Planar Laser-induced Fluorescence Imaging Research Articles

Deep learning-based image segmentation for instantaneous flame front extraction

This paper delves into the methodology employed in examining lean premixed turbulent flame fronts extracted from Planar Laser Induced Fluorescence (PLIF) images at elevated pressures. In such flow regimes, the PLIF signal suffers from significant collisional quenching, typically resulting in image data with low signal-to-noise ratio (SNR). This poses severe difficulties for conventional flame front extraction algorithms based on intensity gradients and requires intense user intervention to yield acceptable results. In this work, we propose Convolutional Neural Network (CNN)-based Deep Learning (DL) models as an alternative to problem specific conventional methods. The pretrained DL models were fine-tuned, employing data augmentation, on a small annotated dataset including a variety of conditions between SNR ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.6 to 2.6 and subsequently evaluated. All DL models significantly outperformed the best-scoring conventional implementation both quantitatively and visually, while having similar inference times. IoU-scores and Recall values were found to be up to a factor ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.2 and ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 2.5 higher, respectively, with ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.15 times improved Precision. Small-scale structures were captured much better with fewer erroneous predictions, becoming particularly pronounced for the lower SNR data investigated. Moreover, by applying artificially modeled noise, it was shown that the range of image conditions in terms of SNR that can be reliably processed extends well beyond the images included in the training data, and satisfactory segmentation performances were found for SNR as low as ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.1. The presented DL-based flame front detection algorithm marks a methodology with significantly increased detection performance, while a similar computational effort for inference is achieved and the need for user-based parameter tuning is eliminated. It enables a very accurate extraction of instantaneous flame fronts in large image datasets where supervised processing is infeasible, unlocking unprecedented possibilities for the study of flame dynamics and instability mechanisms at industry-relevant conditions.

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.

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.

Flame state recognition method of a scramjet based on PLIF image fusion features and an artificial neural network

In this study, a pattern recognition model is proposed to differentiate the flame state of a scramjet using an artificial neural network. The flame images are obtained from a scramjet ground test utilizing planar laser-induced fluorescence (PLIF). By extracting basic features, Hu moments and Zernike moments, the preprocessed flame images are compressed to mine valuable information. In order to reduce redundant input features and improve the efficiency of model operation, the partial least squares (PLS) method is introduced for feature screening and fusion. Then, a back propagation neural network (BPNN) model for multi-flame classification is established and analyzed. Finally, the flame states are determined by comparing the probabilities of different states. In order to optimize the recognition performance, the fusion features are studied and discussed. Experimental results show that when the filtered 11-dimensional features are used as input, the average recognition rate for the four states can reach 97.4%. These results demonstrate the significant potential of integrating PLIF and advanced data analysis methods, thereby broadening their application to intricate combustion fields.

Drag reduction by natural yam mucilage in turbulent flows

Nontoxic bio-polymeric drag reducers are of great practical importance. In this work, a new natural and environmentally friendly drag reducer is introduced, which is extracted from the yam. Yam mucilage solutions are highly shear thinning and are insensitive to temperature variation. Their drag-reducing capability is tested in a water tunnel with the injection of yam mucilage solutions at the bottom wall. The main flow speed varies 0.5–2 m/s, and the corresponding bulk Reynolds number (Re) varied from 11 467 to 45 868. The mean concentration profile from the planar laser-induced fluorescence (PLIF) images and the mean velocity profile from the particle image velocimetry (PIV) images are obtained to explain the drag reduction of the yam mucilage solution. A maximum drag-reduction rate (DR) of 25.27% is achieved. The effects of the solution concentration, the injection rate, and the main flow speed on the drag-reduction efficiency are explored. The DR-log10K fitting curve is linear, consistent with that of the reported polymer drag reducers. The K-scaling laws also imply that the consumption of yam mucilage would be much more than that of polyethylene oxide (PEO) corresponding to similar DR. The further revelation of the effective drag-reducing component within the yam mucilage is believed to promote the efficiency of drag reduction. The yam mucilage is a candidate drag-reducing agent that can be an alternative to existing polymer solutions.

