Simultaneous Planar Laser-induced Fluorescence Research Articles

Characterization of detonation waves by simultaneous OH and NO planar laser-induced fluorescence

This study presents a thorough evaluation of the simultaneous planar laser-induced fluorescence (PLIF) on hydroxyl (OH) radicals and nitric oxide (NO), for characterizing hydrogen detonation fronts. The study combines experimental and numerical results to evaluate the benefits and drawbacks of different visualization strategies, namely OH-PLIF alone, NO-PLIF alone, OH-PLIF + shock det. (comparable to simultaneous OH-PLIF and schlieren), and combined OH- and NO-PLIF. Main findings are: (i) For the first time, simultaneous single-shot OH- and NO-PLIF visualizations are demonstrated on detonation waves for three mixtures with varying stability levels: Mixture (a) 2H2-O2-3.76Ar, Mixture (b) 2H2-O2-3.76N2, and Mixture (c) 3H2-CH4-3.5O2-3.76Ar; (ii) the simultaneous visualization evidences the self-similarity of both techniques, with a clear interlock of OH-PLIF imaging into NO-PLIF imaging. This means that the structure of the OH-reaction zone, typically visualized from OH-PLIF, can be accurately described from NO-PLIF imaging alone, from which induction zones can also be visualized. In addition, both can clearly depict the wrinkling of the front and the presence of unburned pockets; (iii) the effectiveness of the four visualization strategies at characterizing the detonation wave of a H2-O2-3.76Ar mixture is determined using ZND and spectroscopic simulations. Interestingly, the induction zone (Δi) is more accurately predicted by NO-PLIF (3% error) than with the combined OH + shock det. (10% error). This study highlights the potential of NO-PLIF diagnostic in characterizing detonation waves with low levels of confinement (i.e., d/λ>>>1) both qualitatively (e.g., reaction zone structure) and quantitatively (e.g., Δi measurements) using a single diagnostic. Additionally, the simultaneous OH- and NO-PLIF diagnostic may offer significant advantages in characterizing the detonation waves in engine-relevant conditions (i.e., in complex geometries with unburned pockets or local quenching events), as combining both diagnostics enables to discriminate unburned pockets and local quenching events.

On the effects of walking speed, crowd density and human-to-source distance on pollutant dispersion in indoor spaces

The effects of walking speed, crowd density and human-to-source distance on pollutant dispersion in two scaled room models are investigated using simultaneous planar laser-induced fluorescence and particle-image velocimetry techniques. For a small 3 m high room, where the length-scales of the people and room are comparable, the walking motions significantly influenced the macro room mean flow patterns. This has a strong effect on the scalar dispersion properties as the magnitudes of the advective scalar fluxes are often comparable or larger than the turbulent scalar fluxes. As such, the scalar dispersion properties are case specific. For a large 9 m high room, the walking motion influenced only the local mean flow field. The increase in walking speed and crowd density improves the efficiency in which the scalar is transported and mixed with fresh ambient fluid out of the measurement plane (i.e. along the direction of the motion), leading to scalar-free zones observed on the opposite side of the room from the ventilation outlet. The area of the scalar-free zone increases with an increase in the walking speed and crowd density. The advective scalar fluxes are more sensitive to the human motion than the turbulent components, and as the mixing efficiency improves, the advective fluxes show a greater weakening with increased distance from source. The concentration PDFs in the near-source region can be described by the exponential function where the expected value at the 99% percentile can be derived as C99/crms′=4.61, which agreed well with the experimental measurements of 4.1 to 5.9.

