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

In the emerging field of plasma-assisted ammonia (NH 3) combustion, the evolution of key intermediate species has rarely been reported. This work establishes a simultaneous measurement system of laser-induced fluorescence (LIF) for hydroxyl (OH) and quantitative two-photon-absorption LIF for atomic oxygen (O), to explore the OH and O dynamics in an NH 3 /air flame affected by a nanosecond (ns) pulsed plasma discharge. Firstly, with the plasma on, the molar fraction of O is quantified to reach 8.7 × 10–3 in the burnt zone, about two orders of magnitude higher than that without plasma. In addition, the OH LIF signal intensity is four times higher, indicating a significant kinetic enhancement. Then, the spatial characteristics of OH and O are discussed and compared, showing remarkable discrepancy. The discrepancy between them indicates that O production is dominated by plasma kinetics, however, the OH production, primarily stemming from reactions between O and NH 3 /H 2 O, still depends on parameters associated with combustion kinetics. We further study the temporal dynamics of O and OH. It is concluded that O and OH peaks at 1.75 μs are mainly attributed to the pathway of quenching of the excited species. After that, O and OH start to decay but show significant differences between unburnt and burnt zones, which are characterized by a single-exponential decay and a bi-exponential decay, respectively. In the unburnt zone, the OH decay is much slower than the O decay due to the diverse pathways for OH production. In the burnt zone, the bi-exponential decay of O and OH can essentially be regarded as a process in which the NH 3 /air reactive system reaches chemical equilibrium. At this stage, the impacts of the excited species from the plasma gradually diminish and combustion kinetics dominates alone. [ABSTRACT FROM AUTHOR]

The impacts of the pulse repetition frequency (PRF), number of pulses, and energy per pulse in a train of nanosecond discharge pulses on the ignition of a flowing lean premixed methane–air mixture are investigated using numerical simulations. A phenomenological plasma model coupled with a compressible reacting flow solver is used for these simulations. The simulation strategy has been well validated by comparing the experimental schlieren and OH planar laser induced fluorescence (PLIF) results with the numerical schlieren (i.e. , density gradient) and OH density profiles, respectively. The characteristics of the ignition kernels produced by each discharge pulse and their interaction with each other as functions of the PRF are investigated. Three regimes were defined in the literature based on this interaction of the ignition kernels — fully coupled, partially coupled, and decoupled. This study uses numerical simulations to probe into the constructive and destructive effects, that ultimately determine ignition success, in these different regimes. The complete overlap of kernels and the complete lack of synergy between kernels produced by consecutive pulses are attributed to the success and failure of ignition and flame propagation in the fully coupled and decoupled regimes, respectively. In the partially coupled regime, the convection heat loss driven by the shock-turned-acoustic wave of the next discharge pulse, on the kernel produced by the previous discharge pulse, in addition to diffusion losses, contribute to ignition failure. However, the expansion of the next kernel in a region of higher average temperature and radical concentration created by the previous kernel could help to bridge the gap between the two kernels and result in successful ignition. The important parameters of energy per pulse, number of pulses, and equivalence ratio affect the competition between these constructive and destructive effects, which eventually determines the ignition success in this regime. Finally, the change in the nature of interaction between consecutive kernels from decoupled to partially coupled, at the same frequency but with different energies per pulse, is also shown. Novelty and significance statement This study presents large eddy simulation (LES)-based results on the impact of the pulse repetition frequency (PRF), number of pulses, and energy per pulse, on the success of plasma assisted ignition of a flowing lean premixed methane–air mixture. This is the first simulation work to show direct validation based on both schlieren and OH density from the experiments of Lefkowitz et al. (2021). This is also the first work which identifies and explains the constructive and destructive effects to explain the reduced ignition probability in the partially coupled regime observed in Lefkowitz and Ombrello (2017). The role of the shock-turned-acoustic wave produced by every subsequent discharge pulse, on the previous kernel in a pulse train (destructive); and the assistance provided by the previous kernel to the next kernel (constructive), has been shown quantitatively. The use of a compressible solver is imperative to identify this destructive effect. The change in the regime boundaries defined by the PRF, by changing the energy deposition, is also shown. [ABSTRACT FROM AUTHOR]

This study reports an experimental investigation of quantitative Nitric Oxide (NO) distribution in both premixed and non-premixed NH 3 /H 2 -air flames using a counterflow burner at atmospheric pressure. One-dimensional (1D) NO laser-induced fluorescence (LIF) spectroscopy and Raman/Rayleigh spectroscopy were conducted to accurately resolve the quantitative 1D NO profile in terms of mixture fraction, temperature, and physical space. We calibrated a saturated NO-LIF model in 5 premixed lean H 2 /N 2 /NO-air flames with different seeded NO levels in a McKenna burner and validated its accuracy in three H 2 N 2 NO-air counterflow diffusion flames. The overall uncertainty of NO quantification was less than 90 ppm. Our measurements were compared with simulations using different ammonia chemical kinetic models, revealing that current models have over 30% uncertainty in predicting peak NO concentrations (mole fraction) in 1D non-premixed and premixed flames and over 100% uncertainty in lower temperature regions. In premixed flames, measured NO concentrations fell within the intermediate range of current chemical kinetic models at lean and stoichiometric conditions, but were lower than the models at rich conditions. In non-premixed flames, all models overestimated the peak NO concentrations by more than 1000 ppm. It is noted that the measured peak NO concentrations increased with higher NH 3 /H 2 ratios (from 4/6 to 8/2), strain rates (from 80 to 140 1/s), and N 2 dilution ratios in a 1:1 NH 3 /H 2 mixture (from 0 to 30%). Although most models could qualitatively predict the trends, they were inaccurate in quantifying NO. Additionally, the measured width of the NO profile in mixture fraction space expanded with increasing NH 3 /H 2 ratio, N 2 dilution ratio, and strain rate. While models could qualitatively predict this behavior, they consistently underestimated NO in the fuel-rich, lower-temperature region, resulting in a narrower NO profile width. The Manna model showed a better prediction of NO distribution in the fuel rich portion of non-premixed flames, accounting for NH 3 NO interactions at lower temperatures. These findings highlight the critical need to improve models to accurately predict NO concentrations in ammonia-containing flames and their behavior in fuel rich regions. [ABSTRACT FROM AUTHOR]

This work aims to find the benefits, limitations, and costs (in terms of plasma energy and NO x emissions) of plasma-assisted ignition-stabilized combustion (PAISC). The experiments are performed on a custom-made methane-air jet flame burner with two coaxial flow inlets. Dielectric barrier discharge (DBD) configuration is set up in the outer flow channel which acts as four ignition sources. The DBDs are operated by bursts of positive voltage pulses of 10 ns duration and 24 kV amplitude. In this way, four ignition kernels are created in flow simultaneously. The ignited kernels expand while blowing out with the flow which is followed by new ignition kernel initiation during the next burst. This cycle is repeated continuously and complete burning of the fuel is ensured by proper interactions between ignited kernels. The PAISC dynamics are visualized using OH* chemiluminescence images captured at 10 kHz framerate. Along with visualization measurements, we measure CO and NO x emissions using an industrial gas analyzer. The gas analyzer measurements are complemented by two-photon laser-induced fluorescence (TALIF) measurements of CO. We find the optimal plasma parameters to optimize CO, and NO x emissions and the effective flame length. A parametric study for various flow parameters is performed and combustion operation regimes are identified. The main highlight of this work is quasi-complete combustion at flow speeds of up to 4 times the blowout flow speed, which costs less than 10 ppm@3%O 2 plasma-produced NO x and has a plasma energy consumption of less than 1% of the combustion heat release. Novelty and Significance Statement Various researchers have demonstrated plasma's ability to enhance combustion at near-lean-blowout limits in the past. This work presents a novel plasma-assisted ignition stabilized combustion (PAISC) strategy to achieve quasi-complete combustion at flow speeds much above blowout flow speeds. Pulsed dielectric barrier discharge plasma is used to ignite methane-air flow as it is a fast, repetitive, and energy-efficient ignition source. We study PAISC dynamics using high-speed visualization along with CO, NO x emissions, and plasma energy for various plasma and flow parameters. From these analyses, we optimize plasma parameters first and then discuss the benefits, limitations, and costs of PAISC at optimal plasma conditions. We believe that the presented strategy will motivate future combustor design with a wider range of operability, especially for high-speed propulsion applications. [ABSTRACT FROM AUTHOR]

Laser diagnostics has been one of the most powerful tools in advancing state-of-the-art combustion research over the last five decades. Prof. Marcus Aldén, one of the most well-known pioneers in this field, has contributed many influential original publications over more than 40 years of his research career. In this paper, we will review some selected contributions, with emphasis on optical techniques developed and applied by Marcus and the group in Lund under his leadership, which are expected to play important roles in facing the challenge of the transition to a carbon-neutral energy system. A brief biography of Marcus is presented in the introduction section, including a summary of his academic services and achievements. The following sections are organized into chapters on different well-recognized techniques, i.e. , laser-induced fluorescence, Raman and Rayleigh scattering, thermographic phosphors, coherent anti-Stokes Raman spectroscopy, resonant four-wave mixing, and additional novel technical developments and approaches. The description often follows different categories, including technique development, activities using relatively mature techniques for studies of combustion phenomena, and applications of the most mature techniques in practical devices. The last part of the paper includes a short interview with Marcus for his comments, suggestions, and thoughts on the future challenges and opportunities in this field. This article, on the one hand, is an acknowledgment of the outstanding contribution of Marcus to the field of combustion research and, on the other hand, intends to provide a valuable review of laser diagnostics in combustion research. [ABSTRACT FROM AUTHOR]

