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In this study, we apply particle image velocimetry (PIV), hot-wire anemometry (HWA), and large-eddy simulation (LES) to identify and characterize a key mechanism by which high-intensity turbulence measured in the “Hi-Pilot” burner is generated. Large-scale oscillation of the high-velocity jet core about its own mean axial centerline is identified as a dominant feature of the turbulent flow field produced by this piloted Bunsen burner. This oscillation is linked to unsteady flow separation along the expanding section of the reactant nozzle and appears stochastic in nature. It occurs over a range of frequencies (100–300 Hz) well below where the turbulent kinetic energy (TKE) spectrum begins to follow a – 5/3 power law and results in a flow with significant scale separation in the TKE spectrum. Although scale separation and intermittency are not unusual in turbulent flows, this insight should inform analysis and interpretation of previous, and future studies of this unique test case.

Nanosecond-gated laser-induced breakdown spectroscopy (n-LIBS) has been used to quantify fuel mole fraction (ΧC2H4) in the cavity flameholder of a model high-speed combustor. The measurement locations selected, in the vicinity of the cavity shear layer, have low fuel concentrations. Previous n-LIBS measurements showed unexpectedly high values at these locations with expected low average fuel mole fraction (ΧC2H4) and intermittent fuel presence; thus, an effort was undertaken to understand potential sources of error with n-LIBS at low ΧC2H4. An improved direct spectrum matching (DSM) calibration matrix was thus constructed, focusing on low ΧC2H4 for both reacting and non-reacting conditions. Comparisons were made between n-LIBS measurements using two different laser systems for plasma generation, and the insensitivity of the signal to the specific laser was demonstrated for the first time. Uncertainty of the n-LIBS measurement technique at two different gas densities was analyzed through the processing of spectra acquired at known values of XC2H4. Measurements in the windtunnel, around the shear layer of the cavity flameholder, were conducted with a Mach-2 or Mach-3 freestream, and comparisons were made between n-LIBS results—using two DSM calibration matrices with different increments in XC2H4—and numerical results employing dynamic hybrid Reynolds-averaged Navier-Stokes and large-eddy simulation (DHRL). The high-resolution DSM matrix produced slightly lower XC2H4 than those from the coarse matrix, and comparisons with the simulation showed good agreement overall. For Mach-3 conditions, comparisons were made for both a non-reacting and a reacting cavity flameholder. Here, results showed a systematic difference between measured and simulated values of the mean XC2H4 within the shear layer but good (even excellent) agreement within the upper portion of the recirculation zone. For reacting condition, four reduced chemical kinetic models were implemented with the DHRL technique and compared to n-LIBS results. Overall, the four kinetic models produced a substantial range of results (e.g., temperatures and reaction progress), showing the importance of a validated diagnostic technique like n-LIBS for model testing.

New imaging capabilities for topology in prevaporized, liquid-fuel flames are presented with the application of CH-radical planar laser-induced fluorescence (PLIF) using the C2Σ+-X2Π (v′=0, v″=0) band. Imaging of turbulent flame structure for ethanol, n-heptane, n-dodecane, and kerosene (JP-8) fuels has been conducted in jet flames using a piloted McKenna burner with a central jet tube. This work presents the first application of CH PLIF for the study of full flame structure and curvature effects with prevaporized liquid fuels, particularly n-dodecane and kerosene. Liquid fuel structure is also compared to results with methane and ethylene. Results show that good signal-to-noise ratio (SNR) is achieved for all fuels, and the method is feasible in the presence of a dilute droplet field, despite the resonant nature of the technique. Reaction-zone structures clearly depict evidence of nonequidiffusive (Le ≠ 1) effects: strong variation in CH fluorescence is observed in regions of high curvature. Simultaneous excitation of the OH radical is also possible with prevaporized liquid fuels, for combined imaging of the reaction and heat-release zones. In kerosene flames, with aromatic species, fuel fluorescence may also be simultaneously captured, in addition to that from CH and OH, providing information on vapor localization.

