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.

LES and RANS Spray Combustion Analysis of OME3-5 and n-Dodecane

Clean-burning oxygenated and synthetic fuels derived from renewable power, so-called e-fuels, are a promising pathway to decarbonize compression–ignition engines. Polyoxymethylene dimethyl ethers (PODEs or OMEs) are one candidate of such fuels with good prospects. Their lack of carbon-to-carbon bonds and high concentration of chemically bound oxygen effectively negate the emergence of polycyclic aromatic hydrocarbons (PAHs) and even their precursors like acetylene (C2H2), enabling soot-free combustion without the soot-NOx trade-off common for diesel engines. The differences in the spray combustion process for OMEs and diesel-like reference fuels like n-dodecane and their potential implications on engine applications include discrepancies in the observed ignition delay, the stabilized flame lift-off location, and significant deviations in high-temperature flame morphology. For CFD simulations, the accurate modeling and prediction of these differences between OMEs and n-dodecane proved challenging. This study investigates the spray combustion process of an OME3 − 5 mixture and n-dodecane with advanced optical diagnostics, Reynolds-Averaged Navier–Stokes (RANS), and Large-Eddy Simulations (LESs) within a constant-volume vessel. Cool-flame and high-temperature combustion were measured simultaneously via high-speed (50 kHz) imaging with formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) representing the former and line-of-sight OH* chemiluminescence the latter. Both RANS and LES simulations accurately describe the cool-flame development process with the formation of CH2O. However, CH2O consumption and the onset of high-temperature reactions, signaled by the rise of OH* levels, show significant deviations between RANS, LES, and experiments as well as between n-dodecane and OME. A focus is set on the quality of the simulated results compared to the experimentally observed spatial distribution of OH*, especially in OME fuel-rich regions. The influence of the turbulence modeling is investigated for the two distinct ambient temperatures of 900 K and 1200 K within the Engine Combustion Network Spray A setup. The capabilities and limitations of the RANS simulations are demonstrated with the initial cool-flame propagation and periodic oscillations of CH2O formation/consumption during the quasi-steady combustion period captured by the LES.

Quantitative acetone/toluene planar laser‐induced fluorescence measurements of equivalent ratio

AbstractIn this work, we establish a quantitative relationship between the fluorescence signal intensity of acetone and toluene and their concentration, and acetone/toluene planar laser‐induced fluorescence (PLIF) is developed to measure the concentrations of two components simultaneously, providing an equivalent ratio distribution of the flow field. Due to a thorough consideration of the tracer effect on each other's fluorescence signals, the uncertainty in the PLIF concentration measurement is reduced to 10.4%. Experimental results of OH‐PLIF and acetone/toluene PLIF indicate that the equivalence ratio distribution obtained by acetone/toluene PLIF in cold‐state flow fields can be employed to predict and analyze the trend of equivalence ratio distribution in the combustion field.

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.

Effect of water content in ethanol on flame front wrinkling and curvature in a spark-ignition engine using OH PLIF imaging

The production of ethanol fuel involves separating water and ethanol by distillation and dehydration in an energy intensive process. To reduce the carbon footprint, hydrous ethanol is seen as a promising alternative. This study focused on the effect of water content in ethanol on flame development in a single-cylinder optical spark-ignition engine. Ethanol with 0%, 4% and 10% water content per volume was used with Port Fuel Injection (PFI) and Direct Injection (DI) for homogeneous and heterogeneous mixture preparation. Tests were also conducted with iso-octane for comparison. Flames captured by Planar Laser Induced Fluorescence (PLIF) of OH were spatially filtered to remove scales larger than the flame size, then the flame brush was quantified in terms of thickness, crossing frequency between instantaneous and filtered contours, and curvature. Increasing water content resulted in slower flame growth, with 10% hydrous exhibiting marginally higher flame speed than iso-octane. PFI led to faster flame growth and thicker flame brushes than DI at the same timing after ignition. When the brush thickness was normalised by flame radius, all hydrous fuels and iso-octane exhibited similar values with PFI, whilst anhydrous ethanol exhibited the thickest normalised brush. The alcohols maintained similar normalised thicknesses with DI and iso-octane exhibited the thickest normalised brush. When flames of the same radii were considered with PFI at the same timing, the fuels with sequentially lower laminar burning velocity were associated with larger crossing frequency. For PFI, increasing water content resulted in decreased flame roundness irrespective of size, with the 4% hydrous and iso-octane exhibiting similar values. With DI, anhydrous ethanol flames were rounder than the 4% hydrous when averaged over all cycles, but marginally rounder than the 10% hydrous and iso-octane. Increasing water content widened the curvature distribution and increased the mean with PFI. Flame roundness decreased with increase in mean curvature and Lewis number for both PFI and DI.

