Conditional moment closure and transient flamelet modelling for detailed structure and NO x formation characteristics of turbulent nonpremixed jet and recirculating flames

This study has been mainly motivated to assess computationally and theoretically the conditional moment closure (CMC) model and the transient flamelet model for the simulation of turbulent nonpremixed flames. These two turbulent combustion models are implemented into the unstructured grid finite volume method that efficiently handles physically and geometrically complex turbulent reacting flows. Moreover, the parallel algorithm has been implemented to improve computational efficiency as well as to reduce the memory load of the CMC procedure. Example cases include two turbulent CO/H₂/N₂ jet flames having different flow timescales and the turbulent nonpremixed H₂/CO flame stabilized on an axisymmetric bluff-body burner. The Lagrangian flamelet model and the simplified CMC formulation are applied to the strongly parabolic jet flame calculation. On the other hand, the Eulerian particle flamelet model and full conservative CMC formulation are employed for the bluff-body flame with flow recirculation. Based on the numerical results, a detailed discussion is given for the comparative performances of the two combustion models in terms of the flame structure and NO _x formation characteristics.     

Prediction of NO in turbulent diffusion flames using Eulerian particle flamelet model

The combustion characteristics for the turbulent diffusion flames using the unsteady flamelet concept have been numerically investigated. The Favre-averaged Navier–Stokes equations are solved by a finite volume method of SIMPLE type that incorporates the laminar flamelet concept with a modified k ? ε turbulence model. The NO formation is estimated by solving the Eulerian particle transport equations in a postprocessing mode. Two test problems are considered: CH₄/H₂/N₂ jet flame and CH₄/H₂ stabilised bluff body flame. The temperature and species profiles are well captured by the flamelet model. Two different chemical mechanisms (GRI 2.11 and 3.0) give nearly identical results for temperature and species except NO. The GRI 3.0 gives significantly higher NO levels compared to the GRI 2.11. This is mainly attributed to the difference in NO formation by the prompt mechanism. The NO formation is sensitive to the number of flamelet particles. The NO levels for two test flames do not change when the flamelet particle number exceeds six.     

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 (?_g). Based on variations in ?_g, 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 NO_x capability of the IIST-GS2 burner. CO₂ dilution leads to a reduction in NO_x emission due to both thermal and chemical effects. For ?_g ≥ 0.7 extreme low levels of CO and unburned hydrocarbons (UHC) are observed for both cases. For ?_g ≤ 0.6 the dramatic increase of both CO and UHC maybe due to the lower flame temperatures and shorter flame zone residence times, respectively.     

Second-order conditional moment closure modeling of a turbulent CH4/H2/N2 jet diffusion flame

The second-order CMC model for a detailed chemical mechanism is used to model a turbulent CH₄/H₂/N₂ jet diffusion flame. Second-order corrections are made to the three rate limiting steps of methane–air combustion, while first-order closure is employed for all the other steps. Elementary reaction steps have a wide range of timescales with only a few of them slow enough to interact with turbulent mixing. Those steps with relatively large timescales require higher-order correction to represent the effect of fluctuating scalar dissipation rates. Results show improved prediction of conditional mean temperature and mass fractions of OH and NO. Major species are not much influenced by second-order corrections except near the nozzle exit. A parametric study is performed to evaluate the effects of the variance parameter in log-normal scalar dissipation PDF and the constants for the dissipation term in conditional variance and covariance equations.     

A LES-CMC formulation for premixed flames including differential diffusion

A finite volume large eddy simulation–conditional moment closure (LES-CMC) numerical framework for premixed combustion developed in a previous studyhas been extended to account for differential diffusion. The non-unity Lewis number CMC transport equation has an additional convective term in sample space proportional to the conditional diffusion of the progress variable, that in turn accounts for diffusion normal to the flame front and curvature-induced effects. Planar laminar simulations are first performed using a spatially homogeneous non-unity Lewis number CMC formulation and validated against physical-space fully resolved reference solutions. The same CMC formulation is subsequently used to numerically investigate the effects of curvature for laminar flames having different effective Lewis numbers: a lean methane–air flame with Le_eff = 0.99 and a lean hydrogen–air flame with Le_eff = 0.33. Results suggest that curvature does not affect the conditional heat release if the effective Lewis number tends to unity, so that curvature-induced transport may be neglected. Finally, the effect of turbulence on the flame structure is qualitatively analysed using LES-CMC simulations with and without differential diffusion for a turbulent premixed bluff body methane–air flame exhibiting local extinction behaviour. Overall, both the unity and the non-unity computations predict the characteristic M-shaped flame observed experimentally, although some minor differences are identified. The findings suggest that for the high Karlovitz number (from 1 to 10) flame considered, turbulent mixing within the flame weakens the differential transport contribution by reducing the conditional scalar dissipation rate and accordingly the conditional diffusion of the progress variable.     

