Local Equivalence Research Articles

Experimental study on co-firing characteristics of ammonia with pulverized coal in a staged combustion drop tube furnace

Utilizing ammonia as a co-firing fuel to replace amounts of fossil fuel seems a feasible solution to reduce carbon emissions in existing pulverized coal-fired power plants. However, there are some problems needed to be considered when treating ammonia as a fuel, such as low flame stability, low combustion efficiency, and high NOx emission. In this study, the co-firing characteristics of ammonia with pulverized coal are studied in a drop tube furnace with staged combustion strategy. Results showed that staged combustion would play a key role in reducing NOx emissions by reducing the production of char-NOx and fuel(NH3)-NOx simultaneously. Furthermore, the effects of different ammonia co-firing methods on the flue gas properties and unburned carbon contents were compared to achieve both efficient combustion and low NOx emission. It was found that when ammonia was injected into 300 mm downstream under the condition of 20% co-firing, lower NOx emission and unburnt carbon content than those of pure coal combustion can be achieved. This is probably caused by a combined effect of a high local equivalence ratio of NH3/air and the prominent denitration effect of NH3 in the vicinity of the NH3 downstream injection location. In addition, NOx emissions can be kept at approximately the same level as coal combustion when the co-firing ratio is below 30%. And the influence of reaction temperature on NOx emissions is closely associated with the denitration efficiency of the NH3. Almost no ammonia slip has been detected for any injection methods and co-firing ratio in the studied conditions. Thus, it can be confirmed that ammonia can be used as an alternative fuel to realize CO2 reduction without extensive retrofitting works. And the NOx emission can be reduced by producing a locally NH3 flame zone with a high equivalence ratio as well as ensuring adequate residence time.

Experimental Investigation of Kerosene Droplet Distribution in a Linearized Rotating Detonation Engine

ABSTRACT The atomization and mixing process in a linearized rotating detonation engine supplied with non-premixed kerosene was optically analyzed by experiment. Schlieren, simultaneous LIF/Mie technique and particle/droplet image analysis (PDIA) system were applied to study flow field structure and spray properties under various conditions, including air flow rate, fuel flow rate and flow velocity at injector position. Aerodynamic shearing force and fuel injection pressure contribute to the refinement and uniformity of the spray. The distribution of fuel in the combustor has obvious stratification, which makes the local equivalence ratio higher than the set global equivalence ratio. Compared with full-scale RDE experiments, the detonation wave exists in a dual-wave collision mode. The increased kerosene flow rate refines the droplets, which raises the detonation wave velocity. Furthermore, the detonation wave will propagate more stably due to the slower airflow velocity at higher combustor pressure.

Optical diagnostics of misfire in partially premixed combustion under low load conditions

To clarify the misfire mechanism is important for stabilizing combustion in partially premixed combustion (PPC) under low load. Fuel-tracer planar laser-induced fluorescence (PLIF), formaldehyde PLIF, flame and OH* natural luminosity imaging were utilized to qualify the local equivalence ratio, low-temperature reaction and the high-temperature flame features in an optical engine. Results show that in high direct injection (DI) pressure (1000 bar), due to excessive premixing, the local equivalence ratio in the initial timing of the high temperature heat release (HTHR) is low. Although the auto-ignition flame kernels are formed in high DI pressure, they cannot stably develop, resulting in misfire during the flame development process. In late DI timing (-5 crank angle degree after top dead center, °CA ADTC), since the whole heat release process occurs in the expansion stroke, the in-cylinder temperature and pressure continue decreasing. Although the local equivalence ratio in some regions is high enough, the in-cylinder thermodynamic environment does not support the generation of more auto-ignition flame kernels, thus a small amount of auto-ignition flame kernels can only develop through flame propagation. In short, the misfire of PPC occurs in regions where the equivalence ratio is low or the in-cylinder thermodynamic environment does not further support flame development. Therefore, the trade-off relationship between equivalence ratio and temperature determines the formation of auto-ignition kernels. The local equivalence ratio and temperature distribution near the initial timing of HTHR is the key factor to ensure the subsequent stable combustion. Taking the ambient pressure of 18 bar as an example, the boundary condition where the autoignition kernels are most likely formed or the charge is most likely ignited by the nearby flame kernels is in the range of 0.53–0.62 for equivalence ratio and 740–757 K for temperature. The misfire region most likely appears when the equivalence ratio is lower than 0.49. It can be concluded that the misfire of PPC results from the synergistic effect of local equivalent ratio and temperature. The controlling parameters of injection pressure and injection timing are actually optimizing the suitable combinations of equivalence ratio and temperature to stabilize combustion.

