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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.

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.

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.

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.

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.

The changes in flame structure and burning velocity of premixed n -heptane/air flames associated with ignition chemistry have been investigated in a reactor-assisted turbulent slot (RATS) burner. Two distinct turbulent flame regimes are identified by varying the flow residence time and reactor temperature. A chemically frozen (CF) regime is observed at a reactor temperature of 450 K and a low-temperature ignition (LTI) regime is identified at 650 K. At a reactor temperature of 450 K, the measured turbulent burning velocities ( S T ) exhibit a monotonic trend, proportional only to the turbulent intensity and laminar flame speed ( S L ) calculated with the initial fuel/air mixture. At a reactor temperature of 650 K, S T initially decreases with increasing flow residence times (decreasing turbulent intensity) but then increases once the reactor flow residence time exceeds the LTI delay. Furthermore, S T in the LTI regime exhibits a strong correlation with the extent of low-temperature reactivity (defined by CH 2 O concentration). The species distributions at the exit of the RATS burner after the onset of LTI are quantified by gas sampling-chromatography and used to compute the changes in S L and mixture Lewis number ( Le ), which are shown to substantially change after the onset of LTI. Damkohler's scaling analysis indicates that the increase in S T in the LTI regime originates from an increase in S L , a decrease in Le , and an increase in turbulence intensity due to the heat release from the low-temperature chemistry. To examine the role of ignition chemistry on flame stability, flame flashback measurements have been performed by varying mean jet velocities and n -heptane/air mixture equivalence ratios for reactor temperatures of 450 and 650 K. Measurements at 650 K imply the strong influence of high-temperature ignition on flame stability phenomena.

We describe efforts to develop kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the CH radical for application to premixed flames. The basic approach used here involves excitation and detection of the CH radial via the C 2 Σ + −X 2 Π ( v ′ = 0, v ″ = 0) band, which has transitions in the wavelength range λ ≈ 310–320 nm. Transitions in this band are generally stronger than those in the A−X and B−X bands of CH and the radiative lifetimes are shorter too. Thus, the C−X band should have advantages with regard to CH detectability in atmospheric flames, and we show that good CH-PLIF signal-to-noise and signal-to-background ratios can be attained at a 10-kHz interrogation rate and that the spatial resolution (of the CH layer) is reasonably good as well. Of course, strong OH lines, from the A 2 Σ + −X 2 Π (0,0) and (1,1) bands, lie nearby the CH C−X lines. While this can create some interference in the detection of CH, we demonstrate that OH lines can be avoided, if desired, or excited, if desired. Indeed, easy access to either CH or OH is a substantial benefit of the method outlined herein. Furthermore, we show that simultaneous imaging of CH and OH—using a single laser system and camera—is possible too. We demonstrate the utility of this approach for resolving the flamefront dynamics with 10-kHz measurements in a turbulent, premixed methane–air Bunsen flame: we see intrusion of flame and products into the reactant-zone, which appears to accelerate the consumption of the reactant core, and fingers of flame and reactants that extend into the product-zone and then pinch off and burn out.

The interplay of humidity and non-equilibrium, transient plasma was studied via ignition experiments in a C2H4–air mixture, concentration measurements in humid air, and detailed simulations. Hydroxyl (OH) and ozone (O3) produced via non-equilibrium plasma were characterized in a flowing H2O–air mixture at atmospheric pressure with varying the levels of humidity using planar laser-induced fluorescence (PLIF) and UV absorption, respectively. The OH, which was created in the discharge streamers, peaked at a concentration of ∼5 × 1014/cm3 and then decayed below 1 × 1014/cm3 after ∼100 µs. O3, which is long lived, peaked at a concentration of 1.4 × 1015/cm3. An increase in humidity from X H 2 O ≈ 0.2% to 1% resulted in a monotonic increase in the concentration of OH and a 67% decrease in that of O3. Zero-dimensional Boltzmann modeling of non-equilibrium plasma discharges in humid air showed qualitative agreement with these results and points to the decrease in O concentration (with increasing humidity) as the reason for the decreased O3 concentration. In spite the dramatic decline in X O 3 with increased humidity, there was no strong commensurate effect on ignition and flame propagation in C2H4–air mixtures: Peak pressure rise rate was at its maximum value at X H 2 O = 1% but was only 25% less at X H 2 O = 5%.

