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Simultaneous laser-induced fluorescence of OH radicals and particle image velocimetry, and quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering measurements are used to investigate the effects of Karlovitz number (by varying the equivalence ratio) and residence time (by varying the axial measurement location) on the internal flame structures of lean premixed hydrogen/air turbulent jet flames. The turbulent flow fields, instantaneous macroscopic flame structures, and thermochemical states of a set of lean premixed hydrogen/air turbulent flames with varying initial equivalence ratio of 0.3, 0.4, and 0.45 at a constant bulk velocity of 100 m/s are discussed. With an increasing equivalence ratio, the Karlovitz number derived from the turbulent flow fields decreases rapidly from 7690 to 260 and 100, determined at a downstream location of x / D = 7. At the highest Karlovitz number (i.e., the lowest equivalence ratio), a distributed burning is observed in the jet flame as turbulent transport dominates over molecular mixing, and the effects of differential diffusion and flame curvature are suppressed. With decreasing Karlovitz number, intense burning regions characterized by elevated local equivalence ratio, high water mole fraction, and super-adiabatic flame temperatures are observed in association with positive flame curvature. The same combustion diagnostics are applied to another lean premixed hydrogen/air turbulent flame with an initial equivalence ratio of 0.4 and a bulk velocity of 200 m/s at selected downstream locations of x / D = 3.5, 7, 10.5, and 14 to assess the effects of developing turbulence and residence time. Corresponding Karlovitz numbers are 680, 730, 775, and 690, as the local turbulent intensity changes along downstream locations. While flame structures reveal characteristics towards distributed burning at lower x / D , a locally intense burning region appears at higher x / D together with positive curvature. This is mainly because the turbulence develops with increasing x / D and the flame surface is more disturbed and curved by turbulent eddies with increasing turbulence length scales. The highly diffusive hydrogen is locally concentrated by positively curved flame surfaces, resulting in fuel-rich burning regions at a higher temperature. This indicates that, as velocity fluctuations increase in axial direction, turbulence can also promote thermo-diffusive instabilities to a certain extent, increasing the mixture inhomogeneity in temperature space by interacting with the differential diffusion effect of hydrogen at atmospheric pressure. Novelty and significance The novelty of the current work is the focus on turbulence–chemistry interactions of premixed hydrogen/air jet flames at high turbulence and ultra lean conditions. Comprehensive experimental results including the turbulent flow fields (particle image velocimetry), instantaneous flame structures (laser-induced fluorescence of OH radicals) and internal thermochemical states (quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering imaging) of hydrogen/air jet flames are analyzed and discussed. The data sets with well-characterized boundary conditions and high measurement accuracy, are crucial for validation and development of numerical simulation models. [ABSTRACT FROM AUTHOR]

• A novelty micro-mixing multi-injector burner was constructed for burning high-hydrogen-content CH 4 /H 2 fuels. • Under all experimental conditions, the micro-mixing burner can achieve ultra-low NOx emissions (less than 8 ppm@15 %O 2). • The impact of H 2 content on the flame morphology, flame stability and emission characteristics were investigated. • Distinct mechanisms for flame stabilization between pure methane and high-hydrogen-content CH 4 /H 2 fuels were observed. Micro-mixing combustion technology premixes air and fuel in a microscale channel, enabling efficient mixing within a short distance can preventing flame flashback suitable for H 2 -enriched fuels. This approach effectively mitigates the formation of localized high-temperature zones in the flame and shortens the residence time of fuel in the high-temperature zone, thereby significantly minimizing NOx emissions. In this study, a self-designed micro-mixing multi-injector burner was constructed for burning high-hydrogen-content CH 4 /H 2 fuels. The H 2 / CH 4 volume ratios range from 0:10 to 8:2. The effects of fuel composition and equivalence ratio on flame structure, flame stability, and pollutant emission of the CH 4 /H 2 micro-mixing combustion were investigated experimentally. The results demonstrated that even at an adiabatic flame temperature of 2100 K, the micro-mixing burner can achieve low NOx emissions (less than 8 ppm@15 %O 2). Furthermore, compared to pure methane, highly hydrogen-enriched fuel can exhibit lower NO x emissions under stable combustion conditions. Based on the measurement of the distribution of OH radicals using Planar Laser-Induced Fluorescence (PLIF), and the captured OH* chemiluminescence signals, the impact of H 2 concentration, equivalence ratio and Reynolds number on the flame morphology were investigated. The measurements suggest that hydrogen plays a pivotal role in the formation of OH radicals. For fuels with higher H 2 concentration, the flame height and lift-off height are smaller, and the heat release area is more concentrated. The flame with higher H 2 concentration can be stabilized at a lower equivalence ratio. As the H 2 concentration decreases, the distance between flame jets shortens, leading to gradual coupling and eventual reunion of the jet flames. When the combustion state approaches the lean blowout (LBO) limit, the flame structure of high hydrogen concentration fuel exhibits notable disparities compared to that of pure methane fuel, indicating distinct mechanisms for flame stabilization between these two fuels. [ABSTRACT FROM AUTHOR]

Understanding the turbulence-flame interaction is crucial to model the low-emission combustors developed for energy and propulsion applications. To this end, a novel frame interpolation (FI) method is proposed to better resolve the spatiotemporal evolution of premixed turbulent flame structures. The framework is completely self-supervised, agnostic to optical flow, and driven by leveraging transferrable feature knowledge at lower speeds and adversarial learning to statistically map the flame dynamics across frames. The method is successfully applied on a 10 kHz CH planar laser-induced fluorescence (PLIF) dataset of highly wrinkled premixed flames with turbulent Reynolds numbers (ReT) of 1100, 1400, and 7900, by down-sampling the image sequence to 5 kHz and restoring the sequence back to 10 kHz via FI. All reconstructions recovered important flame events and displayed excellent resemblance of the corrugated CH-layer geometries to that of the ground truths, with average intersection over union (IoU) and structural similarity index (SSIM) scores of 0.49 and 0.82, which are above the high-similarity baselines of 0.36 and 0.75, respectively. The wrinkling parameters (WP) of the flames also matched the ground truths, wherein R2 was roughly 0.95 for ReT = 1100 and 1400 and 0.85 for ReT = 7900 (lower due to the turbulence-induced uncertainties). The FI is further iteratively repeated to 40 kHz on the ReT = 7900 flames to facilitate pocket analysis by confidently linking their origin of formation, thus, enabling distinction from 3D tunnels, and improving statistical characterization of their consumption speeds. Given that the object features do not exhibit highly turbulent motions with regard to the initial time step, the proposed FI method is shown to be highly accurate and useful to analyzing finite-resolution experimental image sets including, but not restricted to, CH-PLIF.

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A La2O3/CeO2 catalyst was synthesized and achieved 19 % C2+ yield with 65 % C2+ selectivity at 650 °C. The doping of CeO2 not only enhanced catalytic performance, but also improved catalyst stability in the presence CO2 and H2O. This study investigated the kinetics on La2O3/CeO2 over a wide range of operating conditions. Planar laser‐induced fluorescence was implemented to directly observe the presence of formaldehyde, as the first time reported in OCM. A packed bed reactor model was constructed based on the dimensions of experimental setup and catalyst characterization. Rather than using a tuned gas phase model, the experimentally validated mechanism NUIGMech1.1 was implemented. The proposed homogeneous‐heterogeneous OCM mechanism kinetic model was validated against experimental data under isothermal conditions, and the results show good agreement with measured species. The rate of production (ROP) was performed to identify the important reactions and prove the necessity of surface reactions for the OCM process. [ABSTRACT FROM AUTHOR]

The effect of a zero-mean-flow turbulent ambient on the geometry of the turbulent/turbulent interface (TTI) of an axisymmetric jet is investigated and compared with the traditional turbulent/non-turbulent interface (TNTI). The ambient turbulence is generated using a random jet array. Orthogonal cross-sections of the jet subjected to different levels of ambient turbulence intensities and length scales are captured using planar laser-induced fluorescence. The enhanced radial transport of concentration from the jet core towards the interface and the increased scalar fluctuations within the jet caused by the ambient turbulence result in steeper mean and root-mean-square scalar conditional jumps across the TTI layer compared with those of the TNTI, respectively. When compared with the quiescent ambient, the mean effect of background turbulence is to stretch and corrugate the interface surface area as evident from the wider probability density of the interface radial position, the lowered occurrence of zero-curvature surface elements, the greater misalignment between the radial and normal unit vectors of the interface, the increased tortuosity and the increased magnitude of the fractal exponent. [ABSTRACT FROM AUTHOR]