Investigation of the three-dimensional structure of low-swirl methane-hydrogen flames with multiple UV cameras-based OH* chemiluminescence tomography

Blending hydrogen with natural gas is useful for reducing CO2 emissions, but this alters the flame structure, creating challenges for flame stability. Numerous researches on methane-hydrogen flame structure have been carried out based on two-dimensional combustion diagnostic techniques such as flame chemiluminescence and planar laser-induced fluorescence. In this paper, a low-cost OH* chemiluminescence imaging system consisting of 8 ultraviolet (UV) cameras is proposed to reconstruct the three-dimensional (3D) structure of methane-hydrogen flames, and validated through use of the correlation coefficient. Further experimental investigations of the low-swirl methane-hydrogen flames are performed by this computed tomography of chemiluminescence (CTC) system. The flame properties are determined to evaluate the effects of various fractions of blended hydrogen on the flame structure, including the flame center of mass, flame surface density and fractal dimension. Results showed that the flame initially has bowl-shape structure. The flame height gradually decreases and the flame cross-sectional area increases as the blended hydrogen fraction increases. Meanwhile, the bottom of the flame moves toward the nozzle outlet. Nevertheless, when the blended hydrogen fraction reaches 50%, the flame transforms into a cup-shaped flame and shows a double-layer flame structure. When the flame is bowl-shaped, the flame center of mass moves away from the nozzle center axis with increasing blended hydrogen fraction, but the fractal dimension and flame surface density are not sensitive to the blended hydrogen fraction. In contrast, when the flame is cup-shaped, the flame center of mass moves towards the nozzle center axis, and the flame surface density and fractal dimension increase significantly, indicating the increase of flame wrinkling. These results reveal that blending hydrogen leads to two distinct flame structures and characteristics in low-swirl flame, also demonstrate that the proposed 3D low-cost CTC system based on multiple UV cameras is well-suited for combustion diagnostics.

On the errors and their mitigation in planar laser induced fluorescence

In planar laser induced fluorescence (PLIF), scalar concentration fields are obtained from fluorescence images of the flow where the scalar is ‘tagged’ with a suitable fluorescent dye and illuminated with a laser sheet. While the fluorescence occurs, the intensity of the laser sheet is attenuated along its passage through the dye field. This paper discusses the effect of correcting for the attenuation of the laser by the dye while obtaining the scalar concentration field from the fluorescent images of the flow. If the attenuation correction cannot be done, the paper discusses three possible calibration scenarios to minimise the error. It is shown that the error in ignoring the attenuation due to turbulent concentration fluctuations cannot be corrected by any means. However, the remaining error can be exactly corrected if it were possible to calibrate with the mean concentration field. It is shown that a uniform concentration field, centred around the mean field, greatly reduces the error when compared to having no attenuation correction but still is significantly more when compared to the use of the average field. Relying on these results, we propose an iterative technique which ideally achieves the accuracy obtainable when calibrating with the mean concentration field, but which requires no reference cell. In addition, the paper explores the removal striations, of a common artefact during PLIF image acquisition with origins in the image optics. A technique to remove the same is proposed and its advantages discussed when compared to available techniques.

Fluid dynamics and mixing in a novel intermittently rotating bioreactor for CAR-T cell therapy: Spin-down from incomplete spin-up

This work explores the fluid flow and mixing within a new type of intermittently rotating bioreactor used for the automated manufacturing of CAR-T cell therapy products; CAR-T is a novel cell-based gene therapy which offers a single-dose cure for several forms of advanced blood cancer. The rotating cylinder bioreactor has a free surface and a liquid height to radius ratio of 0.5. Agitation is achieved via intermittent rotation of the entire vessel around its central vertical axis. No comprehensive engineering characterisation of the fluid dynamics in a bioprocessing context exists to date for this type of bioreactor. The present study examines the fluid dynamics and mixing during spin-down to rest following incomplete spin-up (i.e. the starting state of the fluid is not solid body rotation). Novel Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence (PLIF) data is presented, shedding light on the different transient flow regions inside the intermittently rotating bioreactor, and on the impact spin-up time has on the mixing efficiency of the subsequent spin-down stage. The results presented can be used to inform the design of a custom intermittent agitation pattern for any resembling cylinder bioreactors, the adoption of which could lead to higher yields and reduced costs for the cell expansion step of the manufacturing process.

