LIF Imaging Research Articles

Characterization of a high-pressure flame facility using high-speed chemiluminescence and OH LIF imaging

Characterization of a high-pressure flame facility using high-speed chemiluminescence and OH LIF imaging

Direct LiF imaging diagnostics on refractive X-ray focusing at the EuXFEL High Energy Density instrument.

The application of fluorescent crystal media in wide-range X-ray detectors provides an opportunity to directly image the spatial distribution of ultra-intense X-ray beams including investigation of the focal spot of free-electron lasers. Here the capabilities of the micro- and nano-focusing X-ray refractive optics available at the High Energy Density instrument of the European XFEL are reported, as measured in situ by means of a LiF fluorescent detector placed into and around the beam caustic. The intensity distribution of the beam focused down to several hundred nanometers was imaged at 9 keV photon energy. A deviation from the parabolic surface in a stack of nanofocusing Be compound refractive lenses (CRLs) was found to affect the resulting intensity distribution within the beam. Comparison of experimental patterns in the far field with patterns calculated for different CRL lens imperfections allowed the overall inhomogeneity in the CRL stack to be estimated. The precise determination of the focal spot size and shape on a sub-micrometer level is essential for a number of high energy density studies requiring either a pin-size backlighting spot or extreme intensities for X-ray heating.

Femtosecond two-photon LIF imaging of atomic hydrogen in high-pressure methane-air flames

Atomic hydrogen (H) imaging studies in combustion systems have been limited thus far to atmospheric- or low-pressure flames. The commonly used excitation scheme for two-photon laser-induced fluorescence (TPLIF) of H involves 205-nm deep-ultra-violet (DUV) laser pulses generated using nanosecond (ns), picosecond (ps), or femtosecond (fs) laser systems. Ultrashort pulse, fs excitation schemes are more attractive because of the efficient two-photon excitation and interference-free kHz-rate imaging capability. In this work, we implemented a home-built, direct fourth harmonic generation (FHG) laser system to generate high-energy fs pulses for H imaging in CH4/air flames at pressures up to 10 bar. The high-energy FHG output was capable of overcoming transmission losses of DUV laser pulses through thick optical windows of the pressure vessel. Signal interferences such as photoionization, photolytic production, amplified spontaneous emission, and background chemiluminescence were avoided or minimized during data acquisition and subsequent data processing steps. A quadratic dependence of the H TPLIF signal on the laser energy was observed. The increase in the pressure from 1 to 10 bar showed a rapid decay of the fs H TPLIF signal although it should scale linearly with increasing H number density with pressure ideally at equilibrium conditions. Besides a detailed quenching correction, laser absorption and fluorescence signal trapping remain major issues at elevated pressures. The measured fluorescence signals are compared with the equilibrium H number densities and local H concentrations calculated using the Cantera (0D) and UNICORN (2D) flame codes, respectively, for a range of equivalence ratios, and possible reasons for discrepancies are discussed. Experimentally obtained 1D and 2D spatial distributions of H are well predicted by the UNICORN flame model. Steps towards quantitative single-shot H TPLIF imaging measurements in high-pressure flames are also discussed. Overall, the fs TPLIF imaging technique coupled with a high-efficiency home-built FHG system is a promising diagnostic approach for H imaging in high-pressure flames.

2D imaging of absolute methyl concentrations in nanosecond pulsed plasma by photo-fragmentation laser-induced fluorescence