The ability of the one-dimensional turbulence (ODT) model to serve as a surrogate direct numerical simulation (DNS) is assessed for highly turbulent flames. The ODT model is applied to freely propagating premixed methane–air flames at Karlovitz numbers 10, 10 2 , 10 3 , and 10 4 , and results are compared with DNS. The ODT model solves the conservation equations for momentum, energy, and species on a one-dimensional domain, which corresponds to a streamwise line of sight spanning the DNS domain. The effects of turbulent advection are modeled via a stochastic process, in which the Kolmogorov and reactive length and time scales are explicitly resolved. Molecular transport and chemical kinetics are concurrently advanced in time. Both the ODT and DNS simulations use a 21-species skeletal chemical model for methane combustion. The accuracy of the ODT model is assessed by comparing its predictions of several key characteristics of the flames for each Karlovitz number tested, including the turbulent flame speed and width and the joint probability density functions (jPDFs) of major and selected minor species as well as the heat release rate conditioned on temperature with the results of DNS under comparable conditions. The ODT model is shown to yield qualitative and quantitative agreement with the DNS data for most of the above flame characteristics. Discrepancies are observed primarily for the jPDFs of several minor species examined. Overall, the ODT approach is shown to be an effective surrogate of DNS, potentially useful for guiding chemical reaction model reduction and for assessing the sensitivities of the flame structure and the burning rate to chemistry under highly turbulent conditions. Novelty and Significance : The direct numerical simulations (DNS) of premixed turbulent methane–air flames presented in this work span a uniquely wide range of turbulent intensities, from relatively modest corresponding to Karlovitz number Ka = 10 to ultra-high intensities at Ka = 1 0 4 . This represents virtually the entire range of turbulent intensities that could be encountered in any realistic situation. This is also the first time that such a wide range of conditions is probed for methane in high-fidelity, fully resolved simulations, which use a fully compressible set of flow equations. The one-dimensional turbulence (ODT) model utilizes the same forcing that is present in the DNS enabling a direct comparison between the ODT and DNS. The results show that ODT captures the key features of the DNS results. ODT is shown to be an effective surrogate for DNS and may be useful in guiding chemical reaction model reduction, where many simulations are required. [ABSTRACT FROM AUTHOR]

The effects of low and high approach flow turbulence levels on a large-scale, confined, premixed bluff-body-stabilized flame are experimentally characterized. The flame and flow are imaged using simultaneous 10-kHz-rate formaldehyde (CH 2 O) planar laser-induced fluorescence (PLIF), hydroxyl (OH) PLIF, particle image velocimetry (PIV), and OH* chemiluminescence. Elevated approach flow turbulence intensities of 16 % as compared to a lower value of 3 % affects the global and local flame and flow. The elevated approach flow turbulence results in a 22 % reduction in the length of the recirculation zone, despite the mean flame and mean shear layer locations being similar for both approach flow turbulence levels. Increasing the approach flow turbulence intensity does not lead to the mean flame position moving further into the reactant freestream. Both approach flow turbulence levels result in a similar growth of the flame brush thickness moving downstream until approximately 1.5 bluff-body widths, after which the flame brush for the low and high approach flow turbulence case stops growing and continues to linearly increase, respectively. This study describes the mechanisms by which high approach flow turbulence affects the global and local flame and emphasizes that the turbulent flame characteristics are influenced not only by the freestream approach flow turbulence but also by the bluff-body shear layers, recirculation zone, and wake. [ABSTRACT FROM AUTHOR]

This work compares two distinct methodologies for the removal of fluorescence contributions from Raman spectra collected in NH 3 /H 2 , CH 4 , and NH 3 /CH 4 counterflow premixed and non-premixed flames. The first approach (1-component) combines instantaneous measurements in a Raman-free spectral region and temperature-dependent correction factors calibrated in a single laminar flame, to estimate fluorescence contribution on the Raman signal. The second approach (2-component) requires the simultaneous acquisition of the vertically and horizontally polarized signal to separate the strongly polarized Raman signal from the polarization independent fluorescence. The accuracy, precision, and limitations of each method are thoroughly assessed under various flame conditions. The 1-component approach provides the same accuracy and higher precision with the 2-component approach in NH 3 /H 2 flames where the majority of the fluorescence signal is attributed to a single radical, NH 2. The accuracy of measurements based on the 1-component approach drops for CH 4 and NH 3 /CH 4 flames as the composition of the pool of radicals and soot precursors responsible for the fluorescence signal is flame-dependent and a single flame calibration cannot provide general results. In contrast, the polarization-separation approach exhibits inherent robustness by not requiring prior knowledge of fluorescence signal sources. This approach takes into account the depolarization ratio of each molecule in the Raman response curves. For N 2 , O 2 , H 2 , and H 2 O, the library is constructed based on the difference between polarized and depolarized signals. For NH 3 , the Raman response curve is experimentally calibrated using the difference between these two signals. Building upon previous work, this approach benefits from an improved experimental setup and incorporates the wavelet adaptive thresholding and reconstruction (WATR) denoising algorithm to enhance measurement precision. Successfully applied to NH 3 /H 2 -air and NH 3 /CH 4 flames, the polarization-separation method demonstrates satisfactory accuracy (∼ 0.01 mole fractions, 40 K temperatures) and precision (∼ 0.02 mole fractions, 60 K temperatures) across diverse conditions, regardless of the source of laser-induced fluorescence signal (NH 2 , NO 2 , C 2 , PAH, and other soot precursors). [ABSTRACT FROM AUTHOR]

The turbulent boundary layer behavior in the presence of flame–wall interactions (FWI) has an important role on the mass and energy transfer at the gas/solid interface. Detailed experiments resolving the turbulent boundary layer evolution in the presence of FWI are lacking, which impedes knowledge. This work presents a combination of particle image velocimetry (flow field), dual-pump coherent anti-Stokes Raman spectroscopy (gas temperature), and OH laser induced fluorescence (flame topology) measurements to study the evolution of the boundary layer structure in the presence of FWI. Experiments are conducted in a side-wall quenching (SWQ) burner. Findings reveal that the reacting boundary layer flow adheres to the linear scaling law u + = y + in the viscous sublayer until y + = 5. Beyond y + = 5, the flame modifies the velocity and temperature field such that the u z + streamwise velocity deviates from the viscous sublayer and the law-of-the-wall scaling in the log-layer with u z + being smaller than that of the non-reacting flow (the subscript z refers to the streamwise coordinate and is used throughout this manuscript). As the fluid approaches the flame impingement location at the wall, the gas temperature increases significantly, causing a threefold increase in kinematic viscosity, ν. Although the near-wall streamwise velocity gradient d 〈 U z 〉 / d y | y = 0 mm decreases, the larger increase in ν reduces u z + and leads to the deviation from the law-of-the-wall. Downstream the flame impingement location, ν is relatively constant and u z + values begin to approach those of the law-of-the-wall. Trends are presented for SWQ and head-on quenching flame topologies, and are intended to help development of more accurate wall models. [ABSTRACT FROM AUTHOR]

The current work investigates the formation of Nitric Oxide (NO) in hydrogen–air flames, over a wide range of flame temperatures. The use of hydrogen allows improved focus on the thermal-NO pathway by removing the complexity introduced by the prompt-NO pathway, which has been shown to be an important contributor to inaccurate predictions of absolute post-flame NO concentrations in hydrocarbon flames. This experimental study is conducted at atmospheric pressure using stoichiometric, premixed, laminar stagnation flames. Adiabatic flame temperatures ranging from 1600 K to 2300 K are achieved by varying the argon concentration in air. One-dimensional velocity, temperature, and NO concentration profiles are measured using non-intrusive laser diagnostics: Particle Tracking Velocimetry (PTV), NO multiline thermometry, and NO Laser Induced Fluorescence (NO-LIF), respectively. Results show that the experimental velocity profiles are incorrectly captured by the studied mechanisms, especially at low and high temperatures. This suggests that major inaccuracies are present in the hydrogen oxidation chemistry of the thermochemical models, regardless of their optimisation methodology. Furthermore, NO-LIF profiles show major discrepancies between all the studied mechanisms and the experiments, especially at elevated temperatures. The disagreement stems from an inaccurate description of the base chemistry of the models. These inaccuracies arise specifically from the description of the radical pool driving the flame behaviour and NO formation. This study demonstrates the need for model optimisation on experimental measurements using pure hydrogen 1D flames to obtain an accurate description of the hydrogen oxidation chemistry at play. This would lead to an improved description of the NO x sub-chemistry of any hydrogen, or hydrocarbon, combustion system. [ABSTRACT FROM AUTHOR]

This work demonstrates single-shot, two-dimensional temperature measurements in a premixed linear detonation channel using two-color OH planar laser-induced fluorescence thermometry. Detonation environments result in extreme thermodynamic conditions which create challenges in the resulting spectroscopic behavior. However, thermometry based on the ratio of the P 1 (9)/Q 1 (14) transitions within the A2Σ+←X2Π(1,0) band of OH was determined to be well-suited for detonation environments based on the spectral characteristics over a wide range of conditions. The technique is demonstrated in the post-induction region, focusing on the nominal equilibrium state of the Chapman-Jouguet conditions, on mixtures of stoichiometric H 2 and O 2 diluted with either N 2 or Ar. High-speed chemiluminescence imaging at 2 MHz was used to assess the qualitative dynamics of the detonation structure. The measured temperature fields for the Ar dilution case are relatively spatially uniform, with the distributions mean agreeing well with the theoretical calculation of the Chapman-Jouguet temperature, consistent with preliminary work [Grib et al. , AIAA SciTech Forum, (2021). 2021-0421]. Conversely, the temperature fields for a 50% N 2 dilution case are highly irregular, showing a larger dynamic range in temperature as well as pockets of unburned reactants, which emphasized the significance of this measurement by highlighting various reaction scenarios dictated by detonation waves. The irregularity of the N 2 dilution cases is explained in terms of the scale similarity between the channel size and the detonation cell sizes for the N 2 dilution mixtures, leading to weak or failing detonation modes. Overall, the demonstrated thermometry technique produced a precision (1-σ) of approximately 4% in atmospheric conditions and approximately 7.7% in strong detonation environments. In addition, the accuracy, defined as the percent difference between the mean and CJ temperature, was approximately 1% in the Ar diluted case. The ability to distinguish and quantify detonation burning behavior is promising for employing this approach for application in pressure-gain combustion facilities such as rotating detonation combustors, however care needs to be taken when interpreting the resulting temperature field, particularly near the wave front, due to the precision and validation range of the present line pair. [ABSTRACT FROM AUTHOR]

