OH Fluorescence Intensity Research Articles

OH-PLIF study on the mechanism regulating flame-wall interaction with catalytically active CeO2-ZrO2 coatings

Technically, using catalytically active coatings regulate flame-wall interactions to enhance flame stability in small-scale burners, such as the common ZrO2-based ceramics with doping CeO2 which benefit to improve the high-temperature mobility and reactivity of lattice oxygen as an oxygen source to promote gas phase combustion. Probing by OH-PLIF laser diagnosis, the aim of this study was to investigate the dopant content effect (0–20 wt.% CeO2) on the flame characteristics of methane-air mixtures of varying equivalence ratio (0.9, 1.0, and 1.2) in an adjustable-gap narrow channel (2–12 mm) at a wall temperature range of 373 K to 973 K, including the dynamic flame morphology, flame quenching performance, and near-wall spatial distribution of OH radicals. Results show that the flame-wall distances are strongly correlated with wall temperature, equivalence ratio, and channel scale. After doping CeO2, the measured quenching distances can significantly drop, but do not show a linear downward trend with increasing CeO2 content. The case of 5%CeO2-ZrO2 coating demonstrates a small critical quenching Peclet number at high wall temperatures. The maximum OH fluorescence intensity in flame core areas increases with CeO2 doping into CeO2-ZrO2 at 973 K when the channel clearance is less than 4 mm. However, the absolute concentration of near-wall surviving OH radicals in the presence of 20%CeO2-ZrO2 coating is substantially lower than in the 5%CeO2-ZrO2 case due to differences in the redox properties. In situ diffuse reflectance infrared Fourier transformations spectroscopy tests further indicate that the adsorption quantities of C2H4 and C2H6 species on the coating surface also show a non-monotonous CeO2 dependence, as they are important intermediates in the low-temperature C2 chain reaction of methane.

Temperature measurement based on fluorescence intensity in hydroxyl tagging velocimetry (HTV)

Hydroxyl tagging velocimetry (HTV) is generally used for velocity measurement especially in the high-temperature and high-velocity flow field. In this work, according to the monotonous change of OH fluorescence intensity with temperature, the relative temperature distribution could be observed from the intensity distribution along the hydroxyl fluorescence line. We could obtain the temperature along the photodissociation line by calibrating at two positions. In this way, HTV has the potential for simultaneously achieving velocity and temperature without adding extra diagnostic systems.

A method to convert stand-alone OH fluorescence images into OH mole fraction

Due to the accessibility of the planar laser-induced fluorescence technique, images of OH fluorescence intensity are often used to study the structure of turbulent flames. However, there are differences between the measured OH fluorescence intensity and the actual OH mole fraction. These are often neglected because accurate conversion from fluorescence to mole fraction requires the combined knowledge of all major species mole fractions and temperature, which was rarely achieved in 2-D. Here, a new method to convert images of OH fluorescence intensity into OH mole fraction is proposed. This model relies only on inexpensive 1-D laminar flame calculations and does not require information on major species or temperature. The primary assumption behind the applicability of this model is the local approximation of multi-dimensional flames with 1-D counterflow flames. The method utilizes the fact that both OH mole fraction and OH fluorescence intensity profiles are self-similar with pressure and scalar dissipation rate. Only two empirical constants need to be calibrated using 1-D laminar flame calculations. The model was validated using computed 2-D axisymmetric laminar flames and 3-D turbulent flames computed with LES. The accuracy of the conversion model was estimated to about 8% (for Reynolds number up to Re =66,800), which includes errors due to the 3-D effects that are not included in this method relying on 2-D images. As a proof of concept, the conversion model was finally applied to one single-shot image of OH fluorescence intensity measured with OH-PLIF for syngas at P=4bar and Re =16,700, demonstrating potential applications of this new method. The method was tested for hydrogen, syngas and methane fuels but, for brevity, only syngas results are reported in detail.

