Laser-induced Fluorescence Of Nitric Oxide Research Articles

Nitric oxide laser-induced fluorescence using the fifth harmonic of a broad-band Nd:YAG laser

We explore the prospects of laser-induced fluorescence diagnostics of nitric oxide (NO) using non-tunable fifth-harmonic radiation of a broad-band, ns-pulsed Nd:YAG laser at λ=213\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda = 213$$\\end{document} nm. Typically, 2–5 mJ/pulse of 213-nm radiation is produced by a commercial harmonic generator in this study, with an efficiency of about 1–3% (relative to the input pulse energy). We present spectral results obtained in various environments, ranging from air-based combustion processes at room conditions up to elevated pressure and temperature environments, the latter resembling conditions typical for compression-ignition internal combustion engines. In all cases, the laser-induced fluorescence spectrum shows clear signatures of the NO spectrum, mostly on transitions in the γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma $$\\end{document}-band system (A2Σ+→X2Π\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {A}^2\\Sigma ^+ \\rightarrow X^2\\Pi $$\\end{document}). At higher fluences, multi-photon absorption also gives rise to blue-shifted fluorescence. The fluorescence yield increases with increasing pressure, allegedly due to non-resonant excitation, the efficiency of which increases with increasing pressure broadening. When applied to air-based combustion processes, interference by (hot) oxygen needs to be taken into account. We conclude that the method is a relatively straightforward option to visualize the NO distribution in a broad variety of applications.

Experimental study of nitric oxide distributions in non-premixed and premixed ammonia/hydrogen-air counterflow flames

This study reports an experimental investigation of quantitative Nitric Oxide (NO) distribution in both premixed and non-premixed NH3/H2-air flames using a counterflow burner at atmospheric pressure. One-dimensional (1D) NO laser-induced fluorescence (LIF) spectroscopy and Raman/Rayleigh spectroscopy were conducted to accurately resolve the quantitative 1D NO profile in terms of mixture fraction, temperature, and physical space. We calibrated a saturated NO-LIF model in 5 premixed lean H2/N2/NO-air flames with different seeded NO levels in a McKenna burner and validated its accuracy in three H2N2NO-air counterflow diffusion flames. The overall uncertainty of NO quantification was less than90 ppm. Our measurements were compared with simulations using different ammonia chemical kinetic models, revealing that current models have over 30% uncertainty in predicting peak NO concentrations (mole fraction) in 1D non-premixed and premixed flames and over 100% uncertainty in lower temperature regions. In premixed flames, measured NO concentrations fell within the intermediate range of current chemical kinetic models at lean and stoichiometric conditions, but were lower than the models at rich conditions. In non-premixed flames, all models overestimated the peak NO concentrations by more than 1000 ppm. It is noted that the measured peak NO concentrations increased with higher NH3/H2 ratios (from 4/6 to 8/2), strain rates (from 80 to 140 1/s), and N2 dilution ratios in a 1:1 NH3/H2 mixture (from 0 to 30%). Although most models could qualitatively predict the trends, they were inaccurate in quantifying NO. Additionally, the measured width of the NO profile in mixture fraction space expanded with increasing NH3/H2 ratio, N2 dilution ratio, and strain rate. While models could qualitatively predict this behavior, they consistently underestimated NO in the fuel-rich, lower-temperature region, resulting in a narrower NO profile width. The Manna model showed a better prediction of NO distribution in the fuel rich portion of non-premixed flames, accounting for NH3NO interactions at lower temperatures. These findings highlight the critical need to improve models to accurately predict NO concentrations in ammonia-containing flames and their behavior in fuel rich regions.

Iron oxide nanoparticle synthesis: Simulation-based comparison of laboratory- and pilot plant-scale spray-flame synthesis