Characterizing turbulent non-premixed flame structure and pollutant formation of cracked ammonia jet flames using simultaneous NH and NO PLIF

The combustion of cracked ammonia is crucial for enhancing flame stability and reducing pollutant emissions in ammonia-based combustion systems. This study investigated the flame structure and pollutant formation in cracked ammonia jet flames (CAJFs) to improve our understanding of the turbulence-chemistry interactions in ammonia combustion. A simultaneous NH and NO planar laser-induced fluorescence (PLIF) technique was employed to analyze the flame structure of turbulent non-premixed CAJFs, emulated using ammonia-hydrogen–nitrogen mixtures. Experimental measurements were conducted across a range of pressures (1–5 bar) and cracking ratios (7 %-28 %). The results revealed the significant influence of the cracking ratio on the chemical reactivity and NH-layer characteristics of ammonia-hydrogen flames. NH effectively marked the heat release layer of CAJFs. Reducing the cracking ratio from 28 % to 7 % resulted in increased local extinction-induced NH-layer fragmentation and reduced reaction area. The reaction layer thickness exhibited low sensitivity to the cracking ratio and pressure, while the broadening of the NH layer displayed a slowdown pattern with increasing height, differing from premixed flames. The pollutant NO rapidly formed within the reaction zone, persisting in the outer hot products until reduced by turbulent mixing. The fragmented flame structure promoted decreased NO formation and potentially lower NO concentration. A novel approach of multiplying NH and NO was proposed to reflect the formation characteristics of another important pollutant N2O. The effect of turbulent disturbances on N2O formation needs to be considered in turbulent CAJFs because N2O formation is strongly suppressed when turbulent transport reduces local NO concentration. This research enhances understanding of turbulence-chemistry interactions in cracked ammonia combustion, benefiting ammonia flame stability and pollutant emission control.

Simultaneous measurements of kinetic and scalar energy spectrum time evolution in the Richtmyer–Meshkov instability upon reshock

The Richtmyer–Meshkov instability (Richtmyer, Commun. Pure Appl. Maths, vol. 13, issue 2, 1960, pp. 297–319; Meshkov, Fluid Dyn., vol. 4, issue 5, 1972, pp. 101–104) of a twice-shocked gas interface is studied using both high spatial resolution single-shot (SS) and lower spatial resolution, time-resolved, high-speed (HS) simultaneous planar laser-induced fluorescence and particle image velocimetry in the Wisconsin Shock Tube Laboratory's vertical shock tube. The initial condition (IC) is a shear layer with broadband diffuse perturbations at the interface between a helium–acetone mixture and argon. This IC is accelerated by a shock of nominal strength Mach number $M = 1.75$ , and then accelerated again by the transmitted shock that reflects off the end wall of the tube. An ensemble of experiments is analysed after reshock while the interface mixing width grows linearly with time. The kinetic and scalar energy spectra and the terms of their evolution equation are calculated and compared between SS and HS experiments. The inertial range scaling of the scalar power spectrum is found to follow Gibson's relation (Gibson, Phys. Fluids, vol. 11, issue 11, 1968, pp. 2316–2327) as a function of Schmidt number when the effective turbulent Schmidt number is used in place of the material Schmidt number that controls equilibrium scaling. Further, the spatially integrated scalar flux follows similar behaviour observed for the kinetic energy in large eddy simulation studies by Zeng et al. (Phys. Fluids, vol. 30, issue 6, 2018, 064106) while the spatially varying scalar flux exhibits back scatter along the centre of the mixing layer and forward energy transfer in the spike and bubble regions.

Simultaneous planar laser-induced fluorescence of nitric oxide and hydroxyl radical with a single dye laser in hydrogen flames

Simultaneous, planar laser-induced fluorescence (PLIF) of nitric oxide (NO) and hydroxyl radical (OH) is evidenced with a single dye laser and two intensified CCD (ICCD) cameras. The technique is demonstrated on three premixed turbulent hydrogen-air flames, at atmospheric conditions, with several laser excitation wavelengths within the A2Σ+ − X2Π (0,0) and A2Σ+ − X2Π (1,0) vibrational bands of NO and OH, respectively. Via adjustments of the grating angle of the dye laser, the output wavelengths can be optimized so that both LIF signals are obtained simultaneously, or it can be tuned to reach the most common excitation wavelengths for OH-PLIF (i.e., near 283 or 284 nm) or NO-PLIF (i.e., 225 and 226 nm) alternatively. The flame visualization results, obtained with the different excitation strategies, are consistent for the three flame conditions and enable to characterize both the reaction zones and the nitric oxide formation. With appropriate selection of the wavelength pair, the technique shows its great potential to enhance the ability of existing lasers to simultaneously visualize and quantify distributions of two key species in reacting flows with only a single laser.

