-text available
The influences of inlet geometry modification on flow and combustion dynamics are experimentally investigated in a model supersonic combustion ramjet (scramjet) installed in Mach 4.5 high-enthalpy air freestreams. The inlet upper-lip angle (6, 12, and 20 deg) and the intake contraction ratio (1.3 and 1.9) chosen as the generic inlet design paramete...
Article
This Letter reports the first direct comparison between two-dimensional (2D) and three-dimensional (3D) laser-induced fluorescence (LIF) applied to highly turbulent flames, with the goal of experimentally illustrating the capabilities and limitations of volumetric LIF (VLIF). To accomplish these goals, planar LIF (PLIF) and VLIF measurements were s...
Article
The goal of this work was to contrast and compare the 2D and 3D flame topography of a turbulent flame. The 2D measurements were obtained using CH-based (methylidyne radical-based) planar laser-induced fluorescence (PLIF), and the 3D measurements were obtained through a tomographic chemiluminescence (TC) technique. Both PLIF and TC were performed si...
Article
The changes in flame structure and burning velocity of premixed n-heptane/air flames associated with ignition chemistry have been investigated in a reactor-assisted turbulent slot (RATS) burner. Two distinct turbulent flame regimes are identified by varying the flow residence time and reactor temperature. A chemically frozen (CF) regime is observed...
Article
We describe efforts to develop kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the CH radical for application to premixed flames. The basic approach used here involves excitation and detection of the CH radial via the C2Σ+−X2Π (v′ = 0, v″ = 0) band, which has transitions in the wavelength range λ ≈ 310–320 nm. Transitions in this...
Article
Gas density and fuel mole fraction in a cavity flameholder of a supersonic combustor with ethylene (C2H4) fueling are simultaneously measured using nanosecond gated (10ns) laser-induced breakdown spectroscopy (n-LIBS). Emission spectra from the laser-induced plasma over a range (550nm-830nm) containing multiple emission lines of O, H, N, and C are...
Article
Full-text available
Simultaneous 10 kHz repetition-rate tomographic particle image velocimetry, hydroxyl planar laser-induced fluorescence (OH PLIF), and formaldehyde (CH (Formula presented.) O) PLIF were used to study the structure and dynamics of turbulent premixed flames. The flames investigated span from the classically defined corrugated flamelet regime to condit...
Article
Full-text available
Energy and conversion efficiencies of the carbon dioxide (CO₂) dissociation process at atmospheric-pressure conditions are investigated using a direct-coupled continuous microwave plasma system (MPS). Gas chromatography and mass spectrometer measurements were performed on the gas mixture postplasma treatment, over a range of specific energy inputs...
Article
Full-text available
The effect of continuous volumetric direct-coupled nonequilibrium atmospheric microwave plasma discharge on swirl-stabilized premixed methane-air flames was investigated using quantitative OH planar laser-induced fluorescence and spectrally resolved emission. The plasma discharge was found to influence the dynamics of flame stabilization, i.e., pla...
Conference Paper
Simultaneous 10 kHz repetition-rate tomographic particle image velocimetry (TPIV), hydroxyl planar laser induced fluorescence (OH PLIF), and formaldehyde (CH2O) PLIF were performed in a turbulent premixed flame in the thickened flamelet regime in order to quantify the conditional local turbulent flame speed. The flame speed is computed based on the...
Article
The potential for kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the Methylidyne (CH) radical using its \(C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi\)(v′ = 0, v′′ = 0) band was investigated. We show that due to its strength and the excitation wavelength (~314 nm), which is conveniently generated by a dye laser operating with a red...
Article
This study demonstrates high-repetition-rate planar laser-induced fluorescence (PLIF) imaging of hydroxyl radicals (OH) in flames at a continuous framing rate of 50 kHz. A frequency-doubled dye laser is pumped by the second harmonic of an Nd:YAG laser to generate laser radiation near 283 nm with a pulse width of 8 ns and rate of 50 kHz. Fluorescenc...
Article
Laser-induced breakdown is used for quantitative gas property measurements (gas density and ethylene fuel concentration) in a cavity flameholder in a supersonic crossflow. A plasma is produced by a focused laser beam (Nd:YAG, 532 nm) in the cavity to measure gas properties at the location of the plasma and to ignite cavity flames. Plasma energy (PE...
Article
Turbulent burning velocities, flashback, and auto-ignition of turbulent n-heptane/air flames have been experimentally investigated with kHz-rate planar laser-induced fluorescence (PLIF) imaging. Using a reactor-assisted turbulent slot (RATS) burner and varying mixture equivalence ratios and mean jet velocities, turbulent flame structures and dynami...
Article
This paper examines the structure of microwave (MW)-enhanced flames through 10-kHz imaging. High repetition rate laser diagnostic methods are used to simultaneously record 2-D images of OH laser-induced fluorescence and chemiluminescence within an atmospheric plasma-enhanced flame. Collecting both OH planar laser-induced fluorescence and chemilumin...
Article
OH imaging over a wide range of fuel flow rates for a directly fueled cavity were studied at a high sampling rate wherein highly transient complex flame dynamics were captured. At low fuel flow rates, flame position is determined by the primary recirculation zone, with bias toward the ramp and the shear layer. Flame convection from the main combust...
Article
This study demonstrates high-repetition-rate planar laser-induced fluorescence (PLIF) imaging of both cold (∼300  K) and hot (∼2400  K) nitric oxide (NO) at a framing rate of 10 kHz. The laser system is composed of a frequency-doubled dye laser pumped by the third harmonic of a 10 kHz Nd:YAG laser to generate continuously pulsed laser radiation at...
Article
A plasma-coupled methane-air flame is produced at atmospheric pressure by a microwave plasma source utilizing a tunable waveguide. Laser diagnostics are used to examine the direct-coupled, plasma-ignited, and sustained flame, for multiple flame types and nozzle geometries. OH radical number densities are quantified using planar laser-induced fluore...
Conference Paper
This study examines the structure of various microwave enhanced flames through 10 kHz imaging. High-speed laser diagnostic methods are implemented to simultaneously record two dimensional images of OH laser-induced fluorescence and chemiluminescence within an atmospheric plasma enhanced flame. A tunable, microwave waveguide plasma source ignites an...
Article
Full-text available
A tunable microwave waveguide is used to initiate and enhance combustion by coupling an atmospheric plasma dis-charge to a premixed methane/air flame. The absorbed microwave power ranges from 60 to 150 W, which was generated from a continuous source operating at 2.45 GHz, whereas combustion power ranges from 200 to 1000 W. OH radical number densiti...
Article
The re-ignition phenomenon is observed when fuel/oxidizer is re-introduced into an atmospheric-pressure plasma discharge generated by cutting off the gas flow in a re-entrant microwave-plasma applicator system used for plasma-assisted ignition and combustion research works. Results indicate that, for re-ignition to occur, the electric field must be...
Article
Quantitative images of temperature and hydroxyl (OH) concentrations are presented in plasma-enhanced flames, where a nonthermal microwave plasma discharge is coupled di- rectly with the reaction zone of the flame. The plasma jet is generated through a novel microwave (2.45 GHz) waveguide based a coaxial reactor system. Planar laser-induced fluoresc...
Conference Paper
An atmospheric high-Q microwave waveguide applicator is used to couple electromagnetic energy into an air flow and directly into the reaction zone of a premixed and non-premixed methane/air flame for energetic enhancement. Absorbed microwave powers range from 60 to 150 W while combustion powers range from 200 to 1000 W. OH number densities and temp...

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