Three-dimensionalities in the wake of flow past a circular cylinder executing sinusoidal rotary oscillations about its axis is studied experimentally. The results of water tunnel experiments on a rotationally oscillating cylinder for Reynolds number of 250 with varying amplitude and forcing frequency are discussed. Qualitative studies using hydrogen bubble and laser-induced fluorescence flow visualisation techniques are performed. Observation made for oscillating amplitude, $\theta _{0} = {\rm \pi}/4$ and $\theta _{0}=3{\rm \pi} /4$ , and a low normalised forcing frequency, $FR$ , of 0.75 and 0.5, respectively, confirmed a mode having a spanwise non-dimensional wavelength of $\sim$ 1.8 which is also observed for a rotating cylinder. On increasing forcing frequency this mode disappears and a new mode with a bean-shaped structure and a much smaller spanwise normalised wavelength of $\sim$ 0.8 appears at an $FR$ of 1 and an oscillation amplitude of ${\rm \pi} /2$. This mode remains almost stable until a forcing frequency of $FR=1.4$. At higher forcing frequency, $FR=2.75$ , and oscillation amplitude of $3{\rm \pi} /4$ , a mode with cellular structure and a normalised spanwise wavelength of $\sim$ 1.6 is identified. The cells in this mode flatten up with increasing downstream distance and are shed alternately with respect to the adjacent cell. Certain combinations of forcing parameters resulted in a forced two-dimensionality of the wake. Quantitative studies using hot-wire measurements and particle image velocimetry confirm the presence of these modes and wake characteristics. Wake mode map in the forcing frequency and amplitude plane is presented showing regions of newly discovered modes and wake lock-on boundaries. [ABSTRACT FROM AUTHOR]

This paper describes the measurement methodology for quantifying the instantaneous full 3D scalar dissipation rate (SDR or χ ) in order to characterize the rate of mixing. Measurements are performed in a near field of a jet-in-swirling-coflow configuration. All three components of χ are measured using a dual-plane acetone planar laser-induced fluorescence technique. To minimize noise, a Wiener filtering approach is used. The out-of-plane SDR component ( χ 3 ) is validated by assuming isotropy between axial and azimuthal components of SDR. An optimum laser-sheet separation distance ( Δ s ) is identified by comparing the SDR components on the basis of instantaneous, mean, and probability density function data. The in-plane resolution needs to match the Batchelor scale ( λ B ) for the central difference scheme-based SDR deduction. However, the out-of-plane resolution, Δ s , requirement is different owing to the use of two-point difference based SDR and systematic biases. The optimum Δ s is found to be 2.5 λ B . Finally, measurement guidelines are provided to assess the accuracy of 3D SDR measurements. [ABSTRACT FROM AUTHOR]

Inertial vortical flows can be used as a tool to capture and manipulate microparticles, vesicles, or cells. Current work follows our previous report [Zhang et al., Int. J. Multiphase Flow 150, 104030 (2022)] to study the flow of diluted particle suspension in micro-cross-shaped channels at 20 < Re < 500 by micro-laser-induced fluorescence and high-speed photography. Effects of inlet aspect ratio (α) and Reynolds numbers (Re) on flow regimes and particle capture were studied. Numerical simulation was adopted to reveal vortex breakdown dynamics associated with particle capture. For each α, as Re increases, segregated flows, steady engulfment flows, vortex shedding flows, and unsteady engulfment flows appear in turns. Experimental results demonstrate a flow-induced, Re and α-dependent particle cluster within steady engulfment and vortex shedding flows, and an increase in α decreases the onset Re of the cluster. With increasing Re, an interesting oscillation of the cluster is established, which triggers the escape of captured particles. Simulation results show that the oscillation frequencies of the cluster and fluid are comparable. Although isolated particles display brief recirculating paths under unsteady engulfment flows, the particle cluster disappears. [ABSTRACT FROM AUTHOR]

The mixing process and distribution characteristics of a supercritical endothermic hydrocarbon fuel (EHF) jet injected into a supersonic crossflow were investigated by experimental and numerical methods, respectively. The schlieren system and acetone planar laser-induced fluorescence (PLIF) optical system were used to capture the flow-field structural characteristics and instantaneous plume. The mixture and real gas models were employed to calculate the interaction of a transverse jet and supersonic crossflow and reveal a good accuracy with the experimental results. The mixing efficiency and total pressure loss were analyzed based on the numerical results. The results indicate that the supercritical-state EHF directly changes to a gaseous state as it enters the supersonic crossflow from the injector. The EHF jet plume boundary increases with the increasing momentum flux ratio (q). As the streamwise and spanwise distance increases, the traverse heights and expand width increase, and the EHF jet plume presents a semicircle shape in the cross-sectional plane. With the increase in the traverse direction, the concentration distribution shows a fast and then slow power exponential decreasing law; the highest concentration point starts from the near-wall region and rises in the transverse direction with the flow distance increasing. For the same injection condition, the higher the inflow Mach number, the higher the mixing efficiency. For the same Ma, the mixing efficiency is better for the case with low injection pressure and high injection temperature. The total pressure loss is greater in the higher Ma, and high injection pressure conditions cause greater total pressure loss. [ABSTRACT FROM AUTHOR]

Gas-induced liquid-phase mixing in shallow vessels is important to primary and secondary steel-refining processes and governs the quality of the steel. In the present work, we have investigated the dynamics of meandering bubble plumes and their role in liquid-phase mixing in shallow vessel that corresponds to 1:6 scaled-down model of ladle used in the secondary steel-refining process. The effects of multiple meandering bubble plumes and their interactions on the liquid-phase velocity distribution, recirculatory flow, plume oscillation frequencies, and turbulent kinetic energy are analyzed for different bottom injection configurations using particle image velocimetry measurements. The role of meandering bubble plumes and their interactions in the liquid-phase mixing is analyzed using planar laser-induced fluorescence measurements. ~60% reduction in the mixing time was found with differential flow scheme. Present findings help to understand the contributions of differential flow and bubble plumes interactions to liquid-phase mixing in shallow vessels. [ABSTRACT FROM AUTHOR]

The stabilization mechanism of nozzle-attached flames in laminar non-premixed jets was studied experimentally and numerically. The local gas flow velocity and the flame propagation speed within the mixing layer at flame base were experimentally obtained before the blowout limit was reached. The experimental results show that, for burner-attached flames, the local flow velocity is much less than the burning speed, suggesting that the propagation of partially premixed flame within the mixing layer at flame base is stopped. The thickness of the fuel/air mixing layer at the attached flame base is quite narrow (1.1–1.5 mm) and less than the minimum quenching distance caused by conductive heat loss to the tube (or slit) wall in laminar premixed flame. Heat losses by the conduction from the flame base to the burner wall, and the convection between the flame base and the incoming flow (hereafter referred to as local convection) were studied through simulation. The significant roles of these effects in the stabilization of the attached flame were analyzed. While the heat loss by conduction to the fuel tube is neglected, the partially premixed flame at flame base can propagate along a much narrower mixing layer. On the contrary, a critical thickness of the mixing layer (in which the local partially premixed flame can sustain) exists near the flame base with considering the conductive heat loss. The conductive heat loss to the burner wall plays a relatively important role to flame stabilization without (or with a small amount of) coflow air. As the velocity of coflow air increases, the heat loss due to local convection gradually exhibits significant effects on flame stabilization. In addition, a simple theoretical model is provided based on the balance between the heat released by chemical reaction and the heat lost by conduction and local convection, suggesting clear dependence of the thickness of the fuel/air mixing layer on the local gas flow velocity and the laminar flame speed. The stability behaviors of flame base can be reasonably explained and predicted through such relation. [ABSTRACT FROM AUTHOR]

The present work describes an assessment and validation of the newly developed Very-Large Eddy Simulation (VLES) method for the numerical study of complex non-linear response of a lean premixed flame to acoustic velocity fluctuations. The VLES method is a self-adaptive turbulence method which can achieve seamless switching between RANS, LES, and finally approaching DNS. It is extended to the turbulent combustion simulation in the present study. The selected test case is a bluff-body stabilized lean premixed turbulent flame with acoustic forcing. Turbulence modeling is achieved using the VLES method and the premixed turbulent combustion is modeled using the Thickened Flame Model (TFM) combining a reduced two-step chemical reaction mechanism. Harmonically varying velocity fluctuation is imposed on the inlet to introduce the acoustic forcing. Three types of flow including the cold flow, unforced reactive flow, and forced reactive flow are used to validate the simulation method. The mean velocity, dynamic response of the flame to inlet velocity forcing and the quantitative response of the heat release rate are captured by the present calculations with very reasonable accuracy compared with the experimental data. The present VLES predictions are comparable or even better than the present LES results using the combustion model and the same computational grid. This work thus confirms that the VLES methodology is capable of successfully predicting the response of lean premixed turbulent flame to acoustic forcing. [ABSTRACT FROM AUTHOR]