The methyl radical plays a central role in plasma-assisted hydrocarbon chemistry but is challenging to detect due to its high reactivity and strongly pre-dissociative electronically excited states. We report the development of a photo-fragmentation laser-induced fluorescence (PF-LIF) diagnostic for quantitative 2D imaging of methyl profiles in a plasma. This technique provides temporally and spatially resolved measurements of local methyl distributions, including in near-surface regions that are important for plasma-surface interactions such as plasma-assisted catalysis. The technique relies on photo-dissociation of methyl by the fifth harmonic of a Nd:YAG laser at 212.8 nm to produce CH fragments. These photofragments are then detected with LIF imaging by exciting a transition in the B-X(0, 0) band of CH with a second laser at 390 nm. Fluorescence from the overlapping A-X(0, 0), A-X(1, 1), and B-X(0, 1) bands of CH is detected near 430 nm with the A-state populated by collisional B-A electronic energy transfer. This non-resonant detection scheme enables interrogation close to a surface. The PF-LIF diagnostic is calibrated by producing a known amount of methyl through photo-dissociation of acetone vapor in a calibration gas mixture. We demonstrate PF-LIF imaging of methyl production in methane-containing nanosecond pulsed plasmas impinging on dielectric surfaces. Absolute calibration of the diagnostic is demonstrated in a diffuse, plane-to-plane discharge. Measured profiles show a relatively uniform distribution of up to 30 ppm of methyl. Relative methyl measurements in a filamentary plane-to-plane discharge and a plasma jet reveal highly localized intense production of methyl. The utility of the PF-LIF technique is further demonstrated by combining methyl measurements with formaldehyde LIF imaging to capture spatiotemporal correlations between methyl and formaldehyde, which is an important intermediate species in plasma-assisted oxidative coupling of methane.

Relationship between interface shape and signal waveforms of an Optical-fiber-based Reflective Probe during film-thickness-measurements

Liquid film flow widely appears in the industrial field, affecting the machinery's reliability and lifetime. The notorious erosion in the steam turbines is one example, which derives from coarse droplets atomized by the liquid film on the turbine blade. There are fewer methods for measuring the liquid film in such a condition directly. We have proposed an Optical-fiber-based reflective probe (ORP) for this issue. We demonstrated a thickness measurement for the liquid-film flow driven by airflow and found an uncertainty caused by the local curvature of the interface. In this study, we investigated the signals delivered by the ORP during the measurement of the wavy film flow. The ORP measured the wavy interface simulated by a stainless-steel specimen machined into a sinusoidal waveform. We also conducted high-speed visualization and LIF imaging to confirm their positional relationship; then, we compared the time-series variation of the film thickness and the ORP signals. As a result, we found the ORP has a significant sensitivity to the interface's angle even, which is hard to visualize. Further, the waveform of the ORP signal depends on the intensity distribution of the light emitted from its tip. These are beneficial information for optimization of the ORP for future studies.

High-speed volumetric imaging of formaldehyde in a lifted turbulent jet flame using an acousto-optic deflector

The development of high-speed volumetric laser-induced fluorescence measurements of formaldehyde ($$\hbox {CH}_2\hbox {O}$$-LIF) using a pulse-burst laser operated at a repetition rate of $${100} \hbox { kHz}$$ is presented. A novel laser scanning system employing an acousto-optic deflector (AOD) enables quasi-4D $$\hbox {CH}_2\hbox {O}$$-LIF imaging at a scan frequency of $${10}\hbox { kHz}$$. The diagnostic capability of time-resolved volumetric imaging is demonstrated in a partially premixed DME/air lifted turbulent jet flame near the flame base. Simultaneous imaging of laser beam profiles is performed to account for the laser pulse energy fluctuation and laser sheet inhomogeneity. With the accurate registration of laser sheet positions, the volumetric reconstruction of $$\hbox {CH}_2\hbox {O}$$-LIF signals is performed within a detection volume of $$17.3 \times 11.9 \times {2.3}\, \hbox { mm}^3$$ with an average out-of-plane spatial resolution of $${250}\,\upmu \hbox {m}$$. A surface detection algorithm with adaptive thresholding is used to determine the global maximum intensity gradient by calculating gradient percentiles. The flame topology characteristics are investigated by evaluating the 3D curvatures of $$\hbox {CH}_2\hbox {O}$$ surfaces. Curvatures calculated using 2D data systematically underestimate the full 3D curvature due to the lack of out-of-plane information. The inner surfaces near the turbulent fuel jet exhibit higher probabilities of large mean curvature than the outer surfaces. The saddle and cylindrical structures are dominant on both the inner and outer surfaces and the elliptic structures occur with lower probability. The results suggest that the damping of turbulent fluctuations by the temperature increase through the $$\hbox {CH}_2\hbox {O}$$ region reduces the curvature, but the local structure topology remains self-similar.