Simultaneous planar laser-induced fluorescence measurement of reactant NH3, radical NH, and pollutant NO in ammonia-hydrogen flames using a single dye laser

The stability, local extinction, and NOx production of ammonia-hydrogen (NH3-H2) flames are significantly impacted by turbulence-chemistry interactions. A powerful technique to study these interactions is planar laser-induced fluorescence (PLIF). However, multi-scalar PLIF typically requires complex and expensive systems, with multiple laser sources. This study proposes a novel PLIF technique using a single dye laser to perform simultaneous, single-shot imaging of the reactant NH3, radical NH, and pollutant NO in NH3-H2 flames. According to the excitation scans and the emission spectra of NH3, NH, and NO, three wavelength couples can be used to image the three species simultaneously via two-photon excitation of NH3C'−X (2,0), single-photon excitation of NH A3Π−X3Σ− (0,0) near 304 nm, and single-photon excitation of NO A2Σ+−X2Π (0,1) near 237 nm. Wavelengths near 237 nm and 304 nm are obtained simultaneously with only one dye laser by combining outputs of the frequency-doubling and frequency mixing units. NH3-, NH-, and NO-PLIF imaging performance is analyzed by quantifying the signal-to-noise ratios and detection limits. This technique is then used to visualize the structure of premixed and non-premixed flames over wide ranges of NH3 and H2 concentrations in the fuel blend. Results show that the premixed NH3-H2 flames have a compact structure, while the non-premixed flames exhibit a large gap between NH3- and NH-PLIF layers, where ammonia undergoes significant decomposition before reaching the reaction layer. The inner edge of the NO-PLIF layer overlaps well with the reaction layer represented by the NH-PLIF layer in all flames. The fully developed turbulent structure downstream of the NH3-H2 flame causes pinching-off of the premixed flame front and local extinction of the diffusion flame front.

Effect of CO[formula omitted] dilution on structures of premixed syngas/air flames in a gas turbine model combustor

The impact of CO2 dilution on combustion of syngas (a mixture of H2, CO, and CH4) was investigated in a lab-scale gas turbine model combustor at atmospheric pressure conditions. Two mild dilution levels of CO2, corresponding to 15% and 34% of CO2 mole fraction in the syngas/CO2 mixtures, were experimentally investigated to evaluate the effects of CO2 dilution on the flame structures and the emissions of CO and NOx. All experiments were performed at a constant Reynolds number (Re = 10000). High-speed flame luminescence, simultaneous planar laser-induced fluorescence (PLIF) measurements of the OH radicals and particle image velocimetry (PIV) were employed for qualitative and quantitative assessment of the resulting flame and flow structures. The main findings are: (a) the operability range of the syngas flames is significantly affected by the CO2 dilution, with both the lean blowoff (LBO) limit and the flashback limit shifting towards fuel-richer conditions as the CO2 dilution increases; (b) syngas flames exhibit flame-pocket structures with chemical reactions taking place in isolated pockets surrounded by non-reacting fuel/air mixture; (c) the inner recirculation zone tends to move closer to the burner axis at high CO2 dilution, and (d) the NOx emission becomes significantly lower with increasing CO2 dilution while the CO emission exhibits the opposite trend. The flame-pocket structure is more significant with increased CO2 dilution level. The low NOx emissions and high CO emissions are the results of the flame-pocket structures.