Flamelet models of turbulent premixed combustion assume that (a) turbulent-transport and combustion processes can be decoupled and treated independently, suggesting that preheat and reaction zones remain layer-like and do not become highly fragmented and/or distributed. By further assuming (b) that the scalar-structure (e.g. the distribution of thermochemical quantities, like species mass fraction, vs. a control variable, such as a reaction progress variable) of a turbulent flame is akin to that in an associated laminar flame, detailed chemical properties can be introduce into a simulation at low computational cost via pretabulated flamelet tables derived from laminar flame calculations. The authors previously quantified conditions when assumption (a) remains valid. The present work assesses assumption (b) for turbulent Reynolds numbers that exceed those of previous studies by ∼ 7 × . Namely, planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O), hydroxyl (OH), and methylidyne (CH) acquired jointly with Rayleigh scattering are used to produce joint PDFs (scatter plots) of the former vs. a progress variable (CR) derived from the latter. Regardless of the turbulence level the piloted Bunsen flames considered here were subjected to, peak-normalized conditional mean (CM) profiles obtained from such joint PDFs agree well with profiles derived from laminar flame calculations. Additionally, within the near field of modestly turbulent flames, two-term β-PDFs are found to accurately describe CR-distributions. However, agreement between β-PDFs and CR-distributions worsens with increased turbulence and axial distance from the burner. Small discrepancies between the measured CM profiles and those obtained from laminar calculations also emerged at highly turbulent conditions. For instance, under such conditions, laminar profiles of CH over predict the measurements where 0.6

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.

A fueled cavity in a supersonic crossflow was ignited via a pulse detonator (PD) producing detonation waves that were then decoupled to produce varying degrees of shock-flame separation at the exit of the PD tube. This decoupling allowed for observation of the cavity ignition mechanism, and the key parameters required for successful cavity ignition were identified. Measurements were made using high-frame-rate OH Planar Laser-Induced Fluorescence (PLIF) and schlieren and chemiluminescence imaging. It was shown that the entrainment of high-temperature intermediate species into the forward region of the cavity, immediately behind the step, is the principal criterion for cavity ignition. Both coupled and slightly decoupled detonation cases induced significant OH shedding into the step region, leading to ignition and flame stabilization within the cavity. At conditions where OH shedding into the step region did not occur, cavity ignition was not observed. In coupled and slightly decoupled cases, there is more shedding of OH behind the step due to the greater disturbances created in the flowfield. As the degree of detonation decoupling increases, there is less shedding of OH and therefore a lower likelihood of ignition. Additionally, the time required for cavity combustion to reach its steady-state condition varied with the degree of decoupling of the detonation. Coupled detonation cases were shown to be more disruptive to the cavity and thus required more time to reach steady state than the decoupled cases.

Wavelet-based optical flow velocimetry (wOFV) is used to estimate two-dimensional velocity fields from tracer particle images acquired simultaneously with CH PLIF imaging within two highly turbulent premixed methane-air flames. Unlike conventional correlation-based PIV approaches, the current wOFV approach yields a dense (i.e. one vector per pixel) estimation of the velocity field which offers the possibility of greatly enhanced spatial resolution as compared to correlation-based PIV. Results from the wOFV method and correlation-based PIV are directly compared in terms of instantaneous fields, flow statistics, energy spectra, and derived gradient quantities such as vorticity and 2D strain rate. An assessment of the results indicates an increase in spatial resolution and dynamic range of almost one order-of-magnitude when using wOFV as compared to high-resolution PIV. In fact, the use of wOFV permits resolution of the smallest dissipative scales within the premixed flames with turbulent Reynolds numbers of O(104). A particular focus concerns velocity measurements near and within the CH layers, which are excellent markers of the local flame front but are spatially narrow and intermittent. The high-resolution velocity results from the wOFV method provide an opportunity for improved velocity estimations, derivative quantities, and their statistical characterization at and near the flame front.

This work reports that plasma can simultaneously reduce NOx emission and extend the lean blowoff limits of ammonia flames. The results show dramatic differences from similar work using hydrocarbon fuels in which plasma promoted NOx emission. Therefore, this work could lay the foundation of plasma application in practical ammonia combustion.

For the purpose of employing laser-induced breakdown spectroscopy (LIBS), nanosecond laser pulses (6 ns FWHM) from a standard Q-switched Nd:YAG laser (2nd harmonic) are modulated or chopped—using a novel method utilizing a variable air-pressure optical cell—to limit the inverse-Bremsstrahlung photon absorption process occurring in the breakdown plasma. The resulting plasma does not ignite flammable mixtures, but plasma emission is sufficiently strong to enable the characterization of reactants without strong perturbation (i.e., no ignition or shock wave results from the breakdown plasma). Two strong atomic emission lines, H (656 nm) and N II (568 nm), are chosen to find correlations between the plasma emission spectrum and the fuel concentration; plasma emission is captured after a delay of 20 ns with a gate width of 60 ns, the time over which there are well defined emission peaks. The chopped laser pulse is created within the pressurized optical cell with an initial breakdown plasma, by focusing the beam in high-pressure air. The pulse width of the laser beam transmitted through the cell is dependent on the cell pressure, and for this work, a pulse duration of approximately 600 ps was derived from the cell operated at a pressure of 10 bar. The chopped pulses were used for LIBS, and a 2D concentration distribution of a stoichiometric methane-air flow in ambient air were recorded without combustion reactions initiated by the laser-induced plasma.