Impact of swirl and bluff-body on the transfer function of premixed flames

The frequency response of three lean methane/air flames submitted to flowrate perturbations is analyzed for flames featuring the same equivalence ratio and thermal power, but a different stabilization mechanism. The first flame is stabilized by a central bluff body without swirl, the second one by the same bluff body with the addition of swirl and the last one only by swirl without central insert. In the two last cases, the swirl level is roughly the same. These three flames feature different shapes and heat release distributions, but their Flame Transfer Function (FTF) feature about the same phase lag at low frequencies. The gain of the FTF also shows the same behavior for the flame stabilized by the central insert without swirl and the one fully aerodynamically stabilized by swirl. Shedding of vortical structures from the injector nozzle that grow and rollup the flame tip controls the FTF of these flames. The flame stabilized by the swirler-plus-bluff-body system features a peculiar response with a large drop of the FTF gain around a frequency at which large swirl number oscillations are observed. Velocity measurements in cold flow conditions reveal a strong reduction of the size of the vortical structures shed from the injector lip at this forcing condition. The flame stabilized aerodynamically only by swirl and the one stabilized by the bluff body without swirl do not exhibit any FTF gain drop at low frequencies. In the former case, large swirl number oscillations are still identified, but large vortical structures shed from the nozzle also persist at the same forcing frequency in the cold flow response. These different flame responses are found to be intimately related to the dynamics of the internal recirculation region, which response strongly differs depending upon the injector used to stabilize the flame.     

Multiple mapping conditioning of turbulent jet diffusion flames

The multiple mapping conditioning (MMC) approach is applied to two non-piloted CH₄/H₂/N₂ turbulent jet diffusion flames with Reynolds numbers of Re = 15,200 and 22,800. The work presented here examines primarily the suitability of MMC to simulate CH₄/H₂ flames with varying Re numbers. The equations are solved in a prescribed Gaussian reference space with only one stochastic reference variable emulating the fluctuations of mixture fraction. The mixture fraction is considered as the only major species on which the remaining minor species are conditioned. Fluctuations around the conditional means are ignored. It is shown that the statistics of the mapped reference field are an accurate model for the statistics of the physical field for both flames. A transformation of the Gaussian reference space introduced in previous work on MMC is used to express the MMC model in the same form as CMC. The most important advantage of this transformation is that the conditionally averaged scalar dissipation term is in a closed form. The corresponding temperature and reactive species predictions are generally in good agreement with experimental data. The application to real laboratory flames and the assessment of the new conditional scalar dissipation model for the closure of the singly conditioned CMC equation is the major novelty of this paper. The results are therefore primarily examined with respect to changes of the conditionally averaged quantities in mixture fraction space.     

Effect of Lewis number on premixed laminar lean-limit flames stabilized on a bluff body

In this paper, we present a study on the effect of Lewis number, Le, on the stabilization and blow-off of laminar lean limit premixed flames stabilized on a cylindrical bluff body. Numerical simulations and experiments are conducted for propane, methane and two blends of hydrogen with methane as fuel gases, containing 20% and 40% of hydrogen by volume, respectively. It is found that the Le?>?1 flame blows-off via convection from the base of the flame (without formation of a neck) when the conditions for flame anchoring are not fulfilled. Le?≤?1 flames exhibit a necking phenomenon just before lean blow-off. This necking of the flame front is a result of the local reduction in mass burning rates causing flame merging and quenching of the thin flame tube formed. The structure of these flames at the necking location is found to be similar to tubular flames. It is found that extinction stretch rates for tubular flames closely match values at the neck location of bluff-body flames of corresponding mixtures, suggesting that excessive flame stretch is directly responsible for blow-off of the studied Le?≤?1 flames. After quenching of the neck, the upstream part forms a steady and stable residual flame in the wake of the bluff body while the downstream part is convected away.     