Single-shot imaging of major species and OH mole fractions and temperature in non-premixed H2/N2 flames at elevated pressure

Simultaneous, single-shot imaging of all major species (N2, O2, H2, and H2O), OH, temperature, and mixture fraction is demonstrated for the first time in H2N2 non-premixed jet flames at 12 bar. The spatial distribution of mole fraction is obtained for the four major species by recording images of Raman scattering on four separate back-illuminated CCD cameras. The available field-of-view is 25(H) × 8(V) mm2. Temperature and mixture fraction are derived from Raman scattering images. Images of OH fluorescence intensity are recorded using OH-PLIF and are converted into OH mole fraction by calculating the Boltzmann population distribution and the quenching rate accurately for each pixel. Precision and accuracy are assessed by comparing 2-D Raman/OH-PLIF measurements in laminar flames to 1-D flame computations accounting for differential diffusion. Single-shot precision on N2, O2, H2, and temperature is better than 5%. It is better than 6% and 10% for H2O and OH, respectively. Accuracy lies within these values, except for H2O with 10%. Such good performance of Raman imaging is attributed to (a) the use of non-intensified CCD cameras, (b) wavelet adaptive thresholding (WATR) image denoising, (c) rejection of flame luminosity by a Pockels cell electro-optical shutter with 500 ns gating, and (d) elevated pressure that boosts the Raman signal intensity. Measurements in a turbulent flame (3.6N2:H2 by vol. and Re = 29,000) show that most of the flame's thermochemical structure is accurately captured by unity Lewis number computations, suggesting that effects of differential diffusion are less important above some Reynolds number. This is consistent with expectations and it engenders confidence in the Raman imaging technique. Because images of both mixture fraction and OH mole fraction are available, it is also possible to reconstruct a more accurate scalar dissipation rate by projection onto the 2-D flame front normal.

On the Structure of an Impact Jet with Flow Swirling and Combustion

The PIV and PLIF OH techniques were used to experimentally study the structure of a jet impinging on a flat obstacle with swirling and combustion of a propane–air mixture with an equivalence ratio of 0.7 for a nozzle–obstacle distance H/d = 1 and 3 and in the absence of the obstacle. The swirl ratio was 0.41 and 1.0, and the Reynolds number was 5 000. It is concluded that for both values of the swirl ratio, the presence of the impact surface leads to the formation of an extended central cone-shaped recirculation zones. For H/d = 3 and 2, the OH fluorescence intensity near the impact surface and inside the recirculation zone is significantly reduced. This effect may be caused by a decrease in the temperature of combustion products in the recirculation zone, including in the vicinity of the flame front.

A Numerical and Experimental Study on the Effects of CO2 on Laminar Diffusion Methane/Air Flames

AbstractFlue gas recirculation (FGR) is an effective method to reduce NOx emissions from the combustion of fuels, such as natural gas. Nitrogen, carbon dioxide, and water are the main components of flue gas. Nitrogen is an inert gas, and water can be condensed out of the effluent before FGR. However, recycled CO2 can alter the physical and chemical combustion characteristics of a fuel. This research investigated the effects of CO2 on CH4/air laminar diffusion flames, both experimentally and numerically. Experiments used laser-induced fluorescence to measure OH and CH distributions in the resulting flames, at different CO2 concentrations. Numerical methods were used to investigate the reaction mechanism and predict temperature and species concentration fields, as well as the NOx formation. Experiments showed that the CH fluorescence intensities decreased with the addition of CO2, while the OH fluorescence intensities increased. Both the directed relation graph method and the sensitivity analysis method were used to reduce the GRI-mech 3.0 mechanism. The chemical kinetics of methane combustion were analyzed using the reduced mechanism with the diffusion opposed-flow flame model in the chemkin 4.1 software package to determine the main reactions among the major species. Numerical simulations showed that as the amount of CO2 in the fuel increased, the concentration of CH decreased. These CFD simulations using the reduced mechanism were in agreement with the experimental data. Thus, the reduced mechanism was then used to predict NO concentrations. Numerical simulations showed that as the amount of CO2 in the fuel increased, the concentration of CH decreased, and, as a result, lower amounts of NO were predicted.