Comparison of Refractive Index Matching Techniques and PLIF40 Measurements in Annular Flow.

This paper investigates non-invasive techniques for annular two-phase flow analysis, focusing on liquid film characterization to understand the interfacial phenomena that are crucial for heat and mass transfer. Limited methods allow the study of the temporal and spatial evolution of liquid film, such as Planar Laser-Induced Fluorescence (PLIF). However, this method possesses optical challenges, leading to the need for improved techniques to mitigate refraction and reflection, such as Refractive Index Matching (RIM). This study utilizes an experimental annular flow facility to analyze both RIM and non-RIM PLIF over a range of liquid Reynolds numbers from 4200 to 10,400. Three configurations-PLIF RIM90, PLIF RIM40, and PLIF nRIM40-are compared from both qualitative and quantitative perspectives. In the quantitative analysis, key variables of the liquid film are measured, namely mean film thickness, disturbance wave height, and frequency. Variations in the analyzed variables indicate minor deviations, which are not likely to be caused by the technique used. However, all three methodologies exhibited errors that are estimated to be within a maximum of 10%, with a mean value of approximately 8%.

Temperature field of non-Oberbeck–Boussinesq Rayleigh–Bénard convection in a low aspect ratio cell

A time-resolved experimental investigation was undertaken on the temperature evolution of Rayleigh–Bénard convection (RBC) in a slender convection cell with aspect ratio of Γ=0.1. Experiments were conducted for Rayleigh numbers of Ra=5.3×107, 7.6×107, and 9.5×107 and Prandtl number of Pr≈6 within the non-Oberbeck–Boussinesq (NOB) condition with a temperature difference variation in the range of 30 °C≤ΔT≤40 °C. Measurement of the temperature was by applying time-resolved two-color planar laser-induced fluorescence over the initial 2400 s. Experimental observations showed that the lateral confinement of the convection cell leads to the development of a single large-scale thermal plume instead of multiple plumes. Results showed that contrary to expectations, lateral confinement was found to be ineffective in suppressing temperature oscillations near thermal boundaries. Results also indicated that for Ra=5.3×107, 7.6×107, the temperature oscillations had a frequency of f≈0.028 Hz similar to the frequency of the oscillations in Oberbeck–Boussinesq (OB) RBC. For Ra=9.5×107, however, it was found that the frequency of the oscillations was much lower than the OB RBC with a relatively wide range of the oscillations in the vicinity of f≈0.006 Hz. It is also found that the lateral confinement and formation of singular high-energy thermal plumes leads to an increase in the nonsymmetrical temperature distribution of NOB RBC with a bimodal distribution of the temperature field, deviating significantly from the Gaussian distribution temperature field found in OB RBC.

Experimental Investigation of Superheated Liquid Injection in Quiescent and Supersonic Crossflow

This research delves into liquid fuel injection in scramjet combustors, with a focus on the effects of injector length-to-diameter (L/D) ratios and temperatures, spanning from ambient to superheated states. Efficient atomization within the short residence times of liquid-fueled supersonic combustors is crucial for maximizing combustion efficiency. Initial investigations in a quiescent environment aimed to isolate the impact of injector geometry and temperature, utilizing quantitative analysis and diffused backlit illumination to demonstrate that heating fuel beyond its boiling point significantly enhances atomization and dispersion. A notable finding is that higher L/D ratios led to a quicker initiation of flash boiling. Subsequent analyses under supersonic crossflow, using two-dimensional planar laser-induced fluorescence, support these observations. The study further employs spray structure and spatiotemporal characteristic analyses, coefficient of variation, and proper orthogonal decomposition, revealing enhanced flashing atomization at higher L/D ratios. This underscores the critical role of injector design and temperature in optimizing fuel atomization in scramjet engines, highlighting the nuanced interplay between physical injector characteristics and thermal properties in achieving efficient combustion.