This study analyzes burner scale-dependent effects for iron-oxide nanoparticles synthesis in spray flames through a combined experimental and numerical approach. A laboratory- and a pilot plant-scale synthesis facility generate iron oxide nanoparticles from iron nitrate dissolved in ethanol/2-ethylhexanoic acid solvents on the 0.5 and 15 g/h scale, respectively. Phase Doppler measurements supply initial conditions for spray development in the numerical approach, while in situ OH* chemiluminescence and multi-line nitric oxide laser-induced fluorescence (NO-LIF) thermometry provide experimental data for comparison with simulation results of the pilot plant burner. Ex situ particle sizing by gas adsorption according to Brunauer-Emmet-Teller (BET) and transmission electron microscopy (TEM), along with phase composition determination via X-ray diffraction (XRD), are used to compare products of both burners and the corresponding simulations. The numerical approach employs a Reynolds-averaged Eulerian–Lagrangian description of the flow in combination with a flamelet/progress variable (FPV) combustion and monodisperse particle model to reflect spray-flame synthesis. Despite similar chemiluminescence between experiment and simulation, more significant discrepancies are observed in NO-LIF thermometry and particle sizing. Nanoparticle formation and growth at both burner scales is investigated using the numerical method. Special attention is directed to the high-temperature particle residence time (HTPRT). Notably, the average temperature–time profiles particles experience are almost identical in the burners, although their geometric scales and designs differ substantially. Simulation results show that while the primary particle diameter remains mostly consistent, the pilot-scale burner produces larger particle agglomerates than the laboratory burner. The difference is attributed to an increased particle number concentration during the initial formation of soft agglomerates. The findings demonstrate that, despite retaining a similar HTPRT, the overall flow conditions and liquid spray dispersion, which impact the distribution of the particle number concentration, influence the final agglomerate size.

NO measurements in high temperature hydrogen flames: The crucial role of the hydrogen oxidation chemistry for accurate NO predictions

The current work investigates the formation of Nitric Oxide (NO) in hydrogen–air flames, over a wide range of flame temperatures. The use of hydrogen allows improved focus on the thermal-NO pathway by removing the complexity introduced by the prompt-NO pathway, which has been shown to be an important contributor to inaccurate predictions of absolute post-flame NO concentrations in hydrocarbon flames. This experimental study is conducted at atmospheric pressure using stoichiometric, premixed, laminar stagnation flames. Adiabatic flame temperatures ranging from 1600K to 2300K are achieved by varying the argon concentration in air. One-dimensional velocity, temperature, and NO concentration profiles are measured using non-intrusive laser diagnostics: Particle Tracking Velocimetry (PTV), NO multiline thermometry, and NO Laser Induced Fluorescence (NO-LIF), respectively. Results show that the experimental velocity profiles are incorrectly captured by the studied mechanisms, especially at low and high temperatures. This suggests that major inaccuracies are present in the hydrogen oxidation chemistry of the thermochemical models, regardless of their optimisation methodology. Furthermore, NO-LIF profiles show major discrepancies between all the studied mechanisms and the experiments, especially at elevated temperatures. The disagreement stems from an inaccurate description of the base chemistry of the models. These inaccuracies arise specifically from the description of the radical pool driving the flame behaviour and NO formation. This study demonstrates the need for model optimisation on experimental measurements using pure hydrogen 1D flames to obtain an accurate description of the hydrogen oxidation chemistry at play. This would lead to an improved description of the NOx sub-chemistry of any hydrogen, or hydrocarbon, combustion system.

Simultaneous planar laser-induced fluorescence of nitric oxide and hydroxyl radical with a single dye laser in hydrogen flames

Simultaneous, planar laser-induced fluorescence (PLIF) of nitric oxide (NO) and hydroxyl radical (OH) is evidenced with a single dye laser and two intensified CCD (ICCD) cameras. The technique is demonstrated on three premixed turbulent hydrogen-air flames, at atmospheric conditions, with several laser excitation wavelengths within the A2Σ+ − X2Π (0,0) and A2Σ+ − X2Π (1,0) vibrational bands of NO and OH, respectively. Via adjustments of the grating angle of the dye laser, the output wavelengths can be optimized so that both LIF signals are obtained simultaneously, or it can be tuned to reach the most common excitation wavelengths for OH-PLIF (i.e., near 283 or 284 nm) or NO-PLIF (i.e., 225 and 226 nm) alternatively. The flame visualization results, obtained with the different excitation strategies, are consistent for the three flame conditions and enable to characterize both the reaction zones and the nitric oxide formation. With appropriate selection of the wavelength pair, the technique shows its great potential to enhance the ability of existing lasers to simultaneously visualize and quantify distributions of two key species in reacting flows with only a single laser.