This paper presents the results of two-line OH planar laser-induced fluorescence (PLIF) thermometry in a laminar conical flame of a gas–droplet mixture of ethanol and air. Laminar flow of a droplet-laden ethanol–air uniform mixture was produced by an ultrasonic atomizer in a vessel filled with liquid ethanol. The properties of the two-phase flow at the nozzle exit without combustion were controlled by a time-shift optical sensor. The temperature field was estimated based on the excitation of the (5) and (14) lines of the (1–0) band of the – electronic system. The spatial nonuniformity of the energy distribution in the laser light sheet illuminating the central plane of the flame cone and the change in the pulse energy from frame to frame were compensated using an additional camera recording the laser light intensity distribution in the calibration cuvette. [ABSTRACT FROM AUTHOR]

Görtler vortices (GVs) in dual-incident shock-wave/turbulent-boundary-layer interactions (dual-ISWTBLIs) are experimentally investigated in a Mach 2.48 flow. A double-wedge shock generator with two deflection angles of 8° and 5° is used to produce two incident shock waves (ISWs). Flow structures of the experiments with three different shock-wave distances were visualized by the ice-cluster-based planar laser scattering technique at two orthogonal planes (x–y and x–z planes). The images in the x–y plane present three types of flow patterns of dual-ISWTBLIs corresponding to the first type with a triangle-like separation, the second type with a quadrilateral-like separation, and the third type with two isolated interactions induced by the two ISWs. The images in the x–z plane indicate that the GVs exist in the first type of dual-ISWTBLI originating in the vicinity of the apex of the separation region and cover nearly the whole spanwise range of the reattachment region. By comparison, the GVs intermittently occur in the limited spanwise range of the reattachment region in the second type of dual-ISWTBLI. No GVs are observed in the third type of dual-ISWTBLI because no visible separation is induced under the experimental conditions considered in this situation. In addition, based on the wall-pressure distribution in the former two types of dual-ISWTBLIs, this paper proposes a method to estimate the mean-flow streamline curvature in the reattachment region, thereby obtaining the criteria for the existence of GVs, according to which reasonable explanations for the different distributions of GVs in the two types of dual-ISWTBLIs are provided. [ABSTRACT FROM AUTHOR]

Er3+/Li+ co-doped Y 2 O 3 nanocrystals were synthesized by the sol-gel method. Fluorescence intensity has been enhanced about 270 times under 980 nm laser excitation by combined laser annealing (90 times) and co-doped with Li+ ions (3 times). The doping of Li+ ions increased the asymmetry around Er3+, while laser annealing reduced surface defects. Fluorescence characteristics with the temperature of laser annealed Er3+/Li+ co-doped Y 2 O 3 nanocrystals were studied. An interesting phenomenon that the non-thermal coupling of I 563 /I 799 transformed into the thermal coupling as the temperature rises is found. The possible mechanisms were discussed, and fluorescence thermometry based on I 563 /I 799 showed higher temperature sensitivity than other reports. The effective fluorescence enhancement method and the new fluorescence thermometry energy levels can promote the application of rare earth fluorescent materials. [ABSTRACT FROM AUTHOR]

The current work presents simultaneous, high-speed measurements at 60,000 fields per second of velocity and mole fraction using particle image velocimetry (PIV) and planar laser induced acetone-fluorescence in a Richtmyerâ€"Meshkov instability of an inclined interface (Atwood number, At  = 0.22). Specifically, around 2 ms of temporal evolution of the vortex structures and their associated scalar modes immediately following the interface-reshock interaction is presented. Two initial interface conditions are discussedâ€"(a) a sharp, inclined ‘single mode’ interface and (b) a ‘multi-mode’ interface where small perturbations are imposed on the single mode case. A 2D wavelet decomposition of the scalar flow field shows a highly intermittent distribution of small-scale variance throughout the interface even at late times. These are correlated strongly with the vortex structures and local turbulence intensity, where each small-scale scalar mode is sandwiched between two co-rotating vortex structures. This indicates that the interstitial regions between the vortices are significant hotspots of entrainment, which is then dispersed by the induced, counter-flow velocity fields. The multimode case demonstrates similar organization at large scales, while the scalar field is much more homogeneous at smaller scales. These observations highlight the importance of capturing the early time vortex evolution to accurately estimate any late time intermittency, especially where deposition of intense vorticity on sharp interfaces is present. [ABSTRACT FROM AUTHOR]

This work aimed to test and optimise reactive Planar Laser-Induced Fluorescence (PLIF) methods for the visualisation of the micromixing regions in chemical reactors using standard PLIF and Particle Image Velocimetry (PIV) equipment with the laser source 512 nm. Two methods were tested: (i) an acid–base reaction with fluorescein as the reaction-sensitive tracer and (ii) Fenton's reaction, with Rhodamine B as the reaction tracer. Both test-reactions were studied in stopped-flow equipment to define suitable operational conditions, namely the chemical composition of the inflow streams, the concentration of reagents and fluorophore, and suitable excitation light wavelength. The visualisation of the micromixing regions was tested in a continuous flow reactor with a T-jet geometry. A laser light sheet emitted from an Nd:YAG laser illuminated the axial section of the demonstration reactor. The mixing dynamics and the reaction course were visualised with the acid–base reactive PLIF images. Fenton's reactive PLIF method showed the overall distribution of mixing and reaction regions. The main contribution of this work is benchmarking two methods with costs that enable the visualisation of micromixing regions in continuous high-throughput reactors. [ABSTRACT FROM AUTHOR]

Inclined dense jets are commonly used to mitigate the environmental impacts of brine discharge in coastal desalination plants. Numerous studies have been performed to numerically simulate the sophisticated structures of the flow within this process. However, numerical prediction of the process is still a challenge. The present paper performs a comprehensive numerical study using the large eddy simulation (LES) approach on inclined dense jets oriented at angles 15°, 30°, 45°, 60°, and 75°, with a particular emphasis on the near-wall region, where the most complicated structures of the flow occur. The objective is to evaluate the capability of the LES approach to replicate the mixing processes of the brine jets. Besides numerical simulations, a series of experiments using the planar laser-induced fluorescence technique is carried out to compare each numerical simulation with its experimental correspondence. Both numerical and experimental results are presented in comparative figures compared to previous experimental data. The comparisons indicated that the LES model could reasonably predict the geometrical and mixing characteristics of inclined dense jets; however, the flow features are still underestimated by up to 25%. Moreover, the model could reproduce the local concentration build-up near the impact point. The processes within the near-wall region leading to this local decrease of dilution are discussed in detail. Additionally, a novel criterion is proposed to predict when the flow reaches a quasi-steady-state. The criterion can be used to manage the computational expenses, especially in simulations with high demanding computing power such as LES. Article Highlights: The capability of the LES approach to reproduce the mixing behavior of inclined dense jets was investigated. The flow behavior in the near-wall region was analyzed in detail. The LES approach was able to predict the flow behavior, especially in the near-wall region, with reasonable accuracy. [ABSTRACT FROM AUTHOR]

Global instabilities in swirling flows can significantly alter the flame and flow dynamics of swirl-stabilized flames, such as those in modern gas turbine engines. In this study, we characterize the interaction between the precessing vortex core (PVC), which is the consequence of a global hydrodynamic instability, and thermoacoustic instabilities, which are the result of a coupling between combustor acoustics and the unsteady heat release rate. This study is performed using experimental data obtained from a model gas turbine combustor employing two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at five bar pressure. The flow split between the two streams is systematically varied to observe the impact of flow structure variation on the system dynamics at both non-reacting and reacting conditions. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence and acetone planar laser-induced fluorescence are used to obtain information about the velocity fields, flame and fuel flow behaviour, respectively. Spectral proper orthogonal decomposition and a complex network analysis are used to identify and characterize the dominant oscillation mechanisms driving the system. In the non-reacting data, a PVC is present in most cases and the amplitude of the oscillation increases with increasing flow through the centre nozzle. In the reacting data, three dominant modes are seen: two thermoacoustic modes and the PVC. Our results show that in the cases where the frequency of the PVC overlaps with either of the thermoacoustic modes, the thermoacoustic modes are suppressed. The complex network analysis coupled with a weakly nonlinear theoretical analysis suggests the mechanisms by which this coupling and suppression of the thermoacoustic mode occur. [ABSTRACT FROM AUTHOR]

