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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.

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

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...

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.

Shockwave propagation from focused-laser-induced plasmas was characterized for implementing calibration-free seedless velocimetry. An emission-free probe beam deflection (EF-PBD) technique and short-gated shadowgraph/schlieren imaging were used for the investigation. Nanosecond laser pulses at 532 nm were focused to induce plasmas generating shockwaves from the focal point at atmospheric condition. A continuous-wave (CW) laser beam was set up to pass nearby for detecting the shock arrival at the beam location; the shock diverts the CW-laser beam, which is sensed by the photodiode. The EF-PBD was devised to minimize the influence of the plasma emission that affects the probe beam intensity measured by the photodiode. The probe beam was moved from the focal point to consecutive locations very close to the plasma (every 0.2 mm from 0 to 1.8 mm) for accurately measuring the shock speed in the region adjacent to the plasma. A point-explosion model was employed for predicting the shockwave propagation phenomenon. It was found that the ambient flow velocity around the plasma affects the shockwave arrival time on the probe beam, which suggests the feasibility of a seedless velocimetry technique. When the distance between the probe beam and the plasma is known, the velocimetry does not require calibration. The proposed velocimetry technique was tested successfully in a range of 30–180 m/s ambient flow.

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.

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...

The mechanisms of transient formation and oxidation of soot in an aero-engine model combustor at elevated pressure are studied for the first time using a combination of high-speed simultaneous stereo-PIV and OH-PLIF and results from a recent detailed LES. A combined analysis of experiment and LES shows that the highly transient and intermittent evolution of soot in this combustor is governed by an unsteady interplay of distinct pockets of burned gas in the inner recirculation zone (IRZ) with either relatively rich or relatively lean composition. The former originate from reaction of fuel-rich unburned gas, whereas the latter result from additional admixture of secondary air further downstream. The analysis further enables distinction and localization of premixed and diffusion-type flame fronts within the flame zone. The time-resolved complementary measurements of velocity field and flame structure allow accurate tracking of both the burned gas pockets and soot filaments. It is seen that soot generally forms in the rich pockets if their residence time in the IRZ is sufficient, whereas oxidation occurs in the lean zones carrying OH. Correlating the dynamics of flow field and soot indicates that the intermittency of soot is driven by an intermittent flow of lean burned gas into the IRZ that affects the residence time of rich pockets. The results suggest that the formation of soot might be further reduced by a proper adjustment of secondary air injection aiming at a sufficient and more constant recirculation of lean burned gas. A remarkably good agreement of measured and simulated instantaneous flame structures is observed, indicating that flow field and gas-phase reactions are well predicted by the LES. The experimental insights into the transient mechanisms of soot formation and oxidation, on the other hand, may provide useful input for LES soot models where deviations from measurements are generally larger.

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.

In the present study, the internal flowfield of aerated-liquid fuel injectors is examined through X-ray radiography and X-ray fluorescence. An inside–out injector, consisting of a perforated aerating tube within an annular liquid stream, sprays into a quiescent environment at a fixed mass flow rate of water and nitrogen gas. The liquid is doped with bromine (in the form of NaBr) to create an X-ray fluorescence signal. This allows for reasonable absorption and fluorescence signals, and one or both diagnostics can be used to track the liquid distribution. The injector housing is fabricated from beryllium (Be), which allows the internal flowfield to be examined (as Be has relatively low X-ray attenuation coefficient). Two injector geometries are compared, illustrating the effects of aerating orifice size and location on the flow evolution. Time-averaged equivalent pathlength and line-of-sight averaged density ρ(y) reveal the formation of the two-phase mixture, showing that the liquid film thickness along the injector walls is a function of the aerating tube geometry, though only upstream of the nozzle. These differences in gas and liquid distribution (between injectors with different aerating tube designs) are suppressed as the mixture traverses the nozzle contraction. The averaged liquid velocity (computed from the density and liquid mass flow rate) reveals a similar trend. This suggests that at least for the current configurations, the plume width, liquid mass distribution, and averaged liquid velocity for the time-averaged external spray are insensitive to the aerating tube geometry.

A Quasi-DC (Q-DC) electrical discharge generates a highly transient filamentary plasma in high-speed airflow. Major specific properties of this type of discharge are realized due to a strong coupling of the plasma to the moving gas. The plasma, supplied by a DC voltage waveform, demonstrates a pulsed-periodic pattern of dynamics significantly affecting the flow structure. In this study, the dynamics and plasma parameters of the Q-DC discharge are analyzed in the Supersonic Test Rig (SBR-50) at the University of Notre Dame at Mach number M = 2, stagnation pressure P 0 = (0.9–2.6) × 105 Pa, stagnation temperature T 0 = 300 K, unit Reynolds number ReL = 7–25 × 106 m−1, and plasma power W pl = 3–21 kW. The plasma parameters are measured with current–voltage probes and optical emission spectroscopy. An unsteady pattern of interaction is depicted by high-speed image capturing. The result of the plasma-flow interaction is characterized by means of pressure measurements and schlieren visualization. It is considered that the Q-DC discharge may be employed for active control of duct-driven flows, cavity-based flow, and for effective control of shock wave–boundary layer interaction.