Accurate determination of homogeneous ignition of single solid fuel particles enabled by machine learning

This work presents an experimental investigation of solid fuel combustion in a laminar flow reactor with high-speed laser diagnostics. The main focus lays on the development of image evaluation approaches for accurate determination of single-particle ignition based on optical measurement data. Homogeneous ignition of individual bituminous coal particles is visualized by simultaneous planar laser-induced fluorescence of OH radicals (OH-LIF) and diffuse backlight-illumination (DBI) techniques at 10kHz. Two coal particle sizes of 90–125μm and 160–200μm are investigated in conventional air and oxy-fuel conditions with increasing oxygen concentrations. A comprehensive experimental data set is established, containing in total 1518 single-particle events and high-quality ground truth labels of ignition delay times. The previously proposed structure and signal (SAS) analysis is evaluated. Errors and uncertainties are evaluated in a sensitivity analysis with variation in the threshold selection, implying inherent restrictions of conventional image processing methods. An improved blob detection approach is applied, enabling feature detection at various scales and higher accuracy in predicting ignition delay times. Moreover, two deep learning architectures, namely residual network (ResNet) and feature pyramid network (FPN) are implemented. Both networks are capable of detecting ignition with significantly higher accuracy and precision than other algorithms in this study. Influences of training data and depth of networks on the prediction performance of a trained model are examined. The current study shows that the hierarchical feature extraction of the convolutions networks clearly facilitates data evaluation for high-speed optical measurements and could be transferred to other solid fuel experiments with similar boundary conditions.

Simultaneous visualization of instantaneous unburnt and preheating zones in turbulent premixed flames under transverse acoustic excitations

Instantaneous unburnt and preheating zones of bluff-body stabilized turbulent premixed flames under transverse acoustic excitations were investigated using simultaneous planar laser-induced fluorescence (PLIF) of acetone and CH2O, as well as multi-point hot-wire measurements. The PLIF images show that the unburnt zone marked by acetone images, the preheating zone marked by CH2O images, and the pixel-by-pixel product of acetone/CH2O have an increasing distribution area when slowly enlarging the sound pressure level (SPL). Wrinkled and bent edges of the unburnt and preheating zone can be seen at conditions away from the flame blow-off in the presence of the transverse acoustic excitations, and their sizes and areas increase as the flame blow-off is approached. At conditions near the flame blow-off with enlarging SPL to 123 dB, the flame turns from side to side over time and a large scale of the acetone/CH2O regions can be observed to deflect inside the center product zone, implying that the cold reactants can enter the product zone from the unburnt/preheating zones. The unburnt/preheating mainstream presents strong wrinkles and partial fractures. Such a phenomenon indicates that the local extinction of the shear layer flame can also be facilitated due to the turbulent fluctuation enhanced by the transverse acoustic wave. For a low flow velocity, increasing variations of the unburnt and preheating zones in the presence of the transverse acoustic wave can be revealed. The curvature of the acetone PLIF shows that the unburnt zones are more likely to be wrinkled with an increasing SPL and flow velocity. The root-mean-squared velocity measurements stress that the transverse acoustic wave mainly affects the turbulent premixed flame by enhancing the turbulent fluctuations.

Pilot impact on turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

The impact of pilot flame operation on the combustion of pure methane and hydrogen-enriched methane (H2/CH4: 50/50 in vol%) fuels was investigated in a gas turbine model combustor under atmospheric conditions. The burner assembly was designed to mimic the geometry of an industrial burner, the Siemens DLE Burner, in which a concentric annular ring equipped with pilot flame burners is implemented in the dome of the combustor. Two pilot burner configurations have been investigated: a non-premixed and a partially premixed pilot arrangement. The performance of the pilot burners was evaluated for varying Reynolds number (Re) and H2 enrichment. High-speed OH∗ chemiluminescence imaging, as well as simultaneous planar laser-induced fluorescence measurements of the OH radicals and formaldehyde (CH2O) were used for evaluating the dynamics and structures of the flames for different conditions. Furthermore, emission measurements were carried out to determine the influence of hydrogen dilution on the NOx and CO emission levels. The main findings are (a) the effect of the pilot flame is sensitive to the Reynolds number of the main flame and the type of the pilot flame, (b) the stability range becomes narrower with increasing hydrogen ratio, due to the tendency to flashback, (c) non-premixed pilot flames lower the NOx and increase the CO emissions, albeit rather small differences in the emissions have been detected, and (d) the NOx and CO emissions become significantly lower with increasing hydrogen ratio.