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

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

Multiple species imaging from CFD fused H2O absorption spectral tomography and transfer learning

Laser absorption spectroscopy (LAS) tomography is well-proved in combustion diagnosis but has difficulty especially in the simultaneous imaging of multi-species concentrations. A multiple species imaging method from single species LAS tomography was proposed on the basis of computational fluid dynamics (CFDs) and transfer learning. CFD simulation of the methane/air flat flame was conducted to reveal the relationship among multiple species. A back propagation neural network was pre-trained with the dataset obtained from CFD simulation to predict projection values of OH mole fractions from H2O absorption lines at 7185.6 cm−1 and 7444.4 cm−1. The measurement of flat flame by a single wavelength planar laser-induced fluorescence fused LAS tomography system was conducted for network fine-tuning and experiment verification. Distributions of OH mole fractions in lean-burn conditions and nearly complete combustion conditions were quantitatively reconstructed well, while annulus profiles in fuel-rich conditions were qualitatively retrieved. Reconstructed images with two-fifth experiment data used in the network fine-tuning showed a 31.3% decline in image error compared to those without fine-tuning. This proposed method enables LAS tomography of multiple species via only one species with enough measured projections, and also shows potential in image error reduction by introducing more projections.

Freestream and shear layer effects in bluff-body-stabilized turbulent premixed flames

The effects of low and high approach flow turbulence levels on a large-scale, confined, premixed bluff-body-stabilized flame are experimentally characterized. The flame and flow are imaged using simultaneous 10-kHz-rate formaldehyde (CH2O) planar laser-induced fluorescence (PLIF), hydroxyl (OH) PLIF, particle image velocimetry (PIV), and OH* chemiluminescence. Elevated approach flow turbulence intensities of 16 % as compared to a lower value of 3 % affects the global and local flame and flow. The elevated approach flow turbulence results in a 22 % reduction in the length of the recirculation zone, despite the mean flame and mean shear layer locations being similar for both approach flow turbulence levels. Increasing the approach flow turbulence intensity does not lead to the mean flame position moving further into the reactant freestream. Both approach flow turbulence levels result in a similar growth of the flame brush thickness moving downstream until approximately 1.5 bluff-body widths, after which the flame brush for the low and high approach flow turbulence case stops growing and continues to linearly increase, respectively. This study describes the mechanisms by which high approach flow turbulence affects the global and local flame and emphasizes that the turbulent flame characteristics are influenced not only by the freestream approach flow turbulence but also by the bluff-body shear layers, recirculation zone, and wake.

Study on effects of ethylene or acetylene addition on the stability of ammonia laminar diffusion flame by optical diagnostics and chemical kinetics

Improvement of ammonia combustion characteristics by carbon-based combustion promoters is currently emphasized, but the impact of carbon‑carbon multi-bond structure on ammonia combustion characteristics and reaction mechanisms has not been sufficiently investigated. In order to investigate the mechanism of molecular structures (C=C and C≡C) of combustion promoters on the stability of ammonia flames, the flame morphology, flame structure, NH3-H2 cracking, and C-N cross-reaction of ammonia/ethylene and ammonia/acetylene laminar diffusion flame are investigated based on a Gu¨lder burner by utilizing high-speed photography, planar laser-induced fluorescence (PLIF) and laser spontaneous Raman scattering (SRS) techniques. The results show that acetylene addition is more effective than ethylene to improve the flame stability. Combined with the flame structure analysis, it is shown that C=C mainly enhances the low-temperature reaction stage intensity of laminar flame, while C≡C can both enhance the intensity of low-temperature and high-temperature reaction stages, thereby enhancing the flame combustion exothermic process. Besides, C=C mainly strengthens the exothermic reactions in the upstream of the flame whereas C≡C can strengthen the exothermic reactions in the upstream and downstream of the flame simultaneously. Investigation of the NH3-H2 cracking reactions reveals that C≡C promotes H2 generation mainly by increasing NH3→H2 cracking, whereas C=C promotes H2 generation mainly by increasing NH2→H2 and NH→H cracking, hence improving the flame stability. Further analysis through the reaction flows revels that C=C acts mainly on H-containing species HCN/HNCO whereas C≡C mainly acts on non-H-containing species CN/NCO to influence the reaction pathways, but C-N cross-reaction does not play an important role in the characteristics of ammonia composite combustion (ACC).

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.

An investigation of flow regimes and mixing in a novel arrow-shaped jet reactor

Fluid mixing plays a vital role in the industry of chemical engineering. In this work, a novel arrow-shaped jet reactor by varying the angle of fluid impingement in the inlet channel was investigated. Flow field visualization experiments using Planar Laser-Induced Fluorescence (PLIF) technique were carried out to study various flow regimes and mixing performance. Results show that differentiated flow patterns of engulfment flow regimes appear within the arrow-shaped jet reactor. Particularly, vortex ring shedding was found in the unsteady engulfing flow regime, which promoted the mixing significantly. Besides, the mechanism of the variation of fluid impinging angle on mixing in an arrow-shaped jet reactor was investigated by numerical simulation. The results indicate that the transformation from double-leg vortex to single-leg vortex and the increase of the volume of vortex clusters caused by vortex ring shedding are the main reasons for the improvement of the mixing of engulfment flow regime.

