Laser-induced Fluorescence Method Research Articles

Quantitative estimation of ·OH-radical density from fluorescence decay behavior near the plasma-liquid interface by incident microscopic LIF spectroscopy

The formation and transportation of short-lived species on/within the plasma-liquid interfacial layer plays a crucial role in various applications because of their high chemical reactivity. However, the experimental detection and quantification of these short-lived species, such as ˙OH radicals, at the gas–liquid interface still pose formidable challenges. This study confronts this challenge by employing incident microscopic laser-induced fluorescence (mLIF) method to capture the OH-LIF signals on the interfacial layer at different time moments of the post-discharge phase under high spatial resolution. The temporal evolution of ˙OH density is subsequently quantified by fitting the OH-LIF decay behavior to a reaction-dissolution model. Results reveal that increasing the pulse width serves better to enhance ˙OH generation on liquid surface, reaching a density of 1.25 × 1016 cm−3. Furthermore, the cathode-solution interface demonstrates significantly enhanced ˙OH production compared to the anode-solution interface. These results underscore the efficacy of incident mLIF in quantitatively probing short-lived ˙OH-radical production at the interfacial layer in pulsed-driven plasma-solution interactions, with potential applicability to other reactive species.

Advancements in laser-based spatiotemporal measurements of flow boiling

Recent advancements in laser and imaging systems, as well as in computational processing capabilities, have made quantitative optical imaging, which is often combined with laser illumination, highly adaptable, robust and reliable. Laser-based diagnostic techniques, such as planar laser-induced fluorescence (PLIF), offer the possibility of simultaneous spatiotemporally resolved measurements of temperature fields in the liquid phase at boiling conditions. In this paper, we examine the applicability of two-colour PLIF (2cPLIF), where the ratio between individual fluorescent emissions from uniformly dispersed dyes is used to take temperature measurements in the liquid phase in the presence of moving vapour-liquid interfaces typical of boiling flow. The implementation of 2cPLIF necessitates uniformity in the concentration of different dyes across the flow field. However, in the case of a multiphase flow such as boiling, thermophoresis can lead to inhomogeneous dye distributions. To overcome this challenge, a single-dye multispectral planar laser-induced fluorescence (SDMS-PLIF) method has been developed, which employs fluorescent emissions in different spectral bands of the same dye (Nile Red). The spectral characteristics of Nile Red were measured using a spectrometer to identify its temperature-sensitive bands over a wide range of dye concentrations, from 0.3 to 30 mg/L. Following this, we demonstrate the measurement capabilities of SDMS-PLIF thermography as applied to a boiling flow in a miniaturised vertical square channel, gaining insight into the thermohydrodynamic interactions between vapour bubbles and a heated wall.

Assessing the efficacy of an atmospheric air DC glow discharge system for sustainable nitrogen fixation: a vibrational coherent anti-Stokes Raman scattering study

In addressing the substantial greenhouse gas emissions produced by the energy-intensive Haber–Bosch synthesis, this study investigates the viability of sustainable nitrogen fixation (NF) via low-temperature plasma systems energized by renewable sources. Utilizing vibrational coherent anti-Stokes Raman scattering as a diagnostic tool, we probed the nitrogen rovibrational temperature and population dynamics within a DC glow discharge in atmospheric air, a setting with considerable promise for eco-friendly fertilizer production. Besides, density for atomic N , O , and NO was quantified by laser-induced fluorescence (LIF) or two-photon absorbed LIF methods. Our findings reveal a quasi-equilibrium between rotational and vibrational energy states in the DC glow discharge environment, reaching an approximate value of 3500 K at the discharge core. The discharge parameters, discharge current, air flow rate, and discharge gap influence the rovibrational temperature, density distribution of species of interest, and the NF energy cost. However, the influences induced by these parameters are of limitations. Further analysis implies that the high gas temperature and its induced significant vibrational-rotational and vibrational-translational energy exchange are mainly responsible for the non-ideal NF energy cost.