Characterization of Spray Field for Water-Emulsified Diesel Using a Pressure Swirl Atomizer Under a Nonreacting Environment

Abstract Employing a mixture or an emulsion of water and diesel fuel is considered a way to reduce gas emissions such as NOx and soot in a gas turbine. This study presents detailed experimental results on the spray characteristics of a water–diesel emulsion injected by a pressure swirl atomizer with a 90-degree spray angle and a flow number of 0.58 under a nonreacting environment at high pressure and temperature conditions. Acquiring these data is a key step when configuring a combustor that will employ emulsified fuels. In addition, this study seeks to confirm that the emulsion stays intact when it gets sprayed into the combustor. Furthermore, this study attempts to understand if a water–diesel emulsion prepared by a sonicator improves fuel atomization as compared to a water–diesel mixture prepared by a static mixer, i.e., not a proper emulsion. Tests are conducted in a high pressure and temperature testing facility at two ambient pressures and three ambient temperatures and the water to diesel ratio (W/D) is varied from 11% to 100% by mass. Phase Doppler Particle Anemometry (PDPA) is employed to measure the spray characteristics. Through a backlit high-speed photography, overall spray patterns over different test conditions are visualized. Mie-scattering and planar laser-induced fluorescence imaging are utilized to visualize the mixture field. In general, the results indicate that emulsion stays intact as it gets sprayed into the combustor; and emulsion is a better solution to reduce emissions than a statically mixed mixture.

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.

Ambient pressure operando catalytic characterization by combining PM-IRRAS with planar laser-induced fluorescence and surface optical reflectance imaging

We present a combination of optical operando techniques that allow us to bridge the pressure gap in heterogeneous catalysis. By combining Polarization Modulated - InfraRed Reflection Absorption Spectroscopy (PM-IRRAS) with two dimensional-Surface Optical Reflectance (2D-SOR) and Planar Laser Induced Fluorescence (PLIF), we can simultaneously measure the adsorbed species on the catalyst surface, monitor the surface oxide formation across the catalyst surface and image the gas phase right above the catalyst surface, respectively. In a single measurement, we are able to follow heterogeneous catalytic reactions temporally- and spatially resolved with all three optical techniques, which are additionally supported by Mass Spectrometry (MS). To validate the experimental setup, we perform two experiments studying CO oxidation on Pd(100) at 150 mbar and 910 mbar by ramping the sample temperature. PM-IRRAS and 2D-SOR reveal that the formation of well-defined ultrathin surface oxide coincides with the disappearance of CO adsorption on the surface. At the same time, PLIF and MS confirm the simultaneous transition into a mass-transfer-limited (MTL) regime. A difference between 150 and 910 mbar can be seen in the light-off temperature caused by different partial pressures of CO and in the spatial distribution of the gas cloud across the surface in space caused by gas diffusion. This emphasizes the need for spatially-resolved gas phase diagnostics in heterogeneous catalysis. The combination of all techniques aids our understanding of the gas-surface interaction.

Impact of density stratification and azimuthal velocity on the growth of coherent structures in a convectively unstable swirl flame

Large-scale coherent structures resulting from hydrodynamic instabilities can interact with turbulent swirl flames and lead to combustion instabilities. The present work investigates the impact of density stratification and azimuthal velocity on the growth of coherent structures in a convectively unstable swirl flame. Flame structure and flow field are measured by simultaneous hydroxyl planar laser-induced fluorescence and stereoscopic particle image velocimetry (S-PIV) at a repetition rate of 10 kHz and are analyzed by using the spectral proper orthogonal decomposition (SPOD) and spatial linear stability analysis (LSA). The SPOD reveals that the dominant symmetric and anti-symmetric modes are within the frequency range from 156 to 585 Hz, accounting for more than 25% of the turbulent kinetic energy. The spatial growths of these coherent structures are quantified by the LSA that predicts large growth rates near the nozzle exit with the corresponding frequency band matching well with the SPOD analysis. The LSA results show that both density stratification and azimuthal velocity have little effect on the instability frequencies of the most spatially unstable modes. However, the flame-induced density stratification suppresses the growth of the coherent structures by altering the pressure gradient and viscous diffusion, whereas the azimuthal velocity promotes flow instabilities through the changes in convection and production of the coherent perturbations. The results also suggest that the conventional PIV technique with two-component velocity measurement is inadequate for linear modeling of coherent structures, and the density stratification should also be taken into account in convectively unstable swirl flames.