A flow field reconstruction algorithm based on deep learning is an effective method to detect the evolution of wave structure in a scramjet combustor, which is of great significance for accurately predicting the operating performance of the scramjet. This paper proposes a dual-branch fusion model based on a multi-head attention mechanism to reconstruct the flow field schlieren image in a supersonic combustor. The proposed model adopts a dual-path fusion mode. One branch is composed of transposed convolution and conventional convolution, forming a symmetrical structure for dimension enhancement and feature extraction. The other is formed by a multi-head attention mechanism and a full connection layer in series. It utilizes the same attention mechanism to obtain different sensitive features and enhance the global model perception. The proposed model was trained and tested on a dataset constructed from hydrogen-fueled scramjet experiments in a direct-connected supersonic pulse combustion wind tunnel at Mach 2.5. Numerous experiments prove that the model can effectively reconstruct the basic wave system structure of a complex flow field, and it is in good agreement with the original flow field. The average peak signal-to-noise ratio, structural similarity, and average linear correlation coefficient of the proposed model are reached 20.92, 0.602, and 0.943, respectively, which verify the effectiveness of the proposed model in reconstructing the supersonic flow field. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The main challenge of improving spark ignition (SI) engines to achieve ever increasing thermal efficiencies and near-zero pollutant emissions today concerns developing turbulent combustion under homogeneous ultra-lean premixed mixtures (HULP). This continuous shift of the lean operation limit entails questions on the applicability limits of the combustion models used to date for SI engine design and optimization. In this work, an assessment of flamelet-based models, widely used in RANS SI engines simulations of premixed turbulent combustion, is carried out using an open-source 3D-CFD platform to clarify the applicability limits on HULP mixtures. Two different consolidated approaches are selected: the Coherent Flame Model (CFM) and the Flame Area Model (FAM). Both methodologies are embedded by the authors into the same numerical structure and compared against measurements over a simplified and controlled flame configuration, which is representative of engine-like conditions. The experimental steady-state flame of type "A" of the Darmstadt Turbulent Stratified Flame (TSF) burner is selected for the assessment. This configuration is characterized by flame measurements over a strong shear and mixing layer between the central high-speed CH 4 -air jet and the surrounding slow air co-flow, hence, it represents an interesting controlled condition to study turbulent HULP mixtures. A comparison between computed results and experimental data on trends of mean flow velocity, turbulence, temperature and mixture stratification was carried out. This enabled us to assess that the investigated flamelet-based combustion models failed in providing accurate and reliable results when the flame approaches turbulent HULP mixture conditions, demonstrating the urgency to develop models able to fill this gap. [ABSTRACT FROM AUTHOR]

The temperature measurement of liquids is essential in many fields such as the thermal management of a chemical process or material synthesis. It is relatively easy to measure the liquid temperature on the macroscale, but temperature imaging on the micro/nanoscale is still challenging. Conventional methods such as the use of thermocouples and resistance temperature detectors and laser-induced fluorescence have drawbacks when applied to microfluidic temperature imaging techniques. In the present work, fluorescence anisotropy (FA) was used as a liquid temperature measurement method on the microscale. FA has an advantage over conventional methods of intrinsic normalization of the light intensity, which enables ratiometric measurement even when using a single wavelength from a fluorophore. We measured FA values in liquids of different viscosities and temperatures using a spectrofluorometer having two rotational polarizers, then obtained the temperature sensitivity of FA. The temperature sensitivity of FA was also theoretically investigated using the derivative of Perrin’s equation, which relates FA, viscosity, temperature, fluorescence lifetime, and molecular size. The experimental results show that each molecule has an optimal viscosity range indicating the maximum temperature sensitivity and that fluorescein isothiocyanate–dextran conjugates with smaller molecular weights have higher sensitivity. Also, reasonable agreement between experimental and theoretical results was confirmed. Consequently, it was clarified that the temperature sensitivity of FA can be controlled by labeling to adjust the required viscosity range of the sample solution and that theoretical estimation provides qualitative guidance for FA-based thermometry. [ABSTRACT FROM AUTHOR]

In this work, planar two-photon laser-induced fluorescence (2p-LIF) is applied for the first time to analyze the fluid dependent spray structure and atomization behavior of water and ethanol in a quantitative way. A commercial six-hole DISI (Direct-Injection Spark-Ignition) injector was studied at different injection pressures, operated with liquids containing the LIF dye fluorescein. Specifically for DISI-injectors, the fluid-dependent atomization is very complex and not fully understood due to the cavitating, turbulent nozzle flow that dominates the spray formation. Optical access and analysis of the near-nozzle spray are often challenging due to multiple light scattering in dense regions which is reduced by 2p-LIF measurements using a femtosecond laser. This allows high-contrast spray imaging close to the nozzle, resulting in an improved identification of single liquid structures of the spray. Thus, a higher accuracy of sizing is possible. Compared to water, the ethanol spray shape shows increased cone angles in the nozzle near-field of about 6%, which cannot be explained by classical atomization theory based on aerodynamic breakup. The larger cone angle of ethanol was attributed to its larger viscosity, which could decelerate the flow at the wall of the injection hole, affecting the velocity profile of the emerging jet. The atomization shows a main jet breakup distance of 7–10 mm in which the structure sizes decreased drastically, specifically for water. For the size of the liquid structures in the near-nozzle region, which show dimensions of about 80–130 μm, ethanol exhibited about 2% smaller Feret's diameters than water for the tested time steps at 20 MPa. This effect is even more distinct for other injection pressures and positions at a further distance to the injector. For all investigated conditions and measurement positions downstream of the nozzle, ethanol showed on average about 24% smaller structures compared to the water spray. Although this trend is in accordance with the classical atomization theory based on the aerodynamic breakup mechanism, other effects, such as cavitation and nozzle-flow induced breakup, contribute to this behavior. [ABSTRACT FROM AUTHOR]

The effect of fuel-to-air jet momentum flux ratios (varied from 0.1 to 5.1) in a 6.25 kW Reverse-Cross-Flow (RCF) combustor is investigated at a maximum thermal intensity of 40 MW/m3-atm using methane fuel. Reaction zone is characterised by imaging OH radicals using planar laser-induced fluorescence (OH-PLIF) technique. Reynolds-averaged Navier-Stokes (RANS) simulations are performed at an equivalence ratio of 0.8 using Eddy Dissipation Concept (EDC) model for turbulence-chemistry interactions with detailed reaction mechanism (GRI-Mech 3.0). Numerical results show a qualitative match with the experimental results for averaged OH intensity distribution and NO x and CO emissions. Air jet is observed to deflect more with increase in fuel-to-air jet momentum flux ratio. Flame-front is observed to be located at the periphery of the air jet due to mixing of reactants and hot burned products in the shear layer. Instantaneous OH-PLIF data suggest that the flame-front location changes considerably with time. Both numerical and experimental results show increase in CO emissions and reduction in NO x emissions with increase in fuel-to-air jet momentum flux ratio. This may be linked to unfavorable residence time distribution, despite better mixing at high fuel injection momentum. • Reaction zone is located in the shear layer of the air jet and the surrounding gases. • Flame front location fluctuates considerably with time. • At higher fuel injection momentum, the reaction zone gets deflected significantly. • Low NOx observed for high fuel injection momentum case due to better mixing. • Low CO observed for low fuel injection momentum case. [ABSTRACT FROM AUTHOR]

The impact of pilot flame operation on the combustion of pure methane and hydrogen-enriched methane (H 2 /CH 4 : 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 H 2 enrichment. High-speed OH∗ chemiluminescence imaging, as well as simultaneous planar laser-induced fluorescence measurements of the OH radicals and formaldehyde (CH 2 O) 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 NO x 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 NO x and increase the CO emissions, albeit rather small differences in the emissions have been detected, and (d) the NO x and CO emissions become significantly lower with increasing hydrogen ratio. • Optical measurements were used to study hydrogen-enriched methane/air flames in a lab-scale gas turbine combustor. • Hydrogen-enriched methane/air flames are shown to have a narrower operability range than methane/air flames. • Higher Reynolds number flames tend to be stabilized more upstream and have a wider operability range. • Hydrogen-enriched flames exhibit lower emissions of NOx and CO than methane/air flames. • Partially premixed pilot flame tends to lift off, while non-premixed pilot flames tend to attach to the burner. [ABSTRACT FROM AUTHOR]