This Letter reports the first direct comparison between two-dimensional (2D) and three-dimensional (3D) laser-induced fluorescence (LIF) applied to highly turbulent flames, with the goal of experimentally illustrating the capabilities and limitations of volumetric LIF (VLIF). To accomplish these goals, planar LIF (PLIF) and VLIF measurements were simultaneously performed on turbulent flames based on the CH radical. The PLIF measurements imaged a planar cross-section of the target flames across a 2D field-of-view (FOV) of 42 mm×42 mm. The VLIF measurements imaged the same region in the target flame with a 3D FOV of 42 mm×42 mm×5 mm, with 5 mm being the thickness of the measurement volume. The VLIF signals generated in this volume were captured by five intensified cameras from different perspectives, based on which a 3D tomographic reconstruction was performed to obtain the 3D reconstruction of the CH radical (as a marker of the flame front). The PLIF measurements were then compared to a cross-section of the VLIF measurement to demonstrate the feasibility and accuracy of instantaneous 3D imaging of flame topography and flame surface area in highly turbulent flames.

Two-photon-absorption laser-induced fluorescence of Kr was explored using both nanosecond- and femtosecond-duration laser excitation sources. Fluorescence signals following two-photon excitation at two wavelengths (212.56 nm and 214.77 nm) were compared while varying laser pulse duration, energy, and excitation wavelength as well as pressure and Kr mole fraction in mixtures with nitrogen. Our findings show that stronger fluorescence was observed when the excitation wavelength was tuned to 212.56 nm, regardless of the excitation-pulse duration. Moreover, an approximate 100-fold signal enhancement from nanosecond excitation (∼3 mJ/pulse, 10 ns duration) was observed as compared to femtosecond excitation (∼6 μJ/pulse, 90 fs duration).

This paper examines the structure of microwave (MW)-enhanced flames through 10-kHz imaging. High repetition rate laser diagnostic methods are used to simultaneously record 2-D images of OH laser-induced fluorescence and chemiluminescence within an atmospheric plasma-enhanced flame. Collecting both OH planar laser-induced fluorescence and chemiluminescence allows for observation of OH radicals in the plane of the thin laser sheet as well as volume-integrated excited state emission. A tunable, MW waveguide plasma source-operating at 2.45 GHz and delivering 90-130 W to the flowfield-ignites and sustains a CH4/air flame, whereas laser-induced fluorescence and chemiluminescence are acquired at a sustained framing rate of 10 kHz, using two intensified CMOS cameras and a synchronized laser. Multiple geometries and flames (premixed and nonpremixed) are studied by adjusting gas flow compositions and the plasma applicator nozzle components. A stoichiometric premixed flame configuration produces a divergent flame with large-scale fluctuations and vortex shedding into ambient air and is capable of feedstock flow velocities for combustion-to-plasma power ratios . Another arrangement produces plasma along the initial mixing layer of a nonpremixed flame, yielding a thin cylindrical reaction zone of coincident chemiluminescence and fluorescence. Replacing the fuel with rich premixed gases produces a narrow conical flame anchored by the circular plasma discharge with a little flamefront fluctuation. The high-speed diagnostics capture OH signals in cinematic sequences, providing new understanding of the plasma-assisted flame holding mechanism and allowing for the tracking of individual flow feature development.

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.

A plasma-coupled methane-air flame is produced at atmospheric pressure by a microwave plasma source utilizing a tunable waveguide. Laser diagnostics are used to examine the direct-coupled, plasma-ignited, and sustained flame, for multiple flame types and nozzle geometries. OH radical number densities are quantified using planar laser-induced fluorescence and temperature measured by Rayleigh scattering thermometry. Premixed and nonpremixed flames are studied using both solid and hollow inner conductors in the plasma-applicating nozzle. The plasma source is powered by a continuous 2.45-GHz magnetron producing 360 W of power. Plasma power is controlled by adjusting the reflected microwave power, measured at a dummy load attached to a circulator. Maximum OH radical number densities were quantified as approximately (3 - 5) × 1016 cm-3 for plasma powers around 100 W, with small variation between configurations. The maximum temperatures occurred in the nonpremixed flame, where the plasma is generated in air, reaching values of 3500 K. Temperatures are lower, peaking at 2000 K, when the plasma is generated at the air-fuel boundary or the air-premixed boundary through use of the hollow inner conductor. Additional parameters are adjusted, including flow rates, power level, and equivalence ratio, and the effects are discussed. Nonpremixed configurations are ill suited for flame enhancement, whereas a premixed flow through the hollow electrode best demonstrates nonthermal plasma-assisted combustion.

Simultaneous 10 kHz repetition-rate tomographic particle image velocimetry, hydroxyl planar laser-induced fluorescence (OH PLIF), and formaldehyde (CH $$_2$$ O) PLIF were used to study the structure and dynamics of turbulent premixed flames. The flames investigated span from the classically defined corrugated flamelet regime to conditions at which broadened and/or broken flamelets are expected. Methods are presented for determining 3D flame topologies from the Mie scattering tomography and for tracking features through space and time using theoretical Lagrangian particles. Substantial broadening of the CH $$_2$$ O region is observed with increasing turbulence intensity. However, OH production remains rapid, and the region of OH and CH $$_2$$ O overlap remains thin. Local flame speeds exceeding three times the laminar flame speed are observed in regions of flame–flame interaction. Furthermore, a method of tracking fluid residence time within the CH $$_2$$ O layer is presented and shows that residence time decreases at higher turbulence intensity despite the broader distribution of the CH $$_2$$ O, indicating an increase in local reaction rate.