Four-Line C2*/CH* Optical Sensor for Chemiluminescence Based Imaging of Flame Stoichiometry

In the present work, an optical sensor was developed and calibrated for the purpose of non-intrusive equivalence ratio measurements in combustion systems. The sensor incorporates a unique four-line, single-sensor chemiluminescence imaging-based approach, which relies on the ratio of C2* and CH* radical-species intensities to obtain measurements of equivalence ratios. The advantage of the four-line sensor is the use of additional filtering to mitigate broadband luminescence signals, and its improvements over conventional two-line chemiluminescence diagnostics are discussed. The sensor was calibrated using a premixed bluff-body jet burner with a propane–air flame operating over a wide range of equivalence ratios. The results showed that the four-line processing technique improved the signal-to-noise ratio of the chemiluminescence images for all test cases. Calibrations of C2*/CH* intensity ratio to equivalence ratio were developed for both the four-line and two-line techniques. The calibrations were then used to create maps of local equivalence ratios in the flame-holding region. The maps revealed a non-uniform field of equivalence ratios due to the nature of the radical-species intensity profiles within the flame. Therefore, special consideration is required for calibration in order to accurately quantify equivalence ratios and apply these to diffusion flames.

A complete normal form for everywhere Levi–degenerate hypersurfaces in [formula omitted

2-nondegenerate real hypersurfaces in complex manifolds play an important role in CR-geometry and the theory of Hermitian Symmetric Domains. In this paper, we obtain a complete convergent normal form for everywhere 2-nondegenerate real-analytic hypersurfaces in complex 3-space. We do so by entirely reproducing the Chern-Moser theory in the 2-nondegenerate setting. This seems to be the first such construction for hypersurfaces of infinite Catlin multitype. We in particular discover chains in an everywhere 2-nondegenerate hypersurface, the tangent lines to which at a point form the so-called canonical cone. Our approach is based on using a rational (nonpolynomial) model for everywhere 2-nondegenerate hypersurfaces, which is the local realization due to Fels-Kaup of the well known tube over the light cone. For the convergence of the normal form, we use an argument due to Zaitsev, based on building a canonical direction field in an appropriate bundle over a hypersurface.As an application, we obtain, in the spirit of Chern-Moser theory, a criterion for the local sphericity (i.e. local equivalence to the model) for a 2-nondegenerate hypersurface in terms of its normal form. As another application, we obtain an explicit description of the moduli space of everywhere 2-nondegenerate hypersurfaces.

Synergistic interactions of thermodiffusive instabilities and turbulence in lean hydrogen flames

