Laser-induced Fluorescence Diagnostics Research Articles

Planar laser-induced fluorescence diagnostics of water droplets heating and evaporation at high-temperature

Gas-steam-droplet technologies are widely used in systems operating at high temperatures ranging between 400 and 2000°C, namely: fire-fighting systems, thermal fluid cleaning, fuel compounding, industrial waste gasification and evaporation systems for advanced fuel components, cleaning of thermally loaded surfaces of power equipment, etc. System parameters are usually selected empirically via multiple tests and continuous trial operation of appropriate units, aggregates, assemblies, and installations. This situation is caused by insufficient basic knowledge of conditions and parameters of high-temperature (over 500°C) heating and evaporation of water and water-based emulsions, solutions, and slurries. Limited information is available regarding the evaporation rates dependent on the temperature of gaseous medium. Consequently, the up-to-date evaporation models allow the researchers to achieve adequate values (in good agreement with the experiment) of evaporation rates at air temperatures not exceeding 300–400°C. The paper presents a set of experiments on water droplets with the size ranging from 1 to 2mm, which is used to create the information database on high-temperature evaporation parameters. The approach to measuring the evaporation rate involves observation of the droplet size or more exactly its mean radius, and recording the time of its existence. A high-speed video camera and Tema Automotive software with different tracking algorithms are used for experimental observations. During gas heating, the distribution of highly non-homogeneous and non-steady temperature field in evaporating water droplets is detected by the hardware and software cross-correlation system and Planar Laser-induced Fluorescence optical diagnostics. Instantaneous and medium evaporation rates are computed for the whole period of the droplet lifetime. Highly nonlinear evaporation rate dependences are suggested for gas temperatures and the water droplet surface, size, and time of gas heating. Approximate relationships are described for the prediction of evaporation rates depending on the basic parameters. The evaporation rate is shown to increase several-fold during the heating of a water droplet in a high-temperature gaseous medium. The experimental data are described most accurately by exponential dependences of water evaporation rate versus the airflow temperature and velocity. This result makes it possible to predict the corresponding highly nonlinear dependences of evaporation rate versus the density of convective heat flux and the overall thermal conditions in the industrial chambers, evaporators, heat exchangers, etc. The mathematical expressions obtained can be used to identify the effective conditions of water droplet heating in a large group of high-temperatures applications as well as for developing high-temperature liquid evaporation models.

Predicting fluorescence quantum yield for anisole at elevated temperatures and pressures

Aromatic molecules are promising candidates for using as a fluorescent tracer for gas-phase scalar parameter diagnostics in a drastic environment like engines. Along with anisole turning out an excellent temperature tracer by Planar Laser-Induced Fluorescence (PLIF) diagnostics in Rapid Compression Machine (RCM), its fluorescence signal evolution versus pressure and temperature variation in a high-pressure and high-temperature cell have been reported in our recent paper on Applied Phys. B by Tran et al. Parallel to this experimental study, a photophysical model to determine anisole Fluorescence Quantum Yield (FQY) is delivered in this paper. The key to development of the model is the identification of pressure, temperature, and ambient gases, where the FQY is dominated by certain processes of the model (quenching effect, vibrational relaxation, etc.). In addition to optimization of the vibrational relaxation energy cascade coefficient and the collision probability with oxygen, the non-radiative pathways are mainly discussed. The common non-radiative rate (intersystem crossing and internal conversion) is simulated in parametric form as a function of excess vibrational energy, derived from the data acquired at different pressures and temperatures from the literature. A new non-radiative rate, namely, the equivalent Intramolecular Vibrational Redistribution or Randomization (IVR) rate, is proposed to characterize anisole deactivated processes. The new model exhibits satisfactory results which are validated against experimental measurements of fluorescence signal induced at a wavelength of 266 nm in a cell with different bath gases (N2, CO2, Ar and O2), a pressure range from 0.2 to 4 MPa, and a temperature range from 473 to 873 K.

