CH4-air Flames Research Articles

Spatiotemporal Surface Temperature Measurements Resolving Flame-Wall Interactions of Lean H2-Air and CH4-Air Flames Using Phosphor Thermometry

AbstractSpatiotemporal wall temperature (Twall) distributions resulting from flame-wall interactions of lean H2-air and CH4-air flames are measured using phosphor thermometry. Such measurements are important to understand transient heat transfer and wall heat flux associated with various flame features. This is particularly true for hydrogen, which can exhibit a range of unique flame features associated with combustion instabilities. Experiments are performed within a two-wall passage, in an optically accessible chamber. The phosphor ScVO4:Bi3+ is used to measure Twall in a 22 × 22 mm2 region with 180 µm/pixel resolution and repetition rate of 1 kHz. Chemiluminescence imaging is combined with phosphor thermometry to correlate the spatiotemporal dynamics of the flame with the heat signatures imposed on the wall. Measurements are performed for lean H2-air flames with equivalence ratio Φ = 0.56 and compared to CH4-air flames with Φ = 1. Twall signatures for H2-air Φ = 0.56 exhibit alternating high and low-temperature vertical streaks associated with finger-like flame structures, while CH4-air flames exhibit larger scale wrinkling with identifiable crest/cusp regions that exhibit higher/lower wall temperatures, respectively. The underlying differences in flame morphology and Twall distributions observed between the CH4-air and lean H2-air mixtures are attributed to the differences in their Lewis number (CH4-air Φ = 1: Le = 0.94; H2-air Φ = 0.56: Le = 0.39). Findings are presented at two different passage spacings to study the increased wall heat loss with larger surface-area-to-volume ratios. Additional experiments are conducted for H2-air mixtures with Φ = 0.45, where flame propagation was slower and was more suitable to resolve the wall heat signatures associated with thermodiffusive instabilities. These unstable flame features impose similar wall heat fluxes as flames with 2–3 times greater flame power. In this study, these flame instabilities occur within a small space/time domain, but demonstrate the capability to impose appreciable heat fluxes on surfaces.

Enhancement of lean blowout limits of swirl stabilized NH3-CH4-Air flames using nanosecond repetitively pulsed discharges at elevated pressures

This study experimentally examines the impact of nanosecond repetitively pulsed (NRP) discharges on lean premixed methane-ammonia-air swirl flames at pressures up to 4 bar. The results reveal the extension of flame blowout limit by 10–12 % under atmospheric conditions and 5–6 % at 4 bar, when NRP discharges are applied. This decrease in the plasma effect could be attributed to the detrimental impact of strong shock waves at high pressure, destabilizing the flame. NH3-CH4-air flames shows higher lean blowout limits in comparison to CH4-air flames. NRP discharges exhibit consistent impact on the lean blowout limit enhancement across ammonia fractions (20–60 %), irrespective of pressure conditions. These results are obtained for a ratio of NRP discharge power to flame thermal power of 0.63 % and constant pulse repetition frequency of 30 kHz.

Influence of the Pilot Flame on the Morphology and Exhaust Emissions of NH3-CH4-Air Swirl Flames Using a Reduced-Scale Burner at Atmospheric Pressure