Interactions of thermodiffusive instabilities and turbulence have been investigated by large-scale Direct Numerical Simulations (DNS) in this work. Two DNS of turbulent premixed lean hydrogen/air flames have been performed in a slot burner configuration at the same jet Reynolds number of Re=11,000 and Karlovitz number of Ka≈15 using a detailed chemical mechanism. Realistic transport models are employed in one case, which features the characteristic patterns of thermodiffusively unstable flames, such as strong variations of the heat release and super-adiabatic temperatures. In the other case, the diffusivities of all species are set equal to the thermal diffusivity (unity Lewis numbers assumption) and thermodiffusive instabilities are therefore suppressed.The local burning of the turbulent flame without thermodiffusive instabilities is similar to an unstretched laminar flame and the turbulent flame speed increases only due to the increase of flame surface area in agreement with previous studies for flames at similar conditions. In contrast, the thermodiffusively unstable flame features a strong enhancement of the turbulent flame speed, which is not only caused by flame wrinkling, but is greatly increased due to significant variations of the local reaction rates. These are caused by variations of the local equivalence ratio due to the differential diffusion of hydrogen. A comparison with a thermodiffusively unstable laminar flame at the same conditions reveals that the variations of the local equivalence ratio and local reaction rates are significantly enhanced in the turbulent flame due to higher fluctuations of curvature and an enhanced average strain rate induced by turbulence. Thus, turbulence and thermodiffusive instabilities show synergistic effects, which are reflected in a significantly higher fuel consumption rate per flame surface area. The flame surface area generation, which is governed by the tangential strain and the flame propagation in curved flame segments, is also different in the two cases. Most noteworthy, the tangential strain rate is shown to be determined by the smallest turbulent structures in both turbulent flames and to be unaffected by the thermodiffusive instability mechanism. However, thermodiffusive instabilities lead to a production of flame surface area in convexly curved flame segments, featuring the formation of tongue-like structures that penetrate into the unburned gas, which do not exist in the turbulent flame with unity Lewis numbers. This is linked to an enhancement of the flame displacement speed with curvature in the thermodiffusively unstable flame, while in the absence of instabilities, a reduction of the flame displacement speed with increasing curvature is observed, leading to a destruction of flame surface area.These findings suggest that thermodiffusive instabilities are sustained in turbulent flows and even show synergistic interactions with turbulence, which needs to be accounted for in turbulent combustion models.

Fundamental physics with ESPRESSO: Constraints on Bekenstein and dark energy models from astrophysical and local probes

Dynamical scalar fields in an effective four-dimensional field theory are naturally expected to couple to the rest of the theory's degrees of freedom, unless some new symmetry is postulated to suppress these couplings. In particular, a coupling to the electromagnetic sector will lead to spacetime variations of the fine-structure constant, $\alpha$. Astrophysical tests of the space-time stability of $\alpha$ are therefore a powerful probe of new physics. Here we use ESPRESSO and other contemporary measurements of $\alpha$, together with background cosmology data, local laboratory atomic clock and Weak Equivalence Principle measurements, to place stringent constraints on the simplest examples of the two broad classes of varying $\alpha$ models: Bekenstein models and quintessence-type dark energy models, both of which are parametric extensions of the canonical $\Lambda$CDM model. In both cases, previously reported constraints are improved by more than a factor of ten. This improvement is largely due to the very strong local constraints, but astrophysical measurements can help to break degeneracies between cosmology and fundamental physics parameters.

Numerical study of lubricant oil drop induced pre-ignition in engines

Pre-ignition in engines has been the subject of current research with the advent of boosted engines for reduced fuel consumption and emissions. Lube-oil-drop-induced pre-ignition was observed in large natural gas engines which find application in the marine industry. In order to understand the mechanism of pre-ignition in the present study, a computational simulation of pre-ignition by lube-oil drops was performed for a production natural gas marine engine. The engine grid was generated using ICEM tool. For CFD simulations, an in-house version of KIVA4 code was employed. Oil throw-off into the combustion chamber was modeled by a lube oil stripping model, which sets the criteria for oil drops to be stripped from the piston rings/crevice regions and enter the combustion chamber. To capture the ignition caused by the stripped oil drops precisely, single particle ignition cell (SPIC) model was used that utilizes computations of thermal and chemical changes in refined grid cells for the lube oil containing computational cells. For modeling chemical kinetics, a reduced lube oil reaction mechanism previously developed was used. Factors affecting the lube oil stripping process such as thickness, velocity, and temperature of the lube film and its subsequent pre-ignition behavior were studied and discussed. Higher initial thickness, higher film temperature and higher initial film velocity all lead to earlier stripping of lube-oil film into the combustion chamber. Once the film is stripped, keeping the local lube vapor equivalence ratio low is the key to prevent pre-ignition.

