Laser-induced Fluorescence Thermometry Research Articles

Dual-tracer laser-induced fluorescence thermometry for understanding bubble growth during nucleate boiling on oriented surfaces

Nucleate boiling is perhaps one of the most efficient cooling methodologies due to its large heat flux with a relatively low superheat. Nucleate boiling often occurs on surfaces oriented at different angles; therefore, understanding the behavior of bubble growth on various surface orientations is of importance. Despite significant advancement, numerous questions remain regarding the fundamentals of bubble growth mechanisms on oriented surfaces, a major source of enhanced heat dissipation. This work aims to accurately measure three-dimensional (3D), space- and time-resolved, local liquid temperature distributions surrounding a growing bubble on oriented surfaces that quantify the heat transfer from the superheated liquid layer during bubble growth. The dual tracer laser-induced fluorescence thermometry technique combined with high-speed imaging captures transient 2D temperature distributions within a 0.3 ºC accuracy at a 30 μm resolution. The results show that the temperature close to the heated surface and bubble interface exhibits an acute transient behavior at the time of bubble departure, and the growing bubble works as a pump to remove heat from the surface with a temperature difference of up to 10 °C during its growth and departure. The experimental results are compared with data available in the literature to validate the accuracy of the technique. It was found that the heat transfer coefficient close to the bubble interface and heater is approximately 1.3 times higher than the heat transfer coefficient in the bulk liquid.

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

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

Heat transfer near a growing bubble during nucleate boiling using dual-tracer laser-induced fluorescence thermometry

Boiling heat transfer associated with bubble growth is perhaps one of the most efficient cooling methodologies due to its large latent heat during phase change. Despite the significant advancements, numerous questions remain regarding the fundamentals of bubble growth mechanisms, which is a major source of enhanced heat dissipation. This work aims to accurately measure three-dimensional (3D), space and time-resolved, local liquid temperature distributions surrounding a growing bubble to quantify the heat transfer in the superheated liquid layer during bubble growth. The dual tracer laser-induced fluorescence thermometry technique combined with high-speed imaging captures transient 2D temperature distributions, that will render 3D temperature distributions by combining multiple 2D layers, within a 0.3 °C accuracy at a 30 μm resolution. Two fluorescent dyes, fluorescein and sulforhodamine B, were used to measure transient temperatures, by account of their temperature-sensitive emissions. The results show that the temperature close to the heated surface and bubble interface exhibits an acute transient behavior at the time of bubble departure. The growing bubble works as a pump to remove heat from the surface with a peak temperature difference of up to 10 °C during its growth and departure. The experimental results were compared with previously reported studies to validate the accuracy of the technique. It was found that the heat transfer coefficient close to the bubble interface and heater is approximately 1.3 times higher than the heat transfer coefficient in the bulk liquid.

Multi-line OH-LIF for gas-phase temperature and concentration imaging in the SpraySyn burner

Gas-phase temperature and OH concentration images are measured in the SpraySyn nanoparticle-synthesis standard burner using multi-line OH laser-induced fluorescence (LIF) thermometry. In this burner, a spray flame fed by a combustible nanoparticle precursor solution is stabilized by a surrounding axisymmetric lean premixed laminar methane/oxygen pilot flame. For measuring OH rotational temperature, the laser is scanned across the 282.05–282.17 nm range and LIF excitation spectra are fitted for each image pixel by simulated spectra. Temperature maps are determined for the pilot flame and the ethanol spray flame with variable dispersion gas flow rates of oxygen used in the two-fluid nozzle. The addition of 2-EHA (2-ethylhexanoic acid) to the solution was found to increase the flame temperature. Laser attenuation across the flame was measured using simultaneous recordings of the fluorescence of uranine/water solution from two dye cuvettes illuminated with the laser sheet before and after the flame and the effect of attenuation was quantified. Absolute OH concentration distributions are determined by combining the LIF images, laser attenuation measurements, and derived temperature distributions. The temporal stability of the flame is investigated for various operating conditions by statistical evaluation of recorded instantaneous OH-LIF images.