This work presents an experimental study on the influence of the pilot flame characteristics on the flame morphology and exhaust emissions of a turbulent swirling flame. A reduced-scale burner, inspired by that fitted in the AE-T100 micro gas turbine, was employed as the experimental platform to evaluate methane (CH4) and an ammonia-methane fuel blend with an ammonia (NH3) volume fraction of 0.7. The power ratio (PR) between the pilot flame and the main flame and the fuel composition of the pilot flame was investigated. The pilot power ratio was varied from 0 to 20% for both fuel compositions tested. The NH3 volume fraction in the pilot flame ranged from pure CH4 to pure NH3 through various NH3–CH4 blends. Flame images and exhaust emissions, namely CO2, CO, NO, and N2O were recorded. It was found that increasing the pilot power ratio produces more stable flames and influences most of the exhaust emissions measured. The CO2 concentration in the exhaust gases was roughly constant for CH4-air or NH3–CH4–air flames. In addition, a CO2 concentration reduction of about 45% was achieved for XNH3 = 0.70 compared with pure CH4, while still producing stable flames as long as PR ≥ 5%. The pilot power ratio was found to have a higher relative impact on NO emissions for CH4 than for NH3–CH4, with measured exhaust NO percentage increments of about 276% and 11%, respectively. The N2O concentration was constant for all pilot power ratios for CH4 but it decreased when the pilot power ratio increased for NH3–CH4. The pilot fuel composition highly affected the NO and N2O emissions. Pure CH4 pilot flames and higher power ratios produced higher NO emissions. Conversely, the NO concentration was roughly constant for pure NH3 pilot flames, regardless of the pilot power ratio. Qualitative OH-PLIF images were recorded to further investigate these trends. Results showed that the pilot power ratio and the pilot fuel composition modified the flame morphology and the OH concentration, which both influence NO emissions.

Flame Transfer Functions for Turbulent, Premixed, Ammonia-Hydrogen-Nitrogen-Air Flames

Abstract Ammonia is a promising hydrogen and energy carrier but also a challenging fuel to use in gas turbines, due to its low flame speed, limited flammability range, and the production of NOx from fuel-bound nitrogen. Previous experimental and theoretical work have demonstrated that partially dissociated ammonia (NH3/H2/N2 mixtures) can match many of the laminar flame properties of methane flames. Among the remaining concerns pertaining to the use of NH3/H2/N2 blends in gas turbines is their thermoacoustic behavior. This paper presents the first measurements of flame transfer functions (FTFs) for turbulent, premixed, and NH3/H2/N2-air flames and compares them to CH4-air flames that have a similar unstretched laminar flame speed and adiabatic flame temperature. FTFs for NH3/H2/N2 blends were found to have a lower gain than CH4 FTFs at low frequencies. However, the cutoff frequency was found to be greater, due to a shorter flame length. For both CH4 flames and NH3/H2/N2 flames, the confinement diameter was found to have a strong influence on peak gain values. Chemiluminescence resolved along the longitudinal direction shows a suppression of fluctuations when the flame first interacts with the wall followed by a subsequent recovery, but with a significant phase shift. Nevertheless, simple Strouhal number scalings based on the flame length and reactant bulk velocity at the dump plane result in a reasonable collapse of the FTF cutoff frequency and phase curves.

Numerical study on the extinction dynamics of partially premixed meso-scale methane-air jet flames with hydrogen addition

In this paper, a model of partially premixed jet flames that sustained above a meso-scale short tube was established for an individual flame port of domestic gas stoves. The effects of hydrogen addition (volume ratio β = 0%, 10%, 20%, 30%) on the extinction dynamics of CH4-air jet flames were numerically investigated. It is found that flame oscillation occurs once (β = 10% and 20%) or twice (β = 30%) in the extinction process. Moreover, the larger of β, the longer the extinction process can sustain. Analysis was performed in terms of both chemical effect and thermal effect. As to the chemical effect, in the first place, the reaction rate decreases as the inlet velocity is reduced. As a result, the consumption rate of O2 will be less than the supply rate from the incoming mixture, which makes the O2 concentration in the flame center increase. On the other hand, the amount of H radicals increases with the increase of β, and when the O2 content at the flame center reaches a “critical point”, the key elementary reaction “H + O2 ↔ O + OH” will be enhanced and consequently the total reaction rate will also be intensified. After that, the consumption rate of O2 will be larger than the supply rate due to the reduced flow rate of incoming mixture. The total heat release rate will decrease sharply and extinction occurs. As regards the thermal effect, it is revealed that heat recirculation effect (indirect preheating effect) lags behind the variation of the reaction zone (i.e., flame), thus, it has a negligible impact on flame oscillation. In contrast, the preheating temperature in the vicinity of flame front (named as “direct preheating effect”) exhibits a similar variation tendency with the total heat release rate of the flame. And the larger of β, the more remarkable of the direct preheating effect can be. In summary, due to the chemical effect and thermal effect caused by hydrogen addition, the flame can survive for a longer time with fluctuation during the extinction process.