Research on combustion performance of a micro-mixing combustor for methane-fueled gas turbine

This research proposes an H-class micro-mixing gas turbine combustor prototype with a thermal power of 1.15 MW, which employs 40 exterior jet-in-crossflow micro-mixing nozzles with a tapered round outlet to reduce pollutant emissions and prevent the flashback. These nozzles are located by a distribution arrangement in order to improve the temperature distribution uniformity of field flow. To respond to off-design operation of gas turbine, the combustion performance of the micro-mixing combustor prototype is investigated numerically at the rated fuel temperature of 473 K and other ones of 373, 423, 523 and 573 K. The results indicate that NO emission has a relatively high augment as the fuel temperature decreases, which is due to the rising local equivalence ratio. The variation trends of the temperature distribution uniformity, combustion efficiency and total pressure loss are not obvious as the fuel temperature changes. Based on the rated and off-design combustion performance at different radial planes, this research finally proposes an optimal relationship between the axial liner length and micro-mixing nozzle outlet diameter.

Corks, involutions, and Heegaard Floer homology

Building on the algebraic framework developed by Hendricks, Manolescu, and Zemke, we introduce and study a set of Floer-theoretic invariants aimed at detecting corks. Our invariants obstruct the extension of a given involution over any homology ball, rather than a particular contractible manifold. Unlike previous approaches, we do not utilize any closed 4-manifold topology or contact topology. Instead, we adapt the formalism of local equivalence coming from involutive Heegaard Floer homology. As an application, we define a modification \Theta_\mathbb{Z}^{\tau} of the homology cobordism group which takes into account an involution on each homology sphere, and prove that this admits a \mathbb{Z}^\infty -subgroup of strongly nonextendable corks. The group \Theta_\mathbb{Z}^{\tau} can also be viewed as a refinement of the bordism group of diffeomorphisms. Using our invariants, we furthermore establish several new families of corks and prove that various known examples are strongly nonextendable. Our main computational tool is a monotonicity theorem which constrains the behavior of our invariants under equivariant negative-definite cobordisms, and an explicit method of constructing such cobordisms via equivariant surgery.

A local equivalence scale and its information basis

We identify an information basis that is necessary and sufficient for a local equivalence scale, constructed using the indifference surface associated with a poverty level of well-being. It makes less restrictive assumptions about preferences for households with different characteristics than widely used approaches to this problem. Our approach is best suited for poverty comparisons, where calibration of means-tested transfers for different households is an important practical problem.

Multi-channel gliding arc plasma-assisted ignition in a kerosene-fueled model scramjet engine

A multi-channel gliding arc (MCGA) plasma was utilized to ignite a kerosene-fueled model scramjet engine with an inflow of Ma2.92. High-speed photography from the top and side view of the scramjet combustor, high-speed CH* chemiluminescence imaging coupled with the electrical measurements were used to investigate the ignition process. A three-dimensional RANS simulation was used to show the distributions of velocity, temperature, and local equivalence ratio of the supersonic cold flow. In the ignition process, the formation of the flame kernel mainly depends on the discharge characteristics of the MCGA plasma, whereas the flame propagation process is mainly determined by the cavity flow. The MCGA plasma is more likely to produce a large area of the flame kernel in the spark-type discharge with the long arc column and overlaps of each channel gliding arc. The larger flame can be generated subsequently by the merging of the flame kernels ignited by the MCGA plasma, which develops into the resident flame. The resident flame is unable to be formed in a relatively high local equivalence ratio, and it is hard to spread to the back of the cavity along the shear layer due to the large velocity gradient with a relatively low temperature. The resident flame is formed several times by the MCGA plasma ignition, which increases the possibility of the flame spreading to the back of the cavity to form the mainstream flame. The local equivalence ratio can significantly affect the cavity ignition of the kerosene-fueled scramjet combustor.