Effect of gas components on the post-discharge temporal behavior of OH and O of a non-equilibrium atmospheric pressure plasma driven by nanosecond voltage pulses

OH radicals and O atoms are two of the most important reactive species of non-equilibrium atmospheric pressure plasma (NAPP), which plays an important role in applications such as plasma medicine. However, experimental studies on how the gas content affects the post-discharge temporal evolutions of OH and O in the noble gas ns-NAPP are very limited. In this work, the effect of the percentages of O2, N2, and H2O on the amounts of OH and O productions and their post-discharge temporal behaviors in ns-NAPP is investigated by laser-induced fluorescence (LIF) method. The results show that the productions of OH and O increase and then decrease with the increase of O2 percentage. Both OH and O densities reach their maximum when about 0.8% O2 is added. Further increase of the O2 concentration results in a decrease of the initial densities of both OH and O, and leads to their faster decay. The increase of N2 percentage also results in the increase and then decrease of the OH and O densities, but the change is smaller. Furthermore, when the H2O concentration is increased from 100 to 3000 ppm, the initial OH density increases slightly, but the OH density decays much faster, while the initial density of O decreases with the increase of the H2O concentration. After analysis, it is found that OH and O are mainly produced through electron collisional dissociation. O(1D) is critical for OH generation. O3 accelerates the consumption processes of OH and O at high O2 percentage. The addition of H2O in the NAPP considerably enhances the electronegativity, while it decreases the overall plasma reactivity, accelerates the decay of OH, and reduces the O atom density.

Development of thermally assisted OH PLIF temperature measurement method based on a single femtosecond laser

Recently, OH planar laser-induced fluorescence (PLIF) using the broadband, ultrashort femtosecond-duration (fs-duration) and the thermally assisted vibrational transfer in excited state has been investigated in flames. In this present work, we first measured temperature by thermally assisted OH laser-induced fluorescence (TALF) method with a single ultrashort broadband fs laser. In the experiment, the fs excitation of OH at ultraviolet wavelength is followed by fluorescence detection from two different vibrational bands. The ratio of two measured (1–0) and (0–0) band fluorescence is calibrated with calculated temperature using Chemkin PRO PRIMIX. The calibrated results are used in measuring temperature distributions in different laminar flames. It is found that TALF method using the fs laser can detect 2D temperature distribution in the burnt area with high OH fluorescence signal. However, OH chemiluminescence brings inevitable noise at the flame front that the TALF method does not perform well. And because (1–0) band fluorescence is so weak, the noise from the camera sensor and imaging intensifier (I.I.) remains at the measured temperature imaging. In conclusion, quantitative temperature measurement based on OH TALF based on a single broadband, ultrashort fs laser can be applied in laminar flames with high frequency by a simple experiment setup.

A Study on the Oil Film Thickness Between the Lower Rail of Oil Control Ring and Lower Flank of Oil Control Ring Groove of an Engine

Abstract Lubricating oil consumption (LOC) of engines causes particulate matter in exhaust gas and abnormal combustion like pre-ignition which becomes a serious problem in hydrogen engines. Lubricating oil transports upward into the combustion chamber of an engine via the sliding surface, the gap, and the side and back of a piston ring. The target of this study was to clarify the mechanism of oil flowing between an oil control ring lower flank and the groove which was the first entrance of lubricating oil supplied from the crankshaft. Oil film thickness at the lower flank of the oil control ring was measured by laser-induced fluorescence method using optical fibers embedded in the lower flank of the ring groove. The measured oil film thickness was compared with forces acting on the lower rail of three-piece type oil control ring. The oil film thickness change was well explained using those forces and it was found that friction force at the sliding surface and pressure in the ring groove were dominant on the oil film thickness under all operating conditions tested in this study.

