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Grout R.W.,Combustion Research Facility | Gruber A.,Sintef | Yoo C.S.,Ulsan National Institute of Science and Technology | Chen J.H.,Combustion Research Facility
Proceedings of the Combustion Institute | Year: 2011

A reactive transverse fuel jet in cross-flow (JICF) configuration is studied using three-dimensional direct numerical simulation (DNS) with detailed chemical kinetics in order to investigate the mechanism of flame stabilization in the near field of a fuel jet nozzle. JICF configurations are used in practical applications where high mixing rates are desirable between the jet and the cross-flow fluids such as fuel injection nozzles and dilution holes in gas turbine combustors. This study examines a nitrogen-diluted hydrogen transverse jet exiting a square nozzle perpendicularly into a cross-flow of heated air. Improved understanding of the flame stabilization mechanism acting downstream of the transverse fuel jet will enable the formulation of more reliable guidelines for design of fuel injection nozzles which promote intrinsic flashback safety by reducing the likelihood of the flame anchoring at the injection site. The core of the heat release is located near the trailing edge of the fuel jet, at approximately 4 nozzle diameters away from the wall, and is characterized by the simultaneous occurrence of locally stoichiometric reactants and low flow velocities in the mean.The location where the most upstream tendrils of the flame are found is in the region where coherent vortical structures originating from the jet shear layer interaction are present. Instantaneously, upstream flame movement is observed through propagation into the outer layers of jet vortices. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.

Magnotti G.,Combustion Research Facility | Cutler A.D.,George Washington University | Danehy P.M.,NASA
Applied Optics | Year: 2013

This work describes the development of a dual-pump coherent anti-Stokes Raman spectroscopy system for simultaneous measurements of the temperature and the absolute mole fraction of N2, O2, and H2 in supersonic combusting flows. Changes to the experimental setup and the data analysis to improve the quality of the measurements in this turbulent, high-temperature reacting flow are described. The accuracy and precision of the instrument have been determined using data collected in a Hencken burner flame. For temperatures above 800 K, errors in the absolute mole fraction are within 1.5%, 0.5%, and 1% of the total composition for N2, O2, and H2, respectively. Standard deviations based on 500 single shots are between 10 and 65 K for the temperature, between 0.5% and 1.7% of the total composition for O2, and between 1.5% and 3.4% for N2. The standard deviation of H2 is ∼10% of the average measured mole fraction. © 2013 Optical Society of America.

Heaven M.C.,Emory University | Barker B.J.,Los Alamos National Laboratory | Antonov I.O.,Combustion Research Facility
Journal of Physical Chemistry A | Year: 2014

Understanding the influence of electrons in partially filled f- and d-orbitals on bonding and reactivity is a key issue for actinide chemistry. This question can be investigated by using a combination of well-defined experimental measurements and theoretical calculations. Gas phase spectroscopic data are particularly valuable for the evaluation of theoretical models. Consequently, the primary objectives of our research have been to obtain gas phase spectra for small actinide molecules. To complement the experimental effort, we are investigating the potential for using relativistic ab initio calculations and semiempirical models to predict and interpret the electronic energy level patterns for f-element compounds. Multiple resonance spectroscopy and jet cooling techniques have been used to unravel the complex electronic spectra of Th and U compounds. Recent results for fluorides, sulfides, and nitrides are discussed. (Figure Presented). © 2014 American Chemical Society.

Bohlin A.,Combustion Research Facility | Kliewer C.J.,Combustion Research Facility
Journal of Chemical Physics | Year: 2013

Coherent anti-Stokes Raman spectroscopy (CARS) has been widely used as a powerful tool for chemical sensing, molecular dynamics measurements, and rovibrational spectroscopy since its development over 30 years ago, finding use in fields of study as diverse as combustion diagnostics, cell biology, plasma physics, and the standoff detection of explosives. The capability for acquiring resolved CARS spectra in multiple spatial dimensions within a single laser shot has been a long-standing goal for the study of dynamical processes, but has proven elusive because of both phase-matching and detection considerations. Here, by combining new phase matching and detection schemes with the high efficiency of femtosecond excitation of Raman coherences, we introduce a technique for single-shot two-dimensional (2D) spatial measurements of gas phase CARS spectra. We demonstrate a spectrometer enabling both 2D plane imaging and spectroscopy simultaneously, and present the instantaneous measurement of 15 000 spatially correlated rotational CARS spectra in N2 and air over a 2D field of 40 mm2. © 2013 AIP Publishing LLC.