• The pre-chamber jet and reaction zone are visualized simultaneously by acetone PLIF and OH* chemiluminescence imaging, respectively. • The relationship between the pre-chamber jet and main chamber ignition region is explored. • The effects of global λ on PCC is evaluated with the same pre-chamber fueling. • The pre-chamber jets discharged from the upper row and lower row orifices are compared. • The spatial relationship between the pre-chamber jet and the reaction zone is clarified. We studied the relationship between pre-chamber jet and main chamber ignition in the pre-chamber combustion (PCC) of an optical engine, fueled with methane and equipped with an active pre-chamber with two rows of orifices. Acetone planar laser-induced fluorescence (PLIF) and OH* chemiluminescence imaging techniques were simultaneously applied to visualize the pre-chamber jet and the reaction zone in the main chamber, respectively. The pre-chamber fueling was constant and the main chamber fueling was increased to form an ultra-lean case and a lean case with global excess air ratios (λ) of 2.3 and 1.8, respectively. Results indicate that a higher pressure difference between pre-chamber and main chamber (▵ P) produces larger pre-chamber jet penetration speed; the maximum pre-chamber jet penetration speed appears at timing around the peak ▵ P. Over enrichment of the pre-chamber charge reduces the peak ▵ P and thus does not favor a faster pre-chamber jet discharge. In addition to the main pre-chamber jet, a weaker post jet discharge process is visualized; the former is due to the pre-chamber combustion while the latter due to the ▵ P fluctuation and the cylinder volume expansion. The post pre-chamber jet is accompanied by a post reaction zone in the ultra-lean case (λ =2.3) and there are two unburned regions in the main chamber: one is around the pre-chamber nozzle and the other between the adjacent reaction zones. These two unburned regions are consumed by flame propagation in the lean case (λ =1.8). The weak pre-chamber jet from the upper-row orifice does not produce any distinct reaction zone, indicating that the pre-chamber orifice location and arrangement on the nozzle also matters in the pre-chamber design. The pre-chamber jet penetration length is longer than that of the reaction zone during pre-chamber discharge; the penetration length difference between the pre-chamber jet and reaction zone decreases with increasing main chamber fueling. [ABSTRACT FROM AUTHOR]

The stagnant plate influence on spatial distributions of polycyclic aromatic hydrocarbons (PAHs), soot and hydroxyl radical (OH) in the surrogate fuel (80% n-heptane and 20% toluene in volume) partially premixed flame on a McKenna burner were originally investigated in terms of different plate heights above the burner (H p) and plate temperatures (T p). Both PAHs and OH distributions were qualitatively measured by laser-induced fluorescence (LIF) and soot volume fraction field was quantitatively measured by two-color laser-induced incandescence (2C-LII). It is found that the LIF intensity of 2&3-ring aromatics (A2&A3) and 4-ring aromatics (A4) and soot volume fraction firstly increase and then reduce when the H p is set from 35 to 10 mm. The variation tendency transition happens between H p = 20 and 25 mm. In contrary, OH-LIF intensity gradually decreases as the H p decreases. On the other hand, the T p has an opposite effect on the A2&A3-LIF, A4-LIF and soot volume fraction variation tendency at two H p. At H p = 15 mm, A2&A3-LIF, A4-LIF and soot volume fraction increase as the T p increases. However, A2&A3-LIF, A4-LIF and soot volume fraction decrease with the rise of the T p at H p = 35 mm. Interestingly, OH-LIF intensity always increases as the T p rises at any H p. Therefore, it can be concluded that the PAHs-LIF intensity and soot volume fraction depend on the H p and T p , and the maximums of them would be reached at the specific H p and T p. While both the H p and T p have a monotonic effect on the OH-LIF intensity. [ABSTRACT FROM AUTHOR]

Thermochemical interaction – represented by CO mole fraction and gas phase temperature measurements – between flame and cooling air is investigated in a close-to-reality effusion-cooled single sector model gas turbine combustor. To investigate the influence of effusion cooling air mass flow on the thermochemical state, a parametric study is conducted. Temperature measurements are performed using ro-vibrational N 2 coherent anti-Stokes Raman spectroscopy (CARS). CO mole fraction is measured by means of quantitative CO two-photon laser-induced fluorescence (CO-LIF) using a temperature dependent calibration acquired in an adiabatic pressurized laminar flame. Significantly different thermochemical states are observed in the inner and outer shear layer of the swirl stabilized flame. Within the primary zone, increasing cooling air mass flow leads to decreased CO concentrations. Close to the effusion cooled liner, the interaction varies with axial coordinate. In the outer recirculation zone, increased CO mole fractions were measured with increasing cooling air mass flow, indicating occurrence of chemical quenching in the late oxidation branch in the CO-T diagram. Further downstream, processes are dominated by mixing and CO concentrations decrease with the amount of supplied effusion cooling air. To our best knowledge, this is the first time that these effects has been shown experimentally. [ABSTRACT FROM AUTHOR]

This article presents a flame-resolved transient simulation of the Cavaliere et al premixed flame experiment [1] to investigate the mechanisms that lead towards lean blowoff/blowout (LBO). The computational domain includes the swirler to capture the unsteady turbulent 3D velocity field generated. The computational grid design intent is to minimize the use of the subgrid scale models in order to resolve most of the turbulence scales in both the fresh and the burnt gases as well as the flame front thickness throughout the computational domain, resulting in a structured mesh with a grid count of 236 million cells. A transient sequence corresponding to a step change of equivalence ratio Φ from 0.7 to 0.55 is modeled. The combustion chemistry mechanism consists of a single step overall reaction. The computational results are compared to experimental data and an overall good agreement is observed, except for the latest times of the LBO sequence. The validated computational results are analyzed with dynamic mode decomposition (DMD) technique, flow field visualizations, signals time-traces and space-time diagrams of a posteriori reconstructed OH fields to investigate the underlying mechanism leading to lean blowoff for this flame. In addition, an evaluation of known state of the art LBO mechanisms reported in literature is carried out. It includes the precessing vortex core, flame sheet holes, inner recirculation zone (IRZ) dynamics, and heat losses. The analysis shows that a key phenomenon leading to lean blowout for the present configuration is associated with the convective motion of cooler combustion products into the IRZ as the equivalence ratio is decreased. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of turbulent non-premixed jet flames of ethylene/nitrogen where the nitrogen is substituted with different proportions of hydrogen and/or ammonia. The focus is largely on the effects of hydrogen and ammonia on soot production in turbulent flames. A combination of pointwise, laser-induced fluorescence in the visible and UV bands (LIF-UV–visible), and laser-induced incandescence (LII) is used to measure soot precursors and soot along the flame centerline. All signals are collected at a repetition rate of 10 Hz with fast photomultiplier tubes to resolve the time decay. In a separate experiment, joint imaging of LIF-OH CH is also performed at a repetition rate of 10 kHz. Hydrogen substitution is found to increase the production of soot, whereas ammonia substitution inhibits soot formation. The peak mean soot volume fraction is almost a factor of 3 lower in the 25% ammonia case in comparison to the 25% nitrogen case. The mean signal decay time constant decreases with ammonia substitution, implying the formation of smaller soot nanoparticles. The mean signal decay time constant remains unaffected with hydrogen substitution. Measured peaks in LIF-CH and LIF-OH are reduced with ammonia substitution but only in regions upstream of where soot is formed. Further downstream in the sooting region, neither OH nor CH appear to be affected by the substitution of N 2 with H 2 or NH 3. [ABSTRACT FROM AUTHOR]

This work examined combustion modes in a fundamental, axisymmetric, supersonic combustor at flight equivalent Mach numbers ranging from 7.5 to 9.0. Ethylene was injected at a variety of mass flow rates to examine both scram-mode, jet wake stabilized and dual-mode combustion. At higher flight Mach number conditions, the fuelling rate required to transition from scram-mode to dual-mode combustion increased. Distributions of the OH radical were observed using planar laser-induced fluorescence (PLIF) in a cross-sectional plane immediately downstream of the combustor exit, and analyzed according to their variation in the radial and circumferential dimensions. It was observed that radial centroids of the ring-like OH PLIF signal for the scram-mode cases approached the centerline linearly with increasing equivalence ratio, but the dual-mode cases appeared to randomly fluctuate in the observed exhaust cross-section. The distributions of scram- and dual-mode cases were clustered in specific regions on plots of circumferential variation as a function of radial centroid, with a jet wake stabilized case appearing between the two clusters. These clusters may be used to help identify scram- and dual-mode combustion in future experiments. [ABSTRACT FROM AUTHOR]