Leading edge stabilisation of vertical boundary layer diffusion flames

The leading edge stability of vertical boundary layer diffusion flames established over a 3D-printed porous gas burner is revealed through Planar Laser-Induced Fluorescence imaging of the OH radical (OH-PLIF). Flame stability is studied by premixing methane and ethylene with increasing volume fractions of bromotrifluoromethane (CF3Br/Halon-1301) to increase the characteristic chemical timescale. Blow-off occurs at 15.7% and 37.3% CF3Br addition for methane and ethylene respectively, which are remarkably large limits compared to other flame configurations. As CF3Br is added, the flame stand-off distance increases and the reaction zone broadens, thereby increasing the flame length. This accelerates the buoyancy-induced flow ahead of the leading edge, promoting O2 entrainment into the flame anchor. Consequently, the radical scavenging effects on the flame anchor reactivity are dampened, resulting in the flame re-anchoring slightly downstream along the plate. Towards extinction, the flame shortens dramatically due to efficient catalytic cycling and radiative quenching at the flame tip resulting from the brominated species and excessive soot formation. This reduces the O2 mass flux into the kinetically dampened flame anchor resulting in blow-off extinction. Therefore, blow-off is controlled by both the leading edge reactivity and trailing edge length. Methane is shown to be considerably more sensitive compared to ethylene to CF3Br owing to its larger flame speed. These results demonstrate that fire-induced buoyancy greatly increases blow-off limits when using chemically active or inert agents in vertical wall fire configurations.

Verification and validation of the open-source plasma fluid code: Zapdos

Zapdos is an open-source finite element plasma fluid solver based on the MOOSE multiphysics framework. This paper outlines Zapdos verification, benchmarking, and validation efforts for 1D and 2D RF capacitively-coupled plasma discharge models for mid-range pressures (0.1 - 1 Torr). The verification process involved using the method of manufactured solutions to assess Zapdos spatial and temporal error convergence. L2 errors ranged from 10−2 to 10−4, while the convergence's slope were in agreement with the predicted slopes for the tested variable and time integration orders. The benchmarking process involved comparisons to previously results from the validated finite element code, LSODI [1–3]. These works included 1D and 2D simulations for a range of plasma parameters (densities, temperatures, voltage, etc.). For the 1D cases, Zapdos and LSODI results were in very good agreement. In the 2D cases, variable behaviors matched, with slight discrepancies in peak values. The validation process involved comparisons to experimental works including electron density measurements by microwave interferometry [4] and metastable density measurements by planar laser-induced fluorescence imaging [5]. Results shown reasonable agreement at higher pressure, with results starting to diverge at low pressures. Probable causes for this diverges are the limitation of the fluid assumption for plasmas at low pressure, or the need for more robust boundary conditions. Overall, Zapdos shown reasonable results for the verification, benchmarking, and validation efforts, and Zapdos can be downloaded at https://github.com/shannon-lab/zapdos. Program summaryProgram Title: ZapdosCPC Library link to program files:https://doi.org/10.17632/j76jr9m66p.1Developer's repository link:https://github.com/shannon-lab/zapdosLicensing provisions: LGPL-2.1Programming language: C++, PythonSupplementary material:shannon-lab.github.io/zapdosNature of problem: For the plasma computational community, multiphysics packages are needed for the highly couple physics seen in plasma problems (e.g. chemistry, fluid flow, heat transfer, electromagnetic, etc.). One limiting factor for these types of computational research is the high cost to purchase the licenses needed for established multiphysics packages, such as COMSOL. An additional problem that can arise is the limited ability to add new physics to these existing frameworks, in such a way that new physics can be easily coupled to current models. For any new software that wants to tackle these problems, the question of verification and validation for these codes needs to be addressed.Solution method: Zapdos is an open-source finite element plasma fluid solver based on the MOOSE multiphysics framework. MOOSE (Multiphysics Object Oriented Simulation Environment) solves highly nonlinear, tightly coupled sets of partial differential equations (PDEs) and houses multiple physics applications that can be easily coupled together. Zapdos, along with the MOOSE ecosystem, is all free, open-source, and completing customizable with no black box components. Zapdos has gone through verification and validation for applications common in the plasma computational community.Additional comments including restrictions and unusual features: Zapdos, along with MOOSE and other MOOSE applications, require an Unix based operating system.

