NO PLIF Imaging Research Articles

Characterization of detonation waves by simultaneous OH and NO planar laser-induced fluorescence

This study presents a thorough evaluation of the simultaneous planar laser-induced fluorescence (PLIF) on hydroxyl (OH) radicals and nitric oxide (NO), for characterizing hydrogen detonation fronts. The study combines experimental and numerical results to evaluate the benefits and drawbacks of different visualization strategies, namely OH-PLIF alone, NO-PLIF alone, OH-PLIF + shock det. (comparable to simultaneous OH-PLIF and schlieren), and combined OH- and NO-PLIF. Main findings are: (i) For the first time, simultaneous single-shot OH- and NO-PLIF visualizations are demonstrated on detonation waves for three mixtures with varying stability levels: Mixture (a) 2H2-O2-3.76Ar, Mixture (b) 2H2-O2-3.76N2, and Mixture (c) 3H2-CH4-3.5O2-3.76Ar; (ii) the simultaneous visualization evidences the self-similarity of both techniques, with a clear interlock of OH-PLIF imaging into NO-PLIF imaging. This means that the structure of the OH-reaction zone, typically visualized from OH-PLIF, can be accurately described from NO-PLIF imaging alone, from which induction zones can also be visualized. In addition, both can clearly depict the wrinkling of the front and the presence of unburned pockets; (iii) the effectiveness of the four visualization strategies at characterizing the detonation wave of a H2-O2-3.76Ar mixture is determined using ZND and spectroscopic simulations. Interestingly, the induction zone (Δi) is more accurately predicted by NO-PLIF (3% error) than with the combined OH + shock det. (10% error). This study highlights the potential of NO-PLIF diagnostic in characterizing detonation waves with low levels of confinement (i.e., d/λ>>>1) both qualitatively (e.g., reaction zone structure) and quantitatively (e.g., Δi measurements) using a single diagnostic. Additionally, the simultaneous OH- and NO-PLIF diagnostic may offer significant advantages in characterizing the detonation waves in engine-relevant conditions (i.e., in complex geometries with unburned pockets or local quenching events), as combining both diagnostics enables to discriminate unburned pockets and local quenching events.

Simultaneous planar laser-induced fluorescence measurement of reactant NH3, radical NH, and pollutant NO in ammonia-hydrogen flames using a single dye laser

The stability, local extinction, and NOx production of ammonia-hydrogen (NH3-H2) flames are significantly impacted by turbulence-chemistry interactions. A powerful technique to study these interactions is planar laser-induced fluorescence (PLIF). However, multi-scalar PLIF typically requires complex and expensive systems, with multiple laser sources. This study proposes a novel PLIF technique using a single dye laser to perform simultaneous, single-shot imaging of the reactant NH3, radical NH, and pollutant NO in NH3-H2 flames. According to the excitation scans and the emission spectra of NH3, NH, and NO, three wavelength couples can be used to image the three species simultaneously via two-photon excitation of NH3C'−X (2,0), single-photon excitation of NH A3Π−X3Σ− (0,0) near 304 nm, and single-photon excitation of NO A2Σ+−X2Π (0,1) near 237 nm. Wavelengths near 237 nm and 304 nm are obtained simultaneously with only one dye laser by combining outputs of the frequency-doubling and frequency mixing units. NH3-, NH-, and NO-PLIF imaging performance is analyzed by quantifying the signal-to-noise ratios and detection limits. This technique is then used to visualize the structure of premixed and non-premixed flames over wide ranges of NH3 and H2 concentrations in the fuel blend. Results show that the premixed NH3-H2 flames have a compact structure, while the non-premixed flames exhibit a large gap between NH3- and NH-PLIF layers, where ammonia undergoes significant decomposition before reaching the reaction layer. The inner edge of the NO-PLIF layer overlaps well with the reaction layer represented by the NH-PLIF layer in all flames. The fully developed turbulent structure downstream of the NH3-H2 flame causes pinching-off of the premixed flame front and local extinction of the diffusion flame front.