The Precessing Vortex Core (PVC) dynamics is investigated in a laboratory scale swirl stabilized spray combustor. This helical structure commonly found in swirling flows used to stabilize flames in many combustion systems has a notable impact on the structure of the flow, spray and flame formed by the injector unit. The dynamics of the PVC is examined by combining laser sheet illumination of flame front tracers filmed with a high speed camera, Proper Orthogonal mode Decomposition (POD) and wavelet analysis. In distinction with most applications of laser sheet illumination, in which the flow is seeded with oil droplets that vanish at a relatively low temperature when crossing the flame front, the present implementation relies on micronic tin oxide (SnO 2) particles that vanish at much higher temperatures. This is well suited to flows where fresh reactants are already at high temperature or where hot gases are recirculated as is the case in most swirling injection configurations. A first set of experiments carried out with stagnation point premixed flames is used to determine the temperature (T v ≈ 1770 K) at which the tin dioxide particles vanish and to identify the chemical reaction that controls this process. Because the light scattered by the solid particles is quite intense, the method can be used at very high repetition rates. It is then applied at a 100 kHz frame rate to a swirling injection configuration in a situation where the flame is well stabilized and the combustor does not exhibit thermo-acoustic instabilities. It is found that the PVC takes the form of a double helix with brief intermittent switching to a single, followed by a triple helicoidal geometry. The POD-wavelet analysis indicates that these moments correspond to flashback events that randomly occur and during which the PVC characteristics change for a few milliseconds at a time. In a second set of experiments the chamber length is augmented giving rise to a combustion instability coupled by a longitudinal acoustic mode at a frequency that is an order of magnitude lower than the PVC frequency. The planar slices of the flame reveal a large cyclic motion of the lower edge of the flame in the inner recirculation zone. It is found that the PVC is modulated both in amplitude and frequency, the modulating frequency corresponding to that of the thermoacoustic oscillation. The double helix PVC is also observed to vanish in a random manner. [ABSTRACT FROM AUTHOR]

Ozone (O 3) has been shown to accelerate low and high temperature chemistry and is a promising strategy to effectively enhance and control combustion and ignition processes, such as for engines. There has, however, been a lack of experimental investigation of O 3 enhancement effects on low temperature chemistry and premixed cool flames. This study undertook a detailed experimental and numerical assessment of O 3 enhancement effects on ignition behavior, propagation speeds, flame temperatures, and CH 2 O production of n -heptane cool flames as functions of O 3 addition. Zero-dimensional simulations were used to gain insights into the effects of O 3 on two-stage ignition behavior and gradual suppression of NTC phenomena. Propagation speeds of O 3 -enhanced cool flames were experimentally determined using a flame lift-off technique, and one-dimensional simulations utilizing several full and reduced mechanisms were performed for comparison. Enhancement by O 3 versus enhancement by preheating were experimentally compared and analyzed to evaluate relative effectiveness of these enhancement strategies. Finally, downstream temperature measurements and formaldehyde planar laser-induced fluorescence imaging (CH 2 O PLIF) were performed at different O 3 enhancement levels to further elucidate O 3 impacts, and numerical simulations were performed to support experimental insights. Results of this study reveal O 3 impacts on low temperature kinetics and resulting impacts on low temperature ignition and cool flames. [ABSTRACT FROM AUTHOR]

Accurate and detailed data are vital for fundamental understanding of turbulent combustion. However, studies of turbulent combustion often suffer from measurement sparsity or high simulation cost. In the present work, for the first time, a physics-informed neural networks (PINNs) framework is established for three-dimensional high-resolution reconstruction of turbulent combustion with synthetic sparse data. The performance of the PINNs is evaluated on two different configurations of turbulent flames without and with a mean shear, including freely propagating planar premixed combustion and slot-jet premixed combustion. The reconstructed fields of velocity, temperature, and species mass fractions are compared with the high-fidelity direct numerical simulation (DNS) data, and both qualitative and quantitative analyses are performed for the model performance evaluation. The results show that by constraining with the residuals of the governing equations and limited information of sparse data, the proposed PINNs can recover the majority of flow and flame structures in a physics-informed way, even when noise has been added to the sparse data. It is noted that the effectiveness of the models can be influenced by various factors such as the size of the sparse dataset, noise levels, and complexity of turbulence/flame interactions. These factors should be carefully evaluated and addressed to ensure reliable predictions. Overall, this study highlights the potential of PINNs for data assimilation and provide new insights for the development of physics-informed methods in combustion research. Novelty and significance In the present work, a physics-informed neural networks (PINNs) framework for turbulent combustion has been established and the possibility of PINNs for high-resolution reconstruction of various field data in turbulent combustion was explored, which is the first of its kind. The velocity, temperature and species mass fractions were simultaneously reconstructed from limited pointwise sparse data, which were compared with the high-fidelity direct numerical simulation data with good agreements. The proposed PINNs have the capability to recover the flow and flame structures for turbulent flames without and with mean shear layers. Various factors such as the size of the sparse dataset, noise level, and complexity of turbulence/flame interactions on the model performance were also assessed. The study highlights the potential of PINNs for data assimilation and provide new insights for the development of physics-informed methods in combustion research. [ABSTRACT FROM AUTHOR]

Global quench (GQ), a complete extinction, of downwardly-propagating and centrally-ignited premixed ammonia/air flames by intensive near-isotropic turbulence generated in a central region of fan-stirred cruciform burner is explored at wide ranges of the equivalence ratio (ϕ = 0.7–1.25), the r.m.s. turbulence fluctuation velocity (u ′ = 0–8.4 m/s), and the ignition energy (E ig = 50–500 mJ) using pin-to-pin stainless-steel electrodes at 2-mm spark gap. Both central ignition flame kernels and downward propagation flames from the top vessel of the cruciform burner interacting with the central uniform intense turbulence are recorded to determine the required critical u ′ c and/or Ka c for GQ, where Ka is a turbulent Karlovitz number. We find that the low-reactivity ammonia/air laminar flames cannot propagate downwardly from the top vessel at too lean (ϕ = 0.7) and too rich (ϕ = 1.25) conditions. Results show that the large downward propagation flames are much harder to be globally quenched by turbulence than the small central ignition kernels for spherical flames. The downward flame GQ criterion is independent of E ig , while the central ignition GQ criterion is influenced by E ig (the higher E ig , the larger u ′ c). Contrary to the common notion that the maximum critical u ′ c for flame GQ occurs near ϕ ≈ 1, we discover that the richer the downward propagation ammonia/air flame is, the larger u ′ c is. Such a surprising survival capability of rich downward propagation ammonia/air flames to GQ by intense turbulence is due to the nearly linear increase of H 2 mole fraction percentage in the upstream products of the top tube of the cruciform burner from less than 1 % at ϕ = 0.9 to about 6 % at ϕ = 1.2. These turbulent downward propagation flame fronts with filiform structures prior to GQ reveal the morphology of distributed-like turbulent flames. [ABSTRACT FROM AUTHOR]

Effects of increased pressure on the thermochemistry of turbulent flame-wall interaction (FWI) were investigated within this study. Quantitative, pointwise measurements of three state parameters, i.e., gas-phase temperature, CO 2 and CO mole fraction, were performed by means of dual-pump coherent anti-Stokes Raman spectroscopy (CARS) and two-photon laser-induced fluorescence (LIF) of CO. The flame front was simultaneously tracked by planar OH-LIF imaging. The measurements were carried out in a fully premixed methane-air flame in an enclosed side-wall quenching burner test rig under atmospheric and pressurized conditions (3 bar absolute pressure). An evaluation of near wall flame front topologies showed that the interaction of the flame with the wall is predominantly characterized by one single, extended reaction zone, similar to a so-called head-on quenching configuration. The dominance of this quenching scenario occurred most likely due to the confined burner configuration preventing an unhindered expansion of the hot exhaust gases. Thermochemical states were studied using pairwise correlations of temperature, CO 2 and CO mole fraction. Regarding the measurements at atmospheric pressure, an overall good agreement with other studies on atmospheric FWI in a similar unconfined burner was observed considering differences concerning the flow field and the burner configuration. For the thermochemistry measurements at elevated pressure, similar trends as for the atmospheric measurements were identified. A comparison of both operating cases suggested that FWI processes were more strongly affected by heat losses at elevated pressure, but differences are moderately pronounced. To the authors' best knowledge, these experiments are the first attempt to explore the near-wall thermochemistry of FWI processes at elevated pressure using multi-parameter measurements. As the near-wall measurements were further complicated by effects arising with increasing pressure, e.g., intensified beam steering and reduced length scales, implications on the application of the laser diagnostics are reported. [ABSTRACT FROM AUTHOR]

Fuel additives are widely used to optimize the combustion process, reduce the harmful emissions, and improve the performance of engines. Among the metal-based additives, ferrocene is soluble in most fuels, including biodiesel. Furthermore, it is easily available and has high stability, low toxicity, and high lipophilicity. It can evenly disperse in biodiesel without causing any change in the physicochemical properties. Therefore, it can serve as an effective combustion catalyst for biodiesel. In this study, the effect of ferrocene as a combustion catalyst on the pyrolysis and combustion performance of Jatropha biodiesel is investigated. The coupled technique of thermogravimetry-Fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS) is used to analyze the evolution characteristics of multicomponent gaseous products generated during the pyrolysis and high-temperature oxidation of Jatropha biodiesel (JB100) and Jatropha biodiesel + ferrocene (JBF). The laminar premixed combustion flame and pollutant emissions of JB100 and JBF are examined using a variety of techniques, including OH planar laser-induced fluorescence (OH-PLIF), flue gas analysis, spectral analysis, transmission electron microscopy, etc. The results reveal that after adding ferrocene, the activation energy for the pyrolysis reaction of JB100 decreases by 4.66 kJ/mol and the complete weight loss temperature decreases by 10 ℃. Further, the addition of ferrocene reduces the endothermic heat by 0.38 mW/mg during the initial stage of the high-temperature oxidation reaction of JB100, and the amounts of final products CO 2 and H 2 O are increased. Compared with JB100, JBF shows an enhanced OH group intensity in the combustion flame and a lower soot spectral peak. Furthermore, compared with JB100, under JBF combustion, the pollutant PM2.5 emissions decrease by 13.3 %, PM10 emissions decrease by 13.2 %, PM> 10 particulate matter emissions decrease by 19.3 %, CO emissions decrease by 6.8 %, NOx emissions increase by 21.2 %, and the size of soot particles decreases by 20.8 %. [ABSTRACT FROM AUTHOR]