Towards laser-induced fluorescence of nitric oxide in detonation

Towards laser-induced fluorescence of nitric oxide in detonation

Quantitative laser-induced fluorescence of NO in ammonia-hydrogen-nitrogen turbulent jet flames at elevated pressure

Ammonia is a very promising carbon-free fuel, but its combustion is prone to generate a large amount of harmful nitric oxide (NO). Designing NO reduction strategies for ammonia flames requires computational fluid dynamics and accurate kinetic mechanisms. However, there are currently no available experimental data that can be used to validate models describing chemistry-turbulence interactions in ammonia flames. This study introduces two non-premixed turbulent jet flames that emulate some features of the cracked ammonia combustion at 5 bar, relevant to micro gas turbines. These ammonia-hydrogen-nitrogen jet flames feature well-controlled boundary conditions and are particularly amenable to modeling. A one-dimensional NO laser-induced fluorescence (NO-LIF) method was implemented and combined with 1-D Raman spectroscopy to measure the NO mole fraction quantitatively. To avoid laser absorption by ammonia, excitation in the A2Σ+−X2Π (0–1) band near 236 nm was chosen instead of the more conventional (0–0) band near 226 nm. Due to the unsteady nature of turbulent jet flames, offline NO-LIF measurements are not useful, and undesirable interferences from Rayleigh scattering and oxygen LIF were instead quantified and removed using the temperature and major species mole fractions measured with Raman spectroscopy. NO quantification algorithms were optimized and validated first with a laminar NH3–H2–N2 counterflow flame, and then applied to the turbulent jet flames. Results show that a large amount of NO (∼1500 ppm) is produced in the NH3–H2–N2 jet flames. Increasing the ammonia cracking ratio from 14% to 28% reduces the NO concentration in laminar and turbulent NH3–H2–N2 flames. Data also shows that turbulence smear effects of differential diffusion. To the best of our knowledge, this is the first available database featuring quantitative measurements of spatially-resolved NO mole fraction in turbulent NH3–H2–N2 flames at a practically-relevant pressure. This unique database can be used in the future to validate turbulent combustion models for such flames.

Simultaneous detection of three chemical species (NO, O, O2) using a single broadband femtosecond laser

Simultaneous nitric oxide (NO) laser-induced fluorescence (LIF), atomic oxygen (O) two-photon LIF (TPLIF), and molecular oxygen (O2) LIF were demonstrated using a single broadband femtosecond (fs) laser in methane- and hydrogen-fueled flames. An amplified Ti:Sapphire laser with ∼80-fs pulse duration was used to pump an optical parametric amplifier (OPA) to generate 226.1-nm pulses having 2.3 nm (∼450 cm−1) bandwidth to excite all three species simultaneously. The specific excitation transitions are NO A-X (0,0) system, O 3p3P←←2p3P two-photon transition, and O2B3Σu−−X3Σg− Schumann-Runge system. Detailed emission characterization was performed using a 1D imaging spectrometer to identify suitable emission bands for interference-free detection of each species. A simultaneous detection system consisting of a visible and a UV camera provided the optimal detection of all three species. Imaging studies revealed high concentrations of NO, O, and O2 at the edge of the flame cone of Bunsen flames because NO and O are produced at the hot flame front. O2 LIF signal was present only at the flame front because of the strong temperature dependence of the O2 Schumann-Runge system. Direct imaging of O TPLIF enabled single-shot as well as high-fidelity shot-averaged line images in stable laminar flames. Equivalence ratio scans in the CH4/air Hencken flames showed good agreements for O and O2 with Cantera equilibrium predictions performed using GRI-Mech 3.0. Although the NO measurements deviated from the equilibrium calculations on the fuel-rich side, it agreed well with previously reported NO LIF measurements in a similar flame. This discrepancy is likely due to Prompt NO, which was not considered in the simulation. NO LIF measurements as a function of height-above-the-burner in the H2/air Hencken flames agreed well with calculated NO mole fractions using the UNICORN flame code. The present study lays the foundation for single-laser imaging of three critical flame species in NO formation pathways in combustion systems.

Induction zone length measurements by laser-induced fluorescence of nitric oxide in hydrogen-air detonations

This experimental study aims at presenting a proof-of-concept for induction zone length (Δi) measurements in H2-air detonations by using the laser-induced fluorescence (LIF) of nitric oxide (NO) technique. The proof-of-concept is evidenced on a stoichiometric H2-air detonation, propagating in a mixture initially at 18 kPa and 293 K, by using an excitation wavelength at 225.120 nm and seeding the H2-air mixture with 1900 ppm of NO. Both, experiments in an optical detonation duct with a rectangular section and numerical simulations with an in-house code, KAT-LIF, are conducted. After verifying that the NO seeding has a negligible effect on the detonation structure, the main results are the following. A single correlation between the NO-LIF signal evolution and the Δi is determined numerically, based on the LIF signal decay at the end of the induction zone. The Δi measurements have a satisfactory 2% theoretical accuracy, determined from comparisons between Zeldovich-von Neumann-Döring (ZND) and KAT-LIF simulations, and a ±160−μm experimental uncertainty. An evolution of the induction zone length as a function of the cellular cycle is observed experimentally but, due to line of sight integrated chemiluminescence imaging of the detonation structure, a correlation between induction length and cellular cycle could not be achieved. However, statistical analysis of the results revealed that the evolution of the experimental induction lengths well correlates with the ZND and KAT-LIF simulations.