On the flow physics and vortex behavior of rectangular orifice synthetic jets

Synthetic jet actuators possess a continuous jet like behavior in its far region and have found wide-scale engineering applications since it allows momentum transport to the flow system without any net mass transfer across the flow boundaries. The case of a non-axisymmetric synthetic jet is particularly significant since it is affected by the differential shear layer at the orifice exit, that depends on its aspect ratio. However, despite exhaustive research on both continuous and synthetic jets, very few studies have experimentally investigated the case of rectangular orifice synthetic jets, focusing on the effect of aspect ratio of the orifice as well as the actuation frequency upon the vortex behavior and the flow physics. In particular, the intriguing phenomenon of vortex bifurcation has mostly been reported only for an individual vortex or for a plain jet. Yet, in a train of vortex rings, such as that obtained in a synthetic jet, the occurrence of vortex bifurcation can be expected, although the flow physics in the wake of individual vortex rings is significantly different. The present study experimentally investigates a rectangular orifice synthetic jet at different orifice aspect ratios and actuation frequencies, focusing on exploring the conditions at which vortex bifurcation occurs, through LIF imaging and Hot-film measurements. The primary objective of these experiments is to provide a qualitative physical insight into the synthetic jet ejected from a rectangular orifice (through LIF imaging), as well as to quantitatively explore the experimental conditions that promote different flow structures (through velocity time trace, time-averaged velocity profiles and power spectral density measurements), particularly the bifurcation of vortex rings. Our experiments indicate that the phenomenon of vortex bifurcation is observed during the axial switching of vortex rings, but only in a narrow range of experimental conditions. Further, the velocity measurements have ascertained that the two prominent reasons behind this bifurcation process are a large disparity in the velocities of the vortex core and the center of vortex ring, as well as the time lag in which the separation distance between the counter-rotating vortices decrease gradually to zero.

In-Cylinder LIF Imaging, IR-Absorption Point Measurements, and a CFD Simulation to Evaluate Mixture Formation in a CNG-Fueled Engine

In-Cylinder LIF Imaging, IR-Absorption Point Measurements, and a CFD Simulation to Evaluate Mixture Formation in a CNG-Fueled Engine

Two-phase SLIPI for instantaneous LIF and Mie imaging of transient fuel sprays.

We report in this Letter a two-phase structured laser illumination planar imaging [two-pulse SLIPI (2p-SLIPI)] optical setup where the "lines structure" is spatially shifted by exploiting the birefringence property of a calcite crystal. By using this optical component and two cross-polarized laser pulses, the shift of the modulated pattern is not "time-limited" anymore. Consequently, two sub-images with spatially mismatched phases can be recorded within a few hundred of nanoseconds only, freezing the motion of the illuminated transient flow. In comparison with previous setups for instantaneous imaging based on structured illumination, the current optical design presents the advantage of having a single optical path, greatly simplifying its complexity. Due to its virtue of suppressing the effects from multiple light scattering, the 2p-SLIPI technique is applied here in an optically dense multi-jet direct-injection spark-ignition (DISI) ethanol spray. The fast formation of polydispersed droplets and appearance of voids after fuel injection are investigated by simultaneous detection of Mie scattering and liquid laser-induced fluorescence. The results allow for significantly improved analysis of the spray structure.