Influence of fuel injection parameters at low-load conditions in a partially premixed combustion (PPC) based heavy-duty optical engine

The study examined the effects of second injection timings and fuel injection pressures (FIPs) in a double-injection based partially premixed combustion (PPC) using a single-cylinder heavy-duty optical diesel engine equipped with two different configurations, namely all-metal and optical. The thermodynamic analysis of energy balance, exhaust emissions, and particulates characterization was conducted in an all-metal engine configuration. For the same base engine architecture, high-speed natural combustion luminosity, cool-flame, and electronically excited hydroxyl (OH*) chemiluminescence, and planar laser-induced fluorescence (PLIF) imaging of formaldehyde (HCHO-PLIF) were applied in the optical configuration. Additionally, 1D modeling using GT-Power was utilized to examine the distribution of heat transfer losses through the individual cylinder boundaries. The experiments were conducted at low-load conditions at a fixed fuel mean effective pressure (MEPfuel) using a primary reference fuel with a volumetric mixture of 50% n-heptane and 50% iso-octane (PRF50). The results demonstrated that for a fixed first injection, the second injection timing of −6°CA aTDC is the most favorable condition due to increased gross indicated efficiency, reduced exhaust losses, decreased uHC/CO/soot emissions with a slight compromise on heat losses and NOx emissions. Among the tested FIPs of 1200, 1500, and 1800 bar, 1500 bar showed higher efficiency, and overall lower energy losses and exhaust emissions. At higher FIPs, the overall flame development process revealed reduced luminous intensity with dispersed signals formed downstream of the nozzle due to increased premixed combustion. For 1800 bar FIP, both cool-flame and HCHO signals showed an increased rate of reaction zones close to the bowl-wall region from where the OH radicals developed. Not only the initial low- to high-temperature reaction transition was faster for 1800 bar FIP but the signal decay of HCHO signals by OH radicals was also rapid in the late cycle. These findings explained the root cause of increased heat transfer losses and higher uHC/CO emissions at 1800 bar FIP.

Strategy for instantaneous formaldehyde (CH2O) imaging by planar laser-induced fluorescence (PLIF) in a scramjet with intense flame emissions

Conducting CH2O PLIF imaging in scramjets is challenging due to strong flame emissions (FEs). The FE interference is comparable to the CH2O fluorescence in the wavelength range of the fluorescence. To deal with this challenge, we proposed a CH2O PLIF imaging strategy considering CH2O excitation at 353.06 nm and FE reduction based on capturing two frames with a reasonable time delay. The fluorescence efficiency with the 353.06 nm excitation was shown to be approximately four times that with the excitation of ∼355 nm in the linear regime, where the two wavelengths corresponded to full-width-at-half-maximum line widths of ∼0.16 and ∼1 cm−1, respectively. During the two frames being captured, the FE background remained nearly constant. Then collecting the CH2O fluorescence in the first frame and subtracting the second frame from the first substantially reduced FE interference. The signal-to-background ratio in the raw image was approximately 1–2, but it could be increased to 10 using the proposed strategy. This strategy enables CH2O PLIF imaging to be conducted in scramjets and scramjet-like combustion facilities with intense FE interference.

High-speed formaldehyde planar laser-induced fluorescence and schlieren to assess influences of injection pressure and oxygen concentration on Spray A flames

The present study investigates the effect of injection pressure and ambient oxygen concentration variations on the Engine Combustion Network Spray A flame. The two-stage ignition and combustion processes for individual injections are characterized using a high-speed planar laser-induced fluorescence (PLIF) and schlieren imaging diagnostic at high pressure (near 6 MPa). For PLIF measurements, we use a pulse-burst-mode Nd:YAG laser capable of producing a 6 ms long 355-nm pulse-train with 300 pulses at 70 mJ/pulse, separated by 20 µs. The PLIF imaging at high-acquisition rate offers unique insights into understanding the spatiotemporal distribution of formaldehyde (CH2O) and polycyclic aromatic hydrocarbons (PAH). High-speed line-of-sight schlieren with a pulsed infrared light emitting diode (LED) is applied simultaneously with PLIF to visualize the spray penetration, low- and high-temperature ignition, and turbulent flame dynamics, while in-vessel pressure is used in concert with the optical diagnostics to assess the ignition delay time of the high-temperature ignition events. The PLIF and line-of-sight measurements, documenting the position and timing for formation and consumption of CH2O and the inception and distribution of PAH downstream with changes in ambient oxygen concentration and injection pressure, provide a unique database for detailed evaluation of single-event (e.g. LES) or ensemble-average CFD.