Numerical investigation of boundary layer flashback of CH[formula omitted]/H[formula omitted]/air swirl flames under different thermal boundary conditions in a bluff‐body swirl burner

One of the major challenges of applying hydrogen (H2) enriched fuels in industrial combustion systems is the risk of flashback that can cause considerable damage on combustors and affect pollutant emissions. While flashback has been extensively investigated, comparatively less analysis is made to understand the impact of thermal boundary conditions on flashback of bluff-body swirling H2 enriched flames. The aim of this work is to fill a part of this gap through large-eddy simulations of premixed 95%H2/5%CH4-air flame flashback in a bluff-body swirl burner. The results show that flashback characteristics are very sensitive to the bluff-body thermal boundary condition. At Twall=350 K, flashback is led by a large-scale swirling flame tongue (Mode I) that can cause the deflection of streamlines ahead of the flame front and is responsible for flashback Mode I, while a small region of negative velocity induced by small-scale flame bulges cannot lead to a net upstream propagation of the flame front. However, when treating the bluff-body as an adiabatic wall, the flashback is led by multiple small-scale flame bulges, and the circumferential motion of the lowest flame tip is negligible (Mode II). These small-scale bulges can cause large reverse flow pockets and formation of a stagnation point ahead of the preheat zone that facilitates flashback, similar to non-swirling channel flashback. This is the main mechanism responsible for flashback Mode II. At Twall=500 K, flashback is led by different structures, switching between Mode I (upstream propagation of swirling flame tongue) to Mode II (upstream propagation of non-swirling flame bulges). Furthermore, it is found that as the boundary heat loss increases, the axial flashback speed decreases, while the azimuthal flashback speed increases due to the presence of Mode I. The results open up the possibility of extending the flashback limit of H2 enriched swirling flames by controlling the thermal condition of bluff body.

A detailed CO2(1B2) chemiluminescence chemical kinetics model for carbon monoxide and hydrocarbon oxidation

The CO2(1B2) - CO2(X1Σ+g) transition is a source of chemiluminescence from CO and hydrocarbon premixed flames and can be used as a diagnostic; however, its chemistry is not well known due to its broadband nature. Although several attempts have been made to model CO2(1B2) chemiluminescence, none performs well in hydrocarbon flames. We propose a new detailed kinetic model for CO2(1B2) chemiluminescence, based on shock tube experiments in the literature and on opposed flame data presented here. The mechanism consists of 26 reactions which describe the formation of the lower excited state molecule CO2(3B2) (R1), the inter-system crossing reaction between CO2(3B2) and CO2(1B2) (R2), CO2(1B2), the formatting reaction path in hydrocarbon flames (R3), CO2(1B2) radiative quenching (R4) and collisional quenching of CO2(3B2) and CO2(1B2) (R5-R26). The reaction rates constants of R1 and R3 within ± 60% and ± 32% uncertainty, respectively, were determined as follows:CO + O + M = CO2(3B2) + M.k1 = 1 × 1013exp(-10/RT) cm6 mol2 s−1, KJ mol−1.CH + O2 = CO2(1B2) + H.k3 = 8 × 1010 cm3 mol−1 s−1.The mechanism was evaluated against several shock tube experiments at low and elevated pressures and also the CO2(1B2)/OH* chemiluminescent intensity ratio for premixed CH4-air and C3H8-air opposed flames measured in the current study. The comparison showed good agreement for CO2(1B2) temporal profiles for CO-based, CH4-based and C2H4-based mixtures. The prediction of temperature dependence of the CO2(1B2) peak intensity for the CH4-based mixture at both low and elevated pressures was much improved relative to previous models. The CO2(1B2)/OH* chemiluminescent intensity ratio for premixed CH4-air flames predicted by the new model agrees quite well with experiment data, while a small discrepancy remains for C3H8-air flames. Overall, the developed CO2(1B2) chemiluminescence model reproduces, a wide range of experimental data and extends knowledge of CO2(1B2) chemistry.

