Planar Laser-induced Fluorescence Measurements Research Articles

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

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

Flame features and oscillation characteristics in near-blowout swirl-stabilized flames using high-speed OH-PLIF and mode decomposition methods

Flame features and dynamics are important to the explanation and prediction of a lean blowout (LBO) phenomenon. In this paper, recognition of near-LBO flame features and oscillation characterization methods were proposed based on flame spectroscopic images. High-speed planar laser-induced fluorescence measurements of OH were used to capture unique dynamic features such as the local extinction and reignition feature and entrained reactant pockets. The Zernike moment demonstrated a good performance in recognition of stability and near-LBO conditions, though the geometric moment had more advantages to characterize frequency characteristics. Low-frequency oscillations, especially at the obvious self-excited oscillation frequency around 200 Hz, were found when approaching an LBO condition, which can be expected to be used as a novel prediction characteristic parameter of the flameout limit. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) were used to conduct dynamic analysis of near-LBO flames. POD modes spectra showed the unique frequency characteristics of stable and near-LBO flames, which were basically in line with those at the heat-release frequency. The primary POD modes demonstrated that the radial vibration mode dominated in a stable flame, while the rotation mode was found to exist in a near-LBO flame. Analysis of modal decomposition showed that flame shedding and agminated entrained reactant pockets were responsible for generating self-excited flame oscillations.

Experimental analysis of a non-isothermal confined impinging single plume using time-resolved particle image velocimetry and planar laser induced fluorescence measurements

In this study, the experimental results of the flow behavior of a non-isothermal confined impinging single plume were analyzed. Experimental measurements were performed in the upper plenum of a high-temperature gas-cooled reactor (HTGR) that was scaled, designed, and assembled at Texas A&M University. The goal was to investigate flow mixing, thermal stratification, and plume impingement in the upper plenum of the HTGR under a loss-of-forced-coolant accident and provide a high-fidelity experimental database for validating computational fluid dynamics (CFD) and system codes. Time-resolved particle image velocimetry (TR-PIV) and planar laser-induced fluorescence (P-LIF) measurements were performed in the central plane of the plume flow in the upper plenum. A total power of 0.4 kW of heat was applied to the system. Convergence was achieved for the statistical results from the TR-PIV and P-LIF snapshots. The TR-PIV velocity vectors presented statistical characteristics of the plume flow, such as the mean velocity, root-mean-square fluctuating velocity, and Reynolds stress. The results revealed that the flow reached the maximum velocity far from the jet inlet at y/Dj=2. Furthermore, two-point velocity cross-correlation was performed on the TR-PIV velocity vector fields. Finally, the statistical results were calculated from the P-LIF temperature snapshots and the mean temperature. Furthermore, the temperature profiles were plotted. It was observed that the flow reached the maximum temperature near the plume inlet and decreased downstream.

Acetone PLIF visualization of the fuel distribution at plasma-enhanced supersonic combustion

This study examines the mixing and flameholding characteristics of a plasma stabilized planewall supersonic combustor. Fuel injection, ignition, and flameholding are provided by Plasma Injection Modules (PIMs) installed in a M = 2, 76.2 × 76.2 mm duct. Two configurations were explored: the first involves a plasma filament collocated within a single transverse fuel jet, while the second involves combustion stabilization using a series of three PIMs. Unburned fuel distributions were assessed with acetone planar laser induced fluorescence (PLIF), where acetone vapor was seeded into the gaseous fuel. PLIF measurements were performed for both streamwise and spanwise laser sheet orientations. Supporting datasets were collected including schlieren imaging, chemiluminescent imaging, and pressure distributions. PIM actuation is found to increase jet penetration, crossflow expansion, and initiate fuel jet breakup. The average jet cross sectional area is shown to increase by >20% with the addition of plasma. PLIF imaging at PIM stabilized combustion revealed a significant unburned fuel concentration in a local separation region near the PIMs, and identified the minimum plasma pulse duration for flowfield transition from initial conditions to the combustion pattern.

Structure and scalar correlation of ammonia/air turbulent premixed flames in the distributed reaction zone regime