Characterizing the SpraySyn burners with MMC-LES

This paper presents a detailed validation of a modeling framework and its implementation for the simulation of flame spray pyrolysis (FSP) using different nozzle geometries of the so-called SpraySyn burner. Gas, liquid and particulate dynamics are compared against experimental data from literature as well as our own laser-Doppler anemometry, phase-Doppler anemometry and multi-line OH laser-induced fluorescence thermometry imaging measurements. The modeling framework consists of large eddy simulations (LES) coupled to the sparse-Lagrangian multiple mapping conditioning (MMC) model, a Lagrangian spray solver and a sectional description of the population balance equation. Simulations start downstream of the burner exit planes where turbulent inlet data for the gas and liquid phases are generated by independent LES that use a 1-Fluid method to capture the dynamics of the liquid jet break-up. The gas and liquid dynamics are validated and analyzed for an ethanol spray flame in the SpraySyn1 and SpraySyn2 burners. In these burners iron(III) oxide particulates are synthesized from iron pentacarbonyl (IPC)-ethanol solutions with varying IPC concentrations. The results demonstrate the very good predictive capabilities of the MMC modeling framework for the gas and liquid phases. The predictions of the particulate formation are validated by comparison of elastic light scattering (ELS) signals from experiments against synthetic ELS signals calculated from the simulations. Results are of reasonable accuracy for the SpraySyn1 FSP series but indicate an imbalance between particulate growth and dilution with the surrounding gas. Predictions for the SpraySyn2 FSP series are consistent with SpraySyn1 results and indicate an increased product particulate size due to an increased residence time downstream of the heat release zone.

Spectral characterization of fluorescein species in aqueous solution for laser-induced fluorescence thermometry

Spectral characterization of fluorescein species in aqueous solution for laser-induced fluorescence thermometry

Two-dimensional laser-induced fluorescence thermometry in a medium- to high-enthalpy CO2 flow

Ground-based experimental facilities are needed to qualify materials subjected to aerothermodynamic loads during an entry into the Martian atmosphere. In the present work, the focus is on quantifying the temperature field in a medium- to high-enthalpy CO_{2} flow (from 4.5 to {10.5}, hbox {MJ}/hbox {kg}) of a plasma wind tunnel using one- and two-dimensional two-photon absorption laser-induced fluorescence (CO-TALIF) thermometry at a CO number density in the range of 1–2 times 10^{17} cm^{-3}. However, the measurement method developed here can also be used to study satellite propulsion systems. Due to absorption by air, a two-photon excitation scheme of the CO molecule at 115 nm (VUV) was used. Experimental and simulated excitation spectra are compared for temperature determination. For spectrum simulation in the B^1varSigma ^+leftarrow X^1varSigma ^+ system, an existing program was extended and a polynomial regression correlation was developed for signals with low signal-to-noise ratios. After the first one-dimensional temperature measurements using CO-TALIF in the free-stream to quantify the radial temperature profile for three conditions in the medium- to high- enthalpy range, respectively, two-dimensional measurements in a steady state test chamber at room temperature were carried out. For the two-dimensional investigations, a light sheet optic was designed to generate a light sheet free of divergence in width and thickness. The results of the one- and two-dimensional temperature measurements performed at medium- to high-enthalpy conditions provided plausible results concerning both the values and the course in radial and axial direction, the latter being, to the authors’ knowledge, the first two-dimensional temperature measurements under such conditions using CO-TALIF.

