Nitric Oxide Planar Laser-induced Fluorescence Research Articles

Simple, Fast Nitric Oxide Planar Laser-Induced Fluorescence Model for Computational Fluid Dynamics Applications

The current work demonstrates the efficacy of a simple and fast model for nitric oxide (NO) laser-induced fluorescence (LIF). The model is validated with experimental LIF spectra computed from captured images of fluorescence from a gas cell filled with a known amount of NO. These data are obtained for several segments of the NO fluorescence spectrum. The selected segments encompass LIF lines with rotational quantum numbers appropriate for low-to-moderate temperature flows. The spectra obtained using the current model are also compared to those obtained from other theoretical models, namely, LIFBASE and LINUS. The current model is based on a previously developed version of a two-level fluorescence model and is developed specifically for applications in computational fluid dynamics (CFD) where rapid on-the-fly evaluations are required due to the potentially large number of grid cells for which LIF is being computed. Good agreement between the experimental and theoretical results provides confidence in current LIF modeling for use with CFD. An example application of the current model in a Mach 6.5 hypersonic mixing flowfield is also presented.

Megahertz-Rate Imaging of Hypersonic Boundary-Layer Instabilities in a Mach 10 Shock Tunnel

The transition of the boundary-layer state from laminar to turbulent on a 7° sharp cone in Mach 10 flow is measured using several high-speed optical diagnostics capable of time-resolved measurements of the second-mode instability with high spatial resolution. Specifically, megahertz-rate nitric oxide planar laser-induced fluorescence (NO PLIF) imaging is done at 1 MHz to provide a non-path-integrated visualization of second-mode associated structures in the transitioning boundary layer. A recent variation of focused laser differential interferometry (FLDI), namely, linear array FLDI (LA-FLDI), is also applied for multipoint measurements of frequency content in the transitioning boundary layer. From each data set, the frequency of the most unstable second-mode instability is obtained along with the phase velocity of the second-mode waves. Measurements show good agreement relative to each other and also with surface pressure sensors and high-speed schlieren imaging. Furthermore, the 1 MHz NO PLIF imaging demonstrated here shows great potential for visualization of transitional and turbulent boundary layers when path-integrated diagnostics cannot be used.

CH and NO planar laser-induced fluorescence and Rayleigh-scattering in turbulent flames using a multimode optical parametric oscillator.

An optical parametric oscillator (OPO) is developed and characterized for the simultaneous generation of ultraviolet (UV) and near-UV nanosecond laser pulses for the single-shot Rayleigh scattering and planar laser-induced-fluorescence (PLIF) imaging of methylidyne (CH) and nitric oxide (NO) in turbulent flames. The OPO is pumped by a multichannel, 8-pulse Nd:YAG laser cluster that produces up to 225 mJ/pulse at 355 nm with pulse spacing of 100 µs. The pulsed OPO has a conversion efficiency of 9.6% to the signal wavelength of ∼430nm when pumped by the multimode laser. Second harmonic conversion of the signal, with 3.8% efficiency, is used for the electronic excitation of the A-X (1,0) band of NO at ∼215nm, while the residual signal at 430 nm is used for direct excitation of the A-X (0,0) band of the CH radical and elastic Rayleigh scattering. The section of the OPO signal wavelength for simultaneous CH and NO PLIF imaging is performed with consideration of the pulse energy, interference from the reactant and product species, and the fluorescence signal intensity. The excitation wavelengths of 430.7 nm and 215.35 nm are studied in a laminar, premixed CH4-H2-NH3-air flame. Single-shot CH and NO PLIF and Rayleigh scatter imaging is demonstrated in a turbulent CH4-H2-NH3 diffusion flame using a high-speed intensified CMOS camera. Analysis of the complementary Rayleigh scattering and CH and NO PLIF enables identification and quantification of the high-temperature flame layers, the combustion product zones, and the fuel-jet core. Considerations for extension to simultaneous, 10-kHz-rate acquisition are discussed.

Tunable narrow-linewidth 226  nm laser for hypersonic flow velocimetry.

We report on the development and application of a novel, to the best of our knowledge, all-solid-state tunable narrow-linewidth 226 nm UV laser system. The laser system consists of three parts: a tunable single-frequency Ti:sapphire 787 nm laser, a single-frequency long-pulse-width flattop-shaped 532 nm laser, and a nonlinear frequency transformation system. The 532 nm laser is a sum-frequency mixed with the second harmonic of the 787 nm laser to produce the 226 nm laser. The maximum output pulse energy at 226 nm is 3 mJ. Nitric oxide planar laser-induced fluorescence velocimetry is demonstrated in the China Aerodynamics Research and Development Center's FD14 hypersonic shock tunnel using this 226 nm laser system. It is proven that this laser is convenient for high-resolution molecular tagging fluorescence spectroscopy.