Xenon tracers for cost effective laser induced fluorescence of alternative propellant Hall thrusters

One of the limiting factors to developing plasma thrusters on alternative propellants is the cost associated with changing the diagnostic tools, which are often propellant-dependent. For laser induced fluorescence (LIF), which is typically used for ion velocity distribution measurements to determine ion trajectories and potential profiles, either new lasers need to be bought, which are tuned to the wavelength of the new element’s excitation level, or a costly tunable laser is required. A method to use existing LIF setups designed for xenon on any propellant has been demonstrated on a Hall thruster operating on krypton. In the demonstration test, a small amount of xenon (0.01%–4%) was mixed with the main krypton propellant for use as a diagnostic tracer, and xenon ion velocities were measured while also monitoring changes in the mean discharge current and oscillations. High signal-to-noise ratios in LIF data acquired along the channel centerline were obtained with tracer gas fractions ≤1% that negligibly affected the thruster operation. These results and comparison of the emission spectra of xenon and other common propellants suggest that the tracer LIF method should be broadly applicable to LIF measurements in Hall thrusters operating on alternative propellants.

Investigation of the dielectric recovery process of vacuum arc in double breaks by planar laser-induced fluorescence

The multibreak vacuum circuit breaker uses multiple short gaps to interrupt the fault current, greatly improving the dielectric strength, and is a viable method to realize high-voltage interruption. The metal vapor distribution near the current zero is crucial for the dielectric recovery process in the multibreak vacuum circuit breaker. Due to the complicated dielectric construction and the interaction between the breakers, the vacuum arc inevitably deviates from the axisymmetric distribution during the interruption process. The traditional diagnosis method limited to 0D or 1D is not sufficient to study the real distribution of metal vapor near the current zero. To address these issues, we developed a planar laser-induced fluorescence method to measure the 2D distribution of copper vapor near the current zero by detecting 510.6 nm fluorescence intensity. The results indicate that for the butt contacts, the copper vapor is diffused in the gap of the high-voltage break and aggregated on the cathode surface of the low-voltage break. The axial magnetic field and transverse magnetic field affect the 2D copper vapor distribution and eliminate the inconsistency, which is achieved by affecting the motion of charged particles and the ionization-recombination process. Furthermore, the copper vapor density exhibits a positive dependence on the arc current, and the magnetic field impacts the density increase rate and distribution mode.

Three-dimensional flow velocity determination using laser-induced fluorescence method with asymmetric optical vortex beams

Laser-induced fluorescence (LIF) Doppler spectroscopy using an optical vortex beam with an asymmetric intensity distribution, referred to as aOVLIF, is proposed as a new method to measure plasma flow velocity. LIF spectra were calculated numerically using typical laboratory low-temperature plasma parameters, and it was revealed that an ion flow across the beam produces a frequency shift of the spectra. This method also has the capability of temperature measurements. The propagation effects of asymmetric optical vortex beams are discussed assuming an actual experiment, and it is found that the sensitivity to the transverse flow velocity is approximately unchanged. The aOVLIF method, which exploits the inhomogeneous phase structure of optical vortices, can be applied to the determination of three-dimensional velocity vectors and promises to enhance the usefulness of conventional LIF spectroscopy using plane waves.

Pixel-dependent laser-induced fluorescence method for determining thin liquid film thickness distribution

Thin liquid films on free surfaces or inside restricted spaces are widely encountered and can have significant effects in heat transfer, coating, biofilm growth, and lubrication. The thickness and distribution of these thin liquid films are important determinants of performance, but cannot easily be measured accurately. This paper presents a methodological study on determining the thickness of thin liquid films by means of laser-induced fluorescence. A model of fluorescence imaging and measurement is established based on the fluorescence energy transmission. A novel pixel-dependent laser-induced fluorescence method is then proposed and compared with commonly used methods using experimental test results. The accuracy and precision of the measurements are discussed in detail. It is found that the measurement performance is greatly influenced by the nonlinearity, heterogeneity, and random noise of the optical system and imaging technique. The proposed pixel-dependent laser-induced fluorescence method effectively eliminates or ameliorates the effects of these factors and enhances the measuring accuracy.