Laser-induced fluorescence for ITER divertor plasma

Laser-induced fluorescence diagnostics on ITER will be used for local measurements of helium density (nHeI) and ion temperature (Ti) in the divertor area. The diagnostics is combined with divertor Thomson scattering via common laser injection and signal collection optics. Multichannel filter polychromators with design similar to those developed for Thomson scattering will be used to perform spectral analysis of the fluorescent signals. Both the fluorescent signals and the background radiation are estimated using the collisional-radiative models developed for He I and He II. The estimations demonstrate the feasibility of measuring nHeI with the temporal resolution 20ms and accuracy ΔnHeI<20% in both the He and DT regimes. The possibilities of different approaches for the Ti measurements are discussed.

LIF diagnostics of hydroxyl radical in a methanol containing atmospheric-pressure plasma jet**Project supported by the National Natural Science Foundation of China (Grant Nos. 11465013 and 11375041), the Natural Science Foundation of Jiangxi Province, China (Grant Nos. 20151BAB212012 and 20161BAB201013), and the International Science and Technology Cooperation Program of China (Grant No. 2015DFA61800).

In this paper, a pulsed-dc CH3OH/Ar plasma jet generated at atmospheric pressure is studied by laser-induced fluorescence (LIF) and optical emission spectroscopy (OES). A gas–liquid bubbler system is proposed to introduce the methanol vapor into the argon gas, and the CH3OH/Ar volume ratio is kept constant at about 0.1%. Discharge occurs in a 6-mm needle-to-ring gap in an atmospheric-pressure CH3OH/Ar mixture. The space-resolved distributions of OH LIF inside and outside the nozzle exhibit distinctly different behaviors. And, different production mechanisms of OH radicals in the needle-to-ring discharge gap and afterglow of plasma jet are discussed. Besides, the optical emission lines of carbonaceous species, such as CH, CN, and C2 radicals, are identified in the CH3OH/Ar plasma jet. Finally, the influences of operating parameters (applied voltage magnitude, pulse frequency, pulsewidth) on the OH radical density are also presented and analyzed.

Effects of non-equilibrium plasmas on low-pressure, premixed flames. Part 1: CH* chemiluminescence, temperature, and OH

In this paper, we investigate the effects of nanosecond, repetitively-pulsed, non-equilibrium plasma discharges on laminar, low-pressure, premixed, burner-stabilized hydrogen/O2/N2 and hydrocarbon/O2/N2 flames using CH* chemiluminescence and quantitative OH laser-induced fluorescence (LIF) diagnostics. Two different plasma sources, both of which generate uniform, low-temperature, volumetric, non-equilibrium plasma discharges, are used to study changes in chemiluminescence, temperature, and OH concentration when non-equilibrium plasmas are directly coupled to conventional hydrogen/hydrocarbon oxidation and combustion chemistry. Qualitative imaging of CH* chemiluminescence indicates that during plasma discharge, the luminous flame zone is shifted upstream towards the burner surface with little change in the CH* zone thickness. For the same plasma discharge and flame conditions, quantitative results using spatially-resolved OH LIF and multi-line, OH-LIF thermometry show significant increases in ground-state OH concentrations in the preheating zones of the flame. More specifically, for a particular axial position downstream of the burner surface, the OH concentration increases, which can be viewed as an effective “shift” of the OH profiles towards the burner surface. The increase in OH concentration is conceivably due to an enhancement of the lower-temperature kinetics including O atom, H atom and OH formation kinetics and temperature rise due to the presence of the low-temperature, non-equilibrium plasma.

Quantitative OH Planar Laser Induced Fluorescence Diagnostics of Syngas and Methane Combustion in a Cavity Combustor

The current work reports quantitative OH species concentration in the cavity of a trapped vortex combustor (TVC) in the context of mixing and flame stabilization studies using both syngas and methane fuels. Planar laser induced fluorescence (PLIF) measurements of OH radical obtained using a Nd:YAG pumped dye laser are quantified using a flat flame McKenna burner. The momentum flux ratio (MFR), defined as the ratio of the cavity fuel jet momentum to that of the guide vane air stream, is observed to be a key governing parameter. At high MFRs ~4.5, the flame front is observed to form at the interface of the fuel jet and the air jet stream. This is substantiated by velocity vector field measurements. For syngas, as the MFR is lowered to ~0.3, the fuel-air mixing increases and a flame front is formed at the bottom and downstream edge of the cavity where a stratified charge is present. This trend is observed for different velocities at similar equivalence ratios. In case of methane combustion in the cavity, where the MFRs employed are extremely low at ~0.01, a different mechanism is observed. A fuel-rich mixture is now observed at the center of the cavity and this mixture undergoes combustion. On further increase of the cavity equivalence ratio, the rich mixture exceeds the flammability limit and forms a thin reaction zone at the interface with air stream. As a consequence, a shear layer flame at the top of the cavity interface with the mainstream is also observed. The equivalence ratio in the cavity also determines the combustion characteristics in the case of fuel-air mixtures that are formed as a result of the mixing. Overall, flame stabilization mechanisms have been proposed, which account for the wide range of MFRs and premixing in the mainstream as well.