In this paper, the dynamics of diffusion-thermal oscillations of burner-stabilized methane–air flames are numerically and experimentally investigated. High speed imaging and laser induced fluorescence of OH radicals are used as experimental techniques, while a hierarchy of models is employed in numerical simulations: a system of ordinary differential equations describing dynamics of flame location and temperature, models with one-step and detailed reaction mechanisms. It is demonstrated that for small (near the neutral stability boundary) and moderately relaxational oscillations, all three approaches are able to describe the flame dynamics however with different levels of detailization. This is due to the fact that in these regimes the flame front structure remains similar, although perturbed, to that observed in freely propagating combustion waves. However, this flame structure breaks in the regime of highly relaxational oscillations, which can only be described within the detailed reaction mechanisms. The combustion front splits into the high and low-temperature reaction zones, which are shown to be highly and weakly sensitive to the diffusive-thermal pulsations. The high temperature reaction zone is blown away and extinguishes in the course of flame pulsation, and the low temperature zone remains near the burner surface and causes re-ignition of the combustion front. Prospects of further investigations are discussed. [ABSTRACT FROM AUTHOR]

Quasi-instantaneous 3D flame surfaces were reconstructed in a turbulent Bunsen flame via a laser scanning technique based on an acoustic-optic-deflector (AOD). Instantaneous flame surface areas were reconstructed from a series of flame contours at different vertical planes via droplet Mie scattering with a Nd:YLF high-frequency laser at 527 nm. Mean total flame surface areas 〈 A 〉 were estimated and compared with estimated area demands based on total mass flow rates of fuel/air mixture divided by density and unstrained laminar flame speed, m ˙ / (ρ u s L). Flame surface density (FSD, both 2D and 3D) and flame surface curvatures were also measured and compared with those acquired using 3D cross-planar methods at particular locations. Differences of up to 67% were measured at a given location in the flame brush, with 3D FSD values generally higher than corresponding 2D measurements. The present measurements are the first comparisons of FSD and 3D curvatures comparing scanning and orthogonal cross-planar techniques allowed the measurement of 2D flame contours on two orthogonal planes, leading to improved accuracy in the determination of principal curvatures in 3D flame surfaces. Turbulent Bunsen flame surfaces were reconstructed by a high frequency scanning technique. Values of the 3D flame surface area were for the first time measured and compared to the required area for burning the total incoming mass flow rate. Values of local 3D FSD were measured via the scanning technique and for the first time compared to corresponding measurements using cross-planar technique. True 3D principal curvatures of flame surfaces were measured by combining scanning and cross-planar techniques to provide a sufficient accuracy. [ABSTRACT FROM AUTHOR]

Propagation of premixed flames having thick reaction zones in rapidly-varying, small-scale, zero-mean, spatio-temporal periodic flows is considered. Techniques of large activation energy asymptotics and homogenization theory are used to determine the effective Lewis number L e eff and the effective burning speed ratio S T / S L , which are influenced by the flow through flow-enhanced diffusion. The resultant effective diffusivity matrix is, in general, neither a scalar nor a diagonal matrix and therefore induces anisotropic effects on the propagation of multi-dimensional flames. As the flow Peclet number P e becomes large, the flow-enhanced fuel diffusion coefficient and the thermal diffusivity behave respectively like (P e L e) σ and P e σ , where L e is the Lewis number and σ ≤ 2 is a constant which depends on the flow and the direction of flame propagation. The maximal value σ = 2 is achieved for steady, unidirectional, spatially periodic shear flows, while for steady two-dimensional square vortices, we have σ = 1 / 2. In general, the constant σ is determined by solving a linear partial differential equation. The scaling laws for the diffusion coefficients lead to corresponding scaling laws for the effective Lewis number and the effective burning speed ratio of the form L e eff ≃ L e 1 − σ and S T / S L ∼ (P e / L e) σ / 2 . Effects of thermal expansion and volumetric heat loss on the flame are also briefly discussed. In particular, it is shown that the quenching limit is enlarged by a factor 1 / L e σ for L e < 1 and diminished by the same factor for L e > 1 , due to the flow-enhanced diffusion. The potential implications of the results to better understand turbulent combustion are discussed. A special emphasis is placed on the dependence of the flame on L e in the presence of high-intensity, small-scale flows. In particular, it is shown that this dependence is intimately linked to the flow through Taylor-dispersion like enhanced diffusion, rather than through the traditional molecular diffusion coupled with curvature effects. The flow-dependent effective Lewis number identified may also provide an explanation to the peculiar experimental observation that turbulence appears to facilitate ignition in L e > 1 mixtures and to inhibit it in L e < 1 mixtures. An original study, combining asymptotic analysis and homogenization theory, is applied to describe flame propagation in small-scale, spatio-temporal periodic flow fields. Scaling laws are derived for the effective burning speed and the effective Lewis number for high-intensity small-scale flows, which are useful to better understand the behaviour of turbulent premixed flames in the distributed reaction zone regime. The formula for the flow-dependent effective Lewis number identified herein may explain the peculiar experimental observation that turbulence appears to facilitate ignition in L e > 1 mixtures and to inhibit it in L e < 1 mixtures. The high-intensity small-scale flows are shown to increase the quenching limit due to volumetric heat losses in L e < 1 mixtures and decrease it in L e > 1 mixtures [ABSTRACT FROM AUTHOR]

The combustion of ammonia (NH 3) has received much attention over the last few years due to challenges associated with its low reactivity and the emission of nitric oxides. One way to improve the reactivity of NH 3 is to blend it with (H 2 /N 2) mixture as the product of its dissociation, before introducing it into energy systems. In this study, experimental measurements were carried out on nonpremixed bluff-body stabilized flames to better understand the flame and emission characteristics of NH 3 /H 2 /N 2 flames. Four fuel mixtures at different NH 3 and (H 2 /N 2) ratios were investigated to represent different levels of ammonia cracking. Photography, planar laser-induced fluorescence of OH, thermocouples, and gas analysis techniques were used to understand flame features, reaction zone characteristics, NO, and NH 3 concentration within the flame and at the exhaust. It was observed that a decrease in NH 3 ratio in the mixtures resulted in longer and more stable flame with reduced thermal radiation as compared to NH 3 -rich fuel blends. For the highest NH 3 blend studied, the flames exhibit extinction and re-ignition in the neck zone, as evidenced by OH-planar images and temperature profiles. As the H 2 /N 2 ratio in the fuel mixture is increased, while keeping the Re constant, the momentum flux ratio (jet/co-flow) also increased resulting in a fuel-lean recirculation zone (RZ), and a shift in the maximum temperature and OH region from the outer shear layer to the inner layer next to the central jet. At levels of NH 3 in the fuel mixture above 50% by volume, unburned ammonia slips through the flame and into the exhaust, and the subsequent reburn mechanism resulted in reduced NO emission. CFD simulations using Reynolds-averaged Navier-Stokes (RANS) and the flamelet–progress-variable submodel were conducted and compared with the experimental results. The CFD results helped to qualitatively describe and further explain what was observed in the experiment including the flame appearance, mixing field, and the reaction zone location in the tested flames. [ABSTRACT FROM AUTHOR]

Acoustic and parametric instability in downward propagating premixed flames of low Lewis number (∼0.6) mixture of CH 4 /O 2 /Xe is studied experimentally and analytically. Seven mixtures of increasing laminar burning velocity (S L) at constant equivalence ratio of 0.7 are considered. Flame front history and pressure measurements are reported. Parametric instability of the flame front due to effect of acoustics is observed for mixtures with S L higher than 27 cm/s. Complex stability diagrams are observed for the mixtures. For mixtures of lower S L , a clear condition of wavenumber and acoustic velocity amplitude for the onset of parametric instability is not found in the stability diagram. Rather a wavenumber, k 0 B is observed where instability is observed for all the acoustic velocity amplitudes. The experimentally measured wavenumbers at the onset of parametric instability were found to match with k 0 B. For mixtures of S L > 54 cm/s, wavenumber and acoustic velocity amplitude at the onset of parametric instability were clearly identified in the stability diagram. Experimentally measured wavenumbers were found to lie between critical wavenumber of parametric instability and k 0 B. This work shows a quantitative agreement between theory and experiments and thus proves the applicability of theoretical analysis for low Lewis number mixtures. [ABSTRACT FROM AUTHOR]

Soot formation of n -heptane is experimentally and numerically studied in low-strain rate (K = 50 s−1) counterflow soot formation (SF) flames. Flame temperatures and soot volume fractions at different oxygen mole fractions on the oxidizer stream (xO 2 =0.3–0.45) are measured by using thermocouple-calibrated OH two colors laser-induced fluorescence (2C-PLIF) and laser-induced incandescence (LII) calibrated by light extinction methods, respectively. Mono (MAHs) and polycyclic aromatic hydrocarbons (PAHs) are also qualitatively measured by using the PAHs-LIF technique. Good agreement between measured and predicted maximum flame temperature (T max) and peak soot volume fraction (f v ,peak) is obtained. However, discrepancies in the shape of the entire soot volume fraction profiles along the axial centerline are observed between 1D and 2D simulations. This result is found to be primarily related to the thermophoretic and radial effects in the low strain rate flames investigated, which hamper neglecting the underlying 2D effects. MAH/PAH peak mole fractions and f v ,peak increase from xO 2 =0.3 to xO 2 =0.45 due to the related increase of flame temperature, which fosters n -heptane pyrolysis near the fuel nozzle leading to higher concentrations of C 1 C 4 hydrocarbons. The latter are found to grow to MAHs/PAHs and finally to soot particles through analogous pathways independently of xO 2. However, the higher flame temperature in the sooting region at xO 2 =0.45 leads to soot particles and aggregates characterized by larger sizes and more dehydrogenated than those produced at xO 2 =0.3. [ABSTRACT FROM AUTHOR]