Numerical simulation of CO formation and reduction on flame propagation due to heat loss through the cooled wall

To realize further low carbon monoxide (CO) emissions for industrial gas turbine combustors, the elucidation of the CO formation and reduction mechanism for lean premixed combustion is crucial. In this study, one-dimensional numerical simulations using a detailed reaction mechanism approach and two-dimensional numerical simulations using a detailed reaction mechanism approach or a non-adiabatic flamelet generated manifolds (NA-FGM) approach are applied to CH4-air premixed flames near a cooled wall. The effects of the equivalence ratio (0.5, 0.7 and 1.0), pressure (0.1 and 2.0 MPa), and wall temperature (300 and 750 K) on the CO emissions is investigated. The results indicate that the influence of the wall temperature is the most significant on the CO reduction rate. For the lower wall temperature, the CO reduction rate in the vicinity of the cooled wall becomes lower, thereby has a risk to increase the CO emissions. At the pressure of 2.0 MPa and the equivalence ratio of 0.5, the CO reduction rate at a wall temperature of 300 K is only 16.8% of that at a wall temperature of 750 K. This suggests that the control of heat loss through the wall could be key to effectively reducing the CO emissions from combustors.

Investigation of Swirl Stabilized CH4 Air Flame with Varied Hydrogen Content by using Computational Fluid Dynamics (CFD) to Study the Temperature Field and Flame Shape

In the current paper, numerical simulations of the combustion of turbulent CH4-H2 are presented employing the standard k-epsilon and the RNG k-epsilon for turbulence closure. The Fr-ED concept is carried out to account for chemistry/ turbulence interaction. The hydrogen content is varied in the fuel stream from 0% to 100%. The numerical solutions are validated by comparison with corresponding experimental data from the Combustion Laboratory of the University of Milan. The flow is directed radially outward. This method of fuel injection has been already been explored experimentally. The results show that the structure of the flame is described reasonably and both standard k-ɛ and RNG k- ɛ models can predict the flame shape. The general aspect of the temperature profiles is well predicted. The temperature profiles are indicating a different trend between CH4 and CH4/H2 fuel mixtures.

Propulsive effect of microwave-induced plasma jet on spark ignition of CO2-diluted CH4-air mixture

To explore the application potential of microwave-assisted spark ignition (MAI) under exhaust gas recirculation conditions, the ignition performance of MAI under CO2 dilution is tested with CH4-air premixed spherical flames. Experiments are carried out in a constant volume combustion chamber with CO2 dilution ratio of 0–20% and equivalence ratio varied from 0.5–1.0. The microwave pulse repetition frequency (PRF) and trigger delay are altered to evaluate the interaction time window between microwave and flame kernel according to the occurrence of bright spot. Results show that MAI extended the lean limit from 0.7 to 0.6 when the mixture is diluted with 8% CO2. At equivalence ratio 1.0, the CO2 dilution limit is extended from 8% in SI mode to 20% in MAI mode. Delaying the microwave trigger beyond 1 ms, the bright spot no longer occurs implying that the time window is about 1 ms. The time window is extended to 2 ms with 1 kHz microwave triggered synchronously with spark, and increasing the microwave PRF to 10 kHz prolongs the time window to 3 ms. However, as CO2 added, the time window is obviously shortened and no more than 1 ms, indicating that CO2 dilution accelerates the decay of electron density in the flame kernel, so that the enhancement is mainly reflected in the first microwave pulse with 1 kHz PRF. The single microwave pulse of 1 kHz induces stronger plasma jet propelling the flame front, and its direction directly decides the ignition success rate near ignitable limit. On the whole, under high ratio of CO2 dilution, increasing the intensity and controlling the direction of microwave-induced plasma jet is beneficial to MAI performance.