Effect of Gas Flow Rate on Distribution of Active Species and Dynamics of an Atmospheric RF- (Bio) Plasma Jet

A fast imaging technique applied for RF plasma indicates that the plasma jet propagation dynamics, operating even at megahertz frequency includes the so-called plasma bullet phenomenon. Broadband emission and intensity of lines corresponding to excited Ar and OH are registered. The spatial distribution of ground state OH is also detected by means of laser-induced fluorescence diagnostics.

Spatial profiles of electron and metastable atom densities in positive polarity fast ionization waves sustained in helium

Fast ionization waves (FIWs), often generated with high voltage pulses over nanosecond timescales, are able to produce large volumes of ions and excited states at moderate pressures. The mechanisms of FIW propagation were experimentally and computationally investigated to provide insights into the manner in which these large volumes are excited. The two-dimensional structure of electron and metastable densities produced by short-pulse FIWs sustained in helium were measured using laser-induced fluorescence and laser collision-induced fluorescence diagnostics for times of 100–120 ns after the pulse, as the pressure was varied from 1 to 20 Torr. A trend of center-peaked to volume-filling to wall-peaked electron density profiles was observed as the pressure was increased. Instantaneous FIW velocities, obtained from plasma-induced emission, ranged from 0.1 to 3 × 109 cm s−1, depending on distance from the high voltage electrode and pressure. Predictions from two-dimensional modeling of the propagation of a single FIW correlated well with the experimental trends in electron density profiles and wave velocity. Results from the model show that the maximum ionization rate occurs in the wavefront, and the discharge continues to propagate forward after the removal of high voltage from the powered electrode due to the potential energy stored in the space charge. As the pressure is varied, the radial distribution of the ionization rate is shaped by changes in the electron mean free path, and subsequent localized electric field enhancement at the walls or on the centerline of the discharge.

Influence of air diffusion on the OH radicals and atomic O distribution in an atmospheric Ar (bio)plasma jet

Treatment of samples with plasmas in biomedical applications often occurs in ambient air. Admixing air into the discharge region may severely affect the formation and destruction of the generated oxidative species. Little is known about the effects of air diffusion on the spatial distribution of OH radicals and O atoms in the afterglow of atmospheric-pressure plasma jets. In our work, these effects are investigated by performing and comparing measurements in ambient air with measurements in a controlled argon atmosphere without the admixture of air, for an argon plasma jet. The spatial distribution of OH is detected by means of laser-induced fluorescence diagnostics (LIF), whereas two-photon laser-induced fluorescence (TALIF) is used for the detection of atomic O. The spatially resolved OH LIF and O TALIF show that, due to the air admixture effects, the reactive species are only concentrated in the vicinity of the central streamline of the afterglow of the jet, with a characteristic discharge diameter of ∼1.5 mm. It is shown that air diffusion has a key role in the recombination loss mechanisms of OH radicals and atomic O especially in the far afterglow region, starting up to ∼4 mm from the nozzle outlet at a low water/oxygen concentration. Furthermore, air diffusion enhances OH and O production in the core of the plasma. The higher density of active species in the discharge in ambient air is likely due to a higher electron density and a more effective electron impact dissociation of H2O and O2 caused by the increasing electrical field, when the discharge is operated in ambient air.