Investigation of mixing processes of effusion cooling air and main flow in a single sector model gas turbine combustor at elevated pressure

Mixing processes between main flow and effusion cooling air are investigated in an effusion cooled, swirl-stabilized pressurized single sector gas turbine combustor using advanced laser diagnostics. Quantitative planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) and planar laser-induced fluorescence of nitric oxide, seeded to the effusion cooling air, (NO-PLIF) are employed in the primary zone and close to the effusion cooled liner. This data is used to identify mixing events at three stages of premixed combustion, i.e. mixing before reaction, mixing during reaction and mixing after reaction. A parametric study of swirl and cooling air mass flow is conducted to investigate the mutual interaction between flame and cooling air. Within the primary zone, a significant radial asymmetry of OH concentration is observed. This asymmetry is partly explained by the presence of effusion cooling air within the unburned fresh gas, leading to lowered OH concentration within local reaction zones and their post-flame equilibrium concentration. Near the effusion cooled liner, adiabatic mixing after reaction is the dominant process across all investigated operating conditions. Notable mixing before reaction is only observed for the first effusion hole on the center line at low swirl conditions.

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.

Quantitative mixture fraction imaging of a synthetic biogas turbulent jet propagating into a NO-vitiated air co-flow using planar laser-induced fluorescence (PLIF)

A new approach for quantitative mixture fraction imaging in the turbulent mixing of a fuel jet and a high temperature oxidizer co-flow was developed by means of planar laser-induced fluorescence of nitric oxide (NO-PLIF). Unlike existing strategies, the new approach is based on seeding NO in the oxidizer stream. The method was first evaluated during the laminar mixing of methane in air and $$\hbox {CH}_4/\hbox {CO}_2$$ blends (synthetic biogas) in air, at room temperature. Mixture fraction measurements were validated against Rayleigh scattering imaging. Then, the measurements were extended to the turbulent mixing of a cold fuel jet issuing into a NO-seeded, hot air co-flow. By seeding the NO in the air stream, high signal-to-noise ratios were achieved at locations around the stoichiometric mixture fraction. Additionally, a stable in situ calibration region is available at every measurement location, which contributes to reduce the uncertainty. Results showed that the approach is not suitable for CH $$_4$$ /air mixtures due to the lack of sensitivity of the fluorescence signal to the mixture fraction. For biogas/air mixtures, the addition of CO $$_2$$ increased the response of the fluorescence signal to the mixture fraction, making the measurements feasible. The stoichiometric mixture fraction can be fully resolved for each biogas blend. Two-dimensional instantaneous mixture fraction measurements were feasible with an acceptable uncertainty. The high spatial resolution of the measurement was of the same order of the smallest scale in the concentration field (i.e., Batchelor scale).

Absolute SiO concentration imaging in low-pressure nanoparticle-synthesis flames via laser-induced fluorescence

In this paper, we present a strategy for imaging measurements of absolute concentration values of gas-phase SiO in the combustion synthesis of silica, generated from the reaction of hexamethyldisiloxane (HMDSO) precursor in a lean (ϕ = 0.6) hydrogen/oxygen/argon flame. The method is based on laser-induced fluorescence (LIF) exciting the Q(42) rotational transition within the A1Π − X1Σ (1, 0) electronic band system of SiO at 231 nm. Corrections for temperature-dependent population of the related ground state are based on multi-line SiO–LIF thermometry utilizing transitions within the A1Π − X1Σ (0, 0) electronic band around 234 nm. Corrections for local collisional quenching are based on measured effective fluorescence lifetimes from the temporal signal decay using a short camera gate stepped with respect to the laser pulse. This fluorescence lifetime measurement was confirmed with additional measurements using a fast photomultiplier. The resulting semi-quantitative LIF signal was photometrically calibrated using Rayleigh scattering from known gas samples at various pressures and laser energies as well as with nitric oxide LIF. The obtained absolute SiO concentration values in the HMDSO-doped flames will serve as a stringent test case for recently developed flame kinetic mechanisms for this class of gas-borne silicon dioxide nanoparticle synthesis.