Bambha R.P.,Combustion Research Facility | Dansson M.A.,Combustion Research Facility | Schrader P.E.,Combustion Research Facility | Michelsen H.A.,Combustion Research Facility
Applied Physics B: Lasers and Optics | Year: 2013

We have measured time-resolved laser-induced incandescence (LII) from combustion-generated mature soot extracted from a burner and (1) coated with oleic acid or (2) coated with oleic acid and then thermally denuded using a thermodenuder. The soot samples were size selected using a differential mobility analyzer and characterized with a scanning mobility particle sizer, centrifugal particle mass analyzer, and transmission electron microscope. The results demonstrate a strong influence of coatings on the magnitude and temporal evolution of the LII signal. For coated particles, higher laser fluences are required to reach signal levels comparable to those of uncoated particles. The peak LII curve is shifted to increasingly higher fluences with increasing coating thickness until this effect saturates at a coating thickness of 75 % by mass. These effects are predominantly attributable to the additional energy needed to vaporize the coating while heating the particle. LII signals are higher and signal decay rates are significantly slower for thermally denuded particles relative to coated or uncoated particles, particularly at low and intermediate laser fluences. Our results suggest negligible coating enhancement in absorption cross-section for combustion-generated soot at the laser fluences used. Apparent enhancement in absorption with restructuring may be caused by less conductive cooling. © The Author(s) 2013.

Patterson B.D.,Combustion Research Facility | Gao Y.,A-D Technologies | Seeger T.,A-D Technologies | Seeger T.,University of Siegen | Kliewer C.J.,Combustion Research Facility
Optics Letters | Year: 2013

We introduce a multiplex technique for the single-laser-shot determination of S-branch Raman linewidths with high accuracy and precision by implementing hybrid femtosecond (fs)/picosecond (ps) rotational coherent anti-Stokes Raman spectroscopy (CARS) with multiple spatially and temporally separated probe beams derived from a single laser pulse. The probe beams scatter from the rotational coherence driven by the fs pump and Stokes pulses at four different probe pulse delay times spanning 360 ps, thereby mapping collisional coherence dephasing in time for the populated rotational levels. The probe beams scatter at different folded BOXCARS angles, yielding spatially separated CARS signals which are collected simultaneously on the charge coupled device camera. The technique yields a single-shot standard deviation (1ó) of less than 3.5% in the determination of Raman linewidths and the average linewidth values obtained for N2 are within 1% of those previously reported. The presented technique opens the possibility for correcting CARS spectra for time-varying collisional environments in operando. © 2013 Optical Society of America.

O'Connor J.,Combustion Research Facility | Lieuwen T.,Georgia Institute of Technology
Physics of Fluids | Year: 2012

This work investigates the response of the vortex breakdown region of a swirling, annular jet to transverse acoustic excitation for both non-reacting and reacting flows. This swirling flow field consists of a central vortex breakdown region, two shear layers, and an annular fluid jet. The vortex breakdown bubble, a region of highly turbulent recirculating flow in the center of the flowfield, is the result of a global instability of the swirling jet. Additionally, the two shear layers originating from the inner and outer edge of the annular nozzle are convectively unstable and rollup due to the Kelvin-Helmholtz instability. Unlike the convectively unstable shear layers that respond in a monotonic manner to acoustic forcing, the recirculation zone exhibits a range of response characteristics, ranging from minimal response to exhibiting abrupt bifurcations at large forcing amplitudes. In this study, the response of the time-average and fluctuating recirculation zone is measured as a function of forcing frequency, amplitude, and symmetry. The time-average flow field is shown to exhibit both monotonically varying and abrupt bifurcation features as acoustic forcing amplitude is increased. The unsteady motion in the recirculation zone is dominated by the low frequency precession of the vortex breakdown bubble. In the unforced flow, the azimuthal m = -2 and m = -1 modes (i.e., disturbances rotating in the same direction as the swirl flow) dominate the velocity disturbance field. These modes correspond to large scale deformation of the jet column and two small-scale precessing vortical structures in the recirculation zone, respectively. The presence of high amplitude acoustic forcing changes the relative amplitude of these two modes, as well as the character of the self-excited motion. For the reacting flow problem, we argue that the direct effect of these recirculation zone fluctuations on the flame response to flow forcing is not significant. Rather, flame wrinkling in response to flow forcing is dominated by shear layer disturbances. Recirculation zone dynamics primarily influence the time-average flame features (such as spreading angle). These influences on the flame response are indirect, as they control the transfer function relating shear layer fluctuations and the resulting flame response. © 2012 American Institute of Physics.