Instantaneous structures of turbulent premixed ammonia/air flames on a piloted jet burner were investigated using simultaneous planar laser-induced fluorescence (PLIF) and Rayleigh thermometry measurements. Two-dimensional spatial distributions of temperature were simultaneously measured with those of the NH radical and the NO pollutant, respectively. Experiments were conducted for stoichiometric flames under five jet velocities. All flames are located in the distributed reaction zone regime of the Borghi-Peters diagram with the Karlovitz (Ka) number ranging from 274 to 4720. The NH PLIF images are used to characterize the fuel consumption layer of the reaction zones since the formation of NH is associated with the consumption of ammonia. The NH PLIF results show that under all flame conditions investigated, the NH layer remains thin in the proximity of the burner while it becomes progressively thickened and distorted by turbulence with increasing turbulent intensity and axial distance. For flames with Ka of 274, the NH layer essentially remains thin, while at Ka of 590 or higher, significant broadening of the NH layer is observed. Probability density functions (PDFs) of the NH layer thickness show that the NH layer can be broadened by 3 – 4 times as the flames are developed downstream. The broadening of the NH layer is considered to indicate that the flames are in the distributed reaction zone regime. The boundary between the thin-reaction zone regime and the distributed reaction zone regime occurs at a much larger Ka than that in methane/air flames. The broadening of the NH layer is due to the penetration of the turbulent eddies and the merging of flame branches. The latter occurs mainly near the flame tips. NO is shown to exist in a wide region in space, across the preheat zone, reaction zone, and postflame zone. NO formation occurs mainly in the reaction zone, however, it is transported by turbulence eddies to the preheat zone and by convection to the postflame zone. The temperature measurements indicate that the preheat zone is broadened in all flames investigated. The broadening of the preheat zone is moderately sensitive to the Ka number while it is more sensitive to the integral length scale of the flames.

Simultaneous multiple laser beam intensity profile correction and its application to a vitiated bluff body combustor field.

Simultaneous high-speed stereo-particle image velocimetry, OH planar-laser-induced fluorescence (PLIF), and CH2O PLIF measurements in a vitiated bluff body combustor are considered. An ex situ, simultaneous, time-resolved laser sheet intensity profile correction procedure is introduced. This procedure is easily implemented experimentally and is capable of correcting multiple sheets at the same time. As a proof of concept, the procedure is applied to perform correction of the CH2O PLIF images in vitiated and unvitiated conditions. The challenges associated with CH2O PLIF under these combustor operating conditions are also discussed.

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

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

Planar laser-induced fluorescence measurement of the angular pattern of the cone-shaped spray

In the present work, a PLIF-based approach for a large cone-shaped spray patternation ispresented and experimental results of its application are discussed. The patternation approach is based on simple time-averaged PLIF imaging. Special attention is paid to the image and data processing in order to simplify a patternation results interpretation. Experimental testing was performed on a water spray, formed by centrifugal nozzle at atmospheric ambient pressure. Circumferential relative concentration profiles in spray crosswise plane are presented in the paper and their validity is analysed.

Beyond Taylor\u2019s hypothesis: a novel volumetric reconstruction of velocity and density fields for variable\u2011density and shear flows

This work presents a novel numerical procedure for reconstructing volumetric density and velocity fields from planar laser-induced fluorescence (PLIF) and stereoscopic particle image velocimetry (SPIV) data. This new method is theoretically and practically demonstrated to provide more accurate 3D vortical structures and density fields in high shear flows than reconstruction methods based on the mean convective velocity. While Taylor’s hypothesis of frozen turbulence is commonly applied by using the local mean streamwise velocity, the proposed algorithm uses the measured local instantaneous velocity for data convection. It consists of a step-by-step reconstruction based on a mixed Lagrangian–Eulerian solver that includes the 3D interpolation of scattered flow data and that relaxes the Taylor’s hypothesis by iterative enforcement of the incompressibility constraint on the velocity field. This methodology provides 3D fields with temporal resolution, spatial resolution, and accuracy comparable to that of real 3D snapshots, thus providing a practical alternative to tomographic measurements. The procedure is validated using numerical data of the constant-density channel flow available on the Johns Hopkins University Turbulence Database (JHTDB), showing the accurate reconstruction of the 3D velocity field. The algorithm is applied to an experimental dataset of PLIF and SPIV measurements of a variable-density jet flow, demonstrating its capability to provide 3D velocity and density fields that are more consistent with the Navier–Stokes equations compared to the mean flow convective method.Graphic abstract

Physical space analysis of cross-scale turbulent kinetic energy transfer in premixed swirl flames

Understanding the cross-scale kinetic energy transport in turbulent flames is critical for physically-accurate modeling. This paper presents an experimental analysis of cross-scale kinetic energy transport in turbulent premixed swirl flames at Karlovitz numbers between 10 and 20 via physical space filtering. High-resolution tomographic particle image velocimetry and formaldehyde planar laser induced fluorescence measurements are used to obtain 3D velocity fields, as well as estimates for the progress variable and density fields. Filtering these fields at scales in the range of the laminar flame thickness (0.9<Δ/δL0<2.2) allows the kinetic energy transfer across the filter scale to be quantified. Mean kinetic energy transfer from sub-filter scales to larger scales (i.e. back-scatter) occurs internal to the flame structure across the range of conditions studied, with the maximum back-scatter occurring towards the center of the flame. Compared to equivalent non-reacting cases, considerably more back-scatter occurred in the reacting flows across all cases studied. At lower Karlovitz number, the magnitude of the back-scatter monotonically increased with increasing filter scale. At higher Karlovitz number, the back-scatter magnitude saturated at filter scales around Δ/δL0≈1.8. A scaling is proposed that quantifies the relative significance of the swirl-induced pressure gradient relative to the flame-induced pressure gradient on the pressure-work source of kinetic energy, indicating that swirling flow can significantly affect the kinetic energy dynamics. Overall, the results demonstrate that combustion significantly impacts the cross-scale kinetic energy transfer at scales in the range of the laminar flame thickness for conditions and configurations of practical relevance. These conclusions have implications for LES modelling strategies, providing evidence that models should allow for up-scale energy transfer in the vicinity of the flame.