Modelling of Conditional Scalar Dissipation Rate in Turbulent Premixed Combustion

In turbulent premixed flames, for the mixing at a molecular level of reactants and products on the flame surface, it is crucial to sustain the combustion. This mixing phenomenon is featured by the scalar dissipation rate, which may be broadly defined as the rate of micro-mixing at small scales. This term, which appears in many turbulent combustion methods, includes the Conditional Moment Closure (CMC) and the Probability Density Function (PDF), requires an accurate model. In this study, a mathematical closure for the conditional mean scalar dissipation rate, <Nc|ζ>, in Reynolds, Averaged Navier–Stokes (RANS) context is proposed and tested against two different Direct Numerical Simulation (DNS) databases having different thermochemical and turbulence conditions. These databases consist of lean turbulent premixed V-flames of the CH4-air mixture and stoichiometric turbulent premixed flames of H2-air. The mathematical model has successfully predicted the peak and the typical profile of <Nc|ζ> with the sample space ζ and its prediction was consistent with an earlier study.

Simultaneous imaging of H and OH in flames using a single broadband femtosecond laser source

Atomic hydrogen (H) and hydroxyl radical (OH) are two key intermediate species in the combustion of hydrocarbon fuels. For example, the spatial and temporal distribution of H is useful to better understand the ignition/extinction processes, whereas OH distribution profiles can yield flame structure and reaction zone in different flames conditions. In the present study, simultaneous laser-induced fluorescence (LIF) measurements of H and OH are obtained using a single ultrashort femtosecond-duration laser pulse near λ=307.7 nm for 3pLIF of H. Measurements are performed in CH4-air and C2H4-air flames stabilized over the Hencken burner. Recently developed three-photon LIF (3pLIF) technique is used for H, whereas the same excitation laser pulse can simultaneously excite OH A2Ʃ+←X2Π (0,0) transition. Red-shifting the excitation wavelength to 307.7 nm as opposed to commonly used two-photon excitation using 205 nm deep-ultra-violet (DUV) laser pulses enables the reduction of absorption losses of excitation laser pulse energy when transmitting through thick optical windows of high-pressure vessels, for example. Furthermore, the efficiency of the fs wavelength generation scheme of commercial optical parametric devices is approximately a factor of two higher near 307.7 nm as compared to 292 nm for 3pLIF of H. In this study, H and OH number densities have been reported for a range of flame equivalence ratios in CH4/air and C2H4/air flames established over the Hencken burner. Simultaneous imaging of H and OH in the premixed Bunsen flame is presented. Prospective improvements for increasing the signal-to-noise and hence potential kHz-rate simultaneous single-shot imaging are also discussed.

Effects of CO2 dilution on partially premixed CH4-air flames in swirl and bluff body stabilized combustor

The effect of CO2 dilution on the flame characteristics and pollutant emission of a partially premixed CH4-air flame in a confined bluff body and swirl influenced flowfield is investigated using optical and laser diagnostic methods. The non-premixed burner produced a converging-diverging flowfield at the burner exit and a lifted flame is produced at all test cases, with an upstream movement of the flame with decreasing global equivalence ratios (ϕg). Based on variations in ϕg, two flame stabilization modes – bluff body influenced and swirl stabilized – with a transition mode in-between is observed for the cases with (flame FB) and without dilution (flame FM). The characteristics of the heat release zone are influenced by dilution, with the FB flames being longer and also less intense when compared to FM flames. Pollutant measurement at 30 mm downstream from the combustor exit highlighted the ultra-low NOx capability of the IIST-GS2 burner. CO2 dilution leads to a reduction in NOx emission due to both thermal and chemical effects. For ϕg ≥ 0.7 extreme low levels of CO and unburned hydrocarbons (UHC) are observed for both cases. For ϕg ≤ 0.6 the dramatic increase of both CO and UHC maybe due to the lower flame temperatures and shorter flame zone residence times, respectively.