Identification of marine microplastics based on laser-induced fluorescence and principal component analysis

Although the laser-induced fluorescence method shows great potential for microplastic particle detection, overlapping fluorescence signals make accurate type and proportion identification difficult. This paper presents the identification of marine microplastics based on laser-induced fluorescence and principal component analysis. This method works by measuring the fluorescence spectra of water-containing microplastic samples irradiated with a 405-nm laser, which are then analyzed using the principal component analysis (PCA) method. The nine types of microplastics were differentiated based on their positions in the PCA score plot. The mixed sample was positioned between the pure microplastic samples. The component ratio determines its position relative to that of the pure microplastic samples. The first two principal components of the mixed microplastics were linearly dependent. Natural seawater had less influence on the detection, and a mass concentration as low as 0.03% was detected.

Impact of pilot flame and hydrogen enrichment on turbulent methane/hydrogen/air swirling premixed flames in a model gas turbine combustor

This work investigates the impact of pilot flame and fuel composition on the structures and stabilization of swirling turbulent premixed methane/hydrogen/air flames in a lab-scale gas turbine model combustor. Simultaneous measurements of the velocity field and OH radicals distribution in the combustor were conducted using particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF) methods, respectively. Flames under stable and close to lean blow-off (LBO) conditions were studied for two fuel mixtures, with a hydrogen mole ratio of 0 and 50 % in the hydrogen/methane mixture, respectively. The studied flames were at a constant Reynolds number of 20,000 with different equivalence ratios. Two pilot-to-global fuel ratios were investigated (2 % and 6 %) while keeping the pilot-to-global air ratio constant at 2 %. Data for non-piloted flames were also acquired for comparison. The pilot flames were shown to extend the operability range. The LBO equivalence ratio of the main flame decreased with increasing fuel mass flow rate in the pilot flames due to the increased amount of hot gases with high concentrations of OH radicals in the outer recirculation zone (ORZ), which significantly enhanced the stabilization of the main flame. The stable flame reaction zone was in the high-speed shear layer between the ORZ and the inner recirculation zone (IRZ). When approaching LBO, the reaction zone was pushed downstream to the IRZ and subsequently decreased the size of IRZ, indicating a strong flow/flame interaction. Hydrogen enrichment was shown to reduce the LBO equivalence ratio. When close to LBO, the OH radicals in the hydrogen-enriched flames were observed in isolated pockets due to differential diffusion, which enhanced resilience to LBO. The flame front curvature, mean progress variable, and flame surface density were calculated from the acquired OH-PLIF data to quantify the impact of fuel composition and pilot flames on the flame structures.

‘Self-calibration’ method for TALIF measurement of O atoms by full photofragmentation of O3 using a 226 nm UV laser

Quantification of atomic oxygen through the method of two-photon absorption laser-induced fluorescence (TALIF) is common in the fields of plasma fundamental research and application treatments. Fluorescence signal calibration is required to absolutely quantify the O amount and normally achieved with the help of TALIF measurement of a known-density Xe gas. In this study, an alternative calibration method is proposed based on the full photofragmentation (FPF) of O3 with a known density in a known gas composition by the same UV laser beam as that of the TALIF detection of atomic O. This is achieved by an equivalent amount of O fragment contributing the same fluorescence intensity as that of the O3 FPF-TALIF process under the same experimental conditions. The validity of this calibration method is proved by comparing it to the TALIF measurement of the Xe gas. It provides a ‘self-calibration’ method for the TALIF detection of O atoms without any need to change the laser optical arrangements including the laser wavelength. In addition, it only requires a gas flow with known O3 density through the studied medium reactor or chamber (such as plasma discharges). Detailed theoretical and practical principles of this self-calibration approach are presented and discussed in this study.