The dynamics of using a pulse detonation wave to ignite a supersonic cavity flameholder was investigated. Fueling conditions ranging from lean to rich were investigated. Simultaneous 40-kHz schlieren imaging, OH planar laser-induced fluorescence, and chemiluminescence imaging were used to elucidate the fluid dynamics and combustion physics throughout the transient disruption of the ignition event. For moderate and rich fueling conditions, the ignition event stabilized combustion away from the detonation plume until the detonation products were fully exhausted. Ignition of leaner conditions required a low-level flame kernel to persist in the cavity step until the recirculating cavity flow field was re-established. The initial ignition mechanism was shown to be primarily driven by hot products from the detonation plume shedding into the low velocity region near the aft facing step in the cavity, followed by propagation and stabilization of a flame to a favorable region of the cavity. Quenching of cavity combustion at lean overall cavity mixtures was shown to be due to temporary backpressuring of cavity fuel injectors and cavity evacuation from the detonation pressure impulse.

Nanosecond (ns) laser pulses are modulated by seeding electrons on the laser beam path. The seed-electrons are from auxiliary ns-laser-induced breakdown (ALIB), and the ALIB is induced by a focused 1064-nm pulse, which is split after the frequency-doubling that generates the 532-nm pulse; therefore, the 532-nm and 1064-nm pulses are synchronized. The slowly converging (focal length = 500 mm) 532-nm pulse is re-directed to transmit through the region in where the ALIB-generated electrons reside. The seed-electrons from the ALIB then absorb the 532-nm photons via the inverse-Bremsstrahlung photon absorption (IBPA) process. The number density of the seed-electrons on the 532-nm beam path (ne,ALIB) is controlled by varying 1) the 532-nm pulse arrival time at the ALIB region (ΔPAT) after the 1064-nm pulse triggers the ALIB and 2) the location of the 532-nm beam relative to the core of the ALIB; the electron number density in ALIB is highly non-uniform and evolves in time. Electron-seeded laser-induced breakdown (ESLIB) occurs when ne,ALIB is sufficiently high. The 532-nm beam convergence (controlled by the focusing lens) is adjusted so that the breakdown does not occur without the electron seeding. The ESLIB immediately stops the transmission of the trailing edge of the laser pulse acting as a fast shutter, and ne,ALIB above a threshold can cut the pulse leading edge to modulate the 532-nm laser pulse.

Abstract In this paper, an experimental study of the non-reacting turbulent flow field characteristics of a piloted premixed Bunsen burner designed for operational at elevated pressure conditions is presented. The generated turbulent flow fields were experimentally investigated at atmospheric and elevated pressure by means of high-speed particle image velocimetry (PIV). The in-nozzle flow through the burner was computed using large-eddy simulation (LES), and the turbulent flow field predicted at the burner exit was compared against the experimental results. The findings show that the burner yields a reasonably homogeneous, nearly isotropic turbulence at the nozzle exit with highly reproducible boundary conditions that can be well predicted by numerical simulations. Similar levels of turbulence intensities and turbulent length scales were obtained at varied pressures and bulk velocities with turbulent Reynolds numbers up to 5300. This work demonstrates the burner’s potential for the study of premixed flames subject to intermediate and extreme turbulence at the elevated pressure conditions found in gas turbine combustors. Graphical abstract

Three-scalar subgrid-scale (SGS) mixing in turbulent coaxial jets is investigated experimentally. The flow consists of a centre jet, an annulus and a co-flow. The SGS mixing process and its dependence on the velocity and length scale ratios of the annulus flow to the centre jet are investigated. For small SGS scalar variance the scalars are well mixed and the initial three-scalar mixing configuration is lost. For large SGS variance, the scalars are highly segregated with a bimodal scalar filtered joint density function (f.j.d.f.) at a range of radial locations. Two competing factors, the SGS variance and the scalar length scale, play an important role for the bimodal f.j.d.f. For the higher velocity ratio cases, the peak value of the SGS variance is higher, thereby resulting in stronger bimodality. For the lower velocity ratio cases, the wider mean SGS variance profiles and the smaller scalar length scale cause bimodal f.j.d.f.s over a wider range of physical locations. The scalar dissipation rate structures have similarities to those of mixture fraction and temperature in turbulent non-premixed/partially premixed flames. The observed SGS mixing characteristics present a challenging test for SGS mixing models as well as provides an understanding of the physics for developing improved models. The results also provide a basis for investigating multiscalar SGS mixing in turbulent reactive flows.