Autoignition-controlled flame initiation and flame stabilization in a reacting jet in crossflow

This paper describes an analysis of the mechanisms of autoignition-controlled flame initiation and flame stabilization in a nonpremixed jet in crossflows, using simultaneous high-speed (10 kHz) tomographic particle image velocimetry, OH-PLIF and line-of-sight flame emissions. Measurements are conducted on a turbulent, transverse, reacting propane jet issued into a crossflow generated by combustion of natural gas at an equivalence ratio of 0.4 with the crossflow velocity of 10 m/s, the crossflow temperature of 1350 K and the jet momentum flux ratio of 41. While several prior studies have analyzed the lifted character of the flame in similar configurations, we show that several dynamic processes precede the leading edge of the lifted diffusion flame, including formation and evolution of “autoignition kernels”, “flame kernels” and “flame fragments”. “Autoignition kernels”, i.e., discrete compact reaction zones with the peak hydroxyl (OH) fluorescence intensity below that of the diffusion flame, initiate preferably at bulges along the jet periphery where the strain rates and the scalar dissipation rates are lower. The autoignition kernel grows in both size and the OH-fluorescence intensity as it convects downstream. An autoignition kernel transitions into a propagating flame kernel, which quickly gets distorted and elongated in the direction of the principal expansion strain rate to form a flame fragment. Neighboring flame fragments merge with each other and with the downstream diffusion flame via edge-flame propagation. Merging of upstream flame fragments with the downstream diffusion flame results in an upstream advancement of the diffusion-flame front. The diffusion flame front is intrinsically unsteady because of the rather random formation and evolution of autoignition kernels, flame kernels and flame fragments, presumably due to the stochastic velocity, the strain rate and mixture-fraction oscillations.

Turbulent non-premixed ethanol–air flame experimental study using laser diagnostics

A turbulent non-premixed ethanol spray flame is characterized through experiments using planar laser-induced fluorescence (PLIF) of the OH radical and Mie scattering of fuel droplets distribution. The spray burner generates a stable flame with the use of a bluff body. The OH fluorescence intensity demarcates the reaction zone. Initially, different water spray loadings are compared, allowing determining the spray characteristics. Then, two combustion situations are studied emphasizing the instantaneous distribution of the droplet cloud with respect to the reaction zone. The instantaneous and average OH images allow identifying local flame extinction. In the first situation, the spray is surrounded by flame, and complete droplet evaporation occurs before combustion, which represents a classical behaviour. In the second situation, droplets interact strongly with the combustion process leading to an unusually large extinction.

OH-PLIF investigation of wall effects on the flame quenching in a slit burner

To examine the effects of the wall on flame quenching, an OH-PLIF investigation of a premixed methane/air flame was conducted with a slit burner between two parallel walls. Three types of materials, i.e., stainless steel 304, silicon, and zirconia ceramics, were tested at wall temperatures of 300 and 600°C. The quenching process, captured by an intensified charge-coupled device (ICCD) camera, showed different critical distances for stable flames that became unstable and were quenched for the three materials at the same temperature. It is interesting to note that, at a higher wall temperature, the flame is lifted before finally quenching, while this did not happen at 300°C. By analyzing the maximum OH fluorescence intensity in the flame extracted from the OH image, we have not found a distinct relation between the maximum OH fluorescence intensity and the quenching distance. In some cases, the flame can sustain a very weak OH intensity. Conversely, we also obtained the OH fluorescence intensity close to wall and within 0.6mm from the surface. We found that the trend of the OH fluorescence intensity close to wall correlates with the quenching characteristics very well. At the same wall temperature, a greater OH intensity close to wall results in a shorter quenching distance and vice versa. The wall made of zirconia ceramics demonstrates the greatest OH intensity close to the wall and, thus, the shortest quenching distance, while of the quenching distance in the case of STS 304 demonstrates the opposite trend. Additionally, we also calculated the non-dimensional chemical action of the surface based on the OH-PLIF data, which can demonstrate the differences in the chemical quenching characteristics for the three types of materials.

The influence of oxidizer content in the solid propellant on the combustion condition in the flame

We have reported on the detection of the axial distribution at the height of the flame of OH radials in an alcohol burner, in premixed methane/oxygen, and in solid propellant flames using laser-induced fluorescence (LIF) spectroscopy. In this experiment, the influence of substances with different oxidizer contents on the OH fluorescence intensity was investigated. The results indicated that the oxidizer content exerted a strong influence on the combustion condition in the flame.