Scalar dissipation rate and scales in swirling turbulent premixed flames

Simultaneous Rayleigh scattering and OH LIF imaging measurements of temperature and OH were used to investigate the properties of turbulent premixed flames, including the nature of the 2D thermal structures and scalar dissipation rate in the Cambridge/Sandia swirling bluff body stabilized flames, with and without the effect of swirl. Swirl creates enhanced turbulence as well as outer flow entrainment, and disrupts the pre-flame zone significantly, whilst the high temperature reaction zone as marked by OH remains relatively intact. In particular, the temperature at the location of maximum OH gradient shows very low variance across the flame region.The 2D image analysis of OH and temperature shows that the corresponding 2D gradients are aligned up to a distance of half the laminar flame thickness away from the flame front, deviating significantly in the case of swirling flames beyond that region. As in previous investigations in diffusion flames, the mean width of the observed thermal structures increases from 300 to 600 µm near the flame, with a main mode around the laminar flame thermal width in the unswirled case. The correlation between 2D thermal dissipation and variance of the reaction progress variable extracted from the images shows a direct proportionality, with a slope which agrees well with theory in the region of high turbulence away from the base. At the base of the flame where turbulence is low, the local scalar dissipation becomes a function of the local temperature via the thermal diffusivity.

Near-kHz 3D tracer-based LIF imaging of a co-flow jet using toluene

This work demonstrates tracer-based, high-repetition-rate planar (15 kHz) and three-dimensional (940 Hz) laser-induced fluorescence imaging. An off-the-shelf, pulsed, frequency-quadrupled Nd:YAG laser at 266 nm is used as the excitation light source, and a high-frame-rate intensified relay optic with a slow P46 phosphor coupled to a CMOS camera is used to image the fluorescence. Four different tracers are investigated (3-pentanone, acetone, anisole, and toluene) and relative signal levels are measured in the potential core of a laminar co-flow jet. Resulting SNR values range from 6 to 44 for the different tracers, and relative signal levels and SNR for each tracer are provided as an engineering-basis for tracer-based imaging diagnostic design. It was found that signal levels from anisole (relative to toluene) are about ten times less than suggested by other literature, owing to uncertainty in the reported absorption cross sections. Using toluene as a tracer and a custom-made piezo-actuated steering optic to scan the laser sheet, 3D LIF imaging at 940 Hz is demonstrated by visualizing a co-flow jet mixing with ambient air.

Femtosecond two-photon LIF imaging of atomic species using a frequency-quadrupled Ti:sapphire laser

Femtosecond (fs)-duration laser pulses are well suited for two-photon laser-induced-fluorescence (TPLIF) imaging of key atomic species such as H, N, and O in gas-phase reacting flows. Ultrashort pulses enable efficient nonlinear excitation, while reducing interfering photochemical processes. Furthermore, amplified fs lasers enable high-repetition-rate imaging (typically 1–10 kHz) for capturing the dynamics of turbulent flow fields. However, two-dimensional (2D), single-laser-shot fs-TPLIF imaging of the above species is challenging in most practical flow fields because of the limited ultraviolet pulse energy available in commercial optical parametric amplifier (OPA)-based tunable laser sources. In this work, we report the development of an efficient, fs frequency-quadrupling unit [i.e., fourth-harmonic generator (FHG)] with overall conversion efficiency more than six times greater than that of commercial OPA-based systems. The development, characterization, and application of the fs-FHG system for 2D imaging of H atoms in flames are described in detail. The potential application of the same laser system for 2D imaging of N and O atoms is also discussed.

Probing Species Formed by Pilot Injection During Re-Compression in a Controlled Auto-Ignition Engine by H2CO LIF and Chemiluminescence Imaging

Probing Species Formed by Pilot Injection During Re-Compression in a Controlled Auto-Ignition Engine by H₂CO LIF and Chemiluminescence Imaging

Experimental Investigation of a Gasoline-to-LPG Converted Engine Performance at Various Injection and Cylinder Pressures with Respect to Propane Spray Structures

Power reduction when converting a gasoline engine to propane can be mitigated by designing an injection system so the heat required for evaporation of the propane is drawn from the intake air. Air is cooled and densified, resulting in volumetric efficiency increase. LPG sprays were imaged using Mie and LIF imaging techniques from a port fuel injector, and from long and short connecting pipes. Images were taken in an optically-accessed pressure chamber at atmospheric pressure and fuel pressures of 1.5 MPa. Images of the pipe-coupled injection spray show significant evaporation in the pipe, whose amount depend on the length and diameter of the pipe. The duration of the LPG pulse at the manifold end is, for 300mm pipes, five times the original duration at the injector, and even greater for 600mm pipes. The narrow sprays and the amount of evaporation that occurs before the fuel enters the manifold explains the differences in engine torque and in-cylinder mixture temperature with the different systems.