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.

Flame structure and burning velocity of ammonia/air turbulent premixed flames at high Karlovitz number conditions

This paper presents experimental studies of the structures and burning velocities of premixed ammonia/air jet flames at high Karlovitz (Ka) number conditions. Simultaneous planar laser-induced fluorescence (PLIF) imaging of imidogen (NH) and hydroxyl (OH) radicals was performed to investigate the local flame structure and Laser Doppler Anemometry (LDA) measurements were employed for extracting complement relevant turbulent quantities from the flow field. All the selected cases are located in the regime of distributed reaction zones (DRZ) in the Borghi-Peters diagram, with a maximum Karlovitz (Ka) number and turbulent intensity (u′/SL) up to 1008 and 240, respectively. The OH- and NH-PLIF data were used to determine the flame surface density, flame-surface area ratio, and turbulent burning velocity (ST). The main findings include: (a) The NH layer remains thin and continuous over the investigated range of turbulent intensity and Karlovitz number, and the thickness keeps constant statistically without any significant broadening by turbulent eddies; (b) Spatial correlations of the NH and OH radicals show that overlap of NH and OH layers always exists in a thin region where OH has a weaker signal intensity; (c) The ratio of turbulent to laminar burning velocity (ST/SL) shows a nearly linear increase with turbulent intensity, while the ratio of wrinkled flame surface area to that of ensemble-averaged flame surface area increases only slightly with turbulent intensities. The slower increase of wrinkled flame surface area with turbulent intensity can be attributed to under-resolution in the current state-of-the-art PLIF experiments, the neglection of 3D flame wrinkles in 2D experiments, and the increase in flame stretch factor at high turbulent intensities.

Time-averaged velocity and scalar fields of the flow over and around a group of cylinders: a model experiment for canopy flows

We conduct a well-controlled model experiment for a wide variety of canopy flows. Examples of these include engineering flows such as wind flow, dispersion of scalars through and over urban areas, and the convective heat transfer in many heat exchangers, as well as natural canopies such as flows through terrestrial or aquatic vegetation. We aim to shed the light on fundamental flow and transport phenomena common to these applications. Specifically, the characteristics of mean flow and scalar concentration characteristics of a turbulent boundary layer flow impinging on a canopy, which comprises a cluster of tall obstacles (this can also be interpreted as a porous obstruction). The cluster is created with a group of cylinders of diameter $d$ and height $h$ arranged in a circular patch of diameter $D$ . The solidity of the patch/obstruction is defined by $\phi$ (the total planar area covered by cylinders), which is systematically varied ( $0.098 \leq \phi \leq 1$ ) by increasing the number of cylinders in a patch ( $N_c$ ). A point source is placed at ground level upstream of the patch and its transport over and around the patch is examined. Time-averaged velocity and scalar fields, obtained from simultaneous planar particle image velocimetry-planar laser-induced fluorescence (PIV-PLIF) measurements, reveal that the characteristics of wake and flow above porous patches are heavily influenced by $\phi$ . In particular, we observe that the horizontal and vertical extent of the wake and scalar concentration downstream of the patches decreases and increases with $\phi$ , respectively. Here, the recirculation bubble is shifted closer to the trailing edge (TE) of the patches as $\phi$ increases, limiting the flow from convecting downstream, decreasing the scalar concentration and virtually ‘extending’ the patch in the streamwise direction. As the bubble forms in the TE, vertical bleeding increases and hence the concentration increases above the patch where the cylinders appear to ‘extend’ vertically towards the freestream.