Experimental Study of EGR Dilution and O2 Enrichment Effects on Turbulent Non-Premixed Swirling Flames

ABSTRACT This paper presents experimental results of the effects of CO2 and H2O dilution on CH4-air flames enriched by oxygen. The study focuses on pollutant emissions, flame stability through the determination of liftoff heights and flow fields by LDV measurements. Different parameters of the burner are studied such as the swirl number, the global equivalence ratio, and the fractions of O2, H2O, and CO2 in the mixture. The fraction of diluents varies from 0% to 20%, O2 enrichment from 21% to 30% (in vol.), and the swirl number from 0.8 to 1.4. The experiments are carried out on a coaxial swirled burner placed in a vertical combustion chamber. The flame is visualized by the chemiluminescence technique on OH* in order to locate the flame front. Measurements of combustion products such as NOx, CO, and CO2 are done using a HORIBA PG250 multi-gas analyzer. Flow fields are measured by LDV measurement for the longitudinal velocity component and its fluctuations. Results show that the CO2 and H2O dilution influences significantly the flame characteristics. With dilution the liftoff height increases but the flame remains stabilized. With O2 enrichment the liftoff height decreases and flame stability is enhanced. If we therefore want to dilute more, we should enrich with oxygen. The increase of dilution rate induces a decrease in NOx emission and exhaust gas temperature and an increase in CO emissions. LDV measurements showed the longitudinal velocity distribution of flow and its fluctuations in reactive and non-reactive conditions. Note that the presence of the flame induces an increase in the longitudinal velocity downstream of the flow due to the expansion of gases.

Measurements of laminar burning velocity and Markstein length in outwardly-propagating spherical SNG-air premixed flames at elevated pressures

A study has been conducted to measure unstretched laminar burning velocities and burnt Markstein lengths for outwardly-propagating spherical SNG-air premixed flames at elevated pressures. The range of flame radius and extrapolation models (LM, NM I and NM II) were first optimized for CH4-air premixed flames at normal temperature. The presently measured unstretched laminar burning velocities yielded the minimal errors when Aramco 2.0 and NM II were adopted for the optimized range of flame radius. The capability of predicting Markstein length with a composite model was investigated by using different definition of flame thickness and non-dimensional thermal conductivity for four iso-temperature surfaces. The theoretical Markstein lengths with appropriately defined flame thickness and non-dimensional thermal conductivity best trace the presently measured ones for the iso-temperature surface of 600 K.Unstretched laminar burning velocities against equivalence ratio were measured and compared with the predicted ones with Aramco 2.0 for SNG-air premixed flames. Except 0.1 MPa, the predicted ones were deviated appreciably from measured data when chamber pressure increases. The capability of predicting unstretched laminar burning velocities against equivalence ratio was improved via modifying several kinetic data based on sensitivity analysis to Aramco 2.0 Mechanism. The capability of predicting Markstein lengths was also tested for SNG-air premixed flames at elevated pressures up to 0.7 MPa. Choosing the iso-temperature surface of 600 K excellently traced the presently measured data when flame thickness and non-dimensional thermal conductivity are appropriately defined, while several issues are raised up as future works.

The effects of aspect ratio on CH4/air flame stability in rectangular mesoscale combustors

The effects of aspect ratio (α = 1, 2, 3 and 4) of mesoscale rectangular channels on CH4-air flame stability were numerically studied. The results show that at relatively high inlet velocity, the flame is convex (with respect to the upstream) when α ≤ 3; instead it shifts to a W-shaped appearance at α = 4. In addition, the flame blowout limit exhibits a non-monotonic variation and it peaks at α = 3. Our analysis reveals that when α is raised from 1 to 3, both of the heat loss ratio and hydrodynamic perturbation rate are reduced, which favors a larger blowout limit. However, for the channel with α = 4, the perturbation rate of hydrodynamic instability is significantly increased at the flame tip because of the increased wave number, which results from the W-shaped flame front. Consequently, the intensified hydrodynamic instability will lead to a reduction in the flame blowout limit of the channel with α = 4.