Structure of partially premixed n-heptane–air counterflow flames

To avoid the complexities associated with the droplet/vapor transport and nonuniform evaporation processes, a fundamental investigation of liquid fuel combustion in idealized configurations is very useful. An experimental–computational investigation of prevaporized n-heptane nonpremixed and partially premixed flames established in a counterflow burner is described. There is a general agreement between various facets of our nonpremixed flame measurements and the literature data. The partially premixed flames are characterized by a double flame structure. This becomes more distinct as the strain rate decreases and partial premixing increases, which also increases the separation distance between the two reaction zones. The peak partially premixed flame temperature increases with increasing premixing of the fuel stream. The peak CO₂ and H₂O concentrations are relatively insensitive to partial premixing. The CO and H₂ peak concentrations on the premixed side increase as the fuel-side equivalence ratio decreases. These species are transported to the nonpremixed reaction zone where they oxidize. The C₂ species have peaks in the premixed reaction zone. The concentrations of olefins are ten times larger than those of the corresponding paraffins. The oxidizer is present in partially premixed flames throughout the combustion system and there are no regions characterized by simultaneous high temperature and high fuel concentration. As a result, pyrolysis reactions leading to soot formation are greatly diminished.     

A skeletal mechanism for flame inhibition by trimethylphosphate

On the basis of a multi-step kinetic mechanism for flame inhibition by organophosphorus compounds including more than 200 reactions, a skeletal mechanism for flame inhibition by trimethylphosphate was developed. The mechanism consists of 22 irreversible elementary reactions, involving nine phosphorus-containing species. Selection of the crucial steps was performed by analysing P-element fluxes from species to species and by calculating net reaction rates of phosphorus-involving reactions versus the flames zone. The developed mechanism was validated by comparing the modelling results with the measured and simulated (using the starting initial mechanism) speed and the chemical structure of H₂/O₂, CH₄/O₂ and syngas/air flames doped with trimethylphosphate. The mechanism was shown to satisfactorily predict the speed of H₂/O₂/N₂ flames with various dilution ratios, CH₄/air and syngas/air flames doped with trimethylphosphate. The skeletal mechanism was also shown to satisfactorily predict the spatial variation of H and OH radicals and the final phosphorus-containing products of the inhibitor combustion. Further reduction of the skeletal mechanism without modification of the rate constants recommended in the starting mechanism was shown to result in noticeable disagreement of the flame speed and structure.     

Dilution effects analysis of opposed-jet H2/CO syngas diffusion flames

This paper reported the analysis of dilution effects on the opposed-jet H₂/CO syngas diffusion flames. A computational model, OPPDIF coupled with narrowband radiation calculation, was used to study one-dimensional counterflow syngas diffusion flames with fuel side dilution from CO₂, H₂O and N₂. To distinguish the contributing effects from inert, thermal/diffusion, chemical, and radiation effects, five artificial and chemically inert species XH₂, XCO, XCO₂, XH₂O and XN₂ with the same physical properties as their counterparts were assumed. By comparing the realistic and hypothetical flames, the individual dilution effects on the syngas flames were revealed. Results show, for equal-molar syngas (H₂/CO = 1) at strain rate of 10 s^?1, the maximum flame temperature decreases the most by CO₂ dilution, followed by H₂O and N₂. The inert effect, which reduces the chemical reaction rates by behaving as the inert part of mixtures, drops flame temperature the most. The thermal/diffusion effect of N₂ and the chemical effect of H₂O actually contribute the increase of flame temperature. However, the chemical effect of CO₂ and the radiation effect always decreases flame temperature. For flame extinction by adding diluents, CO₂ dilution favours flame extinction from all contributing effects, while thermal/diffusion effects of H₂O and N₂ extend the flammability. Therefore, extinction dilution percentage is the least for CO₂. The dilution effects on chemical kinetics are also examined. Due to the inert effect, the reaction rate of R84 (OH+H₂ = H+H₂O) is decreasing greatly with increasing dilution percentage while R99 (CO+OH→CO₂+H) is less affected. When the diluents participate chemically, reaction R99 is promoted and R84 is inhibited with H₂O addition, but the trend reverses with CO₂ dilution. Besides, the main chain-branching reaction of R38 (H+O₂→O+OH) is enhanced by the chemical effect of H₂O dilution, but suppressed by CO₂ dilution. Relatively, the influences of thermal/diffusion and radiation effects on the reaction kinetics are then small.     