Active control of jet premixed flames in a model combustor with manipulation of large-scale vortical structures and mixing

Active control of a bluff-body stabilized methane/air jet flame issued from a coaxial nozzle is made by using miniature magnetic flap actuators attached to the outer nozzle. CH radical chemiluminescence, and CO and NO x emissions are measured to assess the flame characteristics. By changing the flapping Strouhal number, the flame stability and CO emission are drastically improved under different equivalence ratio ( ϕ) conditions. At ϕ = 0.72, the optimum Strouhal number for stable combustion is unity, since methane/air mixing is enhanced by large-scale vortices synchronized with the flap motion. On the other hand, at a lower equivalence ratio of ϕ = 0.48, the optimum Strouhal number is much larger than unity; with small-scale vortices, the premixed combustion is stabilized by stratified mixing. In addition to acetone and OH-PLIF, a new two-line OH-PLIF is employed for flame temperature measurement. The longitudinal flame temperature distribution is obtained from conditional-averaged OH fluorescence intensities at the OH front taken with two different excitation lines. CO and NO x emission characteristics of the controlled flame are discussed on the basis of the local fuel and OH distributions and the flame temperature.

Universal calibration of quantitative OH-LIPF measurements in hydrocarbon flames at elevated pressures

Quantitative measurements of radical concentrations are important in studies of the structure of flames. In this research, a quantitative analysis was performed using laser-induced predissociative fluorescence of OH radicals (OH-LIPF) in high-pressure (1–5 atm) premixed methane-air and propane-air flat flames (ϕ = 0.7–1.3). OH was excited (A2Σ+, ν′ = 3 − X2Π, ν″=0, and P28) using a KrF excimer laser and (3,2) band fluorescence was observed. OH fluorescence intensities were calibrated against the OH concentrations calculated from flame simulation results in the postflame zone using the CHEMKIN premix flame code in conjunction with the GRI-Mech 3.0 reaction mechanism. The accuracy and spatial resolution of temperature measurements are important factors for the correctness of the corresponding flame simulations, especially in the reaction zone near the burner surface. In this work, a carefully constructed thermocouple (R-type, 50 µm) positioning system was used to identify the temperatures above the burner surface. With careful evaluations of quenching rates, Voigt profiles, and normalization against room-air N2 Raman scattering intensity, a universal calibration constant [CT = (1.076 ± 0.174) · 1016 molecules/cm3] was determined. The OH concentrations obtained by flame simulations showed good agreement with the quantitative OH-LIPF measurements in all methane-air flames and fuel lean (ϕ = 0.7−0.8) propane-air flames. However, a 2-fold to 5-fold discrepancy was obtained in propane flames at ϕ > 0.9. This may be caused by the lack of C3 reaction paths in the GRI mechanism and/or the inaccuracy of the thermochemical data for large molecules.

Sight-Premixing Effects on Oxidation/Formation of Polycyclic Aromatic Hydrocarbon in Counterflow Flames

This paper considers experimentally the effect of the oxygen addition to the fuel flow given to the oxidation/ formation characteristics of polycyclic aromatic hydrocarbon (PAH) in a methane-air, counterflow, nonpremixed (diffusion) flame. The amount of oxygen added in the fuel flow is 0.0-15.0% of range in a volumetric flow rate, and a corresponding equivalence ratio is 10 or more. A quartz microsampling probe was inserted into the flame to extract the test gas, and the local amount of lighter hydrocarbons including three or four carbons (C3 and C4, respectively), and PAH was investigated using a gas chromatograph-mass spectrometer. Distribution of temperature, the emissive power (near 650 nm) of a luminous flame, and OH fluorescence intensity were separately measured. On the conditions used in this experiment, it turned out that soot yield and detected PAH mole fraction decreased with the increase in the added amount of oxygen. The depletion degree became weaker gradually and their inhibition by an oxygen addition was reaching the ceiling. Additionally, the peak in the profile of the mole fraction of PAH approached the elevated-temperature zone with an increase in the added amount of oxygen. These results suggest that the promotion of PAH formation is somehow associated with the increase in temperature. The reason for such leveling off is considered as follows. Oxidation of the benzene in the fuel flow is promoted by the increase in the added amount of oxygen, and the components of C3 and C4 are produced. If these arrive at the elevated-temperature zone, oppositely, benzene is produced via C3 and C4. Consequently, a difference will be lost in the total amount of PAH generated.