This work reports the measurements of three-dimensional (3D) flame topography and curvature of a premixed turbulent Bunsen flame at a rate of 5 kHz, using a technique combining chemiluminescence and tomography. Line-of-sight images of chemiluminescence (termed projections) of the target flame were recorded by six cameras from different orientations simultaneously at 5 kHz. Based on these projections, a tomography algorithm reconstructed the 3D flame structure, based on which 3D curvature was then calculated. Due to the 3D nature of the data, statistics of flame properties can then be extracted both in temporal and spatial domains. Probability density function (PDF) of flame topography was extracted from a series of 3D measurements, and the PDFs of the flame at different spatial locations were examined. Furthermore, the instantaneously measured 3D flame topography also enabled the calculation of 3D flame curvature (or 2D curvature along an arbitrary orientation). The PDFs of curvature in 2D and 3D were then extracted and compared. These results provide quantification of the flame surface shape in 3D (cylindrical, elliptic, or hyperbolic), illustrating the utility of 3D diagnostics to fully resolve the dynamics of turbulent flames.

UV Raman scattering is applied to measure fuel/air mixing and combustion of a Mach-2 air stream flowing over a step-ramp cavity fueled with 70% methane/30% hydrogen. Average and RMS fluctuations of temperature and major species profiles as well as scatter plots of simultaneous temperature and chemical scalars are determined from single-shot Raman scattering measurements along a 6-mm line transverse to the main supersonic flow. In the fuel-rich regions of the pilot cavity, the 248-nm KrF laser induces broadband fluorescence interference that reduces the number of analyzable Raman spectra; nonetheless, on the whole, a significant fraction of the spectra were reducible. In the cavity, hydrogen fuel reacts quickly resulting in a uniform water concentration in the recirculation zone. Methane reacts slowly to carbon monoxide/carbon dioxide in the cavity, leading to non-uniform concentrations of these species. Mean and instantaneous mixture fraction data inside the shear layer were indicative of oxygen transport across the shear layer. Temperature, water, and oxygen fluctuations are fairly constant throughout the combustor due to recirculation/turbulent transport across the shear layer and the slow reaction of methane.

This work reports the study of the ignition processes in a Mach-2 cavity combustor based on three-dimensional (3D) measurements at 20 kHz. The 3D measurements were obtained by a combination of tomographic chemiluminescence and fiber-based endoscopes. Measurements of 3D flame and flow properties were reported under two fueling conditions of the combustor. The properties included 3D volume, surface area, shape factor, and 3D3C (three-dimensional and three-component) velocity of the ignition kernel. These results clearly distinguished the ignition stage from the stable combustion stage of the combustor and enabled the determination of a transition time to quantify both stages. The analysis of the change of the ignition kernel's shape, when combined with the 3D3C velocity measurements, also illustrated flame-flow interactions in the cavity combustor. These results demonstrated the utility of the 3D diagnostics to overcome some of the limitations of established planar diagnostics and to resolve the dynamics of high-speed combustion devices both spatially and temporally.

Staged combustion has been explored for power generation gas turbine engines to increase engine efficiency with minimal contribution to pollutant formation. Secondary fuel injection into the vitiated flow from the primary combustion process is one approach. In this work, advanced diagnostic measurements were performed on an experimental representation of such a system, with a transverse jet injection into a swirling vitiated crossflow. High-repetition-rate simultaneous particle image velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) were performed at three measurement planes perpendicular to the jet axis. The measurements provided qualitative as well as quantitative information on the evolution of complex flow structures and transient events such as re-ignition, local extinction and vortex-flame interactions in the turbulent reacting flow. Transverse jets composed of H 2 mixed with N 2 and premixed natural gas and air were injected through a tube protruding into the crossflow. The vitiated crossflow is produced by a low swirl burner (LSB). The crossflow exhibits considerable swirl at the location of the transverse jet injection. Two momentum flux ratios, J = 3 and J = 8 were employed to study the effect of momentum flux ratio on the stabilization of reaction fronts. The time-averaged flow field show a pair of steady wake vortices, which were found to be much larger for J = 8 than for the J = 3 case. The region with the maximum time-averaged OH-PLIF intensity was found to be localized between the wake vortex pairs. The wake Strouhal number was found to be invariant with respect to the momentum flux ratio. Based on the experimental data, it is hypothesized that the shear layer and wake field vortices play a significant role in stabilizing a steady reaction front within the near-wake region of the jet.