The existence of a turbulent/turbulent interface (TTI) has recently been verified in the far wake of a circular cylinder exposed to free-stream turbulence (Kankanwadi & Buxton, J. Fluid Mech., vol. 905, 2020, p. A35). This study aims to understand the physics within the TTI. The wake boundary, approximately 40 diameters downstream of a circular cylinder subjected to grid-generated turbulence, was investigated through simultaneous cinematographic, stereoscopic particle image velocimetry and planar laser induced fluorescence experiments. With no grid placed upstream of the cylinder, the behaviour of the resultant interface, our closest approximation to a turbulent/non-turbulent interface, exactly matched what is observed in existing literature. When background turbulence is present, viscous action is no longer the only method by which enstrophy is transported to the background fluid, unlike for turbulent/non-turbulent interfaces. The presence of rotational fluid on both sides of the TTI allows the vorticity stretching term of the enstrophy budget equation to be the dominant actor in this process. The role of viscosity within a TTI is greatly diminished as the vorticity stretching term takes over responsibilities for enstrophy production. The turbulent strain rate normal to the TTI was found to be enhanced in the interfacial region. Decomposing the vorticity stretching term into components aligned with the three principal strain-rate directions, it was found that the term most aligned with the interface-normal direction contributed to the largest share of enstrophy production. This indicates that better 'organised' vorticity on the wake side of the interface yields the enstrophy amplification leading to the previously discovered enstrophy jump across the TTI by Kankanwadi & Buxton (J. Fluid Mech., vol. 905, 2020, p. A35). [ABSTRACT FROM AUTHOR]

In this study, we use the Algebraic Flame Surface Wrinkling (AFSW) model to conduct numerical simulations of the PSI (Paul Scherrer Institute) high pressure, turbulent premixed Bunsen flame experiments. The AFSW model was implemented in the open-source computational fluid dynamics solver OpenFOAM, and the numerical simulations were performed using a two-dimensional axial-symmetric model with the standard k–ɛ turbulence model with wall functions. The numerical simulations were performed for two different fuel/air mixtures, namely, 100% CH4 volumetric ratio and 60% CH4 + 40% H2 volumetric ratio. The thermophysical and transport properties of the mixture were calculated as a function of temperature using the library Cantera (an open-source suite of tools for problems involving chemical kinetics, thermodynamics, and transport processes), together with the GRI-Mech 3.0 chemical mechanism. It was found that the outcome of the AFSW model implemented in OpenFOAM was in good agreement with the experimental results, quantitatively and qualitatively. Further assessment of the results showed that, as much as the chemistry, the turbulence model and turbulent boundary/initial conditions significantly impact the flame shape and height. [ABSTRACT FROM AUTHOR]

A practical experimental method is proposed to investigate thermal transport by characterizing the motion of plasma flows through a x-ray spectroscopic technique using tracers. By simultaneously measuring multiple parameters, namely, the mass-ablation rate, the temporal evolution of plasma flow velocities and trajectories and the temperature, it is possible to observe a variety of physical processes, such as shock wave compression, heating by thermal waves, and plasma thermal expansion, and to determine their relative importance in different phases during the irradiation of CH and Au targets. From a comparison with hydrodynamic simulations, we find significant differences in the motion of the plasma flows between CH and Au, which can be attributed to different sensitivities to the thermal transport process. There are also differences in the ablation and electron temperature histories of the two materials. These results confirm that velocities and trajectories of plasma motion can provide useful evidence in the investigation of thermal conduction, and the approach presented here deserves more attention in the context of inertial confinement fusion and high-energy-density physics. [ABSTRACT FROM AUTHOR]

To examine the effects of water washing and acid-based deashing on the structure as well as combustion characteristics of Zhundong lignite, this study used high-speed planar laser-induced fluorescence measurements (OH-PLIF) with a specific burner and characterization techniques including XRD, BET, and ICP-AES as the method. The results indicated that both water washing and deashing can remove the ash and the main mineral alkali metals. Compared with RC, the total amount of OH release from WWC and UCC was increased attributing to the addition of combustibles and volatiles. The ignition of WWC and UCC was faster than that of RC, while the burning rate followed the opposite order. Experimental result confirms that the alkali metal and alkaline earth metals play a catalytic role in coal combustion. Meanwhile, the increase of the BET surface area and the reduction of ash are conducive to heat transfer, which is favorable for the ignition of pulverized coal. [ABSTRACT FROM AUTHOR]

• A S-PLIF system is designed to detect the interface structures of horizontal gas–liquid intermittent flow. • The effects of the interface fluctuations and detached bubbles on the S-PLIF70&90 results are investigated. • Hydrodynamic parameters are detected by paired parallel-wire conductance sensors and compared with previous correlations. • The interface structures and hydrodynamic parameters are uncovered as the flow condition changes. Horizontal gas–liquid two-phase flows widely exist in a variety of industrial fields such as the environment, chemical industry and energy. Intermittent flow, characterized by the quasi-periodic alternation motion of liquid slugs and large-scale Taylor bubbles, is the most prevalent flow pattern in horizontal gas–liquid two-phase flows. Comprehensive research on the hydrodynamic behavior of gas–liquid intermittent flow is essential for revealing the mass and heat transfer characteristics between phases, thereby establishing physical models of flow parameters. In this study, a structured planar laser-induced fluorescence (S-PLIF) system and paired parallel-wire conductance sensors (PWCSs) are designed to detect the gas–liquid interface structures and the hydrodynamic parameters. The S-PLIF system consists of a laser, a visualization box, a Ronchi ruling-plate and a pair of high-speed cameras. An optical linear prism forms a laser sheet in the transverse direction and the middle of the test section, and the cameras are positioned at 70 ° and 90 ° respectively to the plane of the laser sheet, referred to as S-PLIF70 and S-PLIF90. Under the laser excitation, bright and dark stripes generated by the Ronchi ruling-plate are deflected at the gas–liquid interface. The influence of total reflection can be effectively avoided by identifying the deflection position of the stripes, allowing for accurate detection of the gas–liquid interfacial structures. The effects of the interface fluctuations and detached bubbles on the S-PLIF70&90 results are investigated. Fluctuation and evolution characteristics of the gas–liquid interface are analyzed by probability density function (PDF) and power spectral density (PSD). Meanwhile, hydrodynamic parameters, i.e., Taylor bubble nose and tail velocity, Taylor bubble and slug length, and slug frequency, are detected by the PWCSs, and compared with previous correlations. The evolution characteristics of the interfacial structures and hydrodynamic parameters are uncovered as the flow condition changes. [ABSTRACT FROM AUTHOR]

In this study, the dissolution of a single oxygen bubble on a solid surface, here Titanium alloy Ti64, in ultrapure water with different oxygen undersaturation levels is investigated. For that purpose, a combination of shadowgraph technique and planar laser-induced fluorescence is used to measure simultaneously the changes in bubble geometry and in the dissolved oxygen concentration around the bubble. Two different wettabilities of the Ti64 surface are adjusted by using plasma-enhanced chemical vapor deposition. The dissolution process on the solid surface involves two distinct phases, namely bouncing of the oxygen bubble at the Ti64 surface and the subsequent dissolution of the bubble, primarily by diffusion. By investigating the features of oxygen bubbles bouncing, it was found that the boundary layer of dissolved oxygen surrounding the bubble surface is redistributed by the vortices emerging during bouncing. This establishes the initial conditions for the subsequent second dissolution phase of the oxygen bubbles on the Ti64 surfaces. In this phase, the mass transfer of O 2 proceeds non-homogeneously across the bubble surface, leading to an oxygen accumulation close to the Ti64 surface. We further show that the main factor influencing the differences in the dynamics of O 2 bubble dissolution is the variation in the surface area of the bubbles available for mass transfer, which is determined by the substrate wettability. As a result, dissolution proceeds faster at the hydrophilic Ti64 surface due to the smaller contact angle, which provokes a larger surface area. [Display omitted] • Mass transfer around an oxygen bubble interacting with solid surfaces was quantified. • Dissolved oxygen and bubble shape were measured by combined PLIF and shadowgraphy. • A bouncing and a dissolution phase were identified during bubble-surface interaction. • Stronger redistributed bubble boundary layer after bouncing in more hydrophobic case. [ABSTRACT FROM AUTHOR]