Evaluation of nitric oxide laser-induced fluorescence thermometry techniques in a hypersonic boundary layer

Nitric oxide planar laser-induced fluorescence was performed to measure the wall-normal distribution of static temperature through a hypersonic boundary layer. A 10-degree half-angle wedge model was oriented at a 5-degree angle of attack in the NASA Langley 31-in Mach 10 facility, resulting in a 5-degree flow turning angle and an edge Mach number of 7.6. Nitric oxide was seeded through a spanwise slot into the boundary layer upstream of the imaging region and was excited with a pulsed ultraviolet planar laser sheet. The laser was spectrally scanned across six fluorescence transitions in the (0, 0) band of the $$A^2\Sigma ^+$$–$$X^2 \Pi $$ system. Eighteen thermometry methods were assessed through comparison to predictions of the temperature field from computational fluid dynamics simulations. The effect of spectral resolution and laser linewidth on measurement uncertainty was also investigated. The most accurate technique was spectral peak thermometry, which achieved an accuracy of ± 31.6 K ($$12.6\%$$ error relative to CFD temperature). The spectral peak thermometry technique required a minimum spectral resolution between 0.074 and 0.102 $$ {\mathrm {cm}}^{-1} $$ and a maximum laser linewidth of 0.49 $$ {\mathrm {cm}}^{-1} $$ to extract meaningful temperature information from the spectra.

Visualization of Simulated Fuel–Air Mixing in a Dual-Mode Scramjet

Nitric oxide planar laser-induced fluorescence measurements have been performed in a small-scale scramjet combustor at the University of Virginia Aerospace Research Laboratory at nominal simulated Mach 5 flight. A mixture of nitric oxide and nitrogen was injected at the upstream end of the inlet isolator as a surrogate for ethylene fuel, and the mixing of this fuel simulant was studied with and without a shock train. The shock train was produced by an air throttle, which simulated the blockage effects of combustion downstream of the cavity flameholder. Nitric oxide planar laser-induced fluorescence signal was imaged in a plane orthogonal to the freestream at the leading edge of the cavity. Instantaneous planar images were recorded and analyzed to identify the most uniform cases, which were achieved by varying the location of the fuel injection and shock train. This method was used to screen different possible fueling configurations to provide optimized test conditions for follow-on combustion measurements using ethylene fuel. A theoretical study of the selected nitric oxide rotational transitions was performed to obtain a laser-induced fluorescence signal that was linear with the nitric oxide mole fraction and approximately independent of the pressure and temperature.

Nitric-Oxide Planar Laser-Induced Fluorescence Measurements in the Hypersonic Materials Environmental Test System

Planar laser-induced fluorescence of naturally occurring nitric oxide has been used to provide insight into baseline flow conditions of the Hypersonic Materials Environmental Test System 400 kW arc-heated wind tunnel at NASA Langley Research Center via radial and axial velocity measurements and instantaneous flow-visualization images. This represents both the first flow-tagging velocity measurements and the first application of nitric-oxide planar laser-induced fluorescence flow visualization in an arcjet facility. Results are presented at selected facility run conditions, including some in a simulated Earth atmosphere (75% nitrogen, 20% oxygen, 5% argon) and others in a simulated Martian atmosphere (71% carbon dioxide, 24% nitrogen, 5% argon) for specific bulk enthalpies ranging from 6.5 to . Flow-visualization images reveal the presence of large-scale unsteady flow structures and indicate nitric-oxide fluorescence signal over more than 70% of the core flow for specific bulk enthalpies below about but over less than 10% of the core flow for specific bulk enthalpies above about . Axial velocimetry was performed using molecular tagging velocimetry. Axial velocities of about were measured along the centerline. Radial velocimetry was performed by scanning the wavelength of the narrowband laser and analyzing the resulting Doppler shift. Radial velocities of were measured.

Mars Science Laboratory Reaction Control System Jet Computations with Visualization and Velocimetry

Numerical predictions of the Mars Science Laboratory reaction control system jets interacting with a Mach 10 hypersonic flow are compared to experimental nitric oxide planar laser-induced fluorescence data. The steady Reynolds-averaged Navier–Stokes equations using the Baldwin–Barth one-equation turbulence model were solved using the OVERFLOW code. The experimental fluorescence data used for comparison consist of qualitative two-dimensional visualization images, qualitative reconstructed three-dimensional flow structures, and quantitative two-dimensional distributions of streamwise velocity. Through modeling of the fluorescence signal equation, computational flow images were produced and directly compared to the qualitative fluorescence data. Post processing of the experimental and simulation data enabled the jet trajectory to be extracted for a more quantitative comparison. The two-dimensional velocity fields were reconstructed through interpolation of a series of single-component velocity profiles. Each distribution of single-component velocity was obtained using molecular tagging velocimetry. After validating the numerical model, the numerical solution was further examined to gain insight into hypersonic jet-in-crossflow interaction. Novel nitric oxide planar laser induced fluorescence experiments are proposed based on this analysis.