Local Unitary Equivalence of Generic Multi-qubits Based on the CP Decomposition

The CANDECOMP/PARAFAC (CP) decomposition is a generalization of the spectral decomposition of matrices to higher-order tensors. In this paper we use the CP decomposition to study unitary equivalence of higher order tensors and construct several invariants of local unitary equivalence for general higher order tensors. Based on this new method, we study the coefficient tensors of $3$-qubit states and obtain a necessary and sufficient criterion for local unitary equivalence of general tripartite states in terms of the CP decomposition. We also generalize this method to obtain some invariants of local unitary equivalence for general multi-partite qudits.

Flame characteristics of methane/air with hydrogen addition in the micro confined combustion space

This paper investigated methane/air flame characteristics with hydrogen addition in micro confined combustion space experimentally and computationally. The focus is on the effect of hydrogen addition on the methane/air flame stabilization, the onset of flame with repetitive extinction and ignition (FREI), and the global flame quenching in decreasing continuously combustion space. Furthermore, the effects of hydrogen addition on the flame temperature and the local equivalence ratio distribution were analyzed systematically using numerical simulations. In addition, the effects of hydrogen addition on the concentrations of OH and H radicals, and the critical scalar dissipation rate of local flame extinction were discussed. With a higher hydrogen ratio, the mixing is faster, and the flame is smaller. When the micro confined space is narrower, the heat loss to the combustor walls has a higher impact on the flames. The flames with higher hydrogen ratios have therefore lower peak flame temperatures and lower concentrations of H and OH radicals. The results show that hydrogen addition can effectively widen the stable combustion range of methane/air flames in the micro confined space by about 20% when the hydrogen addition ratio reaches 50%. The frequency and the maximum propagation velocity of FREI flames can be increased as well. The quenching distance of methane/hydrogen/air flames decreases nearly linearly with the increase of hydrogen ratio. This is attributed to the higher critical scalar dissipation rate of local flame extinction in flames with a higher hydrogen ratio.

Non-thermal gliding arc discharge assisted turbulent combustion (up to 80 kW) at extended conditions: phenomenological analysis

ABSTRACT Plasma assisted combustion has been proposed as an efficient technique to enhance combustion, especially under the extreme conditions. For shedding light on the interactions between plasma and turbulent flame at extended conditions, a burner design with integrated electrodes was used to couple a non-thermal gliding arc (GA) discharge to a turbulent flame. The morphology and dynamic behaviors of the GA assisted flame under extended flow rates and gas temperatures were investigated by high-speed video imaging. It is found that two distinct types of flame (named as Flame A and Flame B) can be sustained by the GA discharge depending on the local flow conditions. Flame A was sustained by the GA on stable anchor points, while Flame B moved together with the thin plasma volume of the gliding arc, behaving as an unstable flame. When the fed air gas temperature was increased, Flame A became more stable while Flame B became fragile and extinguished easily. Furthermore, the phenomenological findings under different flow conditions imply typical four flame types for the GA discharge assisted combustion system, including the self-sustained flame at relatively low Reynolds number (Re), the GA sustained stable flame at moderate Re number, the GA sustained unstable flame and the GA assisted auto-ignited and propagating flame at relatively large Re number. In all, the GA discharge seems to provide various effects on combustion depending on the overall turbulence as well as the local equivalence ratio, the gas temperature, etc.

Laser-induced breakdown spectroscopy for local equivalence ratio measurement in opposed jet methane-air flames