• Wall confinement significantly affects flow macrostructures and flame topologies. • Unconfined flames are observed with much lower LBO limits than confined flames. • Unconfined and confined flames approaching LBO exhibit different flame dynamics. • Inhibited re-ignition processes result in higher LBO limits in confined flames. Dimethyl ether (DME) is a promising alternative fuel for gas turbines, considering its renewable nature and potential for large-scale synthesis from CO 2 feedstock. However, there are limited studies on the swirl combustion characteristics of DME, particularly regarding the stability performance. This work investigates the effects of wall confinement on flow, mixing, and lean blowout (LBO) characteristics of partially premixed DME/air swirl flames in a gas turbine model combustor. Flow and flame macrostructures are captured using particle image velocimetry and planar laser-induced fluorescence (PLIF) measurements, respectively. The results show remarkable differences in flame topologies between unconfined and confined flames, especially the formation of inner recirculation zone and outer recirculation zone. The LBO limits are measured over various Reynolds numbers and flame dynamics approaching LBO are captured using 10 kHz OH* chemiluminescence imaging. Unconfined flames have unexpectively lower LBO limits (∼0.4) than the LBO limits of confined flames (over 0.5). Approaching LBO, unconfined flames display a quiet blowout mode featuring swirling spiral blow-off behaviors. In contrast, confined flames exhibit a violent blowout mode featuring large-scale quasi-instability behaviors, including periodic events of local extinction, flame lift-off, and re-ignition. Based on the numerical simulations and simultaneous OH- and CH 2 O-PLIF measurements, the large discrepancy in the measured LBO limits between the two configurations is found to result from different re-ignition processes related to the low-temperature ignition before the final blowout. These findings highlight the crucial roles of wall confinement in determining the stable flame topologies, LBO characteristics, and mechanisms driving the LBO physics for partially premixed DME/air swirl flames. [ABSTRACT FROM AUTHOR]

• High oxygen concentration and low co-flow velocity can make under-ventilated flames. • Blowout fuel velocity increases significantly with under-ventilated flames. • Effective velocity shows good relations with co-flow velocities and liftoff heights. • Mechanism of over-ventilated liftoff flames is related with the turbulent flame speed. Greenhouse gas emissions should be reduced to stop the advance of global warming. Oxy-fuel combustion is one of the methods to reduce CO 2 emissions. This paper aims to study the stabilization of CO 2 -diluted oxy-methane non-premixed jet flames and compare this with the results of methane-air flames. The lower the co-flow velocity and the higher the oxygen concentration in the co-flow, the higher the stability. If the flame stability is strong enough to withstand flame shape changes from the over-ventilated flame to the under-ventilated flame, the flammable range without blowout increases significantly under the under-ventilated flame. Otherwise, by decreasing the liftoff flame stability, the over-ventilated flame cannot change to the under-ventilated flame and blows out near the globally stoichiometric condition. The flame stability of a CO 2 -diluted oxy-fuel flame can differ from that of methane-air flames due to the properties of CO 2 and N 2. The effective velocity proposed by Guiberti et al. shows a good collapse of the dimensionless liftoff height among the O 2 /CO 2 co-flow, but the dimensionless liftoff height under the air co-flow is not on the same line with the O 2 /CO 2 co-flow. This study suggests a modified dimensionless liftoff height equation considering the fuel and co-flow mixture kinematic viscosity instead of the fuel kinematic viscosity and the ratio of the thermal diffusivity of the mixture. The thermal diffusivity ratio does not affect the result of the air co-flow. Additionally, the new coefficient β and turbulent Schmidt number are suggested because these numbers are factors depending on the burner geometry. The modified effective velocity using new coefficients and the modified dimensionless liftoff height show a good collapse for not only O 2 /CO 2 co-flows but also air co-flow. Also, the modified effective velocity shows the possibility of the critical liftoff velocity prediction. [ABSTRACT FROM AUTHOR]

• NO formation has been studied for NH3/DME binary fuel mixtures. • NO concentration profiles were determined with laser-induced fluorescence. • Maximal amounts of NO are formed in mixtures with around 50% NH 3 in the fuel. • Contemporary detailed reaction mechanisms are able to predict NO well. • Chemical interactions between C- and N-species are non-negligible for NO formation. Binary mixture of ammonia (NH 3) and dimethyl ether (DME) has been considered in literature as a potential fuel for practical use. Nitric oxide (NO) is a major product of combustion of NH 3 -containing fuels, and its formation routes have to be comprehensively studied. In this work, concentration profiles of NO were experimentally measured in laminar axisymmetric flames using planar laser-induced fluorescence. The molar percentage of NH 3 in the NH 3 /DME fuel mixture varied from 10% to 60%. Emission levels of NO have reached as much as around 1% for mixtures with around 50% NH 3. NO formation was analyzed with numerical simulations of 1D laminar flames and several detailed kinetic mechanisms. Modeling was performed in Chemkin with the steady-state burner-stabilized and free-propagating planar laminar flame reactor models. It was observed that the most recent versions of the contemporary NH 3 /DME models are able to reproduce the experiments, and their predictions agree with each other due to similarities in the NH 3 submechanisms. Kinetic analysis has revealed some disagreement was observed in terms of how much direct chemical coupling between carbon- and nitrogen-containing species affects NO formation. [ABSTRACT FROM AUTHOR]

• Confinement induces significant differences in flow field and flame macrostructure. • Unconfined and confined flames are stabilized in the shear layer far from LBO. • Unconfined and confined flames exhibit different flame dynamics approaching LBO. • Confinement effects on extinction mechanisms during the LBO process are discussed. This work investigates lean blowout (LBO) dynamics of dimethyl ether (DME)/air premixed swirl flames in a gas turbine model combustor with and without confinement. CH* chemiluminescence imaging and 2 kHz high-speed OH* chemiluminescence imaging are performed to capture the swirl flame macrostructures. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) are used to visualize the flow fields and reaction zones, respectively. Compared to the unconfined flame, the addition of confinement results in the appearance of an outer recirculation zone (ORZ), variation in flame topology, enhanced overall heat release (HR) rate, increased turbulent kinetic energy (TKE), and extended LBO limits. Far from LBO, all flames are stabilized in the shear layer. Approaching LBO, the confinement enhances the flow vortex structures along the shear layers, which is associated with extinction at the flame base and shear layer. However, the unconfined flame is still stabilized in the shear layer due to the weaker flow vortex structures. Different extinction mechanisms during the LBO process are also discussed between the unconfined and confined flames. The unconfined flame suddenly leaves the inner shear layer (ISL) and recedes to the inner recirculation zone (IRZ) within about 100 ms before LBO. Abundant CH 2 O is transported into IRZ mainly by recirculation, which reduces the HR region area and give rise to LBO. For the confined flame, more and more cold reactants gather near the base plate and then are gradually brought into IRZ by ISL vortices, which makes the flame lift from the base plate and decrease the HR region area until LBO happens. [ABSTRACT FROM AUTHOR]

Over the last years, several advanced micro Gas Turbine (mGT) cycle developments have been proposed, aiming at making the mGT more fuel and operational flexible. However, accurate data on real industrial combustors, assessing the performances and emissions of the combustion under unconventional diluted conditions or fuels involved in these novel cycles, are still missing. In this framework, Large Eddy Simulations, who allow to accurately assess the unsteady effects coupled to turbulent-chemistry interaction of reacting flows, offer an opportunity to better assess the combustion behaviour under these specific conditions. However, the computational cost remains much higher compared to RANS simulations. The mesh generation process might be complex, especially when the region of interest is not intuitively known. Therefore, Adaptive Mesh Refinement (AMR) methods allow the mesh to be refined only in the region where finer cells are required to capture important phenomena, i.e. in the flame front. By dynamically refining the mesh all along the simulation, the mesh is optimized in terms of cell quantity and distribution for more accurate results at potentially lower computational costs. In this work, an adaptive mesh refinement method based on a predefined criterion is successfully applied to the LES of a typical industrial mGT combustor, the Turbec T100. The results show that the adaptation strategy allows to automatically generate a dynamic mesh that is able to capture correctly the flame over time for an increased computational cost of 15%. Therefore, the human meshing effort is reduced while the automatic meshing leads to an optimized mesh. • Adaptive Mesh Refinement to reduce human meshing effort. • Large Eddy Simulation of a micro Gas Turbine combustor. • Adaptive Mesh Refinement based on a combustion criterion. • Flame sensor of the Dynamic Thickened Flame model used to track the reaction. • Error-metric-based strategy to trigger the mesh refinement. [ABSTRACT FROM AUTHOR]