LIF Imaging of OH radicals in Atmospheric DC Glow Discharge Using Miniature Gas Flow and Electrolyte Cathode

ABSTRACTIn this paper, we report the spatial distribution of OH radical density in atmospheric-pressure DC glow discharge using a miniature helium flow and an electrolyte cathode. Laser-induced fluorescence imaging was applied for the measurement of the OH radical density. The effect of collisional quenching was considered in obtaining the spatial distribution of the OH density. The spatial distribution of the OH radical density showed that the peak of the OH density was located at a separated distance from the electrolyte surface. However, the OH radicals kept contact with the electrolyte surface. It was suggested that the OH radicals were generated mainly in a region separated from the electrolyte surface and some fraction of the generated OH radicals reached to the liquid phase.

LiF detectors-polycapillary lens for advanced X-ray imaging

A great effort in X-ray imaging studies has been dedicated to the design of novel optics-detector solutions that aims in creating a compact laboratory X-ray microscopy apparatus. Based on our experience in the use of both capillary/polycapillary optical systems and LiF imaging detectors we have recently tested a new combination of these techniques. The potential of the optics both to concentrate and to shape X-ray radiation enforced by the high performances in terms of spatial resolution/dynamic range of LiF imaging detectors allows us to use very simple imaging techniques, like the contact one, particularly suitable for compact imaging systems. In this paper we present first results about high spatial resolution images of transmitted X-rays through a polycapillary semi-lens. Submicron X-ray imaging of thick geological samples was also presented and discussed. These results demonstrate the potentialities of these novel X-ray imaging system for characterization of X-rays optics and for applications in material, life and heart sciences.

Simultaneous measurement of localized heat-release with OH/CH2O–LIF imaging and spatially integrated OH∗ chemiluminescence in turbulent swirl flames

In practical flames such as gas turbine combustors, spatially-integrated OH∗ chemiluminescence (CL) is frequently used as a heat release rate (HRR) indicator – which has been questioned by some authors to be restricted to flames of a limited range of equivalence ratios and low Reynolds numbers – while in lab flames the approach of combined detection of OH and H2CO via LIF is an accepted diagnostic technique. Even when using specialized optics with limited acceptance angle the first method is spatially integrating while the second one allows for spatially resolved imaging. In the present work we retrieved simultaneously HRR-based information via both techniques from the same spatial flame volume, i.e., OH∗–CL radiation is collected exclusively from within the light sheet volume cutting through the flame for LIF imaging. Turbulent premixed swirl flames were investigated with a thermal power up to 30kW to shed light on the still unresolved question if correlations exist between signal intensities derived from both methods in turbulent flames. Measurements were performed in methane/air flames with Reynolds numbers between 6900 and 10,000, equivalence ratios between 0.8 and 1.2, and with a replacement of 20vol% of methane by hydrogen. Although scatter plots of HRR vs. CL intensities cluster in certain regions depending on flame conditions, their large scatter shows that correlations are weak, probably caused by flame stretch and curvature. Depending on flame conditions, correlation coefficients to characterize the scatter plots range between 0.45 and 0.81.