A direct spectrum matching method for laser-induced breakdown spectroscopy is proposed to simultaneously measure gas density and concentration in turbulent reacting environments with improved measurement accuracy. The breakdown spectrum recorded in the target flow is directly matched with a spectrum out of a database consisting of various emission spectra recorded under well-defined conditions in a range of gas density and composition. It is shown that the wavelength, intensity and line width of the atom/ion emission lines in the spectrum indicate atom composition and gas density that are independent of parent molecular species in the target flow. Once a matching spectrum (within 550–830 nm containing O, H, N, and C lines) in the database of a known gas condition is found, the concentration and gas density at the location of the breakdown can be accurately derived. A 532-nm Nd:YAG laser with 10-Hz pulse repetition rate is used to induce breakdown in fuel/air mixtures in a variable pressure combustion chamber to build the spectrum database.In addition, it is used in a cavity flameholder of a model supersonic combustor to measure the gas density and concentration fields in a turbulent reacting environment. All the measurements are completed within 100 ns after laser firing, before breakdown affects the flow and the fast evolving environment alters the breakdown spectrum.

It has been predicted that several changes will occur when premixed flames are subjected to the extreme levels of turbulence that can be found in practical combustors. This paper is a review of recent experimental and DNS results that have been obtained for the range of extreme turbulence, and it includes a discussion of cases that agree or disagree with predictions. “Extreme turbulence” is defined to correspond to a turbulent Reynolds number (ReΤ, based on integral scale) that exceeds 2800 or a turbulent Karlovitz number that exceeds 100, for reasons that are discussed in Section 2.1 . Several data bases are described that include measurements made at Lund University, the University of Sydney, the University of Michigan and the U.S. Air Force Research Lab. The data bases also include DNS results from Sandia National Laboratory, the University of New South Wales, Newcastle University, the California Institute of Technology and the University of Cambridge. Several major observations are: (a) DNS now can be achieved for a realistic geometry (of the Lund University jet burner) even for extreme turbulence levels, (b) state relations (conditional mean profiles) from DNS and experiments do tend to agree with laminar profiles, at least for methane-air and hydrogen-air reactants that are not preheated, and (c) regime boundaries have been measured and they do not agree with predicted boundaries. These findings indicate that the range of conditions for which flamelet models should be valid is larger than what was previously believed. Additional parameters have been shown to be important; for example, broken reactions occur if the “back-support” is insufficient due to the entrainment of cold gas into the product gas. Turbulent burning velocity measurements have been extended from the previous normalized turbulence levels (u’/SL) of 24 up to a value of 163. Turbulent burning velocities no longer follow the trend predicted by Shchelkin but they tend to follow the trend predicted by Damkohler. The boundary where flamelet broadening begins was measured to occur at ReTaylor = 13.8, which corresponds to an integral scale Reynolds number (ReT) of 2800. This measured regime boundary can be explained by the idea that flame structure is altered when the turbulent diffusivity at the Taylor scale exceeds a critical value, rather than the idea that changes occur when Kolmogorov eddies just fit inside a flamelet. A roadmap to extend DNS to complex chemistry and to higher Reynolds numbers is discussed. Exascale computers, machine learning, adaptive mesh refinement and embedded DNS show promise. Some advances are reviewed that have extended the use of line and planar PLIF and CARS laser diagnostics to studies that consider complex hydrocarbon fuels and harsh environments.