Contributions of flame thickening and extinctions to a heat release rate marker of intensely turbulent premixed hydrogen-enriched methane-air flames

Influences of the reaction zone thickening and extinctions on a heat release rate marker of extremely turbulent hydrogen-enriched methane-air flames are investigated using simultaneous planar laser-induced fluorescence of formaldehyde molecule and hydroxyl radical as well as separate stereoscopic particle image velocimetry techniques. The fuel-air equivalence ratio is set to 0.7, and the amount of hydrogen-enrichment varies from 0% to 70% by volume. Mean bulk flow velocities ranging from 5 to 35 m/s with Reynolds and Karlovitz numbers of 18 to 2729 and 0.1 to 76.0, respectively, are examined. It is shown that, by increasing the turbulence intensity, the preheat and reaction zone thicknesses can increase to values that are, respectively, 6.3 and 4.9 of the corresponding laminar flames, which were measured experimentally. Broadening of these zones for intensely turbulent hydrogen-enriched methane-air flames is shown experimentally for the first time. Broadening of the reaction zone suggests that the flamelet assumption used for development of the turbulent flame speed formulations may not hold. Thus, a new formulation, which does not utilize the flamelet assumption, is developed and used to calculate a heat release rate marker of the tested flames. It is shown that, at small turbulence intensities, the heat release rate marker values follow those of the local consumption speed, which is developed in the literature based on the flamelet assumption. However, at large turbulence intensities, the estimated heat release rate marker features large values, and the ratio of this parameter to the local consumption speed is consistent with the ratio of the global and local consumption speeds reported in the literature. It is shown that the ratio of the normalized heat release rate marker to the normalized local consumption speed is correlated with the broadening of reaction zone, suggesting that the disparity between the values of the heat release rate marker and the local consumption speed is linked to the reaction zone thickening. It is shown, although the flame thickening increases the heat release rate marker, extinctions decrease this parameter, which may contribute to the characterized bending behavior. It is argued that the amount of bending is related to the inverse of the Damköhler number. Using this, a mathematical formulation that allows for estimation of the heat release rate marker is developed. The predictions of the proposed formulation agree well with the normalized global consumption speeds of past experimental investigations.

Effect of an Oscillating Grid on Hydrodynamics and Gas-liquid Mass Transfer in an Aquarium

This experimental study describes the effect of the oscillating grid on hydrodynamics and mass transfer in an aquarium. The contribution of the two driving elements CO2 and oscillating grid is identified. Depending on the operating conditions, either these two effects add up and promote the circulation and transport of the liquid, or these effects are opposite, the liquid velocity is then reduced. On the other hand, with regard to gas-liquid mass transfer, the use of the grid is beneficial since, under certain operating conditions; the mass transfer coefficient is increased compared to that obtained without the grid. Analysis of the various energy contributions in the unit shows that the presence of the grid is justified only in cases where the CO2 flow rate must remain low. Flow characterization was performed using Particle Image Velocimetry (PIV) technique. The results were compared with previous studies. In order to perform the concentration field measurements by planar laser induced fluorescence (PLIF) technique and simultaneous PIV and PLIF measurements, the test bench was modified. The observations of velocity and concentration fields are in adequacy with the previous studies and allow to validate the bench. The necessary tools have been put in place, the study of mass transfer can continue.

CH and NO planar laser-induced fluorescence and Rayleigh-scattering in turbulent flames using a multimode optical parametric oscillator.