Lift-off region temperature field and planar flow field of a twin-nozzle reacting jet in hot crossflow

Secondary fuel injection affects the combustion organization and flame dynamic in axial fuel staging gas turbines. Here, we experimentally investigate a front-fuel rear-air twin-nozzle configuration in which the fuel and air can be independently injected to stabilize a lifted flame front, control the jet trajectory and tune the combustion mode in a very flexible way. A high repetition two-line formaldehyde planar laser induced fluorescence thermometry technique is used to measure the lift-off preheat zone temperature field of the flame with different amount of secondary air injection. The combustion flow field and the flame front dynamics are investigated using particle image velocimetry and CH* chemiluminescence. When the air to fuel ratio increases from 0 to 3, the preheat zone temperature, which is affected by the local equivalence ratio and injection air cooling, increases from 1300 K to 1600 K and then slightly declines. The lift-off region temperature and flow field dynamics are related to the variation of lift-off height, heat release rate and stability of the reacting jet in hot crossflow.

Experimental Investigation of Flame-Film Cooling Interactions With an Academic Test Rig and Optical Laser Diagnostics

Abstract The development of lighter low-carbon technologies in the aviation sector raises a technological challenge regarding material durability and wall thermal management. This study aims to introduce a new optically accessible test rig to investigate the interaction between a premixed CH4/air turbulent flame and a parietal cooling air film for various blowing ratios. Laser Doppler anemometry (LDA) is implemented to measure the cooling air and the mainstream inflow conditions. The impact of the blowing ratio on the mean flame topology is conducted with OH* chemiluminescence. Results show that the flame-wall distance increases with the blowing ratio and eventually reach a plateau, while the flame length constantly decreases. OH planar laser-induced fluorescence (OH-PLIF) and phosphor thermometry (PT) are performed to assess the near-wall flame structure and the cooling effectiveness. The flame seems to be aerodynamically controlled while the cooled-air momentum has a beneficial impact on the wall temperature.

Liquid-phase temperature in the SpraySyn flame measured by two-color laser-induced fluorescence thermometry and simulated by LES

In the SpraySyn burner (Schneider et al., 2019) a solution of metal salts in a combustible liquid is atomized in a two-fluid nozzle with molecular oxygen as dispersion gas to generate oxide nanoparticles via combustion synthesis. The flame is anchored by a methane (CH4)/oxygen (O2) premixed flat flame stabilized on a porous plate surrounding the spray nozzle. This burner features a standardized design with simple geometry, which provides well-defined boundary conditions for the simulation to support model development and the interaction between experiment and simulation. In this work, two-color laser-induced fluorescence (2cLIF) thermometry based on the fluorescence tracer coumarin 152 dissolved in ethanol is applied to provide temporally averaged but spatially resolved liquid-phase temperature maps. The operating conditions for the spray formation were modified by varying the injection rate and the dispersion gas flow. Liquid-phase temperatures were predicted by large-eddy simulation (LES), where the spray is modeled by a Eulerian-Lagrangian approach based on measured liquid droplet properties (initial liquid-phase temperature, droplet size, and velocity distribution). Droplet size and velocity were measured by phase-Doppler anemometry (PDA). Gas-phase turbulent combustion is modeled with the flamelet-generated manifold (FGM) approach. Droplet temperature distributions were provided by the simulation to assess the systematic measurement error caused by LIF-signal averaging. Experimental and simulated average liquid-phase temperature maps are compared. The simulation results were found to be in good agreement with the experiment by showing a suitable prediction of the droplet heat-up process within the experimentally probed spray region.

Burst-mode 1-methylnaphthalene laser-induced fluorescence: extended calibration and measurement of temperature and fuel partial density in a rapid compression machine