Autothermal reforming of methane (ATR) technique is a promising technology for H 2 production. Soot formed in the combustion region of ATR may deactivate the downstream catalyst used to enhance the reforming reactions. In this work, the effects of CO 2 /CH 4 ratio on soot production and main gas-phase components involved in reforming reactions were investigated by experiment and numerical simulations in laminar inverse coflow diffusion flames, at conditions close to ATR. The CO 2 /CH 4 ratio ranges from 0.27 to 4.90, and the pressure ranges from 1 atm to 10 atm. The oxidant consists 70% O 2 and 30% N 2 to mimic the oxy-combustion in the ATR process. The soot, polycyclic aromatic hydrocarbon (PAHs), OH* distribution, and flame structure were measured using planar laser-induced incandesce, planar laser-induced fluorescence, OH* luminescence, and flame luminescence, respectively. High fidelity simulations with a reduced gas-phase kinetic mechanism and soot aerosol models were also undertaken to supplement the findings. The results show that soot formation is suppressed by a higher CO 2 /CH 4 ratio, while the level of soot reduction is attenuated as the pressure increases. The peak soot volume fraction increases with the flame height, and then levels off. PAHs concentration monotonically increases with flame height. The spatial distributions of PAH and soot along flame height were not well captured by numerical simulations, and the used physical based soot inception model is expected to account for the discrepancy. The peak soot volume fraction correlates with CO 2 /CH 4 ratio by an exponentially decaying function, with the exponent decreasing with increasing pressure. A similar trend of PAHs concentration along with CO 2 /CH 4 ratio was also observed. Non-sooting flame is achievable at elevated pressure if the CO 2 /CH 4 ratio is higher than 4.0, at the expense of lowering the syngas yield in the combustion region. The simulations also predicted that the flame temperature and concentration of H 2 and CO decrease with CO 2 /CH 4 ratio. [ABSTRACT FROM AUTHOR]

Flame synthesis is a powerful and scalable method for generating nanoparticles for a wide range of applications. The chemical interaction of the flame and the precursor combined with the spatial and temporal temperature distribution determine the product properties. For controlled nanoparticle synthesis that can also be scaled to industrial production rates, detailed knowledge of the underlying chemical kinetics and their interaction with the reactive flow is essential. Laser diagnostics has the capability to analyze the process by probing the concentration of important intermediates in shock tubes and reactive flows. The gas-phase synthesis of silica nanoparticles from SiCl 4 in a premixed H 2 /O 2 low-pressure flame reactor is studied by laser-induced fluorescence imaging of SiO mole fractions and temperature. The literature value-based spectroscopy model of SiO used for fitting the LIF spectra are validated based on absorption cross-sections measurements in a shock tube, where SiO is formed under precisely defined conditions (temperature, pressure, mole fraction) using a well-known kinetics mechanism for SiH 4 /CO 2 /Ar decomposition. Based on literature sources, a reaction mechanism is assembled to describe the oxidation of SiCl 4 in the flame, which is then compared to the measured SiO mole fractions distribution to shed light on the current state of the understanding of SiCl 4 combustion chemistry and to direct further refinements. [ABSTRACT FROM AUTHOR]

The challenges in designing high-performance aeronautical combustion systems have not changed significantly over the years, but the increase of stringent regulations and the need to tackle rising fuel prices require new sophisticated analysis processes. To address this concern, the theme of this study was cast in terms of the ability of improving the performances of advanced non-intrusive instrumentation in high-pressure turbulent reactive two-phase flows to enhance the combustion efficiency and reduction of pollutants of new innovative low-NOx injection systems fed with liquid kerosene. In the first part, a Lean Premixed injection system designed for helicopter engines was first studied using the Particle Image Velocimetry (PIV), Phase Doppler Anemometry (PDA) and Planar laser-induced fluorescence (PLIF) of OH, kerosene vapor and NO. These diagnostics were applied individually or in combination for custom-made solutions for accessing to detailed information on the spatial distributions of velocity, droplets size, OH, kerosene and NO molar fractions in pressure conditions ranging from 0.4 to 1.8 MPa. Complementary large-Eddy simulations of these flames were performed using the PCM-FPI tabulated chemistry approach in conjunction with a polydisperse Euler–Lagrangian approach. Comparison of simulations with experiments highlighted the importance of the size distribution of the droplets injected into the flow on the mutual predictions of the aerodynamic field, the structure of the flame and its anchorage point as well as the spatial distribution of the fuel and OH species. The second part of this work was aimed to study the advantages and limitations of the chirped probe pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs-CARS) diagnostic for performing time-series of single-shot temperature measurements on an aircraft multi-point injection system developing two-phase flames under realistic pressure conditions. [ABSTRACT FROM AUTHOR]

In this work, the two-photon laser-induced fluorescence technique (TPLIF) was applied to measure the concentration profile of atomic hydrogen in low-pressure laminar premixed flames. Excitation of H-atoms was performed by two-photon absorption at 205 nm and collecting the fluorescence at 656.3 nm. For the first time in flames, the TPLIF signals from the H-atom have been calibrated using the TPLIF signal from krypton, directly seeded in the flame, excited at 204.13 nm and collecting the fluorescence at 826.5 nm. This method was previously demonstrated in plasma environments and recently applied in our group to calibrate O-atom TPLIF signals using xenon as a standard gas. The calibration requires the measurements of TPLIF signals of H and Kr atoms in a flame, where the quenching rates can be determined from time-resolved LIF measurements. Given the short fluorescence lifetime of H-atom, this last task was particularly challenging. A calibration flame was chosen to minimize collisions and the response time of the detection was determined using a deconvolution method. We found that the quenching rate is fairly constant around 4.6 × 108 s−1 at 5.3 kPa in a large portion of the flame. The calculated quenching rate overestimates the measured value from 35% to 500% depending on the chosen assumption on the dependence of the quenching coefficient with temperature. The quantitative measurement of H-atom mole fraction was carried out in three nitrogen-diluted low-pressure methane flames. The experimental profiles were compared with the calculated ones using chemical modeling. The variation in the experimental H-atom mole fraction in the range of equivalence ratios agrees well with the simulated values. Quantitatively, the calculated mole fractions agree within 30% with the experimental ones. The method is robust but its accuracy is limited by the uncertainty in the knowledge of the ratio of the two-photon cross-sections of Kr and H atoms. Application of this calibration method to atmospheric is discussed. [ABSTRACT FROM AUTHOR]

We present time-domain measurements and modelling of the CH 4 ν 1 vibrational Q-branch, as well as simultaneous temperature acquisition from the CH 4 and N 2 vibrational Q-branches using broadband hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS). Accurate measurements with fs/ps CARS require knowledge of the time-domain response of the probed molecule at different temperatures and pressures. In this work, a fs/ps CARS model was developed for the CH 4 ν 1 Q-branch and validated using time-resolved probe delay scans at pressures between 70 and 600 Torr and temperatures between 295 and 1000 K. A simplified modified exponential energy gap model was used to determine the linewidths of the Raman transitions within the Q-branch. Total Q-branch linewidths for the entire explored parameter space were also determined from the probe delay scans. This enabled quantitative measurements of CH 4 with fs/ps CARS as well as the temperature through the hot bands of the CH 4 ν 1 Q-branch. With a broadband fs laser pulse in a two-beam phase matching scheme, the N 2 Q-branch and CH 4 ν 1 Q-branch were simultaneously measured in a heated cell, and the temperatures from each molecule's spectra was evaluated. The fitted temperatures corresponded well with differences ranging from 4 to 17%. It was also shown that exchanging 90% of CH 4 for N 2 or Ar at 295 K and 500 Torr did not significantly impact the normalized time-domain signal prior to a 100 picosecond probe delay. Spatially resolved one-dimensional (1-D) thermometry using the CH 4 Q-branch was also demonstrated with an imaged spatial resolution of 40 µm. With the developed model, 1-D quantitative measurements of CH 4 concentration and temperature can be measured with time resolution of tens of picoseconds. [ABSTRACT FROM AUTHOR]

Infrared (IR) planar laser-induced fluorescence (LIF/PLIF) exploits the vibrational level transitions of small gas molecules to achieve spatially-resolved spectroscopic information of the species for combustion and flow-field studies. While typical LIF utilizes electronic excitations with ultraviolet or visible lasers, IR LIF presents an alternative for molecules that are not readily accessible with a conventional UV/visible excitation-collection system. In view of the complexities rooting from longer vibrational level lifetimes and more densely-spaced energy levels, a comprehensive vibrational energy transfer model is developed for state-to-state population evolution and transition process characterization, and a typical CO 2 /CO/H 2 O/O 2 /N 2 system relevant to combustion gas mixture is studied by considering a total of 265 energy states and 85,036 transition pathways. The resultant fluorescence signal is thereby studied, with quantitative dependence on local temperature, pressure, gas composition, as well as excitation laser parameters including wavelength, power level and linewidth. Further optimization of collection scheme can be achieved by considering the time-histories and major transition pathways pertaining species evolution on different energy levels. The study develops general modelling and methodology for characterizing the temporal and spectral behavior of a vibrationally-excited molecular system, and demonstrates the potential for quantitative, spatially-resolved diagnostics using IR LIF/PLIF. [ABSTRACT FROM AUTHOR]