[Display omitted] • An in situ observation system for naphthalene hydrothermal reaction was built. • OH radicals were observed in hydrothermal environments. • OH radicals were proposed to optimize the hydrothermal reaction pathway of PAHs. Hydrothermal decomposition of coal or biomass for hydrogen production is a clean and efficient technology. Comprehending the hydrothermal decomposition mechanism of Polycyclic Aromatic Hydrocarbons (PAHs) is pivotal for advancing biomass or coal hydrothermal reaction technology. Traditional non-in situ measurement methods present challenges in thoroughly exploring the intricacies of PAHs' hydrothermal reaction mechanisms. To address this issue, this study establishes an in situ visualization platform leveraging planar laser-induced fluorescence (PLIF) technology. Fluorescence signals emanating from naphthalene are meticulously measured in situ across diverse temperature conditions (450 °C-650 °C). The decomposition rate characteristics of PAH in hydrothermal process are thereby elucidated. A proportional increase in the reaction rate of naphthalene with elevated temperatures, concomitant with an augmented risk of coking. Moreover, the generation of hydroxyl radicals (OH) during the hydrothermal decomposition process was observed in the in situ visualization platform. Afterwards, to promote the generation of OH radicals and improve the conversion of PAHs, small molecule additives (formic acid, acetic acid, methanol, and ethanol) were added into the reactions. The experimental results indicate that the addition of small molecules increases the conversion rate of PAH and significantly reduces coking. These radicals effectively occupy reactive sites on naphthalene, thereby suppressing the coking proclivity of PAHs. This work provides a novel method to investigate the mechanism of hydrothermal process and offers a potential way to improve the conversion efficiency. [ABSTRACT FROM AUTHOR]

The detonation front's unstable structure leads to an unsteady and three-dimensional (3D) phenomenon that renders the study of the cell cycle challenging. Traditionally, fundamental studies are carried out in narrow channels where the detonation behavior is very peculiar (quasi two-dimensional with velocity deficit). In this study, we propose a fully experimental approach to study the cell cycle in the case of multicellular detonations. The cell cycle is characterized through three techniques: systematic and statistical analysis of soot foil, planar laser-induced fluorescence on nitric oxide, and Rayleigh scattering. These techniques provide measurements for cell size, local induction length, and local shock speed, respectively. The work is carried out in the 2 H 2 -O 2 -3.76Ar and the 2 H 2 -O 2 -3.76N 2 mixtures at 293 K, and 20 kPa and 25 kPa, respectively. These conditions ensure that the cell pattern is considered being between regular and weakly irregular, thus, a shot-to-shot reconstruction of the cell cycle is possible. The cell widths follow a normal distribution, from which a quantitative parameter (2 σ / λ) is proposed to assess the cell regularity, experimentally. The evolution of the speed and the local induction length are reconstructed along the cell cycle. The results agree with the available data for narrow channels and constitute the first of their kind for 3D detonation (i.e., multicellular in the transverse dimension). Two methods are proposed to analyze the local induction length δ i and compare it to the available literature (experimental and numerical studies). The technique can be applied to mixtures where the mean cell width is a meaningful parameter from highly regular to irregular mixtures. Novelty and Significance statement For the first time, combined soot-foils, NO-PLIF, and Rayleigh scattering measurements were used to reconstruct and characterize the cellular cycle of multicellular detonations using a 2 H 2 -O 2 -3.76Ar and 2 H 2 -O 2 -3.76N 2 mixtures at 293 K, and 20 kPa and 25 kPa, respectively. This constitutes the first combined measurements of local induction length and local front speed in a multicellular configuration, where the number of cells in the section is large, i.e., the cell width (λ) is small compared to all dimensions of the rig. This study provides two methodologies to extract an experimental Δ i , which is a quantity that can be compared with the ZND theory. Such measurements were not achievable in multicellular conditions until now. The measurement of the Δ i demonstrates that a fully experimental correlation between this experimental Δ i and λ can be obtained for the first time. These new results are important for the quantitative validation of chemical kinetic schemes for predictive simulations of detonations. [ABSTRACT FROM AUTHOR]

• ISD combustion of hydrogen-enriched jet-fuel. • OH-CH 2 O-fuel-PLIF and NTLAF temperature imaging. • Effects of H 2 enrichment on flame propagation stability. • H 2 favors the suppression of local flame instability and temperature fluctuations. • H 2 inhibits CO and CO 2 emissions but promotes NO x emission. Hydrogen (H 2) and jet fuel co-firing is regarded as a promising transition strategy of decarbonization in aviation industry. Considering the simplicity and safety during highly hydrogen enrichment conditions, the inverse swirl diffusion (ISD) combustion holds research value. Given the current lack of understanding for the flame properties of H 2 -enriched pre-evaporative jet fuel ISD co-combustion, in this work, the simultaneous planar laser-induced fluorescence (PLIF) of hydroxyl (OH), formaldehyde (CH 2 O), and fuel, as well as the non-linear excitation regime two-line atomic fluorescence (NTLAF) thermography imaging, are used to study the effects of hydrogen enrichment on ISD flame intricate structure and temperature field. The results show the positive effects of H 2 enrichment. The ISD flames display a blue hue and the increase in doping hydrogen helps the flames stabilize under global-lean conditions with almost no CO emission and low CO 2 emission at the chamber exit. And the H 2 enrichment contributes to inhibiting the CH 2 O formation upstream, reducing the generation of isolated reaction islands and promoting the strength of the flame wings, favoring the suppression of flame instability. In contrast, when at low H 2 enrichment rate, the shift of the heat release zone and high-temperature zone, and the enlargement of the cold zone, are found in flame upstream. The analysis of the probability distribution functions (PDF) of temperatures confirms the effective effects of hydrogen on inhibiting the local flame fluctuations. Although the increase in the overall temperature level due to the hydrogen addition would lead to a promotion in NO x generation, the discussion on the structural analysis and flame propagation stability of ISD flames demonstrates the significance of H 2 enrichment for the achievement of leaner low-NO x combustion of ultra-H 2 -enriched jet fuel in the future. [ABSTRACT FROM AUTHOR]

• The supersonic premixed cavity-stabilized combustion was realized in a gaseous ethylene-fueled scramjet with a transverse injection scheme. • The combustion stabilization mechanism of the cavity-stabilized supersonic premixed flame was analyzed through OH-LIF and CH* photography. • The transient flame morphology of the cavity-stabilized supersonic premixed flame was investigated from the aspect of flame spreading angle and flame front position abstracting from high-speed photography results. With the overwhelming trend of fast and economic aviation, it is becoming increasingly difficult for fuel to burn stably and efficiently in a constrained space and time period in the combustor of a hypersonic air-breathing vehicle. Conventional supersonic diffusion flame runs the risk of blowing out in the incoming flow at such high velocity. Upstream fuel injection of the flame holder is gaining popularity due to its prominent advantage in improving mixing efficiency, and this fuel injection method contributes to the organization of supersonic premixed combustion. In order to study the combustion characteristics of the supersonic premixed flame, corresponding experiments are carried out on a direct-connected scramjet platform with the inflow total temperature of 1700 K and the inflow Mach Number of 2.0. Gaseous ethylene with an equivalence ratio of 0.15 was injected transversely 670 mm upstream of a cavity. Optical measurements including laser-induced fluorescence on the hydroxyl radical (OH-LIF), CH*chemiluminescence photography, and high-speed photography were performed to investigate the steady-state and transient combustion characteristics of the cavity-stabilized supersonic premixed flame. From steady-state OH-LIF and CH* results, the flame could be divided into reactant zone, preheating zone, intense reacting zone, and product zone. It could be inferred that the preheating zone induced by the hot product in the cavity acts as a radical source and heat source for combustion stabilization in the cavity. In addition, the transient flame morphology analysis was carried out from the high-speed photography results. The flame base position and flame spreading angle were abstracted from the binarized image. The results showed that there existed a subtle flame flashback phenomenon for the flame base, whereas the flame spreading angle maintained a relatively constant value with a small deviation. The frequency-domain analysis revealed that both the flame base position and flame spreading angle reached the peak amplitude in the direct current component (0 Hz), exhibiting no explicit high-amplitude resonance component within 1500 Hz compared to those in the diffusion combustion mode. In all, the supersonic premixed flame presented a stabilized flame morphology. [ABSTRACT FROM AUTHOR]

• Detailed analysis of flame structures in cracked ammonia jet flames. • Insights into N 2 O formation in ammonia flames through NH and NO multiplication. • Impact of cracking ratio on chemical reactivity, flame stability, and NO emissions. • PLIF-derived statistics contribute to validating turbulence-chemistry interactions. 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 N 2 O. The effect of turbulent disturbances on N 2 O formation needs to be considered in turbulent CAJFs because N 2 O 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. [ABSTRACT FROM AUTHOR]