The effect of ozone (O 3 ) addition on laminar flame speeds ( S L ) across a wide pressure range was investigated experimentally and numerically using three fuels, CH 4 , C 2 H 4 and C 3 H 8 . Enhancement of S L due to O 3 addition was consistently observed for CH 4 and C 3 H 8 mixtures over a range of lean to rich equivalence ratios, based on comparisons of S L measured with and without O 3 addition. For both fuels, simulation results agree with experimental results, with the best predictions at near stoichiometric conditions and the largest discrepancies for fuel-rich cases. A significant increase in the S L enhancement was observed at elevated pressures: the enhancement in the measured S L for a stoichiometric CH 4 /air mixture with 6334 parts per million (ppm) O 3 addition increased from 7.7% at atmospheric pressure to 11% at 2.5 atm. Elevated pressure both promotes O 3 decomposition, which provides O atoms, and suppresses diffusion of H, which reduces the influence of the O 3 +H=OH+O 2 reaction. Together, these lead to the increased S L enhancement with pressure. In contrast to the results for the two saturated hydrocarbons, both detrimental and beneficial effects due to O 3 addition were observed for the unsaturated hydrocarbon fuel, C 2 H 4 in this study. With O 3 addition, C 2 H 4 /air S L decreased at room temperature and pressure, owing to the heat loss induced by the exothermic ozonolysis reaction between O 3 and C 2 H 4 in the mixing process, but increased as the ozonolysis reactions were minimized when reactants were cooled to 200 K or pressure was decreased below 0.66 atm. These experimental results were successfully explained by a numerical model that includes a new ozonolysis sub-mechanism.

Detailed flame measurements are necessary for the development of accurate chemical kinetics models. This study characterizes quasi one-dimensional, laminar, nearly adiabatic, and freely propagating flames stabilized over a Hencken flat flame burner. Major reactant and product species in lean premixed methane/oxygen/argon and ethylene/oxygen/argon flames were measured through extractive sampling and absorption spectroscopy to obtain spatially-resolved species profiles of oxygen, methane, ethylene, carbon monoxide and carbon dioxide. Experimental results were compared to numerical simulations of one-dimensional, freely propagating, adiabatic flames. Experimental and numerical results for flame speeds and species concentrations were largely in agreement within calculated uncertainty, indicating that data produced using a Hencken burner at sub-atmospheric pressure can be readily compared to numerical simulations without the need for flame or burner temperatures, which are prone to uncertainty. The influence of the extractive probe on flame structure was also investigated, and results suggest that the structure is less influenced by probe intrusion when the flame is highly stretched.

Ignition of an ethylene fueled cavity in a supersonic flow was achieved through the application of two energy deposition techniques: a spark discharge and pulse detonator ( PD ). High-frequency shadowgraph and chemiluminescence imaging showed that the spark discharge ignition was passive with the ignition kernel and ensuing flame propagation following the cavity flowfield. The PD produced a high-pressure and temperature exhaust that allowed for ignition at lower tunnel temperatures and pressures than the spark discharge, but also caused significant disruption to the cavity flowfield dynamics. Under certain cavity fueling conditions a multiple regime ignition process occurred with the PD that led to decreased cavity burning and at times cavity extinction. Simulations were performed of the PD ignition process, capturing the decreased cavity burning observed in the experiments. The PD exhaust initially ignited and burned the fuel within the cavity rapidly. Simultaneously, the momentary elevated pressure from the detonation caused a blockage of the cavity fuel, starving the cavity until the PD completely exhausted and the flowfield could recover. With sufficiently high cavity fueling, the decrease in burning during the PD ignition process could be mitigated. Cavity fuel injection and entrainment of fuel through the shear layer from upstream injection allowed for the spark discharge ignition process to exhibit similar behavior with peaks and valleys of heat release (but to a lesser extent). The results of using the two energy deposition techniques emphasized the importance of cavity fueling and flowfield dynamics for successful ignition.

Laser-induced breakdown is used for quantitative gas property measurements (gas density and ethylene fuel concentration) in a cavity flameholder in a supersonic crossflow. A plasma is produced by a focused laser beam (Nd:YAG, 532 nm) in the cavity to measure gas properties at the location of the plasma and to ignite cavity flames. Plasma energy ( PE ), defined by the laser pulse energy absorbed/scattered in the plasma, and plasma emission spectra are recorded for estimating gas density and species concentration, respectively. To obtain correlations of PE vs. gas density and emission spectra vs. fuel concentration, calibration experiments are conducted using a variable-pressure (0–900 mbar)/temperature (300–900 K) chamber and a Hencken burner installed in a variable-pressure (50–900 mbar) combustion chamber. Total measurement time is sufficiently short, ∼80 ns after laser arrival at the plasma region, to capture the high intensity portion of the emission and to minimize effects of plasma displacement (in the high-speed flow). Specifically, the laser pulse energy incident and transmitted (through the plasma) are measured, and the plasma emission spectra are captured during a 50-ns gate, after an approximate 30-ns time delay (relative to onset of emission from the plasma volume) to avoid strong background emission from the plasma.