Hybrid algorithm for the detection of turbulent flame fronts

This paper presents a hybrid and unsupervised approach to flame front detection for low signal-to-noise planar laser-induced fluorescence (PLIF) images. The algorithm combines segmentation and edge detection techniques to achieve low-cost and accurate flame front detection in the presence of noise and variability in the flame structure. The method first uses an adaptive contrast enhancement scheme to improve the quality of the image prior to segmentation. The general shape of the flame front is then highlighted using segmentation, while the edge detection method is used to refine the results and highlight the flame front more accurately. The performance of the algorithm is tested on a dataset of high-speed PLIF images and is shown to achieve high accuracy in finely wrinkled turbulent hydrogen-enriched flames with order of magnitude improvements in computation speed. This new algorithm has potential applications in the experimental study of turbulent flames subject to intense wrinkling and low signal-to-noise ratios.Graphic abstract

Investigation of dynamic mixedness characteristics of a transverse acoustically excited turbulent jet by high-repetition-rate acetone tracer planar laser-induced fluorescence technique

The dynamic mixedness characteristics of a bluff-body stabilized turbulent jet under transverse acoustic excitations were investigated using high-repetition-rate acetone planar laser-induced fluorescence (PLIF) at 7 kHz and multipoint scanning hot-wire measurements. Acetone mixedness imaging was applied for the turbulent jet to assess the interaction between the turbulent jet and the imposed transverse acoustic excitations at a driving frequency of 50 Hz. The high-repetition-rate acetone PLIF images showed that the acetone mixedness distribution swung left and right frequently under the transverse acoustic excitation, and the deflection angle could reach about 6°. The mixedness area of a turbulent jet flow could also be increased by 13.3% when excited by a transverse acoustic wave. Meanwhile, the sequence of instantaneous acetone PLIF images illustrated how the wrinkled edges were generated when acoustic excitations were imposed. The curvature of the acetone PLIF interface showed that the portion of large curvatures increased by 60% after applying an acoustic wave of 123 dB. Multipoint hot-wire measurements further stressed that the turbulent intensity at the transverse acoustic excitation of 123 dB increased to be about 1.3 times the natural turbulence. The proper orthogonal decomposition results showed that the large and small scales of the jet wrinkles both increased with the sound pressure level. RANS transient simulation also implied that distorted velocity streamlines could be achieved inside the turbulent jet due to the transverse acoustic excitation. They could further lead to inhomogeneous convection and increased mixing between the turbulent jet and the surrounding air.

OH planar laser-induced fluorescence imaging system using a kilohertz-rate 283 nm UV Ti:sapphire laser.

A narrow linewidth Ti:sapphire laser is developed and characterized for the generation of an ultraviolet nanosecond laser pulses for the planar laser-induced fluorescence (PLIF) imaging of hydroxyl (OH). With a pump power of 11.4W at 1kHz, the Ti:sapphire laser produces 3.5mJ at 849nm with pulse duration of 17ns and achieves a conversion efficiency of 28.2%. Accordingly, its third-harmonic generation outputs 0.56mJ at 283nm in BBO with type I phase match. An OH PLIF imaging system has been built; a 1 to 4kHz fluorescent image of OH of a propane Bunsen burner has been captured based on this laser system.

Analysis of Combustion Instability of Methane-Air Premixed Swirling Flame Based on OH-PLIF Measurements.