In this work, tracer-based laser-induced fluorescence (LIF) with the tracer 1-methylnaphthalene is utilized to study temperature and fuel courses in a rapid compression machine (RCM) under high temperature and pressure conditions. A burst-mode Nd:YAG laser at 266 nm is applied for excitation of tracer fluorescence at a frame rate of 7.5 kHz. A high-speed intensified CMOS camera equipped with an image doubler is used for 2-color LIF (2c-LIF) thermometry. With known local temperature, the fuel partial density can be determined using the signal of the channel covering the complete LIF spectrum. Both temperature and fuel partial density are determined during the compression and expansion strokes in nitrogen and air atmospheres. For this purpose, first-time 1-MN LIF calibration measurements in air atmosphere were performed for cylinder pressures up to 2.8 MPa. This significantly extends the calibration data base generated in current calibration cells. Although the LIF signal dropped significantly due to oxygen quenching, first promising measurements of temperature and fuel partial density were conducted in the RCM at relevant equivalence ratios. The influence of the RCM driving gas pressure on the temperature course is shown for cylinder pressures up to 7.4 MPa in nitrogen atmosphere. Although the temperature and concentration fields are very homogeneous at early points in time during compression, inhomogeneities in terms of millimeter-sized hot and cold gas regions were resolved especially near top dead center (TDC) using the present approach. These structures were also visible in the fuel partial density field. These inhomogeneities are due to the heat transfer between the hot gas and the cool walls and are probably also induced by the piston movement. Especially at TDC, the minimum gas temperature is about 300 K lower than the peak temperature in the wall region of the cylinder head. These cool region temperatures are much lower than in piston engines and other RCMs reported in the literature at comparable conditions, which may due to the special design of the present layout of the machine.

Absolute OH density measurements in a CO2–H2O glow discharge by laser-induced fluorescence spectroscopy

Time-resolved absolute OH density measurements in a millisecond-pulsed DC glow discharge from carbon dioxide with water admixture, by laser-induced fluorescence (LIF) spectroscopy, are presented. For this purpose, a novel technique is suggested to calibrate the OH LIF setup by means of two-photon absorption LIF spectroscopy measurements on a CO-filled gas cell. OH densities in the order of 1 × 1018 m−3 are reported at a pressure of 6.67 mbar with a water admixture of 20% and a discharge current of 50 mA. Furthermore, the time evolution of the CO2 conversion is determined from collision energy transfer LIF and validated against the literature. The time-dependent rotational temperature of OH is obtained with LIF thermometry.

Structure and scalar correlation of ammonia/air turbulent premixed flames in the distributed reaction zone regime

Instantaneous structures of turbulent premixed ammonia/air flames on a piloted jet burner were investigated using simultaneous planar laser-induced fluorescence (PLIF) and Rayleigh thermometry measurements. Two-dimensional spatial distributions of temperature were simultaneously measured with those of the NH radical and the NO pollutant, respectively. Experiments were conducted for stoichiometric flames under five jet velocities. All flames are located in the distributed reaction zone regime of the Borghi-Peters diagram with the Karlovitz (Ka) number ranging from 274 to 4720. The NH PLIF images are used to characterize the fuel consumption layer of the reaction zones since the formation of NH is associated with the consumption of ammonia. The NH PLIF results show that under all flame conditions investigated, the NH layer remains thin in the proximity of the burner while it becomes progressively thickened and distorted by turbulence with increasing turbulent intensity and axial distance. For flames with Ka of 274, the NH layer essentially remains thin, while at Ka of 590 or higher, significant broadening of the NH layer is observed. Probability density functions (PDFs) of the NH layer thickness show that the NH layer can be broadened by 3 – 4 times as the flames are developed downstream. The broadening of the NH layer is considered to indicate that the flames are in the distributed reaction zone regime. The boundary between the thin-reaction zone regime and the distributed reaction zone regime occurs at a much larger Ka than that in methane/air flames. The broadening of the NH layer is due to the penetration of the turbulent eddies and the merging of flame branches. The latter occurs mainly near the flame tips. NO is shown to exist in a wide region in space, across the preheat zone, reaction zone, and postflame zone. NO formation occurs mainly in the reaction zone, however, it is transported by turbulence eddies to the preheat zone and by convection to the postflame zone. The temperature measurements indicate that the preheat zone is broadened in all flames investigated. The broadening of the preheat zone is moderately sensitive to the Ka number while it is more sensitive to the integral length scale of the flames.