OH Laser Induced Fluorescence (LIF) and picosecond (ps), broadband Coherent Anti-Stokes Raman Spectroscopy (CARS) are used for time-resolved temperature and time-resolved, absolute OH number density measurements in lean H 2 -air, CH 4 -air, C 2 H 4 -air, and C 3 H 8 -air mixtures in a nanosecond (ns) pulse discharge cell/plasma flow reactor. The premixed fuel–air flow in the reactor, initially at T 0 = 500 K and P = 100 torr, is excited by a repetitive ns pulse discharge in a plane-to-plane geometry (peak voltage 28 kV, discharge gap 10 mm, estimated pulse energy 1.25 mJ/pulse), operated in burst mode at 10 kHz pulse repetition rate. In most measurements, burst duration is limited to 50 pulses, to preclude plasma-assisted ignition. The discharge uniformity in air and fuel–air flows is verified using sub-ns-gated images (employing an intensified charge-coupled device camera). Temperatures measured at the end of the discharge burst are in the range of T = 550–600 K, using both OH LIF and CARS, and remain essentially unchanged for up to 10 ms after the burst. Time-resolved temperature measured by CARS during plasma-assisted ignition of H 2 -air is in good agreement with kinetic model predictions. Based on CARS measurement, vibrational nonequilibrium is not a significant factor at the present conditions. Time-resolved, absolute OH number density, measured after the discharge burst, demonstrates that OH concentration in C 2 H 4 -air, C 3 H 8 -air, and CH 4 is highest in lean mixtures. In H 2 -air, OH concentration is nearly independent of the equivalence ratio. In C 2 H 4 -air and C 3 H 8 -air, unlike in CH 4 -air and in H 2 -air, transient OH-concentration overshoot after the discharge is detected. In C 2 H 4 -air and C 3 H 8 -air, OH decays after the discharge on the time scale of ∼0.02–0.1 ms, suggesting little accumulation during the burst of pulses repeated at 10 kHz. In CH 4 -air and H 2 -air, OH concentration decays within ∼0.1–1.0 ms and 0.5–1.0 ms, respectively, showing that it may accumulate during the burst. The experimental results are compared with kinetic modeling calculations using plasma/fuel chemistry model employing several H 2 -air and hydrocarbon-air chemistry mechanisms. Kinetic mechanisms for H 2 -air, CH 4 -air, and C 2 H 4 -air developed by A. Konnov provide the best overall agreement with OH measurements. In C 3 H 8 -air, none of the hydrocarbon chemistry mechanisms agrees well with the data. The results show the need for development of an accurate, predictive low-temperature plasma chemistry/fuel chemistry kinetic model applicable to fuels C 3 and higher.

Laser Induced Breakdown Spectroscopy (LIBS) is used to measure hydrocarbon fuel concentration in reacting flows. Emission spectra of the plasma induced by a focused-laser beam (Nd:YAG laser at 532 nm) are correlated with hydrocarbon fuel concentration in regions upstream (reactants) and downstream (combustion products) of a flame and adjacent to the combustion reaction zone. Nitrogen (568 nm) and hydrogen (656 nm) atomic emission lines are selected to establish a correlation between the line intensities and fuel concentration. These correlations are effective in a wide range of fuel mole fraction (7–90% methane/air and 5–93% ethylene/air mixtures) and independent of flow velocity. Nevertheless, the correlation depends on gas species in the plasma. Three individual correlations for premixed methane/air, ethylene/air and combustion product gases are established. For the application of the LIBS in high-speed flows, the emission spectrum is captured employing a 10-ns time gate approximately 25 ns after initial emission of radiation (from the probe region). The 25-ns gate delay is chosen to avoid broadband thermal emission from the high-temperature plasma core and achieve high spectrum signal intensity with reasonable signal-to-noise ratio of the atomic emission lines.