A state-of-the-art review on laser-induced fluorescence (LIF) method with application in temperature measurement

Laser-induced fluorescence (LIF) is a robust and vigorous method for non-intrusive measurement of temperature, pressure, concentration, and pH in fluids. The application of LIF for measurement is also extendable to solid and gaseous environments. In this method, the fluorescent molecules are transferred to a higher energy state by absorbing the energy of a laser beam and subsequently undergo spontaneous emission of light in the form of fluorescence during the transition to the ground. The emitted fluorescence is commonly captured using a setup consisting of a solvent, fluorescent material, detecting cameras, lenses, and filters, which enables the determination of the thermophysical parameters of the measuring environment using pertinent mathematical formulae. This method can be used for precise measurements in a wide range of engineering and medical applications. Despite the numerous studies on the utilization of LIF for measurement, there is not a comprehensive review paper with a focus on various types of LIF techniques and their applications in temperature measurement. This paper seeks to address this gap by fully discussing these approaches, demonstrating temperature measurements in different environments and circumstances, and outlining their benefits and drawbacks. Accordingly, one-dye one-color, two-dye two-color, one-dye two-color, one-dye three-color, two-dye three-color, volumetric LIF, and encapsulation techniques are fully discussed as the main LIF approaches and their applications for the temperature measurement in fluid flows, droplets, microfluidics, and gas environments.

Kinetic study of C1 criegee intermediate with diethylamine and ethylamine and their atmospheric implications

This paper investigates the pressure and temperature-dependent kinetics of CH2OO reacting with diethylamine (DEA) and ethylamine (EA) using the OH laser-induced fluorescence method. Pressure independence was observed within the range of 5.4–75 Torr. The rate coefficients for these reactions at 298 K are (2.49 ± 0.39) × 10−11 and (8.14 ± 1.25) × 10−12 cm3 molecule−1 s−1. The activation energies, determined from experiments conducted between 283 and 328 K, are (−2.01 ± 0.08) and (−1.35 ± 0.02) kcal mol−1. These reactions may contribute to the degradation of DEA and EA under specific environmental conditions.

Imaging and investigation with PLIF40 for improved film thickness measurements in annular flow

Gas–liquid annular flow occurs frequently in industrial production processes, and the accurate measurement of film thickness is essential for applying its mass and heat transfer properties. The non-invasive planar laser-induced fluorescence (PLIF) method is widely employed in annular flow investigation. However, both methods of PLIF90 and PLIF70 (the measurement angles between laser and camera are set to 90°and 70°) suffer from total reflection. In this paper, the effect of measurement angle on total reflection, measurement accuracy, and robustness are quantitatively investigated. Accordingly, a PLIF40 method is proposed, which minimizes the influence of total reflection by reducing the measurement angle to 40°. Sub-pixel-based edge detection and liquid film smoothing algorithms are developed to accurately identify liquid films with missing edge details, resulting in a measurement error of 0.09 mm. The flow structure is qualitatively diagnosed by waveform reconstruction and wave height extraction combined with the film thickness distribution. Furthermore, the prediction models for average film thickness are established and compared with the classical prediction models to demonstrate the accuracy and reliability of PLIF40.

Kinetics for the reaction of Criegee intermediate CH2OO with n-butyraldehyde and its atmospheric implications

The Criegee intermediates (CIs), formed through ozone oxidation of unsaturated hydrocarbons, can undergo reactions with various carbonyl compounds in the atmosphere. In this study, we have investigated the 1,3-cycloaddition reaction of n-butyraldehyde with the simplest Criegee intermediate, CH2OO, using the OH laser-induced fluorescence (LIF) method. The experiments were conducted over a temperature and pressure range of 283–318 K and 5–100 Torr, respectively. Our results reveal that the bimolecular rate constant of the reaction is dependent on both temperature and pressure. Specifically, the measured rate constants at 100 Torr are (3.51 ± 0.63), (2.97 ± 0.53), (2.70 ± 0.49), and (2.52 ± 0.45) × 10−12 cm3 molecule−1 s−1 at temperatures of 283, 298, 308, and 318 K, respectively, with a total error of 18%. By fitting the pressure-dependent rate constants according to the Lindemann mechanism, we obtained a high-pressure limit rate constant of (3.05 ± 0.55) × 10−12 cm3 molecule−1 s−1 at 298 K. The Arrhenius plot of the temperature-dependent rate constants yields an activation energy of (−1.71 ± 0.30) kcal mol−1 and a pre-exponential factor of (1.66 ± 0.24) × 10−13 cm−3 s−1 (total error). Furthermore, our findings indicate that the title reaction exhibits a more pronounced temperature-dependent effect compared to that of CH2OO with water dimer and SO2, as well as that of OH with n-butyraldehyde. This observation suggests that the significance of title reaction in the atmosphere increases at lower temperature.