Nitric oxide formation in lean, methane-air stagnation flames at supra-atmospheric pressures

Increasingly stringent regulations are imposed on nitric oxide (NO) due to its numerous, direct and indirect, deleterious effects on human health and the environment. A better control of the post-flame temperature field to contain the thermal (Zel’dovich) route resulted in significant reductions of engine emissions. An improved knowledge of the chemistry and rate of the secondary prompt, NNH, and N2O pathways is now required to decrease nitric oxide emissions further. For this effort, NO laser-induced fluorescence (LIF) measurements are presented for lean (ϕ=0.7), jet-wall, stagnation, premixed flames at pressures of 2, 4, and 8 atm. For all cases, the NO-LIF signal increases rapidly through the flame front, and relatively slowly in the post-flame region where the temperature is too low to sustain the thermal pathway. Nitric oxide mole fractions are inferred from the measurements and show that the pressure has a very weak, monotonic, adverse effect on NO formation. Reaction pathway analyses are applied to flame simulations performed with a thermochemical model capturing the general trends of the data to assess the contribution of each NO formation route. For all cases, the N2O pathway, which proceeds mostly in the flame front region, dominates. This route produces slightly larger amounts of NO at higher pressures, but the variation appears very limited when considering the termolecular nature of its initiation reaction. The thermal route is predicted to progress slowly in the post-flame region, which causes a shallow increase in the NO mole fraction. The NNH and prompt pathways generate small amounts of NO, and their contributions reduce with the pressure. Overall, the thermochemical model predicts that the formation of NO is relatively unaffected by the pressure, which is consistent with the experiments. The experimental dataset reported in this work is made available for the development of thermochemical models.

Enhanced Mixing in Supersonic Flow Using a Pulse Detonator

Mixing enhancement in a crossflow using the transient high-pressure, high-temperature, and high-velocity pulse from a detonation was investigated experimentally. High-frame-rate shadowgraphy and planar laser-induced fluorescence of the nitric oxide molecule showed the structure and time-dependent interaction of the detonation plume with the supersonic flow. The high-momentum flux from the detonation provided significant penetration, with blowdown times of 4–8 ms achieved. Planar laser-induced fluorescence of nitric oxide captured the spanwise structure of the plume and the large counter-rotating vortex structure for enhanced mixing. The upstream jet was shown to be drawn into the pulse detonator’s plume, providing distribution to the core supersonic flow. Significant coupling between a continuous upstream jet and the pulse detonator plume was found and indicated that there was an optimal staging distance for enhanced mixing.

Fluorescence Imaging of Reaction Control Jets and Backshell Aeroheating of Orion Capsule

Planar laser-induced fluorescence of nitric oxide was used to visualize the interaction of reaction control system jet flows on the afterbody of a hypersonic capsule reentry vehicle at the Calspan-University at Buffalo Research Center’s Large Energy National Shock Tunnel I reflected shock tunnel facility. The interaction of pitch and roll jets with the flowfield was investigated. Additionally, thin-film sensors were used to monitor heat transfer on the surface of the model to detect localized heating resulting from the firing of the reaction control system jets. Visualizations of the capsule shear layer using both planar laser-induced fluorescence and schlieren imaging compared favorably. The structure of the roll jet was found to be significantly altered due to interactions with the flowfield. Additionally, the presence of the roll jet appeared to change the nature of the shear layer from steady laminar to unsteady. The pitch jet structure was only disturbed in the far field. Comparison of the planar laser-induced fluorescence jet-fluid visualizations and the surface heat flux distributions indicate that the regions of enhanced aeroheating are not caused by the jet fluid itself impinging on the surface, but rather by the presence of jet-induced horseshoe vortices and shock wave/boundary-layer interactions.

Development, Uniformity and Peak Heating of a Hypersonic Laminar Near-Wake Flow

This paper describes visualisation experiments performed using the planar laser-induced fluorescence of nitric oxide. The focus was to investigate the establishment and cross-stream uniformity of the hypersonic separated flow generated in a pulsed free-piston shock tunnel facility. In particular, the laminar Mach 7.5 flow over a step downstream of a wedge was investigated. The ratio of wedge width to total horizontal distance downstream of the leading edge was 0.71 at the furthest downstream measurement point, and model side plates were not used. Flow images were obtained on successive tunnel runs with different delays after flow onset, to identify the nominal flow establishment time. Furthermore, images were obtained at the nominal facility test time using a laser sheet oriented perpendicular to the step, propagating in the spanwise direction at a height equal to half the step height. These images are compared with thermocouple heat flux measurements obtained both with and without the step and show that ...