In this study, we demonstrate successful development of a predictive model that detects both the fuel-air equivalence ratio (ϕ) and local pressure prior to plasma formation via machine-learning from the laser-induced plasma spectra; the resulting model enables measurement of a wide range of fuel concentrations and pressures. The process of model acquisition is composed of three steps: (i) normalization of the spectra, (ii) feature extraction and selection, and (iii) training of an artificial neural network (ANN) with feature scores and the corresponding labels. In detail, the spectra were first normalized by the total emission intensity; then principal component analysis (PCA) or independent component analysis (ICA) was carried out for feature extraction and selection. Subsequently, the scores of these principal or independent components as inputs were trained for the ANN with expected ϕ and pressure values for outputs, respectively. The model acquisition was successful, and the model’s predictive performance was validated by predicting the ϕ and pressure in the test dataset.

This experimental study examines flow control authority of a quasi-DC (Q-DC) electric discharge in a cavity-based supersonic flow. Testing was performed with Mach 2 airflow using the Research Cell ...

The effects of inlet distortion and shockwave interaction on fuel-air distribution and ignition in a cavity-based flameholder with supersonic flow were investigated using nanosecond-gated laser-ind...

Simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) utilizing R-branch transitions in the C-X (0,0) band were performed at a 10-kHz repetition-rate in a turbulent premixed flame. The CH lines at 310.690 nm (from the R-branch of the C-X band) used here have greater efficiency than A-X and B-X transitions, which allows for high-framerate imaging with low laser pulse energy. Most importantly, the simultaneous imaging of both CH PLIF and PIV is enabled by the use of a custom edge filter, which blocks scattering at the laser wavelength (below ∼311 nm) while efficiently transmitting fluorescence at longer wavelengths. The Hi-Pilot Bunsen burner operated with a turbulent Reynolds number of 7900 was used to demonstrate simultaneous PIV and CH PLIF utilizing this filtered detection scheme. Instances where pockets of products were observed well upstream of the mean flame brush are found to be the result of out-of-plane motion of the flame sheet. Such instances can lead to ambiguous results when interpreting the thickness of reaction layers. However, the temporally resolved nature of the present diagnostics facilitate the identification and proper treatment of such situations. The strategy demonstrated here can yield important information in the study of turbulent flames by providing temporally resolved flame dynamics in terms of flame sheet visualization and velocity fields.

Combustion characteristics of a mesoscale burner array have been studied using several diagnostic and analysis techniques. The array was specifically configured to enhance overall combustion stability, particularly under lean operating conditions, by promoting flame to flame interactions between neighboring elements. The 4 × 4 burner array demonstrated stable operations up to 3 kW and is designed to flexibly accommodate wide range of combustion power outputs by scaling the element dimensions or array size. Flame stabilizing mechanisms were experimentally examined using OH, CH, and CH2O planar laser induced fluorescence (PLIF) of premixed CH4 and air flames at operating equivalence ratios between 0.7 and 1.2. A quantitative measure of flame stability was obtained through dynamic mode decomposition (DMD) analysis of high speed OH-PLIF images. Lean blow off limits and emissions were also characterized across a wider range of equivalence ratios to better understand mesoscale burner array combustion characteristics. Lastly, combustion experiments using liquid fuel, pentane (C5H12), were also carried out. Marked improvement in combustion stability was observed compared to a single swirl-stabilized flame of similar power output. Results indicate mesoscale burner arrays can potentially serve as flexible and scalable next generation propulsion and power generation systems.