A counterpropagating phase-matching geometry is employed for high-spatial-resolution one-dimensional (1D) imaging of temperature and O 2-to-N2 concentration ratio using picosecond pure-rotational coherent anti-Stokes Raman spectroscopy (RCARS) over a large field (20 mm). A single-shot 1D RCARS image of more than 20 mm in length is thus acquired at 300 K in air. High-resolution 1D RCARS flame measurements are demonstrated using a custombuilt burner and a premixed methane/air flame (Φ = 0.6). This phase-matching scheme improves the spatial resolution by approximately 1 order of magnitude when compared to the standard small-angle BOXCARS phase-matching schemes typically employed in CARS measurements. Additionally, for a 20 mm 1D image, signal levels are increased by 10 2 because of the higher irradiance provided in the current scheme. © 2012 Optical Society of America.

Knudsen E.,Stanford University | Richardson E.S.,University of Southampton | Doran E.M.,Robert Bosch GmbH | Pitsch H.,Institute For Technische Verbrennung | Chen J.H.,Combustion Research Facility
Physics of Fluids | Year: 2012

Scalar dissipation rates and subfilter scalar variances are important modeling parameters in large eddy simulations (LES) of reacting flows. Currently available models capture the general behavior of these parameters, but these models do not always perform with the degree of accuracy that is needed for predictive LES. Here, two direct numerical simulations (DNS) are used to analyze LES dissipation rate and variance modelsto propose a new model for the dissipation rate that is based on a transport equation. The first DNS that is considered is a non-premixed auto-igniting C2H4 jet flame simulation originally performed by Yoo et al. [Proc. Combust. Inst.33, 1619-1627 (2011)]10.1016/j.proci.2010.06.147. A LES of this case is run using algebraic models for the dissipation rate and subfilter variance. It is shown that the algebraic models fail to adequately reproduce the DNS results. This motivates the introduction of a transport equation model for the LES dissipation rate. Closure of the equation is addressed by formulating a new adapted dynamic approach. This approach borrows dynamically computed information from LES quantities that, unlike the dissipation rate, do not reside on the smallest flow length scales. The adapted dynamic approach is analyzed by considering a second DNS of scalar mixing in homogeneous isotropic turbulence. Data from this second DNS are used to confirm that the adapted dynamic approach successfully closes the dissipation rate equation over a wide range of LES filter widths. The first reacting jet case is then returned to and used to test the LES transport equation models. The transport equation model for the dissipation rate is shown to be more accurate than its algebraic counterpointthe dissipation rate is eliminated as a source of error in the transported variance model. © 2012 American Institute of Physics.

Chen J.H.,Combustion Research Facility
Proceedings of the Combustion Institute | Year: 2011

The advent of petascale computing applied to direct numerical simulation (DNS) of turbulent combustion has transformed our ability to interrogate fine-grained 'turbulence-chemistry' interactions in canonical and laboratory configurations. In particular, three-dimensional DNS, at moderate Reynolds numbers and with complex chemistry, is providing unprecedented levels of detail to isolate and reveal fundamental causal relationships between turbulence, mixing and reaction. This information is leading to new physical insight, providing benchmark data for assessing model assumptions, suggesting new closure hypotheses, and providing interpretation of statistics obtained from lower-dimensional measurements. In this paper the various roles of petascale DNS are illustrated through selected examples related to lifted flame stabilization, premixed and stratified flame propagation in intense turbulence, and extinction and reignition in turbulent non-premixed jet flames. Extending the DNS envelope to higher Reynolds numbers, higher pressures, and greater chemical complexity will require exascale computing in the next decade. The future outlook of DNS in terms of challenges and opportunities in this regard are addressed. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.

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