A method for determining combustion instability using flame structure parameters is presented. A speaker is used to provide controllable external excitation for the combustion system. The experimental object is a methane-air swirl premixed flame. The flame structure parameters such as height, width, and flame surface density extracted from the hydroxyl planar laser-induced fluorescence image were used to analyze combustion instability at different equivalence ratios (0.8-1.2) and inlet flow rates. It is confirmed that the inflection point of the flame structure parameters corresponds to the evolution of combustion instability verified by the flame transfer function. The results show that with the increase of inlet velocity v, the flame aspect ratio h/b, average OH* concentration, and surface density Σ gradually decrease. The thickness δ of the flame brush shows an increasing trend under the same conditions. With the increase of equivalence ratio Φ, the average OH* concentration and flame surface density Σ increase continuously. The changing trend of flame brush thickness decreases first and then increases to a peak. Finally, it continues to decline after reaching the peak. The flame responds strongly to the sound field when the equivalence ratio is 0.9 and 1.0. In the range of 80-240 Hz, the flame response near 110 and 190 Hz is stronger at each equivalence ratio (0.8-1.0). When the equivalent ratio is 0.9 and 1.0, the amplitude fluctuations of the flame transfer function are much larger than those under other conditions. Meanwhile, the specific performances of the flame structure parameters are that the flame height, average OH* concentration, and flame surface density decrease, and the flame brush thickness increases. These results can be used as a basis for judging combustion instability. This method proves that the parameter information monitored during the flame combustion process can be used to judge the changes in combustion conditions and can adjust the corresponding conditions more accurately and quickly.

Investigation of syngas combustion in a novel ultra-low emission 20-kW two-stage combustor

Two-stage rich-catalytic lean-swirl combustion is investigated in a 20-kW combustor using biomass-derived syngas as fuel. In the first stage, fuel-rich catalytic combustion of syngas (Фfirst = 4) is investigated using FeCr-alloy monolith and platinum (Pt) as a catalyst. The fuel undergoes partial conversion in the catalytic stage. The remaining unoxidized reactants from the first stage are burnt using additional air in the second stage. The flame is stabilized by two concentric swirling streams in the second stage, where the exhaust from the first stage flows through the inner stream, and the outer stream carries air. By changing the swirl direction of the inner stream, co or counter-swirl flame is established, whereas the overall equivalence ratio is changed by increasing the airflow rate. It is found that the flame topology is significantly different for the co-swirl and counter-swirl configurations. Transitions from a compact ‘M’ shaped flame to a shear-layer stabilized ‘V’-shaped flame is observed by changing the overall equivalence ratio. The combination of planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) highlights the differences in the flow field and OH radical distribution, leading to the observed flame shape. The dynamic stability of the combustor is assessed using sound pressure level (decibel) obtained using a microphone and high-speed OH* chemiluminescence. Finally, the CO and NOx concentrations are measured at the combustor exit. The CO emission ranges from 10 ppm to 70 ppm, whereas the NOx emission is negligible in all conditions studied. Thus, the present study successfully demonstrates the feasibility of catalytic and twin-swirl combustion in achieving near-zero emissions while operating with biomass-derived syngas.

Liquid fuel refill dynamics in a rotating detonation combustor using megahertz planar laser-induced fluorescence

The highly dynamic response of a continuously injected liquid fuel jet to rotating detonation waves is a critical parameter in the design, performance optimization, and modeling of a rotating detonation engine (RDE). In this work, this response is spatio-temporally resolved from the fuel injection point to the detonation channel using planar laser-induced fluorescence (PLIF) at imaging rates up to 1 MHz. A rotating detonation combustor (RDC) is operated on hydrogen and air to sustain stable detonation waves that interact in a one-way coupled manner with a single liquid fuel jet that propagates into the combustion chamber with cycle periods of ∼250 µs. Diesel is utilized as a realistic fuel surrogate with higher aromatic compounds to enable fluorescence excitation using the 355 nm third-harmonic output of a burst-mode Nd:YAG laser. By optimizing the technique to accommodate orders of magnitude variations in the fuel density throughout the injection process, the PLIF data enable measurements of (i) the overall refill dynamics after the arrival of the detonation wave, (ii) changes in the liquid spray trajectory with microsecond temporal resolution, and (iii) the time required to reestablish the quasi-steady axial refilling process as a function of peak chamber pressure relative to the air- and liquid-injector pressure drops. As the passage of the detonation wave imparts significant changes in the momentum flux ratio, the qualitative liquid break-up process and spatial distribution of the spray also vary significantly in time. Only as the injection system recovers late in the cycle does the fuel spray eventually return to a quasi-steady position and allow comparisons with theoretical jet trajectories. These data, enabled by ultra-high-speed PLIF imaging, represent some of the first detailed measurements for quantifying the dynamic response and recovery of liquid jets exposed to periodic detonations in an operating RDC.