Laser-induced breakdown spectroscopy (LIBS) has been applied to both non-reacting and reacting opposed jet flows to evaluate the measurement accuracy of instantaneous, local air-fuel ratio. However, the LIBS signal is much weaker in flames than in the corresponding non-reacting gases and more work is required to investigate the potential of this technique for instantaneous measurements in reacting flows and quantify measurement uncertainties. The present study reported local equivalence ratio measurements of methane/air flames in an opposed-jet burner, which allowed independent control of flow rate and air-fuel ratio. Images and emission spectra of laser-induced plasma were investigated simultaneously using both an Intensified CCD (ICCD) and a spectrometer with a high degree of spatial and temporal resolution. The influences of the camera delay time, exposure time and laser pulse energy on the LIBS measurements were investigated as well. The dependence of spectral intensity ratios of H/O and C2/CN on air-fuel ratio was quantified over a wide range of conditions extending from pure air to pure fuel. It was found that H/O and C2/CN intensity ratios depend monotonically on the mole fraction of methane in the ranges of 0.0–0.8 and 0.3–1.0 respectively. The presence of a flame within the laser beam led to significant measurement deterioration relative to the corresponding non-reacting flows. This could lead to increased measurement uncertainty, and was therefore corrected by increasing the laser pulse energy and applying a proposed data processing method; the proposed correction method was able to reduce the equivalence ratio measurement uncertainty to be within 10% for mixtures with methane mole fractions lower than 50% and within 15% for mixtures with higher methane mole fraction. The established correlations between the intensity ratios of H/O and C2/CN with local air-fuel ratio allowed measurement of the spatial gradient of air-fuel ratio across opposed jet diffusion flames. The LIBS measurements of air-fuel ratio in non-premixed flames were finally compared successfully with CHEMKIN simulations, which demonstrates the ability of LIBS to accurately measure the spatial gradient of the air-fuel ratio.

Turbulent flame propagation mechanism of polymethylmethacrylate particle cloud–ammonia co-combustion

Ammonia is a promising energy carrier for realizing a carbon-neutral society. In particular, solid particle cloud–ammonia co-combustion is considered as an efficient and feasible method to reduce CO2 emissions from the thermal power generation sector by using particle-fuel such as solid waste, biomass, and pulverized coal particles in combustors. However, the fundamental turbulent flame propagation mechanism of solid particle cloud–ammonia co-combustion remains unknow. Therefore, the present study intends to investigate and validate the general turbulent flame propagation mechanism of solid particle cloud–ammonia co-combustion. To achieve this aim, silica particle cloud–ammonia–oxygen–nitrogen mixing combustion, silica particle cloud–acetylene–air mixing combustion, and PMMA particle cloud–ammonia–oxygen–nitrogen co-combustion experiments were conducted. The results showed that the turbulent flame propagation velocity of silica particle cloud–gas-fuel–oxidizer mixing combustion is lower than that of pure gas-fuel–oxidizer combustion. However, the comparison of the turbulent flame propagation velocity of PMMA particle cloud–ammonia co-combustion and that of pure ammonia combustion, showed that whether the flame propagation of the co-combustion was higher than that of the pure ammonia combustion was dependent on the equivalence ratio of the ammonia-oxidizer. Therefore, the consistency of the results between the current study of PMMA particle cloud–ammonia co-combustion and the previous study for coal particle cloud–ammonia co-combustion indicates the turbulent flame propagation mechanism of solid particle cloud–ammonia co-combustion is dominated by the negative effect of the heat sink by unburned particles and the local equivalence ratio increment effect in the preheat zone of the flame front by the addition of the volatile matter, and that the positive effect of radiation from soot particles has little effect on the turbulent flame propagation of co-combustion for small-scale flames. Further, the influence of the heterogeneous combustion of char particles on the turbulent flame propagation of solid particle cloud–ammonia co-combustion is minor because of its slow combustion process. Based on the validated turbulent flame propagation mechanism of co-combustion, new numerical simulation models for solid particle cloud–ammonia co-combustion can be developed in the future.

Modular Structure of the Weyl Algebra

We study the modular Hamiltonian associated with a Gaussian state on the Weyl algebra. We obtain necessary/sufficient criteria for the local equivalence of Gaussian states, independently of the classical results by Araki and Yamagami, Van Daele, Holevo. We also present a criterion for a Bogoliubov automorphism to be weakly inner in the GNS representation. The main application of our analysis is the description of the vacuum modular Hamiltonian associated with a time-zero interval in the scalar, massive, free QFT in two spacetime dimensions, thus complementing the recent results in higher space dimensions (Longo and Morsella in The massive modular Hamiltonian. arXiv:2012.00565). In particular, we have the formula for the local entropy of a one-dimensional Klein–Gordon wave packet and Araki’s vacuum relative entropy of a coherent state on a double cone von Neumann algebra. Besides, we derive the type {III}_1 factor property. Incidentally, we run across certain positive selfadjoint extensions of the Laplacian, with outer boundary conditions, seemingly not considered so far.