Planar Laser-Induced Fluorescence of Mars Science Laboratory Reaction Control System Jets

Nitric-oxide planar laser-induced fluorescence was used to visualize the flow in the wake of a Mars Science Laboratory entry capsule with activated reaction control system jets in NASA Langley Research Center’s 31 in. Mach 10 air tunnel facility. Images were processed using the virtual diagnostics interface method, which brings out the three-dimensional nature of the flow visualization data while showing the relative location of the data with respect to the model. Comparison of wind-on and wind-off results illustrates the effect that the hypersonic crossflow has on the trajectory and structure of individual reaction control system jets. The visualization and comparison of both single and multiple activated jets indicate low levels of jet–jet interaction. Quantitative streamwise velocity was also obtained via molecular tagging velocimetry.

Nitric-oxide planar laser-induced fluorescence at 10 kHz in a seeded flow, a plasma discharge, and a flame

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 226 nm for excitation of NO. The laser-induced fluorescence signal is detected using a high-frame rate, intensified CMOS camera, yielding a continuous cinematographic propagation of the NO plume where data acquisition duration is limited only by camera memory. The pulse energy of the beam is ~20 μJ with a spectral width ~0.15 cm(-1), though energies as high as 40 μJ were generated. Hot NO is generated by passing air through a DC transient-arc plasma torch that dissociates air. The plasma torch is also used to ignite and sustain a CH(4)/air premixed flame. Cold NO is imaged from a 1% NO flow (buffered by nitrogen). The estimated signal-to-noise ratio (SNR) for the cold seeded flow and air plasma exceeds 50 with expected NO concentrations of 6000-8000 parts per million (ppm, volume basis). Images show distinct, high-contrast boundaries. The plasma-assisted flame images have an SNR of less than 10 for concentrations reaching 1000 ppm. For many combustion applications, the pulse energy is insufficient for PLIF measurements. However, the equipment and strategies herein could be applied to high-frequency line imaging of NO at concentrations of 10-100 ppm. Generation of 226 nm radiation was also performed using sum-frequency mixing of the 532 nm pumped dye laser and 355 nm Nd:YAG third harmonic but was limited in energy to 14 μJ. Frequency tripling a 532 nm pumped dye laser produced 226 nm radiation at energies comparable to the 355 nm pumping scheme.

NO PLIF imaging in the CUBRC 48-inch shock tunnel

Nitric oxide planar laser-induced fluorescence (NO PLIF) imaging is demonstrated at a 10-kHz repetition rate in the Calspan University at Buffalo Research Center’s (CUBRC) 48-inch Mach 9 hypervelocity shock tunnel using a pulse burst laser–based high frame rate imaging system. Sequences of up to ten images are obtained internal to a supersonic combustor model, located within the shock tunnel, during a single ~10-millisecond duration run of the ground test facility. Comparison with a CFD simulation shows good overall qualitative agreement in the jet penetration and spreading observed with an average of forty individual PLIF images obtained during several facility runs.

Velocity Profile Measurements in Hypersonic Flows Using Sequentially Imaged Fluorescence-Based Molecular Tagging

Nitric-oxide planar laser-induced fluorescence was used to perform velocity measurements in hypersonic flows by generatingmultiple tagged lines thatfluoresce as they convect downstream.Determination of axial velocitywasmade by application of a cross-correlation analysis of the horizontal shift of individual tagged lines. A single interline, progressive scan-intensified charge-coupled device camera was used to obtain two sequential images of the nitricoxide molecules that had been tagged by the laser. The charge-coupled device allowed for submicrosecond acquisition of both images, resulting in submicrosecond temporal resolution as well as submillimeter spatial resolution (0.5 mm horizontal, 0.7 mm vertical). Quantification of systematic errors, the contribution of gating/ exposure duration errors, and the influence of collision rate on temporal uncertainty were made. This velocity measurement technique has been demonstrated for two hypersonic flow experiments: 1) a reaction control system jet on an Orion crew exploration vehicle wind-tunnel model and 2) a 10 deg half-angle wedge with a 2-mm-tall 4-mmwide cylindrical boundary-layer trip. Mean-velocity uncertainties below 30 m=s (2.7% of the measured average velocity) and single-shot uncertainties below 100 m=s (9.7%of themeasured average velocity) have been obtained in regions with optimal signal intensities using this technique.