The current work assesses the impact of turbulence on flame surface characteristics and scalar dissipation rates ( χ C ) of piloted premixed methane-air flames. This assessment is facilitated via the application of planar Rayleigh scattering (PRS) to a subset of the flames considered in Part I (13 in total), which possess turbulent Karlovitz, Damkohler, and Reynolds numbers in the range of 1.1 ≤ K a T , P ≤ 144, 0.24 ≤ D a T , P ≤ 5.79, and 1,200 ≤ R e T , 0 ≤ 35,000, respectively. Instantaneous temperature maps are derived from the PRS images with an accuracy that is conservatively estimated to be within ∼ 3%. Additionally, the fidelity of the collected images was enhanced via the application of a combined wavelet-based and edge-preserving guided filtering scheme that allows scales associated with the peak of the dissipation spectra in the modestly turbulent flames considered here to be fully resolved. After such filtering, the temperature images were converted to reaction progress variable maps ( C T ). Two-dimensional isocontours were extracted from these maps and used to assess the flame surface density (FSD; Σ ) and the in-plane curvature ( κ 2 D ) of the flames considered here. Furthermore, scalar dissipation rate ( χ C T ), its density weighted average ( χ C T ˜ ), and other relevant quantities (e.g., the density-weighted variance of C T : C T ″ 2 ˜ and the generalized FSD: | ∇ C T | ¯ ) were derived from the C T -maps. Subsequently, relationships between these quantities were assessed and compared against theoretical models that aim to capture such relationships through simple expressions. The results indicate that the distribution of κ 2 D values broadens with enhanced turbulence, as does the integrated and peak values of FSD. Moreover, each of these parameters exhibit a strong positive correlation with K a T , P , highlighting the role this non-dimensional parameter plays in dictating flame surface wrinkling/area-generation. Relationships between χ C T , C T ″ 2 ˜ , and | ∇ C T | ¯ , are compared to flamelet-type models proposed by Vervisch et al. as well as by Bray, Swaminathan, Kolla, Chakraborty, and coworkers. Overall, despite the fact that the considered flames exhibit substantial preheat zone broadening and are subjected to extreme levels of turbulence, the proposed theoretical relationships between χ C T , C T ″ 2 ˜ , and | ∇ C T | ¯ show favorable agreement to the measurements. While differences between the measured and theoretical results are observed, the two can be reconciled with minor adjustments to the various constants in these models (i.e., changing them by no more than a factor of ∼ 2). Ultimately, such observations provide support for utilizing flamelet-type models to simulate premixed flames even when they are subjected to extreme turbulence levels. Moreover, the wealth of information provided here can help guide the development of robust yet efficient numerical tools for simulating highly turbulent premixed flames, which will aid in reducing the cost of and time to produce robust, efficient, and clean-burning combustion engines.

A novel calibration-free seedless velocimetry technique utilizing a shockwave generated by a laser-induced plasma (LIP) is proposed and used for measuring velocity profile in a supersonic flow. A nanosecond laser pulse of 532-nm wavelength is focused in the test section of a supersonic wind tunnel to generate the LIP that emanates a shockwave. A continuous-wave laser beam of 658-nm wavelength traverses the region adjacent to the LIP twice between a knife-edge mirror and a retroreflecting mirror. The probe beam is monitored by a fast photodiode after the second reflection from the knife-edge mirror, to record the arrival of the LIP shockwaves at the two probe beam locations; the laser beam deflects when the shockwave reaches the beam, and this deflection is registered on the photodiode signal read by an oscilloscope. It is shown that the shockwave-arrival time is highly sensitive to the convection speed of the flow that carries the shockwave. An empirical point-explosion blast model, determining time-dependent shockwave location, is used to calculate the average convection velocity between the LIP and the farthest probe beam 4.5 mm from the LIP with a precision of better than 8 m/s (1.5%). The empirical model contains two coefficients, a shockwave-propagation coefficient and the flow velocity, which are determined by the shockwave-arrival times and the locations of the two probe beams relative to the LIP. The LIP is positioned along a span-wise direction of the supersonic wind tunnel to measure the velocity profile; the probe beams are parallel with the spanwise direction. Uniform flow velocity is found within the core-flow of the wind tunnel, which well agrees with the velocity estimated by the pressure and temperature measured in the tunnel.

Combustion stability is an important consideration for many energy systems because of its impact on performance and efficiency. Flame dynamics that govern combustion stability are often complex and difficult to resolve, particularly from experimental data. However, recent advances in postprocessing techniques, such as dynamic mode decomposition (DMD), have partially enabled flame dynamic analysis. This study aims to provide a comprehensive measure of combustion stability through a detailed investigation of coherent structures and their energy contents, frequencies, and growth factors from DMD. These results are then correlated to underlying physics and chemistry that drive flame dynamics. Three different data sets were analyzed in this study. Numerically constructed images and OH-planar laser induced fluorescence (OH-PLIF) images of laminar flames were used to characterize stable flame dynamics. OH-PLIF images of acoustically perturbed swirl-stabilized turbulent flames were used to characterize oscillator...

This communication describes and demonstrates an approach to making simultaneous multi-scalar measurements with reduced equipment requirements. Specifically, this letter describes and demonstrates the ability to simultaneously acquire high-quality planar laser-induced fluorescence (PLIF) images of CH2O and either CH, OH, or a combination of CH and OH using a single Nd:YAG-pumped dye-laser system and two intensified cameras. Acquiring these images with common diagnostic equipment was facilitated by exciting strong transitions in the C–X (0,0) band of CH (near 314 nm) and in the A–X (0,0) and (1,1) bands of OH, which are spectrally adjacent. Additionally, Rayleigh scattering images were acquired simultaneously with these PLIF images using a second Nd:YAG laser and an un-intensified camera. However, it would be possible to conduct these measurements with a single, sufficiently energetic Nd:YAG laser.