2D Mixture Fraction Studies in a Hot-Jet Ignition Configuration Using NO-LIF and Correlation Analysis

The problem of measuring mixture fraction (defined here as the fractional part of mass originating from the exhaust gas jet) during the mixing of an unsteady hot exhaust gas jet impinging into a quiescent premixed hydrogen/air mixture is addressed. The diagnostic method is based on planar Laser-Induced Fluorescence of nitric oxide (NO) that is seeded to the hot jet and used as a tag for mixture fraction. The research work presented in this paper focusses on the analytical and experimental data employed for the estimation of mixture fraction using NO as a tracer. Since the NO LIF-signal does not solely depend on the amount of NO, but also on temperature and overall chemical composition, additional information is normally required to transform LIF-signals into mixture fraction values. To provide this additional information, the concept of reduced state spaces is applied: Correlations between state variables (temperature and species) are exploited to establish relationships between the measured signal and the mixture fraction. Simple numerical model simulations are used in combination with spectroscopic calculations to check for correlations between NO-LIF signal and mixture fraction. The result is that sharp correlations between NO-LIF signals and mixture fraction exist, provided that chemical reactions have not yet significantly influenced the flow. These correlations display only little sensitivity with respect to changes of the jet temperature, amount of NO seeded to the jet, and jet velocity. Measurements of NO-LIF were then performed for a non-reacting case before ignition, and mixture fraction maps were obtained from the measured LIF-signals by exploiting the correlations. The resulting data reveal the different driving forces behind mixing at regions near to the jet tip and at the radial shear layers.

Two-component Doppler-shift fluorescence velocimetry applied to a generic planetary entry probe model

This study discusses the development and application of planar laser-induced fluorescence of nitric oxide (NO PLIF) to measure velocities in an axisymmetric hypersonic near-wake flow field around a model planetary-entry vehicle configuration. Shapes and positions of NO spectral lines at every location in the flow are determined over several successive shock tunnel runs. The lines experience Doppler shifts proportional to the local flow velocity component in the direction of the fluorescence-generating laser. A Gaussian line shape function is then fitted to the acquired wavelength-dependent fluorescence measurements, the line center of which is correlated to the time-averaged velocity at each pixel location. The flow field is probed successively by a laser in two orthogonal directions, which yields the velocity magnitude and direction everywhere in the illuminated plane. The accuracy of the measurement technique is discussed, and various strategies to characterize systematic errors are presented. The variation of random uncertainties in different regions of the flow field provides information about the local steadiness of the flow. To the authors’ knowledge, the measurements represent the first two-component velocity map of a hypersonic near-wake flow.

Pressure, Temperature and Velocity Measurements in Underexpanded Free Jets using Laser-Induced Fluorescence Imaging

We report measurements of pressure, temperature, and velocity in an underexpanded jet using planar laser-induced fluorescence of nitric oxide. Ultraviolet transitions near 44, 097.5 cm -1 in the A-X (0,0) system of nitric oxide were excited using narrow linewidth laser radiation generated from an optical parametric system, injection seeded using a distributed feedback diode laser. Planar laser-induced fluorescence images with excellent spatial resolution and signal-to-noise ratio were acquired by tuning the frequency of the laser radiation over the nitric oxide absorption line. The images were corrected on a shot-to-shot basis for fluctuations in the laser spatial profile. Line shapes constructed from the corrected planar laser-induced fluorescence images were used to determine pressure and temperature values along the centerline of the jet. Good agreement between laser-induced fluorescence and previously reported N 2 coherent anti-Stokes Raman scattering measurements was observed. The laser-induced fluorescence measurements also compared well with calculations of pressure and temperature using computational fluid dynamics codes. Velocity was measured in supersonic regions of the flowfield on the basis of the Doppler shift in the nitric oxide absorption lines. Planar laser-induced fluorescence images were acquired using laser sheets propagating at 90 and 45 degrees with respect to the flow direction; velocity was determined from the frequency shift of the absorption lines for these two shifts. The laser-induced fluorescence technique can potentially be applied to obtain instantaneous measurements of thermodynamic properties and multiple velocity components in high-speed turbulent flows.