Planar laser-induced fluorescence thermometry in moderate-temperature flow using OH from photo-dissociation of water vapor

Since planar laser-induced fluorescence (PLIF) temperature measurements in a combustion-flow field can only be performed in regions in which OH is abundant (i.e., the reaction and high-temperature burnout zones), this paper investigates the feasibility of using photolysis of dissolved water to generate additional OH in the low-temperature and non-reaction zones of the flow field. Multi-line fluorescence showed that the generated OH had reached thermal equilibrium within 50 ns after dissociation with a fitting temperature of 300 K in room-temperature air. To further verify the technical accuracy, averaged PLIF images from dissociated OH were calibrated and compared with thermocouple measurements in a heated-flow field, and the average deviation in the 423–823 K temperature range was less than 40 K, with a relative system error better than 5%. The results of this paper confirm the feasibility and accuracy of the photo-fragment PLIF (PF-PLIF) temperature measurement in a low-temperature flow field. This makes it an effective complement to traditional PLIF measurements, with great significance for analysis and model validation of combustion processes in low temperature.

Quantitative feature extraction of turbulent premixed flames by photofragmentation laser-induced fluorescence

Planar laser-induced fluorescence (PLIF) techniques have been widely used for flame structure visualization in recent years. Nevertheless, most of the studies were concerned with qualitative analysis of the obtained images and the quantitative analysis were much fewer. This work presents a quantitative feature extraction method to process the ribbon structures that are captured in images and represent the radical layers in flames. The extracted features include ridge lines, curvatures, and thicknesses. This method was demonstrated by processing the images of methyl (CH3) photofragmentation laser-induced fluorescence (PF-LIF) and formaldehyde (CH2O) PLIF measurements in stoichiometric, premixed methane/air flames. The flames were generated by a McKenna burner under the Reynolds numbers of 1930, 3870, and 5800. The ridge lines followed well with the wrinkled layers of CH3 and CH2O. The CH2O layers were thickened, but the CH3 layers had minor change with increased Reynold numbers.

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

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

Turbulent entrainment into a cylinder wake from a turbulent background

Abstract

Detailed measurements of transient two-stage ignition and combustion processes in high-pressure spray flames using simultaneous high-speed formaldehyde PLIF and schlieren imaging

This study investigates the low- and high-temperature ignition and combustion processes in a high-pressure spray flame of n-dodecane using simultaneous 50-kHz formaldehyde (HCHO) planar laser-induced fluorescence (PLIF) and 100-kHz schlieren imaging. The PLIF measurements were facilitated through the use of a pulse-burst-mode Nd:YAG laser, producing a 355-nm pulse-train with 300 pulses at 70 mJ/pulse, separated by 20-µs, in a 6-ms burst. The high-speed HCHO PLIF signal was imaged using a non-intensified CMOS camera with dynamic background emission correction. The acquisition rate of this HCHO PLIF diagnostic is unique to the research community, and when combined with high-speed schlieren imaging, provides unprecedented opportunity for analysis of the spatiotemporal evolution of fuel jet penetration and low- and high-temperature ignition processes relevant to internal combustion engine conditions. The present experiments are conducted in the Sandia constant-volume preburn vessel equipped with a new Spray A injector. The influences of ambient conditions are examined on the ignition delay times of the two-stage ignition events, HCHO structures, and lift-off length values. Consistent with past studies of traditional Spray A flames, the formation of HCHO is first observed in the jet peripheries where the equivalence ratio (Φ) is expected to be leaner and hotter and then grows in size and in intensity downstream into the jet core where Φ is expected to be richer and colder. The measurements demonstrate that the formation and propagation of HCHO from the leaner to richer region leads to high-temperature ignition events, supporting the identification of a phenomenon coined “cool-flame wave propagation” during the transient ignition process. Subsequent high-temperature ignition is found to consume the previously formed HCHO in the jet head, while the formation of HCHO persists in the fuel-rich zone near the flame base over the entire combustion period.