A well-defined plasma assisted combustion system with novel in situ discharge in a counterflow diffusion flame was developed to study the direct coupling kinetic effect of non-equilibrium plasma on flame ignition and extinction. A uniform discharge was generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The ignition and extinction characteristics of CH4/O2/He diffusion flames were investigated by measuring excited OH∗ and OH PLIF, at constant strain rates and O2 mole fraction on the oxidizer side while changing the fuel mole fraction. It was found that ignition and extinction occurred with an abrupt change of OH∗ emission intensity at lower O2 mole fraction, indicating the existence of the conventional ignition-extinction S-curve. However, at a higher O2 mole fraction, it was found that the in situ discharge could significantly modify the characteristics of ignition and extinction and create a new monotonic and fully stretched ignition S-curve. The transition from the conventional S-curves to a new stretched ignition curve indicated clearly that the active species generated by the plasma could change the chemical kinetic pathways of fuel oxidation at low temperature, thus resulting in the transition of flame stabilization mechanism from extinction-controlled to ignition-controlled regimes. The temperature and OH radical distributions were measured experimentally by the Rayleigh scattering technique and PLIF technique, respectively, and were compared with modeling. The results showed that the local maximum temperature in the reaction zone, where the ignition occurred, could be as low as 900 K. The chemical kinetic model for the plasma–flame interaction has been developed based on the assumption of constant electric field strength in the bulk plasma region. The reaction pathways analysis further revealed that atomic oxygen generated by the discharge was critical to controlling the radical production and promoting the chain branching effect in the reaction zone for low temperature ignition enhancement.

The kinetic effects of low temperature non-equilibrium plasma assisted CH4 oxidation on the extinction of partially premixed methane flames was studied at 60 Torr by blending 2% CH4 by volume into the oxidizer stream of a counterflow system. The experiments showed that non-equilibrium plasma can dramatically accelerate the CH4 oxidation at low temperature. The rapid CH4 oxidation via plasma assisted combustion resulted in fast chemical heat release and extended the extinction limits significantly. Furthermore, experimental results showed that partial fuel mixing in the oxidizer stream led to a dramatic decrease of O concentration due to its rapid consumption by CH4 oxidation at low temperature. The products of plasma assisted CH4 oxidation were measured using the Two-photon Absorption Laser-Induced Fluorescence (TALIF) method (for atomic oxygen, O), Fourier Transform Infrared (FTIR) spectroscopy, and Gas Chromatography (GC). The product concentrations were used to validate the plasma assisted combustion kinetic model. The comparisons showed the kinetic model over-predicted the CO, H2O and H2 concentrations and under-predicted CO2 concentration. A path flux analysis showed that O generated by the plasma was the critical species for extinction enhancement. In addition, the results showed that O was produced mainly by direct electron impact dissociation reactions and the collisional dissociation reactions of electronically excited molecules with O2. Moreover, these reactions involving electron impact and excited species collisional dissociation of CH4 contributed approximately a mole fraction of 0.1 of total radical production. The present experiments produced quantitative species and extinction data of low temperature plasma assisted combustion to constrain the uncertainties in plasma/flame kinetic models.

An atmospheric high-Q re-entrant cavity applicator is used to couple microwave (2.45 GHz) electromagnetic energy directly into the reaction zone of a premixed laminar methane–oxygen flame for flame enhancement. As microwave energy increases, a transition from electric field enhancement to microwave plasma discharge is observed. At low microwave powers (1–5 W), the flame is influenced by an electromagnetic field only. When power is increased, ionization and eventually breakdown of gas molecules result in a plasma plume with significant increase in the flammability limit. 2-D laser induced fluorescence imaging of hydroxyl radicals (OH) and carbon monoxide (CO) are conducted in the reaction zone over this transition, as well as spectrally resolved flame emission measurements to monitor excited state species and derive rotational temperatures using OH chemiluminescence for a range of equivalence ratios ( ϕ = 0.9–1.1) and total flow rates. In the electromagnetic field only phase (1–5 W), flame stability, excited state species, and temperature slightly increased with power while no significant change in OH number density was detected. With the onset of a plasma plume, a significant rise in both excited state species, CO and OH number density was observed. The importance of in-situ fuel reforming in plasma coupled flames is shown through the concentration of CO, which increases ∼18% with 30 W microwave power.