Исследование образования полиароматических углеводородов и углеродных наночастиц при пиролизе этилена с добавками тетрагидрофурана за ударными волнами

Currently, substances are being actively sought that could serve as the alternative fuel or fuel additive reducing formation and emission of the carbon nanoparticles. Here, processes of the polyaromatic hydrocarbons (precursors of the carbon condensed phase formation) and carbon black nanoparticles formation during the ethylene pyrolysis with addition of the tetrahydrofuran were studied by laser-induced fluorescence and laser extinction methods. Spectral dependences of laser-induced fluorescence of the polyaromatic hydrocarbons were obtained by laser-induced fluorescence at various temperatures and pyrolysis stages, and optical density of the reacting gas mixtures at wavelengths of 405 and 633 nm was obtained by laser extinction. Measurements were carried out on a shock tube behind the reflected shock waves in the temperature range of 1,695--2,500 K and pressure range of 2.7--4.1 atm. It is shown that during the ethylene pyrolysis with the tetrahydrofuran addition soot formation process is intensifying, and the temperature range of the carbon nanoparticles formation is expanding. Using the kinetic simulation, it was found that acceleration in the carbon nanoparticles formation was caused by formation of methyl radical and propylene in the tetrahydrofuran pyrolysis

Corroboration of the Toms effect from a frictional drag reducing self-polishing copolymer

A novel frictional drag reducing self-polishing copolymer (FDR-SPC) was first developed by the authors. The FDR-SPC is a special derivative of an SPC that was designed to achieve skin frictional drag reduction in turbulent water flow by releasing polyethylene glycol (PEG) into water through a hydrolysis reaction. Thus, the FDR-SPC coating acts as a continuous medium accommodating countless, molecular-level polymer injectors. However, direct evidence of such PEG release has not yet been demonstrated. Here, we report the results of in situ PEG concentration measurement based on the planar laser-induced fluorescence (PLIF) method. Polyethylene glycol methacrylate (PEGMA) was probed by the fluorescent functional material dansyl, and the fluorescence intensity from dansyl-PEG was then measured to quantify the concentration in the flow. The near-wall concentration of dansyl-PEG is observed to range from 1 to 2 ppm depending on the flow speed, which corroborates the existence of a drag reducing function for the FDR-SPC. In the concurrent measurement of skin friction, the present FDR-SPC specimen exhibited a skin friction reduction ratio of 9.49% at the freestream flow speed U=5m/s. In the comparative experiment of dansyl-PEGMA solution injection, the skin friction was found to decrease by 11.9%, which is in reasonable accordance with that for the FDR-SPC.

Predicting buoyant jet characteristics: a machine learning approach.

We study positively buoyant miscible jets through high-speed imaging and planar laser-induced fluorescence methods, and we rely on supervised machine learning techniques to predict jet characteristics. These include, in particular, predictions to the laminar length and spread angle, over a wide range of Reynolds and Archimedes numbers. To make these predictions, we use linear regression, support vector regression, random forests, K-nearest neighbour, and artificial neural network algorithms. We evaluate the performance of the aforementioned models using various standard metrics, finding that the random forest algorithm is the best for predicting our jet characteristics. We also discover that this algorithm outperforms a recent empirical correlation, resulting in a significant increase in accuracy, especially for predicting the laminar length.