Hydroxyl and Nitric Oxide Distribution in Waste Rice Bran Biofuel-Octanol Flames

Two-dimensional (2-D) visualization of hydroxyl (OH) radical in combustion of biofuel made of waste rice bran oil (called W) mixed with octanol (called O) at different mixture ratios were examined in a laboratory scale facility using planar laser-induced fluorescence (PLIF) diagnostics. Rice bran oil has a composition similar to that of peanut oil, with 38% monounsaturated, 37% polyunsaturated, and 25% saturated fatty acids. The ratio of this biofuel to octanol fuel examined was W90/O10, W75/O25, and W60/O40. The chemical species generated from within the combustion zone were analyzed from the spontaneous emission spectra of the flame in the ultraviolet to visible (Uv-Vis) range. The spatial distribution of Nitric Oxide (NO) and OH, denoted as OH*, were identified from the spectra. Two-dimensional (2-D) distributions of flame temperature were obtained using a thermal video camera. The experimental results showed the temperatures to range from 600 °C to 1400 °C. The highest temperature was obtained using W60/O40 waste/octanol fuel mixture. A practical burner commonly used in Indonesia, called semawar, that have a built-in preheating system was used for the combustion of biofuels.

Technique for measuring laser radiation intensity in biological tissues

AbstractLaser methods, such as laser-induced fluorescence diagnostics, photodynamic therapy (PDT), and hyperthermia, are finding increasing use in medicine. Irradiation can be performed both with and without contact on the tissue surface. In the case of contact irradiation, especially when laser radiation is introduced into biological tissue through an optical fiber, it is important to know the processes taking place at the irradiation fiber end. These processes affect diffusely reflected radiation which returns to the fiber. By analyzing backscattered radiation, we can evaluate the quality of the radiation procedure and the state of the fiber end. The objectives of this study were to develop a method and device for measuring backscattered radiation power and using this method, to determine the time and temperature ranges realized in PDT and hyperthermia.Light propagation is discussed in bent optical fibers. A technique is proposed for measuring laser radiation intensity in the optical fiber bend. Based on this technique, a system was developed for monitoring the laser radiation dose absorbed in biological tissues. We studied samples of bovine liver, muscle and brain tissues. Experiments were performed using a 675 nm, 100–2200 mW continuous wave semi-conductor laser. Laser radiation was delivered through a silica/polymer optical fiber. Data concerning the temperature and transmitted radiation intensity was acquired.Modeling of the light propagation in a bent optical fiber showed that the sensitivity of the method depends on the position of the photodetectors, but is independent of the loop number of the optical fiber. The results of experiments are presented using different types of biological tissues. We obtained the experimental dependencies of backward and transmitted radiation intensities and the temperature of the tissue surface in the irradiated region on the irradiation time measured with a flat-end fiber. The characteristic ranges of tissue heating caused by irradiation were determined for use in clinical practice.The optical parameters of biological tissues change with increasing temperature. This affects the intensity of transmitting radiation and diffuse radiation entering the fiber. The change in the backscattered radiation intensity greatly depend on the temperature of the irradiated area. The control of the irradiation of biological objects provides an efficient delivery of laser radiation to biological tissues and increases hyperthermia and PDT treatment effect.

Metastable density in nitrogen barrier discharges: I. LIF diagnostics and absolute calibration by Rayleigh scattering

The density of the metastable state of the nitrogen molecule is measured by laser-induced fluorescence (LIF) spectroscopy in an asymmetric barrier discharge at 500 mbar in the filamentary mode. The absolute density calibration is performed by comparison of the time-resolved signals from LIF and Rayleigh scattering. Due to the different physics of both processes, the LIF signal is the temporal convolution of the Rayleigh signal with an exponential decay defined by the effective lifetime of the laser excited state. The result is a delay of the LIF signal with respect to the Rayleigh signal and a slower decay of the LIF signal. This is proven experimentally by a pressure variation from 100 to 1000 mbar. Furthermore, the calibration method allows an implicit measurement of the effective lifetimes and the fluorescence yield, which is important at atmospheric pressure due to the dominance of collisional de-excitation (quenching). The calculated densities in the range of 1013 cm−3 are in good agreement with results from the literature.

Time-resolved laser-induced fluorescence diagnostics in a HIPIMS discharge

Laser-induced fluorescence (LIF) combined with resonant optical absorption spectroscopy (ROAS) were utilized for characterization of the Ar-Ti high-power impulse sputtering (HIPIMS) discharge at working pressures of 5 and 20 mTorr. The LIF measurements were performed during the plasma off-time. It was shown that the time-behavior of the measured densities depends strongly on the working pressure in the reactor as well as on the distance from the magnetron target. ROAS measurements indicates that the absolute ground state level density of Ti can reach ≈ 1011 cm−3 (at 20 mTorr, 10 μs pulse, and 12 kW of applied power per pulse). Generally, the obtained results indicate highly non-uniform spatial and temporal behavior of the sputtered species such as Ti and Ti+, which correlate with the results previously obtained in the other magnetron discharges.