A non-equilibrium plasma assisted combustion system was developed by integrating a counterflow burner with a nano-second pulser to study the effects of atomic oxygen production on the extinction limits of methane diffusion flames at low pressure conditions. The production of atomic oxygen from the repetitive nano-second plasma discharge was measured by using two-photon absorption laser-induced fluorescence (TALIF). The results showed that both the atomic oxygen concentration production and the oxidizer stream temperature increased with the increase of the pulse repetition frequency for a constant plasma voltage. The experimental results revealed that the plasma activated oxidizer significantly magnified the reactivity of diffusion flames and resulted in an increase of extinction strain rates through the coupling between thermal and kinetic effects. Numerical computations showed that atomic oxygen quenching strongly depends on the oxidizer stream temperature. The kinetic effect of atomic oxygen production by a non-equilibrium plasma discharge on the enhancement of flame extinction limits was demonstrated, for the first time, at high repetition frequencies with elevated oxidizer temperatures. The reaction paths for radical production and consumption were analyzed. It was concluded that in order to achieve significant kinetic enhancement from atomic oxygen production on flame stabilization, the plasma discharge temperature needs to be above the critical crossover temperature which defines the transition point from radical termination to chain-branching.

This paper presents observations and analysis of the time-dependent behavior of a 10 kW partially pre-mixed, swirl-stabilized methane–air flame exhibiting self-excited thermo-acoustic oscillations. This analysis is based on a series of measurements wherein particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) of the OH radical were performed simultaneously at 5 kHz repetition rate over durations of 0.8 s. Chemiluminescence imaging of the OH * radical was performed separately, also at 5 kHz over 0.8 s acquisition runs. These measurements were of sufficient sampling frequency and duration to extract usable spatial and temporal frequency information on the medium to large-scale flow-field and heat-release characteristics of the flame. This analysis is used to more fully characterize the interaction between the self-excited thermo-acoustic oscillations and the dominant flow-field structure of this flame, a precessing vortex core (PVC) present in the inner recirculation zone. Interpretation of individual measurement sequences yielded insight into various physical phenomena and the underlying mechanisms driving flame dynamics. It is observed for this flame that location of the reaction zone tracks large-scale fluctuations in axial velocity and also conforms to the passage of large-scale vortical structures through the flow-field. Local extinction of the reaction zone in regions of persistently high principal compressive strain is observed. Such extinctions, however, are seen to be self healing and thus do not induce blowout. Indications of auto-ignition in regions of unburned gas near the exit are also observed. Probable auto-ignition events are frequently observed coincident with the centers of large-scale vortical structures, suggesting the phenomenon is linked to the enhanced mixing and longer residence times associated with fluid at the core of the PVC as it moves through the flame.

A supersonic combustor was experimentally investigated using both conventional instrumentation and laser-based diagnostics. Planar laser-induced fluorescence (PLIF) imaging of OH was used in the main section of the combustor to examine flameholding and flame propagation during a series of evaluations at conditions simulating Mach-5.5 flight. Parameters of interest in this study included the angle of the primary fuel injectors, the distribution of fuel throughout the combustor, and the fuel composition. Changes in fuel-injection angle were expected to influence the mixing and combustion processes, and therefore combustor operation. Fuel-distribution variations were expected to modify the flame propagation between flameholding regions. Finally, ethylene and methane were used to examine the suitability of the flameholder designs over a wide range of fuel reactivity. Results suggest that the combustor provides relatively robust flameholding regardless of the fuel used and good flame propagation as long as the fuel distribution provides favorable conditions in the flameholding regions. In addition, the results show that the primary injectors can be useful in controlling certain aspects of combustor operability.

Fluctuations in temperature non-uniformity along the line-of-sight of a diode laser absorption sensor in a model scramjet are found to precede backpressure-induced unstart (expulsion of the isolator shock train). A novel detection strategy combining Fourier analysis of temperature time series to determine low-frequency heat release fluctuations with simultaneous measurements of multiple absorption features of H 2 O to identify temperature non-uniformities was applied to the scramjet combustor. Time-resolved absorption is measured using wavelength modulation spectroscopy for three transitions chosen with different temperature-dependent absorption characteristics. The line-of-sight (LOS)-averaged temperature inferred from the ratio of absorption from one pair of transitions is highly sensitive to low-temperature non-uniformities along the absorption path while the other ratio is less sensitive. The fraction of fluctuations in the range 1 f