Two-dimensional visualization of induction zone in hydrogen detonations

The induction zone length is an important length scale for gaseous detonations, as it is correlated with other characteristic parameters such as the cell size. A good characterization of the induction zone can then lead to a better comprehension of the detonation structure. In this context, this study investigates two-dimensional (2D) visualizations of induction zones in stoichiometric hydrogen-air and hydrogen-oxygen-nitrogen-argon mixtures. The detonation speed for both mixtures is close to the Chapman Jouguet speed, and without wall losses. Planar laser-induced fluorescence (PLIF) technique was employed, by exciting nitric oxide (NO) molecules (i.e., NO-PLIF). In both mixtures, 1900 ppm of NO was added as a tracer to ensure a good signal-to-noise ratio in the von Neumann condition. The NO-PLIF images displayed a variation of the experimental induction length along the cellular cycle. Large and short induction lengths were observed for incident shock and Mach stem, respectively. A total of 2727 experimental induction lengths were extracted from the NO-PLIF images, among 79 individual experiments. Statistical analyses of these data showed that the mixture with lower activation energy exhibited less dispersion of the data compared to the mixture with higher activation energy. Non-equilibrium effects were not observed in the fluorescence profiles and induction length measurements.

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.

Effect of flow distortion on fuel/air mixing and combustion in an upstream-fueled cavity flameholder for a supersonic combustor

This paper describes an experimental study of the effects of an incident shockwave on the flow field, fuel distribution and combustion within a cavity flameholder with upstream fuel injection. Two impingement locations are employed: (1) near the fuel injector (the so-called shock-on-jet case) and (2) on the cavity shear layer (the shock-on-cavity case). Shadowgraph is used to characterize the flow field. Air seeded with nitric oxide (NO) is used as the simulated fuel and the resulting planar laser-induced fluorescence (NO-PLIF) from NO molecules is used to characterize fuel/air mixing while planar laser-induced fluorescence of OH molecules to characterize the actual combustion process. The shadowgraph and NO-PLIF images are compared with a CFD (Computational Fluid Dynamics) solution of the Reynolds-averaged-Navier Stokes (RANS) for assessment and explanation of experimental results of non-reacting tests. The effect of the shock on the cavity shear layer is to control the fuel distribution within the cavity. The effect of the shock on the jet is to force the shear layer deep within the cavity, which results in higher fuel concentrations near the cavity centerline. The shock-on-cavity case causes the shear layer to separate upstream of the cavity. Mixing uniformity is enhanced by the increased breakup of the fuel plume. Combustion is stronger and more uniform with the shock impinging on the cavity, while it is limited to the edges of the cavity with shock impingement on the jet. The greater mixing afforded in the shock-on-cavity case reduces the fuel concentration near the centerline and allows stronger burning in the center of the cavity. Doubling the fuel injection momentum flux ratio does not strongly affect the pattern of fuel distribution in either case, but combustion in the shock-on-cavity case is reduced, because the fuel concentration at the centerline is high.