Numerical study on propagation and NO reduction behavior of laminar stratified ammonia/air flames

In recently developed ammonia combustion technologies such as swirl flow and staged combustion, locally rich or lean pockets of unburned mixtures may occur due to insufficient mixing. This results in a premixed flamelet propagating into a gradually leaner or richer mixture. The present study aims to numerically investigate the propagation of laminar ammonia/air premixed flames in compositionally stratified mixtures. Results indicate that the flame speed in a rich-to-lean stratified mixture is increased from that in corresponding homogeneous mixtures at each local equivalence ratio. In contrast, the stratified flame speed is decreased in a lean-to-rich stratified mixture. The response of the stratified flame speed is attributed to variation in the amount of H2 in the burned gas. In the rich-to-lean stratified flame, an extra amount of H2 diffuses into the reaction zone to enhance chain-branching reactions which produce H radicals. The increased H radicals then promote dehydration reactions, resulting in an increased fuel consumption rate. In the lean-to-rich stratified flame, the opposite process takes place. The above mechanism is similar to the so-called back-support effect observed in methane/air stratified flames. Meanwhile, in both rich-to-lean and lean-to-rich stratified flames, additional NO reduction occurs in the stoichiometric region of the burned gas. This is facilitated by unburned ammonia diffusing from the neighboring rich burned gas mixing with O/H radicals diffusing from the neighboring lean burned gas, resulting in a production of extra NHi radicals which readily reduce NO. Therefore, NO emission in stratified flames is expected to be lower than that estimated from the emission characteristics in homogeneous mixtures. Although similar to the well-known thermal DeNOx mechanism, the current NO reduction process occurs under a much higher temperature due to the abundance of radical species. A similar phenomenon is expected to be observed in a triple flame configuration, which requires future investigations.

Effects of flow–flame interactions on the stabilization of ultra-lean swirling CH[formula omitted]/H[formula omitted]/air flames

Hydrogen (H2) is regarded as a promising fuel to achieve decarbonization of power and propulsion systems. In this context, hydrogen enriched methane (CH4) combustion has attracted considerable attention in the development of low-emission gas turbines. To achieve low NOx emissions and avoid the dangers of flashback, combustion of CH4/H2/air mixtures under lean and/or ultra-lean operating conditions is of critical importance, while ultra-lean flames are prone to combustion instabilities and difficult to stabilize even in a bluff-body swirl burner. In this work, a series of confined lean premixed CH4/H2/air swirling flames with hydrogen enrichment (αH2) ranging from 0 to 80% is investigated under stable and ultra-lean conditions using simultaneous OH-PLIF and PIV measurements. The results suggest that decarbonization of combustion devices requires large volume fractions of H2 in the fuel mixture, e.g.,80% H2 to achieve half CO2 emission per heat of combustion. It is found that there is a flame topology transition when changing equivalence ratio and/or hydrogen enrichment. At a given αH2, the flames with 0 and 40% H2 always show “V” shapes, whereas an evolution from “M” to “V” shape can be observed for the 80% H2 flame when increasing the equivalence ratio. Moreover, at a given ϕ, the flame shape will shift towards “M” shape at αH2 = 80% from “V” shape at αH2 = 0,40%. Furthermore, H2-enriched flames would move to the inner recirculation zone (IRZ) and stabilize there when decreasing ϕ to ultra-lean conditions. Given that hydrogen enrichment can significantly enhance the resistance to flame strain and that under ultra-lean conditions, there is a strong diffusion of hydrogen from the swirling jet to the IRZ where the sufficient residence time and the increase in the local equivalence ratio contribute to the presence of flame pockets and flame stabilization in the IRZ.