MHz-rate nitric oxide planar laser-induced fluorescence imaging in a Mach 10 hypersonic wind tunnel

Nitric oxide planar laser-induced fluorescence (NO PLIF) imaging at repetition rates as high as 1 MHz is demonstrated in the NASA Langley 31 in. Mach 10 hypersonic wind tunnel. Approximately 200 time-correlated image sequences of between 10 and 20 individual frames were obtained over eight days of wind tunnel testing spanning two entries in March and September of 2009. The image sequences presented were obtained from the boundary layer of a 20° flat plate model, in which transition was induced using a variety of different shaped protuberances, including a cylinder and a triangle. The high-speed image sequences captured a variety of laminar and transitional flow phenomena, ranging from mostly laminar flow, typically at a lower Reynolds number and/or in the near wall region of the model, to highly transitional flow in which the temporal evolution and progression of characteristic streak instabilities and/or corkscrew-shaped vortices could be clearly identified.

Nitric Oxide Formation in a Premixed Flame With High-Level Plasma Energy Coupling

This paper presents quantitative planar laser-induced fluorescence (PLIF) imaging of nitric oxide (NO) in a transient-arc direct-current plasmatron igniter using premixed air/fuel mixtures. Quantitative measurements of NO are reported as a function of gas flow rate (20-50 standard cubic feet per hour), plasma power (100-900 mA, 150-750 W), and equivalence ratio (0.7-1.3). Images were corrected for temperature effects by using 2-D temperature field measurements obtained with infrared thermometry and calibrated by a more accurate multiline fitting technique. The signals were then quantified using an NO addition method and spectroscopic laser-induced fluorescence modeling of NO. NO PLIF images and single-point NO concentrations are presented for both plasma-discharge-only and methane/air plasma-enhanced combustion cases. NO formation occurs predominantly through N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (v) + O ¿ NO + N for the plasma-discharge-only case without combustion. The NO concentration for the plasma-enhanced combustion case (500-3500 ppm) was an order of magnitude less than the plasma-discharge-only case (8000-15 000 ppm) due to the reduction of plasma reactions by the methane. Experiments show the linear decay of NO from equivalence ratio 0.8-1.2 under the same flow condition and discharge current.

In-Stream Hypermixer Fueling Pylons in Supersonic Flow

This paper presents results from both computational fluid dynamic and wind-tunnel experiments of in-stream fueling pylons injecting air, ethylene, and methane gas into Mach number 2.0 cold airflow. Three fuel-injection pylons studied include a basic pylon, a ramp pylon, and an alternating-wedge pylon. The latter two pylons introduce streamwise vorticity into the flow to increase mixing action. The computational fluid dynamic solution was accomplished using the commercial code FLUENT®. Three wind-tunnel experimental techniques were used: aerothermal probing, Raman spectroscopy, and nitric-oxide planar laser-induced fluorescence. Four measures reported include streamwise vorticity, total-pressure-loss, mixing efficiency, and flammable plume extent. The ramp and alternating-wedge pylons show decisive increases in mixing capability compared with the basic pylon for a finite distance downstream of the injector. The alternating-wedge pylon exhibits a measurable increase in total pressure loss compared with the basic pylon, and the ramp pylon exhibits a negligible increase in total pressure loss compared with the basic pylon. For comparison, the downstream mixing effectiveness of the three pylons is compared with the downstream mixing effectiveness of a transverse circular wall injector studied in past research. In addition, a qualitative comparison between the computational fluid dynamic and wind-tunnel experimental results is made.

Ultrahigh-frame-rate nitric oxide planar laser-induced fluorescence imaging

We demonstrate the ability to generate ultrahigh frequency burst sequences of deep UV at 226 nm by mixing the optical parametric oscillator signal output at 622 nm with third harmonic at 355 nm from a pulse burst laser system. We obtained 226 nm burst sequences with uniform burst envelopes, and the average pulse energy is approximately 0.5 mJ. Nitric oxide planar laser-induced fluorescence image sequences at ultrahigh (100 kHz) frame rates have been obtained.

Nitric-oxide planar laser-induced fluorescence applied to low-pressure hypersonic flow fields for the imaging of mixture fraction.

The scalar-field imaging of a hypersonic mixing flow is performed in a mixing facility that is shock tunnel driven. The instantaneous mixture-fraction field of a hypersonic two-dimensional mixing layer (M1 = 5.1, M2 = 0.3) is determined with a temperature-insensitive planar laser-induced fluorescence technique with nitric oxide (NO) as the tracer species. Single-shot images are obtained with the broadband excitation of a reduced temperature-sensitivity transition in the A2 sigma+ <-- X2 II(1/2) (0, 0) band of NO near 226 nm. The instantaneous mixture-fraction field at a convective Mach number of 2.64 is shown to be nearly identical to a typical diffusive process, supporting the notion of gradient-transport mixing models for highly compressible mixing layers.