Characterization of a Newly Designed Test Bench for Investigations of Flame–Wall Interaction

Abstract For the thermal design of combustion chambers and turbine blades in jet engines, a detailed knowledge of the combustion and of the heat loads to the walls is necessary. In general, high operating temperatures and reduced combustor size are striven for in order to increase engine efficiency and reduce weight. Consequently, the components are exposed to temperatures above the melting point of the materials and there is a growing risk of incomplete combustion within the combustion chambers. To study these effects, we setup a new test bench for fundamental investigation of chemical near-wall reactions at atmospheric pressure. First results of gaseous, nonpremixed near-wall CH4/air and H2/air flames are presented. Optical methods such as two-line laser-induced fluorescence thermometry and OH* chemiluminescence were applied. Further, the heat release to the wall was determined by means of inverse heat conduction calculation using the data of implemented thermocouples.

Characterization of tracers for two-color laser-induced fluorescence thermometry of liquid-phase temperature in ethanol, 2-ethylhexanoic-acid/ethanol mixtures, 1-butanol, and o-xylene.

The fluorescence spectra of dye solutions change their spectral signature with temperature. This effect is frequently used for temperature imaging in liquids and sprays based on two-color laser-induced fluorescence (2cLIF) measurements by simultaneously detecting the fluorescence intensity in two separate wavelength channels resulting in a temperature-sensitive ratio. In this work, we recorded temperature-dependent absorption and fluorescence spectra of solutions of five laser dyes (coumarin 152, coumarin 153, rhodamine B, pyrromethene 597, and DCM) dissolved in ethanol, a 35/65 vol.% mixture of ethanol/2-ethylhexanoic acid, ethanol/hexamethylsiloxane, o-xylene, and 1-butanol to investigate their potential as temperature tracers in evaporating and burning sprays. The dissolved tracers were excited at either 266, 355, and 532 nm (depending on the tracer) for temperatures between 296 and 393 K (depending on the solvent) and for concentrations ranging between 0.1 and 10 mg/l. Absorption and fluorescence spectra of the tracers were investigated for their temperature dependence, the magnitude of signal re-absorption, the impact of different solvents, and varying two-component solvent compositions. Based on the measured fluorescence spectra, the tracers were analyzed for their 2cLIF temperature sensitivity in the respective solvents. Coumarin 152 showed for single-component solvents the overall best spectroscopic properties for our specific measurement situation related to temperature imaging measurements in spray-flame synthesis of nanoparticles as demonstrated previously in ethanol spray flames [Exp. Fluids61, 77 (2020)10.1007/s00348-020-2909-9].

10 kHz two-color OH PLIF thermometry using a single burst-mode OPO.

10-kHz hydroxyl radical (OH) two-color planar laser-induced fluorescence (TC-PLIF) thermometry was demonstrated with a single burst-mode optical parametric oscillator (OPO) and a single camera. A fast, dual-wavelength switched seed laser enabled a high-energy, high-repetition-rate burst-mode laser to generate two 10-kHz pulse trains at wavelengths of ${\sim}{354.8}\;{\rm nm}$. The two pulse trains are colinear with 3 µs time interval between the pulse pairs. The injection-seeded OPO efficiently converts the burst-mode laser output to 285.62 and 285.67 nm to excite the ${Q}_2({12})$ and ${P}_1({8})$ OH transitions. PLIF images were collected from each of the two excitation transitions, and intensity ratios from the images were used to determine local temperatures. The development of fast, dual-wavelength switching, burst-mode OPO technology significantly reduces the experimental complexity of the high-speed TC-PLIF thermometry and simplifies its implementation in harsh combustion and flow test facilities.

Laser-induced fluorescence thermometry of supercritical CO2 flows inside a micro-channel

This work demonstrates a thermometric technique using laser-induced fluorescence (LIF) in supercritical carbon dioxide flows in a micro-channel. Rhodamine 6G was used as a temperature-sensitive fluorescent dye. The flow conditions were at a pressure of 7.9 MPa and temperature in the range of 23°-90°C. 2D spatial distributions and time-resolved temperature profiles were obtained at this high pressure. Measured LIF signals showed close relations to the temperatures obtained from resistance temperature detectors.