Transient plasma induced production of OH is followed in a quiescent, stoichiometric CH4–air mixture using the planar laser induced fluorescence technique. Ignition and subsequent flame propagation, for both the transient plasma and traditional spark ignition, are observed with a high speed camera (2000 fps). The transient plasma is generated using a 70 ns FWHM, 60 kV, 800 mJ pulse. OH production was confirmed throughout the chamber volume; however, the mean number density was found to decay below 1.3 × 10 14 cm −3 near 100 μs. Nonetheless, ignition induced by transient plasma was decidedly faster than by spark ignition. Using the high speed camera, ignition initiated by transient plasma was found to occur along the length of the anode at approximately 1 ms, leading to the formation of a wrinkled, cylindrically-shaped flame. Analysis of the flame front propagation rates shows that flames ignited by transient plasma propagate essentially at the speed consistent with well accepted literature values for the stoichiometric methane–air mixture. This supports the notion that residue plasma, if any, has little effect on flame propagation.

This paper reports observations of local extinction events characterized by flame holes in the CH profiles that have been gathered during a simultaneous sequential CH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) investigation of a lifted methane-air diffusion flame. Flame bulges are also reported - that are thought to precede extinction events - as well as ''pinched'' regions upstream of the flame bulges. This information is of use to modelers regarding structures interacting with the reaction zone. It is also relevant to those analyzing and modeling breaks in the reaction zones in studies of flame holes and edge flames.

Several previously unreported properties of turbulent premixed flames were measured because they are especially useful for the future assessment of direct numerical simulations and models. These new properties include local stretch rates, a wrinkling parameter, the degree of flamelet extinction, and the reaction layer thickness, which were quantified using simultaneous CH planar laser-induced fluorescence/particle image velocimetry (CH PLIF-PIV) diagnostics. Other reported properties that are useful for model assessment are flame surface density ( Σ ) and global consumption speed, which is one type of turbulent burning velocity. Also measured was the Meneveau–Poinsot stretch efficiency function ( Γ K ) , which plays a central role in the coherent flamelet model. Some images of the flame–eddy interactions show how eddies exert strain and how flamelets “merge.” A highly wrinkled (corrugated) flame with well-defined boundary conditions was stabilized on a large two-dimensional slot Bunsen burner. It was found that the turbulent burning velocity of Bunsen flames depends on the mean velocity U ¯ , which was varied independently of turbulence intensity. It is concluded that conventional relations for the turbulent burning velocity of Bunsen flames are inadequate because they should include two additional parameters: mean velocity U ¯ and burner width W . These parameters affect the residence times of the flame–eddy interactions. A scaling analysis is presented to explain the observed trends. It indicates that if the burner width is sufficiently large, the long flame will experience significant flamelet merging, which is one factor leading to the “bending” (nonlinear behavior) of the burning velocity curve. Images of CH layers show that flame surface area is lost by flamelet merging, but is not lost due to local extinction, as no extinction was observed. The stretch efficiency function increases with increasing integral scale, indicating that large eddies are more efficient in exerting flame stretch than small eddies.

Experiments were performed to examine the stability of hydrocarbon-fueled flames in cavity flameholders in supersonic airflows. Methane and ethylene were burned in two different cavity configurations having aft walls ramped at 22.5° and 90°. Air stagnation temperatures were 590 K at Mach 2 and 640 K at Mach 3. Lean blowout limits showed dependence on the air mass flowrates, cavity geometry, fuel injection scheme, Mach number, and fuel type. Large differences were noted between cavity floor and cavity ramp injection schemes. Visual observations, planar laser-induced fluorescence of nitric oxide, and shadowgraph imaging were used to investigate these phenomena. Cavity ramp injection provided better performance near the lean blowout limit, whereas injection from the cavity floor resulted in more stable flames near the rich limit. Ethylene flames have a wider range of stable operations than methane in all conditions. Lean blowout limits were not significantly different between the Mach 2 and Mach 3 cases at the lean limit; however, variation in Mach number had a measurable effect near the rich limit. Fuel flowrates at ignition were much greater than the lean blowout limit, but showed similar dependence on air mass flowrate.