An optical parametric oscillator (OPO) is developed and characterized for the simultaneous generation of ultraviolet (UV) and near-UV nanosecond laser pulses for the single-shot Rayleigh scattering and planar laser-induced-fluorescence (PLIF) imaging of methylidyne (CH) and nitric oxide (NO) in turbulent flames. The OPO is pumped by a multichannel, 8-pulse Nd:YAG laser cluster that produces up to 225 mJ/pulse at 355 nm with pulse spacing of 100 µs. The pulsed OPO has a conversion efficiency of 9.6% to the signal wavelength of ∼430nm when pumped by the multimode laser. Second harmonic conversion of the signal, with 3.8% efficiency, is used for the electronic excitation of the A-X (1,0) band of NO at ∼215nm, while the residual signal at 430 nm is used for direct excitation of the A-X (0,0) band of the CH radical and elastic Rayleigh scattering. The section of the OPO signal wavelength for simultaneous CH and NO PLIF imaging is performed with consideration of the pulse energy, interference from the reactant and product species, and the fluorescence signal intensity. The excitation wavelengths of 430.7 nm and 215.35 nm are studied in a laminar, premixed CH4-H2-NH3-air flame. Single-shot CH and NO PLIF and Rayleigh scatter imaging is demonstrated in a turbulent CH4-H2-NH3 diffusion flame using a high-speed intensified CMOS camera. Analysis of the complementary Rayleigh scattering and CH and NO PLIF enables identification and quantification of the high-temperature flame layers, the combustion product zones, and the fuel-jet core. Considerations for extension to simultaneous, 10-kHz-rate acquisition are discussed.

Thick reaction zones in non-flamelet turbulent premixed combustion

Internal structure of extremely turbulent flames is investigated experimentally using simultaneous planar laser-induced fluorescence of formaldehyde molecule and hydroxyl radical as well as stereoscopic particle image velocimetry. The mean bulk flow velocity is changed from 5 to 35 m/s. The fuel-air equivalence ratio is 0.7 for all tested conditions. Three different turbulence generating mechanisms leading to a wide range of turbulence intensity with the corresponding Reynolds and Karlovitz numbers ranging from 19 to 2729 and 0.3 to 76.0, respectively, are examined. Preheat and reaction zone thicknesses for the tested flames are calculated and compared with those of the laminar flame to quantify deviation from flamelet behavior. It is shown that the preheat and reaction zone thicknesses increase to values that are about 6.2 and 3.9 times the laminar flame counterparts, respectively. While broadening of the preheat zone is reported in the literature, broadening of the reaction zone for a relatively large diameter Bunsen burner is reported for the first time in this study. The broadening of the reaction zones is related to non-flamelet behavior of the tested flames, and a parameter proposed earlier by the authors is used to quantify this non-flamelet behavior. Swirling strength contours are overlaid on the cold reactants and preheat zones to study the reason for the reported broadening. The results show that positive correlations exist between the preheat/reaction zone thicknesses and the mean value of eddy swirling strength inside the reactants and preheat zone. These correlations as well as the estimated turbulent kinetic energy of these eddies suggest that energetic eddies may potentially penetrate into the preheat and reaction zones causing broadening of these zones.

Internal structure of hydrogen-enriched methane–air turbulent premixed flames: Flamelet and non-flamelet behavior

Internal structure of pure and hydrogen-enriched methane–air flames were investigated experimentally, using simultaneous Planar Laser-Induced Fluorescence of formaldehyde molecule and hydroxyl radical. The fuel–air equivalence ratio and the hydrogen-enrichment percentages were varied from 0.7 to 0.9 and 0% to 50%, respectively. The Karlovitz number was varied between 0.3 and 1, suggesting that the tested experimental conditions pertain to moderate turbulence intensities. A mathematical model that allows for accurate estimation of the heat release rate was developed and guidelines were generated to determine the turbulent flames preheat and reaction layer thicknesses. It was shown that these thicknesses decrease with increasing the fuel–air equivalence ratio and hydrogen-enrichment percentage. Non-flamelet/flamelet behavior of turbulent premixed flames was also investigated using a newly-developed analytical tool. This tool is developed based on the formaldehyde and hydroxyl fluorescence signals deviation from the counterpart synthetic fluorescence signals of the laminar flame simulations. The results show that, for nearly-laminar flames, the flamelet behavior is dominant. However, for both pure and hydrogen-enriched methane–air turbulent premixed flames, deviation from the flamelet behavior (referred to as non-flamelet behavior) is observed.