Mixing properties of coaxial jets with large velocity ratios and large inverse density ratios

An experimental study was conducted to better understand the mixing properties of coaxial jets as several parameters were systematically varied, including the velocity ratio, density ratio, and the Reynolds number. Diameters of the inner and outer jet were also varied. Coaxial jets are commonly used to mix fluids due to the simplicity of their geometry and the rapid mixing that they provide. A measure of the overall mixing efficiency is the stoichiometric mixing length (Ls), which is the distance along the jet centerline where the two fluids have mixed to some desired concentration, which was selected to be the stoichiometric concentration for H2/O2 and CH4/O2 in this case. For 56 cases, the profiles of mean mixture fraction, rms mixture fraction fluctuations (unmixedness), and Ls were measured using acetone planar laser induced fluorescence diagnostics. Results were compared to three mixing models. The entrainment model of Villermaux and Rehab showed good agreement with the data, indicating that the proper non-dimensional scaling parameter is the momentum flux ratio M. The work extends the existing database of coaxial jet scalar mixing properties because it considers the specific regime of large values of both the velocity ratio and the inverse density ratio, which is the regime in which rocket injectors operate. Also the work focuses on the mixing up to Ls where previous work focused on the mixing up to the end of the inner core. The Reynolds numbers achieved for a number of cases were considerably larger than previous gas mixing studies, which insures that the jet exit boundary conditions are fully turbulent.

Exhaust gas recirculation stratification to control diesel homogeneous charge compression ignition combustion

A single-cylinder diesel engine was used to investigate the potential of exhaust gas recirculation dilution stratification as a control technique for homogeneous charge compression ignition combustion with early direct injections. Experimental studies on both all-metal and optically accessible engines were performed to understand the processes involved when exhaust gas recirculation is introduced separately in the intake ports. Laser-induced fluorescence diagnostics were carried out in the optical engine in order to provide fuel and exhaust gas recirculation distributions. The results indicate that depending on the intake configuration, the exhaust gas recirculation stratification can be maintained until late timings corresponding to the combustion event, leading to decreased maxima of heat-release rates, as well as decreased combustion noise levels. This result suggests that exhaust gas recirculation stratification may be used as a control parameter for combustion speed and therefore may contribute to the extension of the homogeneous charge compression ignition operating range. However, although exhaust gas recirculation stratification appears to be an interesting new control technique for homogeneous charge compression ignition combustion, its effect on the combustion was shown to be very sensitive to parameters such as the intake system configuration or the exhaust gas recirculation composition, showing that industrial use of this control technique requires further understanding of the physical phenomena involved.

3-pentanone fluorescence yield measurements and modeling at elevated temperatures and pressures

Measurements of 3-pentanone fluorescence quantum yield (FQY) over a wide range of temperatures and pressures in air and nitrogen bath gases are reported and a comprehensive FQY model in support of quantitative planar laser-induced fluorescence diagnostics at elevated pressures and temperatures is presented. Measurements were made of the FQY for 20 mbar of 3-pentanone in nitrogen and air for pressures between 1 and 25 bar in a high-pressure and high-temperature cell for excitation wavelengths of 248, 266, 277, and 308 nm. The measurements were performed in nitrogen from 298 to 745 K and in air from 298 to 567 K. The 3-pentanone FQY data were used to optimize FQY model parameters, including the oxygen and nitrogen quenching rates and vibrational relaxation cascade parameters for nitrogen and oxygen. This work introduces vibrational energy dependence for cascade parameters, as well as a nitrogen quenching rate. The new 3-pentanone FQY model agrees with the measurements within 10%, as well as with fluorescence signal measurements from optical internal combustion engines at pressures and temperatures up to 28 bar and 1100 K.