The single laser irradiation (SLI) method was adopted to develop a double laser irradiation (DLI) method to investigate the effects of laser-induced structures of downward-propagating flames in a tube on the transition from primary acoustic instability to parametric instability. Previously, the SLI method was effectively used to study the interaction between acoustic oscillation and flame structure by controlling the shape of the flame front. The deformed cellular structure (either concave or convex) of the SLI -induced flame can transition from primary acoustic instability to parametric instability under certain conditions. We conducted experiments in the same combustion tube with C 2 H 4 /O 2 /CO 2 mixtures using both SLI and DLI for comparison. The DLI method forms double cellular structures, while SLI only forms single cellular structures on the flames. It was found that the DLI method was more effective than SLI in generating transition to parametric instability under same total laser power. Furthermore, a linear relationship was found between the area of the deformed structure (irrespective of its dimension) and the corresponding growth rate of acoustic pressure fluctuation during the propagation of the deformed flame. SLI - and DLI -induced deformed structures having the same deformed surface area demonstrate the same growth rate of thermo-acoustic instability regardless of the difference in (ak)2, where a is the amplitude and k is the wavenumber of the deformed structures, respectively. This factor described in the velocity coupling mechanism is important to reveal the growth rate of thermo-acoustic instability due to the variation in the flame surface area. The results revealed that the actual flame surface area, rather than (ak)2 of the laser-induced downward-propagating flame structure, determines the transition in the presence of nonhomogeneous cell distribution induced by SLI and DLI. The total deformed surface area is a more comprehensive criterion for transition to parametric instability. [ABSTRACT FROM AUTHOR]

The nitric oxide (NO) formation in methane (CH 4) flames has been widely investigated, with quite a few kinetic mechanisms available in the literature. However, studies have shown that there are often discrepancies between the simulations using various mechanisms and the experimental results. To elucidate reactions leading to these discrepancies, experiments were designed to measure the NO formation in the post flame zone of CH 4 +O 2 +N 2 flames with the oxygen ratio, x O 2 = O 2 /(O 2 +N 2), varying from 0.2 to 0.27. The experiments were carried out on a heat flux burner at atmospheric pressure and 298 K using saturated Laser-induced fluorescence. The equivalence ratio, ϕ , was changed from 0.7 to 1.6. The corresponding laminar burning velocity, S L , for each condition was also measured using the heat flux method. A comparison was made for the present experimental data and simulation results using the Konnov, Glarborg, NOMecha 2.0, and San Diego mechanisms, and none of them well reproduced the new NO experimental data for all investigated conditions. Numerical analyses show that the increment of NO mole fraction in stoichiometric and fuel-lean flames when the x O 2 increases is mostly defined by the thermal-NO production, which is found to be over-predicted, especially by the Konnov and San Diego mechanisms. The rate constant of reaction NO+N = N 2 +O was derived as k = 1.529 × 10 13 T − 0.0027 exp (185.41 [ c a l / m o l e ] R T) cm3 / mol s over 225–2400 K temperature range. The rate constants of four reactions controlling CH mole fraction profiles and prompt-NO formation were updated based on the analysis of the literature data that yields an improved performance of the Konnov mechanism. [ABSTRACT FROM AUTHOR]

The present study aims to identify cross-reactions of methane and dimethyl ether (DME) at low temperatures. A weak flame in a micro flow reactor with a controlled temperature profile (MFR) for a stoichiometric methane/DME/air mixture with 90% CH 4 and 10% DME (hereinafter expressed as 90%/10% CH 4 /DME) was used and analyzed. The weak flame structure was investigated by chemiluminescence observation and laser-induced fluorescence (LIF) measurements of OH and CH 2 O. Measured chemiluminescence and fluorescence intensity profiles were compared to computations with five detailed chemical kinetic mechanisms. Both experimentally and computationally obtained results showed the weak flame position of 90%/10% CH 4 /DME shifted to lower temperature in MFR compared to that of 100% CH 4 reported earlier, indicating methane reactivity enhancement by the small DME addition. CH 2 O-LIF showed extensive formation of CH 2 O at low temperatures. Each mechanism predicted different levels of CH 2 O formation. The four mechanisms among five which include intermediate-temperature oxidation chemistry for methane , CH 3 → CH 3 O 2 → (CH 3 O 2 H) → CH 3 O → CH 2 O, predicted two-staged formation of CH 2 O at 650 and 730 K, whereas another mechanism predicted single-stage formation of CH 2 O at 650 K. Rate-of-production analysis revealed that OH radicals formed from low-temperature oxidation of DME initiate H-atom abstraction from methane; then intermediate-temperature oxidation chemistry for methane proceeds even at low temperatures. The contribution of intermediate-temperature oxidation chemistry for methane to CH 2 O formation at 730 K is stronger than that at 650 K, causing two-stage formation of CH 2 O. [ABSTRACT FROM AUTHOR]

Internal structure of extremely turbulent flames is investigated experimentally using simultaneous planar laser-induced fluorescence of formaldehyde molecule and hydroxyl radical as well as stereoscopic particle image velocimetry. The mean bulk flow velocity is changed from 5 to 35 m/s. The fuel-air equivalence ratio is 0.7 for all tested conditions. Three different turbulence generating mechanisms leading to a wide range of turbulence intensity with the corresponding Reynolds and Karlovitz numbers ranging from 19 to 2729 and 0.3 to 76.0, respectively, are examined. Preheat and reaction zone thicknesses for the tested flames are calculated and compared with those of the laminar flame to quantify deviation from flamelet behavior. It is shown that the preheat and reaction zone thicknesses increase to values that are about 6.2 and 3.9 times the laminar flame counterparts, respectively. While broadening of the preheat zone is reported in the literature, broadening of the reaction zone for a relatively large diameter Bunsen burner is reported for the first time in this study. The broadening of the reaction zones is related to non-flamelet behavior of the tested flames, and a parameter proposed earlier by the authors is used to quantify this non-flamelet behavior. Swirling strength contours are overlaid on the cold reactants and preheat zones to study the reason for the reported broadening. The results show that positive correlations exist between the preheat/reaction zone thicknesses and the mean value of eddy swirling strength inside the reactants and preheat zone. These correlations as well as the estimated turbulent kinetic energy of these eddies suggest that energetic eddies may potentially penetrate into the preheat and reaction zones causing broadening of these zones. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence (LIF) technique has been widely applied in PAH measurement in sooting flames, but it is still a big challenge to quantitatively identify the specific PAH species due to the complexities of spectral characteristics and calibration method. In this study, we aimed at developing a spectral-resolved LIF analytic model that can obtain PAH mole fractions in flames. A comprehensive LIF spectrum database in the wide temperature and concentration ranges was firstly established by the LIF experiments of gas-phase PAHs in an optical cell. Then, a spectral-resolved LIF analytic model was presented based on linear superposition regulation and an optimization fitting algorithm, and the LIF spectra of PAH components can be decomposed from the LIF spectra of the flame using this model. Finally, the integral fluorescence intensities of each PAH component were calibrated to their mole fractions by the calibration factors at corresponding temperatures. In this way, we can measure the mole fractions of several PAHs. Subsequently, the presented LIF method was applied to investigate the PAH formation characteristics at different flame temperatures. The experimental results showed that the mole fractions of each PAH except naphthalene decreased around 60% to 70% when the temperature increased from 1186 K to 1483 K, while the mole fraction of naphthalene kept nearly constant. The mole fractions simulated by a chemical kinetic model well reproduce the experimental results. According to the chemical kinetic analysis, C 2 H 2 oxidation reaction was promoted and the C 2 H 2 -addition reactions were suppressed by increasing temperatures, which suppressed the PAH formation. [ABSTRACT FROM AUTHOR]

Recently, reconstruction integrating view registration (RIVR) has been demonstrated as an improved method to significantly enhance the accuracy of three-dimensional (3D) measurements in nonreactive flows. This work extended the RIVR method to 3D measurements of highly turbulent reactive flows with two specific goals. The first goal was to examine if the RIVR method can be effectively applied to highly turbulent flame structures, which display distinctively different spatial features from nonreactive flows. This examination of RIVR was performed specifically using two performance metrics, accuracy and spatial resolution. The second goal was to quantify the end benefits the RIVR can bring about on key flame properties involved in turbulence-chemistry interaction, such as flame surface density. The results demonstrated that the RIVR method can effectively enhance reconstruction accuracy of the thin flame front marked by CH radicals in 3D distribution. Compared to past methods, the RIVR method reduced the reconstruction errors by ~48% on average and improved the spatial resolution by ~26% on average. Such accuracy enhancement ultimately led to a ~15% improved accuracy on the determination of the 3D flame surface density as an indicator of the total burning rate. [ABSTRACT FROM AUTHOR]

An experimental and numerical investigation of the thermochemical structure of piloted premixed jet flames was conducted, encompassing laser absorption tomography measurements and large-eddy simulations (LES). The investigation was performed holding laminar flame speed, jet Reynolds number, and surrounding flow conditions constant while considering three different fuel types, namely an alkene, a normal alkane, and an aromatic fuel. Quantitative spatially-resolved thermochemical profiles of carbon monoxide (CO), carbon dioxide (CO 2), and temperature obtained from laser absorption tomography were compared against profiles predicted by the simulations for premixed ethylene-, toluene-, and n -heptane-air flames. Variations in flow structure are observed for the different fuels, highlighting fuel-specific chemical effects on the spatial evolution of the flames. Quantitative agreement of laser absorption tomography and LES results is generally observed for all flames, with larger deviations observed in the nozzle-near region for the higher molecular-weight fuels, indicating potential deficiencies in the turbulent mixing models. To the authors' knowledge, these measurements represent the largest molecular-weight fuels for which quantitative thermochemical data have been reported in a canonical piloted premixed jet-flame configuration. The spatially-resolved experimental measurements of CO, CO 2 , and gas temperature provide valuable data which can be used as validation targets for the development of turbulent combustion models. [ABSTRACT FROM AUTHOR]