Temperature Measurement with a Conditional Two-line OH-PLIF Technique in an Actively Controlled Coaxial Jet Flame

A novel flame temperature measurement technique based on two-line OH planar laser-induced fluorescence (PLIF) is presented. In order to measure temperature of an unsteady flame using a single excitation laser and a single ICCD camera, a new conditional image processing method has been introduced. In this method, ensemble-averaged OH fluorescence intensities at the flame front, which are taken by two excitation lines, are used, and the mean temperature at each streamwise position is calculated. The present thermometry is applied to a methane/air coaxial jet flame, which is actively controlled by periodically driven magnetic flap actuators. The uncertainty interval at 95% coverage is estimated as +130 K and -120 K at the mean flame temperature of 2000 K. The measured mean temperatures of controlled flames coincide with the theoretical adiabatic flame temperatures within the uncertainty intervals. When the stoichiometric mixture is supplied, the mean flame temperature is the highest (-2200K), and thus CO emission is drastically reduced. On the other hand, when a lean flammable limit mixture is supplied, the mean temperature is decreased to 1 600 K, and this results in the increased CO emission.

5009 アセトンOH同時PLIF法による希薄予混合火炎での局所消炎構造の可視化(S50-2 乱流燃焼の実験と数値シミュレーション(2),S50 乱流燃焼の実験と数値シミュレーション)

This paper describes the applicability of acetone-OH simultaneous PLIF approach (scheme) to the diagnostics of the near-extinction flame behavior. This scheme is recently developed by authors and showed qualitative and quantitative accuracy for the laminar flames. By means of one laser and one detector combination (i.e. the simplest PLIF set), this approach gives us three different quantities of the local flamelet ; such as flame curvature, flame thickness, relative OH fluorescence intensity in the laser sheet plane. Successive imaging of locally-quenched features in 2-D plane is achieved and frequent extinction is observed at convex toward burned side. Local curvature-OH correlation shows positive indicating the qualitative consistency with the previous works done by Chen & Bilger (2002) and Thevenin (2005). This fact ensures that the present scheme could have a potential to work on extinction flame imaging and could be powerful tools for controlling the actual combustor.

Experimental and numerical analyses of magnetic effect on OH radical distribution in a hydrogen-oxygen diffusion flame

The effect of magnetic field on OH radical distribution in a hydrogen-oxygen diffusion flame was experimentally and numerically investigated to explore the possibility of combustion control by magnetic force. In experiments, a coaxial type of burner was set between the magnet pieces. Two-dimensional (2D) cross-section distributions of OH* chemiluminescence intensity and OH fluorescence intensity were obtained with spectroscopic techniques using a CCD camera and a Planar Laser-Induced Fluorescence (PLIF) system, respectively. It was clearly seen that the high-density regions of OH* and OH radicals axisymmetrically migrated toward the central axis of the flame due to the influence of the magnetic field. In numerical simulations, such a phenomenon was qualitatively reproduced by solving the equations of reactive gas dynamics and magnetism. As a result, it was found that the magnetic force does not directly and selectively induce the diffusion velocity (the relative velocity) of OH itself. Alternatively, the magnetic force acting on O 2, whose mass density and magnetic susceptibility are much larger than those of other chemical species, causes the change in the mean velocity (the mass-average velocity) of mixture gas to transport the OH radical distribution indirectly and passively.