This paper presents high-fidelity flame structure measurements of premixed methane–air Bunsen flames subjected to extreme levels of turbulence. Specifically, 28 cases were studied with longitudinal integral length scales (Lx) as large as 43 mm, turbulence levels (u′/SL) as high as 246, and turbulent Reynolds (ReT,0) and Karlovitz (KaT) numbers up to 99,000 and 533, respectively. Two techniques were employed to measure the preheat and reaction layer thicknesses of these flames. One consisted of planar laser-induced fluorescence (PLIF) imaging of CH radicals, while the other involved taking the product of simultaneously acquired PLIF images of formaldehyde (CH2O) and hydroxyl (OH) to produce “overlap-layers.” The average preheat layer thicknesses are found to increase with increasing u′/SL and with axial distance from the burner (x/D). In contrast, average reaction layer (i.e. CH- and overlap-layer) thicknesses did not increase appreciably even as u′/SL increased by a factor of ∼ 60. Furthermore, the reaction layer thicknesses (based on the CH images only) did not increase with increasing x/D. The reaction layers are also observed to remain continuous; that is, local extinction events are rarely observed. Although based on a sequence of combined CH–OH PLIF images acquired at a rate of 10 kHz, it is apparent that when instances of local extinction do occur they are the result of cool gas entrainment. The results presented here, as well as those from 12 prior experimental and 9 numerical investigations, do not agree with the predicted Klimov–Williams boundary on the theoretical Borghi Diagram. Thus, a new Measured Regime Diagram is proposed wherein the Klimov–Williams criterion is replaced by a metric that relates the turbulent diffusivity ( D T = u ′ L x ) to the molecular diffusivity within the preheat layer ( D * = S L δ F , L ). Justification for this replacement is based on physical reasoning and the fact that the line defined by DT/D* ≈ 180 accurately separates cases with thin flamelets from those with broadened preheat yet thin reaction layers (i.e. BP-TR flames). Additionally, the results suggest that the BP-TR regime extends well beyond what was previously theorized since neither broken nor broadened reaction layers were observed under conditions with Karlovitz numbers as high as 533, which is five times higher than the theoretical boundary.

This paper considers the modes of plasma-stabilized supersonic combustion in a cavity-based flameholding configuration. The testing was performed in the SBR-50 supersonic facility at the University of Notre Dame under the following conditions: Mach number M = 2, stagnation pressure and temperature P0 = 1–3.2 bar, T0 = 295–750 K, and an ethylene injection rate of ṁC2H4 = 0–8 g/s. A rail type electric discharge is employed in this work, which represents a modification of the previously explored quasi-DC (Q-DC) configuration. Two principally different cases are realized and described in detail: in- or over-the-cavity combustion, and bulk combustion characterized by a significant increase of pressure downstream of the fuel injection ports. Rail discharge performance is compared to the performance of Plasma-Injection Modules (PIMs) using a 1D analysis. Dimensionless analysis of the plasma-assisted combustor performance indicates reasonable agreement of the in-cavity combustion mode with the Ozawa stability diagram, while the plasma-assisted bulk combustion is essentially distinct from this formalism.

This paper describes an experimental study of the effects of an incident shockwave on the flow field, fuel distribution and combustion within a cavity flameholder with upstream fuel injection. Two impingement locations are employed: (1) near the fuel injector (the so-called shock-on-jet case) and (2) on the cavity shear layer (the shock-on-cavity case). Shadowgraph is used to characterize the flow field. Air seeded with nitric oxide (NO) is used as the simulated fuel and the resulting planar laser-induced fluorescence (NO-PLIF) from NO molecules is used to characterize fuel/air mixing while planar laser-induced fluorescence of OH molecules to characterize the actual combustion process. The shadowgraph and NO-PLIF images are compared with a CFD (Computational Fluid Dynamics) solution of the Reynolds-averaged-Navier Stokes (RANS) for assessment and explanation of experimental results of non-reacting tests. The effect of the shock on the cavity shear layer is to control the fuel distribution within the cavity. The effect of the shock on the jet is to force the shear layer deep within the cavity, which results in higher fuel concentrations near the cavity centerline. The shock-on-cavity case causes the shear layer to separate upstream of the cavity. Mixing uniformity is enhanced by the increased breakup of the fuel plume. Combustion is stronger and more uniform with the shock impinging on the cavity, while it is limited to the edges of the cavity with shock impingement on the jet. The greater mixing afforded in the shock-on-cavity case reduces the fuel concentration near the centerline and allows stronger burning in the center of the cavity. Doubling the fuel injection momentum flux ratio does not strongly affect the pattern of fuel distribution in either case, but combustion in the shock-on-cavity case is reduced, because the fuel concentration at the centerline is high.