Spatiotemporally resolved heat transfer measurements in falling liquid-films by simultaneous application of planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography

We present an optical technique that combines simultaneous planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography for the space-and time-resolved measurement of the film-height, 2-D velocity and 2-D free-surface temperature in liquid films falling over an inclined, resistively-heated glass substrate. Using this information and knowledge of the wall temperature, local and instantaneous heat-transfer coefficients (HTCs) and Nusselt numbers, Nu, are also recovered along the waves of liquid films with Kapitza number, Ka=180, and Prandtl number, Pr=77. By employing this technique, falling-film flows are investigated with Reynolds numbers in the range Re = 18–66, wave frequencies set to fw=7, 12 and 17 Hz, and a wall heat flux set to q̇=2.5 W cm−2. Complementary data are also collected in equivalent (i.e., for the same mean-flow Re) flows with q̇=0 W cm−2. Quality assurance experiments are performed that reveal deviations of up to 2–3% between PLIF/PTV-derived film heights, interfacial/bulk velocities and flow rates, and both analytical predictions and direct measurements of flat films over a range of conditions, while IR-based temperature measurements fall within 1 °C of thermocouple measurements. Highly localized film height, velocity, flow-rate and interface-temperature data are generated along the examined wave topologies by phase/wave locked averaging. The application of a heat flux (q̇=2.5 W cm−2) results in a pronounced “thinning” of the investigated films (by 18%, on average), while the mean bulk velocities compensate by increasing by a similar extent to conserve the imposed flow rate. The axial-velocity profiles that are obtained in the heated cases are parabolic but “fuller” compared to equivalent isothermal flows, excluding any wave-regions where the interface slopes are high. As the Re is reduced, the heating applied at the wall penetrates through the film, resulting in a pronounced coupling between the HTC and the film height in thinner film regions. When the imposed wave frequency is increased, a narrower range of HTCs is observed, which we link to the evolution of the film topology and the associated redistribution of the fluid flow upstream of the imaging location, as the liquid viscosity decreases. The local and instantaneous Nu is strongly coupled to the film height and experiences variations that increase as fw is reduced.

Structure and dynamics of CH2O, OH, and the velocity field of a confined bluff-body premixed flame, using simultaneous PLIF and PIV at 10 kHz.

Abstract This study characterizes the structure and dynamics of a confined, bluff-body-stabilized turbulent premixed flame by simultaneously employing formaldehyde (CH2O) and hydroxyl (OH) planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV), at a rate of 10 kHz. The large field-of-view (>170 cm2) CH2O-PLIF is enabled by use of a burst-mode laser delivering 10-kHz pulse trains of 355-nm at 350 mJ/pulse, resulting in a CH2O signal-to-noise of 47:1 during PIV seed flow. Two cases illustrative of the CH2O dynamics are presented. A statistically stationary turbulent combustion case highlights the development of the CH2O layers in space and time. Notably, presumed CH2O-vortex dynamic interactions are observed where the CH2O accumulates broadly near the Kelvin–Helmholtz vortex core and remains thin near the vortex braid, contributing to a distribution of CH2O preheat zone thickness from 1 to 10 times of the calculated laminar value. The second case highlights the CH2O dynamics during a self-excited combustion instability. Two short-duration increases in CH2O are produced during the elevated velocity portion of the acoustic cycle. The first CH2O increase is caused by the reactant mass flux impulse as the velocity starts to increase. The second CH2O increase is the result of the upper and lower shear layers merging downstream and entraining fresh reactants that burn in intense, distributed regions inside the wake. Estimating the time delay between the CH2O and the heat release, it is suggested that the secondary CH2O increase may contribute to damping of the acoustic instability, because of its out-of-phase relationship with pressure, while the first CH2O increase appears to drive the instability.