Dependence of magnetically induced change in oh distribution in a methane-air premixed flame on equivalence ratio

In order to explore the possibility of combustion control by magnetic field, the magnetically induced change in the spatial distribution of OH radicals in a methane-air premixed flame was visualized via planar laser-induced fluorescence measurements. Then equivalence ratio was changed from 0.89 to 1.8 under a constant total flow rate. The results clearly showed that the change in OH fluorescence intensity distribution depended on the equivalence ratio. At equivalence ratios smaller than 1.0 (fuel-lean conditions), the OH concentration was decreased in the downstream region of the flame by the magnetic field, while it was increased in the upstream region. At ratios greater than 1.0 (fuel-rich conditions), the decrease region existed in the peripheral region of the flame, while the increase region was inside the flame. The change rate of the OH distribution tended to become larger as the equivalence ratio became larger. The mechanisms of these phenomena were discussed in conjunction with O 2 flows induced magnetically.

Measurement of Magnetic Effect on OH Distribution in a Hydrogen-Oxygen Inverse Diffusion Flame by Laser-Induced Fluorescence (LIF).

Effects of the gradient magnetic field on the OH radical distribution in a hydrogen-oxygen inverse diffusion flame were investigated by using the laser-induced fluorometry. Since the mass density and magnetic susceptibility of O2 gas are much larger than those of other chemical species, the O2 gas existing both inside and outside the flame is strongly affected by the magnetic force. The results of the laser-induced fluorescence (LIF) measurements clearly demonstrated as two-dimensional images the slight change in the OH fluorescence intensity distribution in the flame. Particularly inside the flame, the influence of the magnetic force acting on the O2 gas provided from the burner was detected by the LIF measurements.

Lewis number effects on premixed flames interacting with turbulent Kármán vortex streets

The effects of Lewis number on the global and local structure of premixed flames interacting with turbulent Kármán vortex streets are experimentally investigated using OH planar-laser-induced fluorescence (PLIF). The OH PLIF results show that over the range of Lewis numbers studied, i.e., Le = 0.21, 0.94 and 1.79, the flame area increases and the flame front is oriented more randomly as Lewis number decreases, while the flame curvature pdfs are unchanged. The relationship between the local flame structure and the local flame curvature is found to be consistent with the results of stretched laminar flame theory. The correlation between the local maximum OH fluorescence intensity and the local curvature tends to level off for large positive curvature ( H > 0.5 mm −1) as U θ S L increases, indicating that the response of the flame to large flame stretch may be non-linear at high U θ S L . The pdfs of peak OH LIF intensity suggest that the mean burning rate of the H 2/He/air flame at U θ S L = 3.3 is increased approximately by 10% in comparison to the undisturbed laminar flame. The present results imply that even though the local flame curvature may strongly influence the local structure and burning rate of nonunity Lewis number flames through the effect of flame stretch on the local burning rate, these variations tend to cancel in the mean due to the linear relationship between local burning rate and curvature for the most probable values of curvature (−0.5 mm −1 < H < 0.5 mm −1) and due to the symmetry and zero mean of the curvature distribution. Therefore, the main effect of turbulence and Lewis number is to wrinkle the flame and produce flame area, while increasing the mean burning rate per unit surface area by relatively small amount through flow strain effects.

OH fluorescence images of the quenching of a premixed flame during an interaction with a vortex

Local quenching of a premixed laminar flame has been imaged using OH planar laser induced fluorescence as the flame interacts with the well-defined strain field of a single laminar vortex. This flame-vortex interaction represents a fundamental “building block” of premixed turbulent combustion. Since the OH concentrations reach a maximum in the flame-front which is 10 times larger than the value in the burnt gas, the measured [OH] profiles indicate the instantaneous shape of the reaction zone. Local quenching is confirmed by the disappearance of the reaction zone. Four different vortex strengths were studied; the weakest vortex causes flame wrinkling only; stronger vortices create pockets of unburned gas. Local quenching occurs at the leading edge of the vortex when the Karlovitz number (based upon the vortex diameter and rotational velocity) exceeds 0.17, which is significantly less than the value 1 predicted previously. Quenching is believed to result from a combination of flame stretch and radiative heat loss in the burnt gas. Effects of flame curvature are quantified; the highest OH fluorescence intensities occur when the flamefront is convex towards the reactants, as expected for these flames for which Lewis number