Laminar diffusion flames present an elementary configuration for investigating soot formation and validating kinetic models before these are transferred to turbulent combustors. In the present article, we present a joint experimental and modelling investigation of soot formation in a laminar co-flow burner. The diffusion flames are analysed with the aid of laser diagnostic techniques, including elastic light scattering (ELS), planar laser-induced fluorescence of OH (OH-PLIF) and line-of-sight attenuation (LOSA), to measure the spatial distribution of soot, gas phase species and the line-of-sight integrated soot volume fraction (ISVF), respectively. The experimental dataset is supplemented by location-specific TEM images of thermophoretically sampled soot particles. The simulation of the sooting flames is carried out with a recently developed discretisation method for the population balance equation (Liu and Rigopoulos, 2019, Combust. Flame 205, 506-521) that accomplishes an accurate prediction of the particle size distribution, coupled with an in-house CFD code. By minimising numerical errors, we ensure that the discrepancies on the modelling side are mainly due to kinetics and are able to carry out an investigation of alternative models. We include a complete set of soot kinetics for PAH-based nucleation and condensation, HACA-based surface growth and oxidation as well as size-dependent aggregation, and consider three different gas phase reaction mechanisms (ABF, BBP and KM2). Based on predictions of the gas phase composition and particle size distribution of soot, modelled counterparts of the laser diagnostic signals are computed and compared with the experimental measurements. The approach of directly predicting signals circumvents the difficulties of explicitly representing the OH concentration in terms of the measured OH-PLIF data and avoids using 'hybrid' modelled and measured values to approximate the OH concentration. Moreover, the LOSA signal is directly converted to the line-of-sight ISVF instead of a measure of local soot volume fraction to avoid tomographic inversion errors. Lastly, the predicted ELS signal is computed in terms of the particle size distribution resolved by the population balance model, thus circumventing the approximation of an integral soot property using a presumed size distribution. While we cannot obtain quantitative agreement between experiments and simulations, the accuracy of the numerical approach and the direct prediction of experimental signals allow us to conduct sensitivity analyses of key empirical parameters and investigate the importance of the PAH chemistry and its influence on the competition between nucleation, condensation and surface growth. [ABSTRACT FROM AUTHOR]

This paper extends the application of laser-induced fluorescence imaging of OH & CH 2 O (LIF-OH-CH 2 O) to provide a measure of heat release zones in turbulent, moderately dense spray flames of ethanol and biodiesel fuels. A custom filter is used to remove interference from stimulated Raman scattering arising from the liquid fragments. Mie scattering is measured jointly on a separate camera to co-locate the fluid fragments with respect to the reaction zones. It is found that while the overall spray flame structure is similar to that of a turbulent gaseous diffusion flame, additional complexities arise due to the continuous release of fresh fuel vapor from the evaporating fragments. Structures referred to as burning rings of different sizes are observed, and these are ignited by interfacing with a hot edge before they grow, propagate, and burnout. Statistics on the occurrence of these rings, with and without clouds of liquid fragments within them, are presented. These findings are common to both ethanol and biodiesel, although measurements in the latter were affected by strong fluorescence interference from the excited components in the biodiesel blend. [ABSTRACT FROM AUTHOR]

Co-firing NH 3 with other fuels is receiving growing interest as a feasible solution to improve its combustion and emission properties. Previous studies mainly focused on NO x emission, and paid less attention to soot formation which is investigated here using a laminar counterflow flame configuration with ethylene fuel (75% by mole) mixed with different proportions of ammonia (between 0 and 25% by mole while the remainder is nitrogen). Soot volume fraction (SVF) was measured using planar laser induced incandescence (PLII). It was found that the addition of ammonia significantly reduced the measured SVF by 4–6% per 1% ammonia addition as compared to the reference flame (25% nitrogen). To rule out temperature effects, the experiments were simulated using Chemkin Pro and it was found that there were negligible differences in temperature between each condition implying that temperature was not responsible for the reduction in SVF. To investigate the chemical effects of ammonia addition, polycyclic aromatic hydrocarbons (PAH) were measured using planar laser induced fluorescence (PLIF) at 4 wavelengths (350 nm, 400 nm, 450 nm, and 500 nm). PLIF intensities at 350 nm is deemed to correlate with PAHs of 2–3 rings and measured profiles at this wavelength were nearly overlapping for all cases. These findings were supported by GC–MS measurements of acetylene and benzene with the latter showing little change in the peak for the cases studied here. At longer wavelengths, PLIF intensities began to show the same trends found for SVF measurements. Additionally, a specific nitrogen detector was used during GC–MS measurements and several nitrogen containing hydrocarbon species were detected with the 25% addition of ammonia. The combined results indicate that the nitrogen containing hydrocarbon species are likely to account for soot reduction, with the precise mechanism yet to be elucidated. [ABSTRACT FROM AUTHOR]

Turbulent non-premixed flames with local extinction and re-ignition exhibit multiple combustion modes including ignition waves, diffusion flames, partially premixed flames, and ignition-assisted partially premixed flames. The mechanisms of local extinction and re-ignition are not well understood and numerical modeling of multi-mode combustion is a challenging task. In this work, a specially designed swirl-burner was used to study local extinction and re-ignition of non-premixed turbulent methane/air flames. High speed Particle Image Velocimetry (PIV) and laser induced fluorescence of OH radicals (OH-PLIF) measurements along with Large Eddy Simulation (LES) were carried out to investigate the mechanisms of extinction and re-ignition processes in the burner. LES is based on a transported probability density function model within the framework of Eulerian Stochastic Fields (PDF-ESF). It is found that local extinction occurs when the scalar dissipation rate around the stoichiometric mixture fraction is high. The characteristic time scale for local extinction and re-ignition in the present flames is an order of magnitude longer than the characteristic time scale of diffusion/extinction of laminar flamelets. There are two mechanisms for flame hole re-ignition in the present flames. First, under low degree of local extinction conditions (i.e., for small flame holes surrounded by flames) the flame hole re-ignition is due to the mechanism of turbulent flame folding. Second, under high degree of extinction conditions (i.e., with large regions of extinction and lifted flames), re-ignition of the locally extinguished flame is due to the mechanism of ignition assisted partially premixed flame propagation. The results show that the PDF-ESF model is capable of simulating the quenching and re-ignition process found in the experiments. [ABSTRACT FROM AUTHOR]

The synthesis and development of novel energetic materials is a costly, challenging process. Rather than synthesizing new materials, cocrystallization provides the potential opportunity to achieve improved properties of existing energetic materials. This work examined the effects of cocrystallization on the deflagration of a 2:1 molar cocrystal of CL-20 and HMX as well as a 1:1 molar cocrystal of CL-20 and TNT. A hydrogen peroxide (HP) solvate of CL-20 as well as a polycrystalline composite of HMX and ammonium perchlorate (AP) were also studied. A physical mixture of each material was also tested for comparison. The burning rate of each material was measured as a function of pressure. Flame structure during self-deflagration was examined using planar laser-induced fluorescence (PLIF) of CN and OH. The burning rate of the HMX/CL-20 cocrystal and the CL-20/HP solvate closely matched that of CL-20, but the burning rate of the TNT/CL-20 cocrystal was between the burning rate of its coformers. All HMX/AP materials had a higher burning rate than either HMX or AP individually and the burning rate of a physical mixture was found to be a function of particle size. The differences in the burning rate of the physical mixtures and composite crystal of HMX/AP can be explained by changes in the flame structure observed using PLIF. Burning rates and flame structure of the cocrystals were found to closely match those of their respective physical mixtures when smaller particle sizes were used (approx. less than 100 µm). The results obtained demonstrate that the deflagration behavior of the coformers is not indicative of the deflagration behavior of the resulting physical mixture or cocrystal. However, changes in the resulting flame structure greatly affect the burning rate. [ABSTRACT FROM AUTHOR]

In the near future, lean-premixed hydrogen combustion technology for low-emission gas turbine engines is expected to be crucial in the effort to mitigate greenhouse gas emissions, in association with energy system decarbonization. In this study, we examine combustion instabilities in lean fully-premixed hydrogen-air flames in a mesoscale multinozzle array; little is currently known about how these flames respond to acoustic perturbations. Several measurement techniques, including phase-synchronized OH* chemiluminescence imaging, OH Planar Laser Induced Fluorescence, acoustic pressure, and the two microphone method, are used, together with reduced-order acoustic modeling, to identify the key physical properties of lean-premixed hydrogen-air flames, in comparison with lean-premixed methane-air flames. We show that extremely compact lean-premixed hydrogen flames are preferentially coupled to higher eigenmodes of a given system, L3 – L6, including approximately 1 kHz high-frequency instabilities, while the instabilities of methane-air flames are predominantly limited to the first longitudinal mode under the same range of operating conditions. This is attributed to the fact that the dynamics of the methane flames are governed by the collective motion of constituent flames, involving a complex process of the emergence, convection, and interaction of large-scale structures. By contrast, the hydrogen flames in the multinozzle configuration oscillate in isolation within a very short distance and without strong flame-to-flame interactions. This is particularly suitable for accommodating high-frequency heat release modulations. The triggering of intense sound generation from lean-premixed mesoscale hydrogen flames is correlated strongly with a combination of flame surface destruction due to front merging and the flames' close proximity to a pressure antinode. These results, for the first time, highlight the key features of self-excited combustion instabilities of mesoscale multinozzle hydrogen-air flames in a well-controlled laboratory-scale experiment, and could pave the way for future carbon-neutral gas turbine combustion technology. [ABSTRACT FROM AUTHOR]