The influences of inlet geometry modification on flow and combustion dynamics are experimentally investigated in a model supersonic combustion ramjet (scramjet) installed in Mach 4.5 high-enthalpy air freestreams. The inlet upper-lip angle (6, 12, and 20 deg) and the intake contraction ratio (1.3 and 1.9) chosen as the generic inlet design parameters are varied. With the inlet geometries systematically modified, flowfields inside the model scramjet are visualized at cold conditions (room temperature) using a planar laser Rayleigh scattering method (wherein laser radiation is scattered by carbon dioxide particles). In addition, combustion dynamics and structures in the scramjet combustor are investigated in high-enthalpy flows with a total temperature of 2600 K capable of autoigniting injected fuels. High-frame-rate time-averaged chemiluminescence (broadband) imaging and hydroxyl planar laser-induced fluorescence are used for this investigation. Incident shock-wave strength, separated flow behavior, bounda...

Recent results have experimentally demonstrated the feasibility of obtaining 3D (three-dimensional) combustion measurements using tomographic VLIF (volumetric laser-induced fluorescence), specifically of the CH radical, representing the flame surface. To elucidate the fundamental capabilities and limitations of the VLIF technique, this work reports an analysis of its performance in terms of signal level, size of the field of view (FOV) in 3D, and accuracy. Compared to the established PLIF (planar LIF) technique that uses a thin laser sheet to excite the target species in a plane, the VLIF technique uses a thick laser slab to excite the target species in a volume. As a result, the VLIF technique involves more performance metrics compared to PLIF, and the relationship between these metrics is also different from that in the PLIF technique. Therefore, both experimental and computational studies were conducted to analyze the performance metrics of VLIF. First, experiments were conducted on well-controlled flames to examine the relationship among excitation energy, signal level, and FOV in 3D. Second, based on these experimental data, numerical simulations were performed to benchmark the VLIF technique under a range of conditions. These results illustrate the relationship among signal level, 3D FOV, and accuracy and are expected to be valuable for the optimal design of the VLIF technique.

In a previous study we investigated three-scalar mixing in a turbulent coaxial jet (Cai et al. J. Fluid Mech., vol. 685, 2011, pp. 495–531). In this flow a centre jet and a co-flow are separated by an annular flow; therefore, the resulting mixing process approximates that in a turbulent non-premixed flame. In the present study, we investigate the effects of the velocity and length scale ratios of the annular flow to the centre jet, which determine the relative mean shear rates between the streams and the degree of separation between the centre jet and the co-flow, respectively. Simultaneous planar laser-induced fluorescence and Rayleigh scattering are employed to obtain the mass fractions of the centre jet scalar (acetone-doped air) and the annular flow scalar (ethylene). The results show that varying the velocity ratio and the annulus width modifies the scalar fields through mean-flow advection, turbulent transport and small-scale mixing. While the evolution of the mean scalar profiles is dominated by the mean-flow advection, the shape of the joint probability density function (JPDF) was found to be largely determined by the turbulent transport and molecular diffusion. Increasing the velocity ratio results in stronger turbulent transport, making the initial scalar evolution faster. However, further downstream the evolution is delayed due to slower small-scale mixing. The JPDF for the higher velocity ratio cases is bimodal at some locations while it is always unimodal for the lower velocity ratio cases. Increasing the annulus width delays the progression of mixing, and makes the effects of the velocity ratio more pronounced. For all cases the diffusion velocity streamlines in the scalar space representing the effects of molecular diffusion generally converge quickly to a curved manifold, whose curvature is reduced as mixing progresses. The curvature of the manifold increases significantly with the velocity and length scale ratios. Predicting the observed mixing path along the manifold as well as its dependence on the velocity and length scale ratios presents a challenge for mixing models. The results in the present study have implications for understanding and modelling multiscalar mixing in turbulent reactive flows.

Turbulent flame dynamics in an ethylene-fueled model scramjet is experimentally investigated in Mach 4.5 freestream flows with a total temperature of 2600 K using a pulsed arc-heated hypersonic wind-tunnel facility at the University of Notre Dame. Gaseous ethylene fuel is injected into the combustor through a supersonic nozzle, and is autoignited by high-enthalpy flows that are compressed and decelerated by a train of shock waves through the scramjet isolator/combustor. The turbulence level of the freestream is manipulated to investigate the influence of freestream turbulence on the flame dynamics in the supersonic combustor. General flame behavior is observed in time-averaged ethylene-flame-chemiluminescence images, whereas the detailed turbulent flame structures are resolved by imaging instantaneous ground-state hydroxyl radical (OH) distributions using the planar-laser-induced-fluorescence technique, wherein a planar laser sheet is projected into the scramjet in the counterstreamwise direction to illum...