Development of a Mach 5 Nonequilibrium-Flow Wind Tunnel

A small-scale Mach 5 blow down wind tunnel, with ample access for optical diagnostics and ability to generate steady-state nonequilibrium flows, has been designed and operated. The wind tunnel uses transverse repetitively pulsed nanosecond discharge, fully overlapped with a transverse DC discharge and operated at high plenum pressures (P0=0.5-1.0 atm) to load internal energy modes of nitrogen and oxygen molecules. The discharge remains stable at energy loadings of up to ~0.1 eV/molecule in nitrogen (discharge power up to 2.5 kW). The wind tunnel generates nonequilibrium nitrogen and air flows with steady-state run time of 5-10 seconds, translational / rotational temperature of T0~300-400 K, and estimated upper bound nitrogen vibrational temperature of Tv0~2,000 K. Internal energy mode disequilibrium in the flow is controlled by injecting nitric oxide, hydrogen, or water vapor into the subsonic flow between the discharge section and the nozzle throat. The effect of energy mode disequilibrium is studied in a flow over a cylinder model placed in the Mach 5 test section. The flow field in the supersonic test section is well predicted by a 3-D compressible Navier-Stokes flow code, indicating good flow quality. The supersonic flow field over the model is visualized by schlieren imaging and NO PLIF imaging, using a burst mode laser operated in the vicinity of 226 nm, at a pulse repetition rate of 10-20 kHz. The laser was operated in the injection-seeded mode, generating narrow linewidth (~0.1 cm -1 ) output for single-line NO excitation in the flow. Nitric oxide was either injected into the flow in the plenum or generated in a repetitively pulsed nanosecond discharge in dry air. Both single-pulse PLIF images and images integrated over 10-50 laser pulses have been obtained. Two single-line NO PLIF images on a NO(X,v˝=0→A,v´=0) transition are used for measurements of 2-D temperature distributions in nitrogen flows in the supersonic test section. Another single-line NO PLIF image on a NO(X,v˝=1→A,v´=1) transition is used to estimate NO vibrational temperature behind the bow shock, TV(NO)=550 ± 100 K. The NO vibrational temperature increases when the energy loading in the discharge is increased. Kinetic modeling calculations indicate that low NO vibrational temperature is due to fairly low vibrational energy loading per nitrogen molecule in the discharge. Schlieren images of a supersonic flow over the cylinder model demonstrate that the shock stand-off distance is reduced by approximately 5% when the discharge in the wind tunnel is in operation and water vapor or hydrogen are injected into the flow between the discharge section and the nozzle throat. This effect is attributed to additional heating of the flow in the plenum during relaxation of vibrationally excited nitrogen in the presence of water vapor or hydrogen.

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.

Investigation of optical fibers for gas-phase, ultraviolet laser-induced-fluorescence (UV-LIF) spectroscopy

We investigate the feasibility of transmitting high-power, ultraviolet (UV) laser pulses through long optical fibers for laser-induced-fluorescence (LIF) spectroscopy of the hydroxyl radical (OH) and nitric oxide (NO) in reacting and non-reacting flows. The fundamental transmission characteristics of nanosecond (ns)-duration laser pulses are studied at wavelengths of 283 nm (OH excitation) and 226 nm (NO excitation) for state-of-the-art, commercial UV-grade fibers. It is verified experimentally that selected fibers are capable of transmitting sufficient UV pulse energy for single-laser-shot LIF measurements. The homogeneous output-beam profile resulting from propagation through a long multimode fiber is ideal for two-dimensional planar-LIF (PLIF) imaging. A fiber-coupled UV-LIF system employing a 6 m long launch fiber is developed for probing OH and NO. Single-laser-shot OH- and NO-PLIF images are obtained in a premixed flame and in a room-temperature NO-seeded N(2) jet, respectively. Effects on LIF excitation lineshapes resulting from delivering intense UV laser pulses through long fibers are also investigated. Proof-of-concept measurements demonstrated in the current work show significant promise for fiber-coupled UV-LIF spectroscopy in harsh diagnostic environments such as gas-turbine test beds.

2D mixture fraction measurements in a high pressure and high temperature combustion system using NO tracer-LIF

Mixture fraction measurements in a jet-in-cross flow configuration at high pressures (15 bar) and temperatures (above 1000 K) were performed using planar laser induced fluorescence of nitric oxide (NO-PLIF) as trace species. The goal was the evaluation of this tracer LIF technique for the characterization of the mixing of fuel and hot exhaust gas in the mixing channel. The fuel (natural gas (NG) or H2/N2/NG mixture) along with the tracer were injected into the crossflow of the exhaust gas and PLIF measurements were performed in different planes. In order to relate the measured NO-LIF signal to fuel concentration and mixture fraction, effects of pressure, temperature and species concentration were taken into account. Numerical calculations and spectroscopic simulations that mimic the experimental conditions were performed to identify excitation schemes that give optimum correlations between the NO-LIF signal and the mixture fraction. The measured NO-PLIF images were transformed into mixture fraction plots using the computed correlations. The paper reports on the experimental challenges encountered during the measurements and the steps taken to overcome those difficulties. Examples of mixture fraction distributions are presented and discussed. The paper concludes with a detailed analysis on the accuracy of the measured mixture fraction values.