Experimental investigation of CH4-air-O2 turbulent swirling flames by Stereo-PIV

This paper presents an investigation of the dynamic characteristics of non-premixed turbulent swirling flames using the stereo-PIV technique. To finely characterize the flow field, experiments are performed in the longitudinal and transverse planes in non-reacting and reacting conditions. The burner configuration studied in this work is a coaxial swirl stabilized burner used in the turbulent regime. The swirler is placed in an annular part supplying the oxidant flow and the central pipe delivers radially a fuel through eight holes symmetrically distributed on the periphery of the tube. The experiments are conducted using a 25 kW parallelepiped combustion chamber, 1 m high and 0.5 m wide, using circulating water as coolant. In the reacting case, the CH4-air flame is oxygen enriched from 21% to 30% in volume. Effects of the swirl number (0.8–1.4) and the global equivalence ratio (0.8–1) on the flows topology and structure are analyzed. Results of the present work concern the 3D mean velocity fields, the turbulence intensities, the swirl number measurements and the entrainment rate along the flow. SPIV results show the three-dimensional aspect of the flow including the recirculation zone and the annular zone with swirling jet effects. The comparison between reacting and non-reacting flows shows that the presence of the flame induces a larger radial flow expansion, higher mean velocities and higher turbulence intensities. The results show a bifurcation of the tangential velocity towards the jet central axis by crossing the flame front. It is also shown that the fluctuations of the tangential velocity are mainly responsible for the highest turbulence kinetic energy levels at the top of the central recirculation zone. The entrainment rate of the surrounding gases is measured by stereo-PIV as a function of the swirl number and equivalence ratio. The results explain one possible mechanism for the decrease of the NOx emissions when the global equivalence ratio increases via an increase of the entrainment rate at the flame base.

Quantitative 2-D OH thermometry using spectrally resolved planar laser-induced fluorescence.

A novel method is presented for quantitative two-dimensional temperature measurement in combustion gases. This method, namely spectrally resolved planar laser-induced fluorescence thermometry, utilizes a high-power, wavelength-tunable and narrow-linewidth CW laser to access the spectral lineshapes of a key combustion intermediate, the hydroxyl radical (OH), and enables high-fidelity and calibration-free quantification of non-uniform temperature fields in complex reacting flows. Specifically, the R1(11)/R1(7) line pair of the OH A2Σ+-X2Π(0,0) rovibronic band was probed with laser radiation near 306.5nm, and their fluorescence ratios were used to infer temperature. Preliminary demonstrations of this thermometry method were performed in a series of burner-stabilized CH4-air flames.

Large-eddy simulation of low-swirl multi-nozzle combustion with co- and counter-swirling arrays

Large-eddy simulation (LES) of CH4-air low-swirl flame was carried out in a multi-nozzle combustor with two burner configurations by using a premixed flamelet model. The multi-nozzle burner includes a co-swirling array where all five nozzles act in the same direction and a counter-swirling array where the center nozzle is in the opposite swirling direction to the outer nozzles. LES results are in good agreement with OH-planar laser-induced florescence data in terms of OH concentrations and combustion progress variables. Numerical results show that the flow of each nozzle is constant before merging. The neighboring flows interact with each other and generate a highvelocity zone with intensive turbulence. The kinetic energy in the interacting region for the co-swirling array is larger than that for the counter-swirling array. After neighboring flow combining, the flow develops into a unified swirling motion similar to a single swirling flow for the co-swirling array, whereas the flow maintains the individual swirling structures for the counter-swirling arrangement. However, the swirling array exerts minimal effect on multi-nozzle combustion in terms of the temperature distributions and combustion progress of premixed low-swirl multi-nozzle flames.

Interference-free hybrid fs/ps vibrational CARS thermometry in high-pressure flames.

Interference-free hybrid femtosecond/picosecond vibrational coherent anti-Stokes Raman scattering (CARS) of nitrogen is reported for temperature measurements of 1300-2300K in high-pressure, laminar H2-air and CH4-air diffusion flames up to 10bar. Following coherent Raman excitation by 100fs duration pump and Stokes pulses, a time-asymmetric probe pulse is used for the detection of spectrally resolved N2 CARS signals at probe delays as early as ∼200-300 fs. This allows for full rejection of nonresonant contributions while being independent of collisions for single-shot precision of ±2% at elevated pressures. The effects of collisions at longer probe-pulse delays are also investigated to determine the feasibility of varying the detection timing from 200fs to 100ps.