The time constant selected for lock-in amplification (LIA) has a crucial impact on observed line shapes in laser heterodyne spectroscopy, in particular in the case of ground-based remote monitoring of trace gas in the atmospheric column using laser heterodyne radiometer (LHR). Conventional simulation could not allow validation of LHR spectra measured in a real and complex atmospheric environment exhibiting large temporal and spatial variability (humidity, temperature, pressure, etc) that impact significantly the measured LHR spectra profiles. High-precision spectral measurement is thus crucial to avoid any spectral distortion resulting from the measurement. In this paper, the impact of LIA time constant on spectral line shape is investigated for LHR operating in continuous laser tuning mode, based on analysis of laboratory heterodyne spectra, in terms of signal-to-noise ratio (SNR), line width broadening, absorption depth and line shift. With respect to the given frequency scanning speed in continuous mode and to the halfwidth of the absorption feature to scan, a reasonable scanning time ΔTscan, the time needed for scanning laser frequency through the halfwidth ΔνHWHM of the absorption line, equal to or longer than 14 times of the LIA time constant τ is concluded in order to efficiently reduce the noise while without significant shift and distortion of the line shape. Experimental validation was carried out using a laser heterodyne absorption spectroscopy approach in the laboratory. Four different combinations of time constants τ and scanning time ΔTscan were used to record heterodyne spectra of a CH4 absorption line near 1242.00 cm−1 in continuous laser tuning mode. An optimal combination of a scanning time of 137 ms with a time constant of 1 ms was obtained. This optimal combination was used for ground-based measurements of CH4 and N2O in the atmospheric column by LHR. The extracted LHR spectrum is in good agreement with a referenced TCCON (Total Carbon Column Observing Network) FT-IR (Fourier-transform infrared) spectrum. [ABSTRACT FROM AUTHOR]

The main goal of this work was to modify the previously developed blade-type planar structure using plasmonic gold nanostars in order to stimulate photofield emission and provide efficient laser control of the electron current. Localization and enhancement of the field at the tips of gold nanostars provided a significant increase in the tunneling electron current in the experimental sample (both electrical field and photofield emission). Irradiation at a wavelength in the vicinity of the plasmon resonance (red laser) provided a gain in the photoresponse value of up to 5 times compared to irradiation far from the resonance (green laser). The prospects for transition to regimes of structure irradiation by femtosecond laser pulses at the wavelength of surface plasmon resonance, which lead to an increase in the local optical field, are discussed. The kinetics of the energy density of photoinduced hot and thermalized electrons is estimated. The proposed laser-controlled matrix current source is promising for use in X-ray computed tomography systems. [ABSTRACT FROM AUTHOR]

Herein, the effects of the angle between nozzles, oil thickness, gas flow ratio, and gas flow rate on mixing time and open eye area in a physical model of a gas‐stirred ladle with dual plugs are studied. The effect of the variables under study is determined using a two‐level factorial design. Particle image velocimetry (PIV) is used to establish, through the analysis of flow patterns, the effect of the studied variables on the hydrodynamics of the system. Results suggest that operating conditions significantly change the flow structure and greatly influence the system's mixing time and open eye area behavior. The Pareto front of the optimized results on mixing time and open eye area is obtained through a multiobjective optimization using a genetic algorithm (NSGA‐II). The results are conclusive in that, for optimal performance, the ladle must be operated using an angle slightly higher than 90° between plugs, a differentiated flow ratio, high oil thickness, and a gas flow rate adjusted according to whether the preference is given to the protection of the bath or to the mixing process. [ABSTRACT FROM AUTHOR]

Pollutant dispersion by a tall-building cluster within a low-rise neighbourhood of Beijing is investigated using both full-scale Large-Eddy Simulation and water flume experiments at 1:2400 model-to-full scale with Particle Image Velocimetry and Planar Laser-Induced Fluorescence. The Large-Eddy Simulation and flume results of this realistic test case agree remarkably well despite differences in the inflow conditions and scale. Tall buildings have strong influence on the local flow and the development of the rooftop shear layer which dominates vertical momentum and scalar fluxes. Additional measurements using tall-buildings-only models at both 1:2400 and 1:4800 scales indicates the rooftop shear layer is insensitive to the scale. The relatively thicker incoming boundary layer affects the Reynolds stresses, the relative size of the pollutant source affects the concentration statistics and the relative laser-sheet thickness affects the spatially averaged results of the measured flow field. Low-rise buildings around the tall building cluster cause minor but non-negligible offsets in the peak magnitude and vertical location, and have a similar influence on the velocity and concentration statistics as the scale choice. These observations are generally applicable to pollutant dispersion of realistic tall building clusters in cities. The consistency between simulations and water tunnel experiments indicates the suitability of both methodologies. [ABSTRACT FROM AUTHOR]

This paper reports experiments on a hydrogen-fueled scramjet performed at different equivalence ratios in a ground pulse combustion wind tunnel with a Mach-2.5 incoming flow. In the non-chemical reaction flow before the fuel was ignited, the flow field was oscillatory, and from the pressure monitor, the oscillation period was 0.07 s and the oscillation amplitude was 0.035 MPa. Schlieren and flame self-luminescence images of the combustor flow were obtained synchronously, and the flow-field structure was stable with the flame concentrated in the shear layer. Deep learning was used to subject the low-resolution combustion flow field to super-resolution analysis to reconstruct a high-resolution flow field. To improve the spatial resolution of the flow field during self-ignition of the hydrogen-fueled scramjet and study the flow mechanism and coupling rule of turbulent fluctuations in the ignition process, a multipath asymmetric residual network (MARN) is proposed based on a single-path super-resolution convolutional neural network (SRCNN) and a residual network model (ResNet_16). The experimental results show that compared with SRCNN and ResNet_16, MARN has the best precision and performance regarding the super-resolution flow field in the self-ignition of hydrogen fuel in terms of the mean peak signal-to-noise ratio, mean structural similarity, and average correlation coefficient as well as being the least complicated. The proposed method offers the possibility of developing lightweight super-resolution models for the flow fields in supersonic combustors; it shows enormous potential for revealing the physical flow of the fuel and air mixture, and it offers accurate forecasts of self-ignition times. [ABSTRACT FROM AUTHOR]

Turbulent fountain flow consists of two distinct stages, the initial 'negatively buoyant jet' (NBJ) stage, and the fully developed 'fountain' stage. The present study investigates both stages of the flow using particle image velocimetry and planar laser-induced fluorescence, over a range of source Froude numbers, 10 ≲ Fro ≲ 30, and Reynolds numbers, 5500 ≲ Reo ≲ 7700. While the velocity and buoyancy profiles in NBJs take similar Gaussian shapes over a wide range of axial locations, this was not observed in fountains. The changing profile shape is most evident in the outer flow (OF) region, while there is a degree of similarity in the inner flow (IF). Entrainment between IF and OF is shown to depend on the local Richardson number, Ri. The fountains are found to have a negative entrainment coefficient, α < 0, for the majority of their height, implying a net radial outflow of fluid from the IF to the OF. An alternative description of entrainment is considered, the 'decomposed top-hat' model, where the radial flow is separated into inflow and outflow components that are then estimated using the present experimental data. The inflow component was found to be proportional to the axial IF velocity, which is similar to the classical description of entrainment in pure jets/plumes, while the outflow depends on the local Ri. Entrainment in NBJs may also be described by this framework, which, despite not having an OF, is still subject to an Ri-dependent radial outflow. [ABSTRACT FROM AUTHOR]

Featured Application: A transition of ignition kernel delay time at the early stages of lean premixed n-butane/air turbulent expanding flame propagation is found, which may be relevant to spark ignition engines operated at lean-burn turbulent conditions. This paper explores the effects of root-mean-square turbulence fluctuation velocity (u′) and ignition energy (Eig) on an ignition kernel delay time (τdelay) of lean premixed n-butane/air spherical flames with an effective Lewis number Le ≈ 2.1 >> 1. Experiments are conducted in a dual-chamber, fan-stirred cruciform burner capable of generating near-isotropic turbulence with negligible mean velocities using a pair of cantilevered electrodes with sharp ends at a fixed spark gap of 2 mm. τdelay is determined at a critical flame radius with a minimum flame speed during the early stages of laminar and turbulent flame propagation. Laminar and turbulent minimum ignition energies (MIEL and MIET) are measured at 50% ignitability, where MIEL = 3.4 mJ and the increasing slopes of MIET with u′ change from gradual to drastic when u′ > 0.92 m/s (MIE transition). In quiescence, a transition of τdelay is observed, where the decrement of τdelay becomes rapid (modest) when Eig is less (greater) than MIEL. For turbulent cases, when applying Eig ≈ MIET, the reverse trend of MIE transition is found for τdelay versus u′ results with the same critical u′ ≈ 0.92 m/s. These results indicated that the increasing u′ could reduce τdelay on the one hand, but require higher Eig (or MIET) on the other hand. Moreover, the rising of Eig in a specific range, where Eig ≤ MIE, could shorten τdelay, but less contribution as Eig > MIE. These results may play an important role to achieve optimal combustion phases and design an effective ignition system on spark ignition engines operated under lean-burn turbulent conditions. [ABSTRACT FROM AUTHOR]