Characterization of a 5-nozzle array using premix/micromix injection for hydrogen

Hydrogen is one of the most promising fuels to decarbonize energy systems since it has a high specific energy, a carbon-free combustion process, and may be produced sustainably through electrolysis. Direct fuel drop-in replacement cannot be done in traditional lean, premixed combustion burners because of the high reactivity of the hydrogen-air mixture and its inherently unstable nature that might lead to flashback and hardware failure. Consequently, new injection strategies and burner geometries are investigated to mitigate these risks. Here we present hydrogen capabilities of a premix/micromix injector that relies on a two-staged fuel injection strategy to offer wide fuel flexible capability (methane to hydrogen) within a single design. Five injectors are placed in a cross-shaped array to simulate a sector found in multi-element combustion systems. Stability and combustion dynamics maps are obtained for the array and OH planar laser-induced fluorescence (PLIF) provides additional insight into the combustion process of these novel burners when placed in an array. The use of micromixing is shown to drastically improve the acoustics of this geometry and expand the flashback limits compared to the fully premixed configuration. Significant variability in flashback limits are observed for different additively-manufactured injector configurations. Phase-averaged OH-PLIF measurements, obtained by registering the acquisition with the acoustics module, and the 3D reconstruction of a partially-premixed flame highlight the complexity of the stabilization mechanisms for these highly three-dimensional, non-axisymmetric flames that may be subjected to large thermoacoustics. This first investigation into premix/micromix clustered injectors demonstrates the importance of better understanding the impact of flame–flame interaction in multi-element combustion systems with micromixed, or partially-premixed, combustion.

High-efficiency narrow-bandwidth KTP optical parametric oscillator for kHz-MHz planar laser-induced fluorescence.

The electronic excitation of key combustion species or flow tagging of chemical species requires a narrowband tunable UV source. In this work, a potassium titanyl phosphate (KTP) burst-mode optical parametric oscillator (OPO) pumped by a 532 nm laser is developed to generate a spectrally narrow signal and an idler output with 1.48 ± 0.19 cm-1 bandwidth without the need for injection seeding. The idler (1410-1550 nm range) is further mixed with 355 or 266 nm to generate 284 or 226 nm for OH or NO planar laser-induced fluorescence (PLIF), respectively, with up to 1.9% conversion efficiency from 1064 nm to the UV. MHz-rate burst profiles are reported, and OH and NO PLIF are demonstrated in a rotating detonation combustor at rates up to 200 kHz.

Liquid and aerated jets behind different pylon configurations in supersonic crossflow

Effects of different pylon configurations on the liquid and aerated jets injected in supersonic crossflow of Mach number 1.65 were investigated experimentally. Pylon configurations with different bottom curvatures at the trailing edge were explored. Two liquid jets with momentum flux ratio of 0.1 and 0.4, and two aerated jets with gas-to-liquid mass ratio of 2% and 4% were selected in the current study. Acetone and air were used as the secondary liquid jet and the aeration gas, respectively. For the atomization quality of the jet, the droplet velocity field was obtained using fluorescence-based particle image velocimetry. The high-speed shadowgraph and cross-sectional planar laser-induced fluorescence experiments were performed for the mixing characteristics, such as penetration height and spreading area. The high-speed images were also used for the dynamic mode decomposition, which revealed dominant structures and their associated frequencies. Curved pylons with 45°and 60°trailing edge curvatures produced higher droplet velocity distribution than the other cases for liquid and aerated jets, respectively. The penetration height of the liquid jet at low momentum flux ratio tends to traverse towards the wall downstream of the injection. Aeration substantially improves the penetration height for the same liquid flow rate due to the annular nature of the aerated jet with a gas-to-liquid mass ratio of 2%. Cross-sectional planar laser-induced fluorescence results revealed a considerable increase in the spreading of the jet for the aerated jets as compared with the low momentum flux ratio liquid jet. This was due to the higher shear layer interaction of the aerated jet than that of the liquid jet. The aerated jets produced 60% higher spreading area than the liquid jet with the same mass flow rate. The curved pylons exhibited at least 10% increment in the spreading area as compared to the standard pylon cases for all the liquid and aerated jets. Three distinct modes were observed in the DMD analysis. The convective mode was the typical first and second modes of all the cases. The shear layer flapping and shear-induced entrainment breakup modes have also been observed with frequencies around 5 kHz.