Effect of High Pressures on the Formation of Nitric Oxide in Lean, Premixed Flames

Abstract Increasingly stringent regulations are imposed on nitrogen oxides emissions due to their numerous negative impacts on human health and the environment. Accurate, experimentally validated thermochemical models are required for the development of the next generation of combustors. This paper presents a series of experiments performed in lean, premixed, laminar, jet-wall stagnation flames at pressures of 2, 4, 8, and 16 atm. To target postflame temperatures relevant to gas turbine engines, the stoichiometry of the nonpreheated methane–air mixture is adjusted to an equivalence ratio of 0.7. One-dimensional (1D) profiles of temperature and NO mole fraction are measured via laser-induced fluorescence (LIF) thermometry and NO-LIF, respectively, to complement previously published flame speed data (Versailles et al., 2018, “Measurements of the Reactivity of Premixed, Stagnation, Methane-Air Flames at Gas Turbine Relevant Pressures,” ASME. J. Eng. Gas Turbines Power, 141(1), p. 011027). The results reveal that, as the pressure increases, the maximum postflame temperature stays relatively stable, and the concentration of NO produced through the flame front remains constant within uncertainty. Seven thermochemical models, selected for their widespread usage or recent date of publication, are validated against the experimental data. While all mechanisms accurately predict the postflame temperature, thanks to consistent thermodynamic parameters, important disagreements are observed in the NO concentration profiles, which highlights the need to carefully select the models used as design tools. The lack of pressure dependence of NO formation that many models fail to capture is numerically investigated via sensitivity and reaction path analyses applied to the solution of flame simulations. The termolecular reaction H+O2(+M)↔HO2(+M) is shown to hinder the production of atomic oxygen and to consume hydrogen radicals at higher pressures, which inhibits the formation of nitric oxide through the N2O pathway.

Planar laser-induced fluorescence thermometry in moderate-temperature flow using OH from photo-dissociation of water vapor

Since planar laser-induced fluorescence (PLIF) temperature measurements in a combustion-flow field can only be performed in regions in which OH is abundant (i.e., the reaction and high-temperature burnout zones), this paper investigates the feasibility of using photolysis of dissolved water to generate additional OH in the low-temperature and non-reaction zones of the flow field. Multi-line fluorescence showed that the generated OH had reached thermal equilibrium within 50 ns after dissociation with a fitting temperature of 300 K in room-temperature air. To further verify the technical accuracy, averaged PLIF images from dissociated OH were calibrated and compared with thermocouple measurements in a heated-flow field, and the average deviation in the 423–823 K temperature range was less than 40 K, with a relative system error better than 5%. The results of this paper confirm the feasibility and accuracy of the photo-fragment PLIF (PF-PLIF) temperature measurement in a low-temperature flow field. This makes it an effective complement to traditional PLIF measurements, with great significance for analysis and model validation of combustion processes in low temperature.

10 kHz 2D thermometry in turbulent reacting flows using two-color OH planar laser-induced fluorescence.

10 kHz two-color OH planar laser-induced fluorescence (PLIF) thermometry was demonstrated in both laminar Hencken flames and turbulent premixed jet flames using two injection-seeded optical parametric oscillators (OPOs) pumped by a high-speed three-legged burst-mode laser. The two burst-mode OPOs generate ∼5mJ/pulse at 282 nm and 286 nm to excite the Q1(5) and Q1(14) transitions of the A2Σ+←X2Π (1,0) system of OH, respectively. PLIF images were collected simultaneously from each of the two transitions and ratios of intensities from the two images were used to determine local temperatures. Analyses of flame curvature, temperature, and the correlation in time of these two quantities are also discussed. The results from this work are promising for the use of this technique in more complex flow environments and at, potentially, even higher repetition rate.