We demonstrate single-shot three-dimensional (3D) flame measurements based on volumetric laser-induced fluorescence (VLIF). For this effort, the excitation laser beam was expanded into a “slab” with a height of 40 mm and thickness of 10 mm to excite the CH radicals in the target flames. The volumetric LIF signals were then simultaneously captured by five intensified cameras, and the images were fed into a tomographic algorithm as inputs to obtain the 3D distribution of CH. The results show that single-shot 3D measurements with sufficient accuracy can be obtained in a volume of 9.3 × 9.3 × 32.7 mm 3 with an excitation pulse energy of ∼10 mJ. With a total of five intensified cameras, 3D measurements were obtained with a voxel size of 0.15 mm over a total of 64 × 64 × 224≈10 6 voxels, which can be further improved by the use of additional cameras or advanced tomographic inversion techniques. Comparison with PLIF measurements suggested that the spatial resolution of the CH-based VLIF technique to characterize the flame surface was about 0.4 mm. Measurements were first performed on stable laminar flames, for comparison and validation against PLIF, and then on turbulent jet flames, to illustrate the feasibility and accuracy of VLIF for instantaneous 3D flame measurements and motivate further investigation.

Ignition processes in scramjet pilot cavities are highly transient events that are influenced by factors including freestream Mach number and inlet geometry, turbulence intensity, cavity geometry, ignition source, and fueling composition and flow rate. In particular, the location of the flame kernel and associated propagation rate of the flame front throughout the cavity can significantly influence the end state of the ignition process. In this work formaldehyde (CH2O) was used as a flame marker to track ignition progress in a plane throughout the span-wise width of the cavity, while chemiluminescence imaging provided path-integrated flame location along the span-wise and axial directions. Planar laser-induced fluorescence (PLIF) excitation utilized the 355 nm frequency-tripled output of an Nd:YAG burst-mode laser operating at 50–100 kHz over 10 ms with available pulse energy up to 80 mJ. Simultaneous CH* chemiluminescence imaging from the top of the cavity was obtained with a high-speed complementary metal-oxide semiconductor camera. A freestream Mach number of 2 with ethylene fuel rates from 55–90 standard liters per minute were examined along with two different ignition sources: a spark discharge and pulse detonator. The resulting formaldehyde PLIF and chemiluminescence images indicate a strong correlation between fueling rate and the delay between the onset of ignition and stable combustion. More importantly, the span-wise propagation rate and structure of the flame front is highly dependent on the fueling rate, burning region, and ignition source.

This work experimentally quantifies the relationship between metrics of the local reaction rate and flame thickness for turbulent premixed flames in the corrugated flamelet and thickened preheat zone/thin reaction zones regime using data from simultaneous 10 kHz tomographic particle image velocimetry (TPIV), hydroxyl planar laser induced fluorescence (OH PLIF), and formaldehyde (CH2O) PLIF. Two reaction rate metrics are presented, which are based on tracking fluid elements in a Lagrangian manner as they traverse from the flame leading edge to the reaction zone. This yields both a fluid residence time in the flame (τc) and a metric of the flame speed (S). τc in the thickened preheat zone regime was reduced by nearly 40% compared with the corrugated flamelet despite the flame being broader, indicating a substantial increase in the local reaction rate. The joint PDFs of S and local flame thickness showed a positive linear correlation between these quantities, which can be explained by turbulent diffusivity arguments. These data provide guidelines for reaction rate models, as well as a quantitative means of comparing local behavior between experiments and simulations.

Gas density and fuel mole fraction in a cavity flameholder of a supersonic combustor with ethylene (C 2 H 4 ) fueling are simultaneously measured using nanosecond gated (10 ns) laser-induced breakdown spectroscopy (n-LIBS). Emission spectra from the laser-induced plasma over a range (550 nm–830 nm) containing multiple emission lines of O, H, N, and C are captured and used to quantify the gas conditions. A direct spectrum matching (DSM) process is employed for the gas property quantification in which an unknown spectrum is matched to a spectrum from a database, constructed through n-LIBS analysis of well-known gas conditions. A back-scattering collection method is implemented to allow for measurements using a single optical access point. Fuel mole fraction and gas density are mapped within a cavity flameholder on planes parallel to the Mach 2 freestream flow above the cavity. With C 2 H 4 injection from the closeout ramp of the cavity toward the front step, a high fuel mole fraction region appears near the front step where density and flow velocity are low, and thus favorable for cavity flame ignition. The fuel mole fraction at a location in the cavity is found to be nearly proportional to fueling rate; the gas density, on the other hand, is not affected by the fueling rate, under the typical operating conditions.