Investigation of NCN and prompt-NO formation in low-pressure C1–C4 alkane flames

Temperature, CH, NCN, and NO profiles were measured for eight low-pressure hydrocarbon flames fueled by methane, ethane, propane, and butane using laser-induced fluorescence (LIF) diagnostics. These measurements were used (1) to assess NCN and prompt-NO formation chemistry across a series of fuels of increasing number of carbons at different equivalence ratios ( ϕ = 1.07 and 1.28); (2) to examine the predictive capabilities of current C1–C4 hydrocarbon and NCN formation/consumption combustion mechanisms on properly capturing prompt-NO formation and (3) to examine the postulation that additional prompt-NO precursors (other than CH) exist for fuels larger than methane. For a given equivalence ratio, the measured peak CH concentration is fairly constant across all four fuels, while both the peak NCN and post-flame NO concentrations steadily increase. Furthermore, it is found that as the fuels increase in number of carbons, i.e., methane to butane, the correlation between the peak NCN and post-flame NO remains high, while the correlation between peak CH and peak NCN and peak CH and post-flame NO becomes increasingly lower. This is especially evident for rich flame cases. The experimental profiles are compared to numerical calculations using two comprehensive kinetic mechanisms suitable for C4 chemistry, where the CH + N 2 → NCN + H reaction is assumed as the only prompt-NO initiation reaction. For the ϕ = 1.28 flame cases, CH is over-predicted using both mechanisms for all four fuels and by as much as 60%, while for the ϕ = 1.07 cases, CH is predicted to within 15% of the experimentally-derived results, although there is some discrepancy concerning the spatial locations of the CH profiles. For both NCN and NO, there is an increasing under-prediction for the ϕ = 1.28 cases as the fuel increases in number of carbons, while for the ϕ = 1.07 cases there is a systematic under-prediction of NCN and NO with a weaker (although evident) fuel dependence. From the experimental results and the comparison to modeling predictions, it is apparent that additional work concerning CH formation and consumption kinetics is necessary to accurately capture the CH concentration profiles across a broad range of conditions. Furthermore the comparisons to the modeling predictions using only a single prompt-NO precursor, CH, indicate a reasonable plausibility that (an) additional prompt-NO precursor (s) exist and become important when considering fuels larger than methane, especially under rich flame conditions. Possible precursors in addition to CH are discussed.

High-current diode with ferroelectric plasma source-assisted hollow anode

The operation of a ferroelectric plasma source-assisted hollow anode (HA) electron source in a vacuum diode powered by an ∼200 kV and ∼400 ns pulsed generator was studied using time- and space-resolved laser induced fluorescence diagnostics. It was found that the plasma ion “temperature” in the vicinity of the HA output grid increases up to ∼15 eV during the accelerating pulse, which is consistent with a model of the potential screening of the grid by the randomly moving ions [Phys. Plasmas 13, 073506 (2006)]. Also it was shown that the increase in the HA plasma potential up to several kilovolts because of the appearance of a noncompensated ion charge in the HA bulk plasma due to electrons fast extraction, leads to explosive emission centers being generated at the HA grid and to nonuniformity in the cross-sectional electron beam current density. Finally, the plasma prefilled mode of diode operation was studied using a simple one-dimensional model of the plasma erosion and the HA plasma electron heating by energetic ions was considered.

Simultaneous two-dimensional laser-induced-fluorescence measurements of argon ions

Recent laser upgrades on the Hot Helicon Experiment at West Virginia University have enabled multiplexed simultaneous measurements of the ion velocity distribution function at a single location, expanding our capabilities in laser-induced fluorescence diagnostics. The laser output is split into two beams, each modulated with an optical chopper and injected perpendicular and parallel to the magnetic field. Light from the crossing point of the beams is transported to a narrow-band photomultiplier tube filtered at the fluorescence wavelength and monitored by two lock-in amplifiers, each referenced to one of the two chopper frequencies.

LIF diagnostics in volume and surface dielectric barrier discharges at atmospheric pressure

In this paper we review our recent work based on Laser Induced Fluorescence (LIF) diagnostics applied to Dielectric Barrier Discharges (DBD) at atmospheric pressures. Emphasis is given to the main issues that have to be faced in this application through the detailed description of the most difficult case, that of CH LIF detection. The application to kinetic studies of LIF diagnostics is then illustrated through the application of LIF to the CN radical and of Optical-Optical Double Resonance (OODR) LIF to the measurement of N2(A3Σ+u) metastable.