Flame chemiluminescence and OH LIF imaging in a hydrogen-fuelled spark-ignition engine

Research into novel internal combustion engines requires consideration of the diversity in future fuels in an attempt to reduce drastically CO2 emissions from vehicles and promote energy sustainability. Hydrogen has been proposed as a possible fuel for future internal combustion engines. Hydrogen’s wide flammability range allows higher engine efficiency with much leaner operation than conventional fuels, for both reduced toxic emissions and no CO2 gases. This paper presents results from an optical study of combustion in a spark-ignition research engine running with direct injection and port injection of hydrogen. Crank-angle resolved flame chemiluminescence images were acquired and post-processed for a series of consecutive cycles in order to calculate in-cylinder rates of flame growth. Laser induced fluorescence of OH was also applied on an in-cylinder plane below the spark plug to record detailed features of the flame front for a series of engine cycles. The tests were performed at various air-to-fuel ratios, typically in a range of φ = 0.50–0.83 at 1000 RPM with 0.5 bar intake pressure. The engine was also run with gasoline in direct-injection and port-injection modes to compare with the operation on hydrogen. The observed combustion characteristics were analysed with respect to laminar and turbulent burning velocities, as well as flame stretch. An attempt was also made to review relevant hydrogen work from the limited literature on the subject and make comparisons were appropriate.

In Vivo, Dual-Modality OCT/LIF Imaging Using a Novel VEGF Receptor-Targeted NIR Fluorescent Probe in the AOM-Treated Mouse Model

Increased vascular endothelial growth factor (VEGF) receptor expression has been found at the sites of angiogenesis, particularly in tumor growth areas, as compared with quiescent vasculature. An increase in VEGF receptor-2 is associated with colon cancer progression. The in vivo detection of VEGF receptor is of interest for the purposes of studying basic mechanisms of carcinogenesis, making clinical diagnoses, and monitoring the efficacy of chemopreventive and therapeutic agents. In this study, a novel single chain (sc)VEGF-based molecular probe is utilized in the azoxymethane (AOM)-treated mouse model of colorectal cancer to study delivery route and specificity for disease. The probe was constructed by site-specific conjugation of a near-infrared fluorescent dye, Cy5.5, to scVEGF and detected in vivo with a dual-modality optical coherence tomography/laser-induced fluorescence (OCT/LIF) endoscopic system. A probe inactivated via excessive biotinylation was utilized as a control for nonreceptor-mediated binding. The LIF excitation source was a 633-nm He:Ne laser, and red/near-infrared fluorescence was detected with a spectrometer. OCT was used to obtain two-dimensional longitudinal tomograms at eight rotations in the distal colon. Fluorescence emission levels were correlated with OCT-detected disease in vivo. OCT-detected disease was verified with hematoxylin and eosin stained histology slides ex vivo. High fluorescence emission intensity from the targeted probe was correlated with tumor presence as detected using OCT in vivo and VEGFR-2 immunostaining on histological sections ex vivo. The inactivated probe accumulated preferentially on the surface of tumor lesions and in lymphoid aggregate tissue and was less selective for VEGFR-2. The scVEGF/Cy probe delivered via colonic lavage reaches tumor vasculature and selectively accumulates in VEGFR-2-positive areas, resulting in high sensitivity and specificity for tumor detection. The combination of OCT and LIF imaging modalities may allow the simultaneous study of tumor morphology and protein expression for the development of diagnostic and therapeutic methods for colorectal cancer.

A comparative analysis of flame surface density metrics inpremixed and stratified flames

Flame surface density (FSD) is an important quantity in premixed turbulent combustion models, many of which use it in order to predict flame behaviour and characteristics. This quantity may be determined using a statistical method based on measurements of temperature and its gradient, or a geometric method based on 2D temperature or LIF imaging. Measurements of FSD obtained using both methods have been reported in the literature, but no direct comparisons have been made between these methods in premixed or stratified flames. This paper compares these methods in both premixed and stratified datasets (up to a stratification ratio of 3) from a slot burner and a swirl burner. Temperature, local equivalence ratio and three dimensional flame normal orientations were obtained from simultaneous cross plane OH-LIF and Rayleigh–Raman line measurements. Three dimensional progress variable gradients, which are used in the statistical FSD calculations, are derived by angle-correcting gradients along the measurement line. Differences between the statistical and geometrical methods of evaluating FSD are shown to be negligible for values of progress variable between 0.2 and 0.9, and differences at extreme values are detailed and explained. This good agreement between methods is demonstrated to hold over a broad range of flame operating condition.