Turbulent transport dissimilarity with modulated turbulence structure in channel flow of viscoelastic fluid

This experimental study investigated the turbulent transport dissimilarity with a modulated turbulence structure in a channel flow of a viscoelastic fluid using simultaneous particle image velocimetry and planar laser-induced fluorescence measurements. An instantaneous dye concentration field with fluctuating velocity vectors showed that mass was transferred by hierarchically large-scale wavy motions with inclination. A co-spectral analysis showed that the spatial phase modulation of the streamwise velocity and dye concentration fluctuations for the wall-normal velocity fluctuation corresponded to the relaxation time. The occurrence of intense dye concentration fluctuation and small streamwise velocity fluctuation in a thin boundary layer caused dissimilar turbulent transport because of the non-zero negative correlation of the streamwise velocity and dye concentration fluctuations for the wall-normal velocity fluctuation only on large scales. This explains the turbulent transport dissimilarity which leads to the zero averaged Reynolds shear stress and non-zero wall-normal turbulent mass flux.

Dynamics of laminar ethylene lifted flame with ozone addition

The effect of ozone (O3) addition on ethylene (C2H4) non-premixed jet flames is investigated. Stable C2H4 lifted flames are established with oxygen/nitrogen co-flow at reduced oxygen content conditions. It is observed in the experiments that after O3 addition, the flame liftoff height could either increase or decrease, depending on the initial value of the flame liftoff height before O3 is added. Formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) measurement shows that prompt ozonolysis reaction between C2H4 and O3 produces large amounts of CH2O upstream of the flame. In contrast to previous studies of O3 addition on lifted flames—with saturated hydrocarbon fuels in which O3 decomposition dominates—the ozonolysis reaction between C2H4 and O3 changes the chemical composition of flow even at room temperature. Such chemical reaction causes the simultaneous increase of both the triple flame propagation speed of lifted flame and the axial jet velocity along the stoichiometric contour, which also therefore changes the dynamic balance between these two values to stabilize the flame. While the increase of triple flame propagation speed tends to decrease the flame liftoff height, the increase of axial jet velocity along the stoichiometric contour tends to have the opposite effect. A competing kinetic-dynamic process forms, and the final location of the flame depends on the degree of ozonolysis reaction, which is determined by the initial flame liftoff height before O3 is added.

Effects of Water Washing and Deashing on the Combustion Characteristics of the Zhundong Lignite

ABSTRACT 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.

Experimental and numerical study on the quenching limits and mechanism of small-scale H2 diffusion flames

The quenching limits of small-scale H2 diffusion flame achieved by microtubes were studied experimentally and numerically. The experimental observation shows that the luminosity of small-scale H2 diffusion flame becomes ambiguous with decreasing fuel jet velocity. However, the OH radical in the flame can be detected clearly by using the planar laser-induced fluorescence (PLIF) measurement. The quenching limits of small-scale H2 diffusion flames at different inner tube diameters were obtained by OH-PLIF measurement and numerical computation with a detailed chemical mechanism. The computed and measured quenching limits show good agreement. The results show that the quenching mass flow rate decreases gradually with decreasing inner tube diameter. A power function relationship exists between the quenching mass flow rate and the inner tube diameter. The analysis of the computational results reveals that the quenching mass flow rate depends on the ratio between heat generation and heat loss, net fuel consumption ratio, the lower heating value of the fuel and total heat loss from the reaction-part of the flame. The decrease in the diameter of the reaction-part of the flame with decreasing inner tube diameter leads to a smaller quenching mass flow rate.

Plasma-assisted stabilization of premixed swirl flames by gliding arc discharges

This paper presents key effects of gliding arc discharges on the stability limits of externally perturbed methane/air swirl flames. The low-frequency flow disturbance mimicking the transient conditions in practical combustor was generated by a flow pulsation system with repetition of ∼5 Hz. The synchronized OH planar laser-induced fluorescence and particle imaging velocimetry (OH-PLIF/PIV) measurements were utilized to visualize the instantaneous flow and flame structures. We find that, with gliding arc discharges, the lean blowout limits in perturbed flames are dramatically extended by up to 60%, which is more pronounced than in non-perturbed flames. OH-PLIF/PIV results show that, the stabilization effect can be explained by the expansion of the inner recirculation zone, wherein the opening angle increases from 52° to 62° due to the plasma enhancement. Time-resolved OH-PLIF images illustrate that, in the presence of gliding arc discharges, the flame response to the flow disturbance consists of three stages, including enhancement, extinction, and re-ignition processes. Furthermore, proper orthogonal decomposition (POD) analysis of OH-PLIF images reveals a considerable increase of mean energy contents and suppression of turbulent fluctuations. It suggests that the enhancement of gliding arc discharges is not a simple superposition of the energy deposit, but a strong non-linear coupling between the plasma and the turbulent flame with closed-loop feedbacks.