LES of oxy-fuel jet flames using the Eulerian Stochastic Fields method with differential diffusion

The Eulerian Stochastic Fields (ESF) Monte Carlo method to solve the transported PDF (TPDF) equation is extended to account for differential diffusion effects by incorporating species individual molecular diffusivities. The method has been applied in Large Eddy simulation (LES) to non-piloted oxy-fuel jet flames at different Reynolds numbers experimentally investigated by Sevault et al. [1]. Due to the high H₂ content in the fuel stream and CO₂ in the oxidizer these flames pose new challenges to combustion modeling as the flame structures are different compared to CH₄/air flames. The simulations show very good agreement with the experiments in terms of mixture fraction conditional mean values for temperature and mayor species on the fuel lean side and the reaction zone, deviations on the fuel rich side are discussed. The trend and location of localized extinction is reproduced well in the simulations, as well as differential diffusion effects in the near field. Additionally, it is shown that a neglect of differential diffusion in the combustion model leads to a lifted flame.     

Effect of Soret diffusion on lean hydrogen/air flames at normal and elevated pressure and temperature

The influence of Soret diffusion on lean premixed flames propagating in hydrogen/air mixtures is numerically investigated with a detailed chemical and transport models at normal and elevated pressure and temperature. The Soret diffusion influence on the one-dimensional (1D) flame mass burning rate and two-dimensional (2D) flame propagating characteristics is analysed, revealing a strong dependency on flame stretch rate, pressure and temperature. For 1D flames, at normal pressure and temperature, with an increase of Karlovitz number from 0 to 0.4, the mass burning rate is first reduced and then enhanced by Soret diffusion of H₂ while it is reduced by Soret diffusion of H. The influence of Soret diffusion of H₂ is enhanced by pressure and reduced by temperature. On the contrary, the influence of Soret diffusion of H is reduced by pressure and enhanced by temperature. For 2D flames, at normal pressure and temperature, during the early phase of flame evolution, flames with Soret diffusion display more curved flame cells. Pressure enhances this effect, while temperature reduces it. The influence of Soret diffusion of H₂ on the global consumption speed is enhanced at elevated pressure. The influence of Soret diffusion of H on the global consumption speed is enhanced at elevated temperature. The flame evolution is more affected by Soret diffusion in the early phase of propagation than in the long run due to the local enrichment of H₂ caused by flame curvature effects. The present study provides new insights into the Soret diffusion effect on the characteristics of lean hydrogen/air flames at conditions that are relevant to practical applications, e.g. gas engines and turbines.     

Stability characteristics and flowfields of turbulent non-premixed swirling flames

A simple, yet representative, burner geometry is used for the investigation of highly swirling turbulent unconfined, non-premixed, flames of natural gas. The burner configuration comprises a ceramic faced bluff-body with a central fuel jet. The bluff-body is surrounded by an annulus that delivers a swirling primary flow of air. The entire burner assembly is housed in a wind tunnel providing a secondary co-flowing stream of air. This hybrid bluff-body/swirl burner configuration stabilizes complex turbulent flames not unlike those found in practical combustors, yet is amenable to modelling because of its well-defined boundary conditions. Full stability characteristics including blow-off limits and comprehensive maps of flame shapes are presented for swirling flames of three different fuel mixtures: compressed natural gas (CNG), CNG–air (1:2 by volume) and CNG–H₂ (1:1 by volume).

It is found that with increased fuel flow, flame blow-off mode may change with swirl number, S_g. At low swirl, the flame remains stable at the base but blows off in the neck region further downstream. At higher swirl numbers, the flames peel off completely from the burner's base. Swirling CNG–air flames are distinct in that they only undergo base blow-off. In the low range of swirl number, increasing S_g causes limited improvement in the blow-off limits of the flames investigated and (for a few cases) can even lead to some deterioration over a small intermediate range of S_g. It is only above a certain threshold of swirl that significant improvements in blow-off limits appear. Six flames are selected for further detailed flowfield and composition measurements and these differ in the combination of swirl number, primary axial velocity through the annulus, U_s, and bulk fuel jet velocity, U_j. Only velocity field measurements are presented in this paper. A number of flow features are resolved in these flames, which resemble those already associated with non-reacting swirling flows of equivalent swirl obtained with the present burner configuration. Additionally, asymmetric flowfields inherent to some flames are revealed where the fluidic centreline of the flow (defined in the two-dimensional (U–W velocity pair) velocity field by the ?ω? = 0 tangential velocity contour), meanders strongly on either side of the geometric centreline downstream by about one bluff-body diameter. Flow structures revealed by the velocity data are correlated to flame shapes to yield a better understanding of how the velocity field influences the flames physical characteristics.     

NH3/H2预混旋流火焰稳定性及燃烧极限实验研究

采用叶轮型旋流燃烧器,选取氢气作为燃料添加剂,研究了掺氢比对氨气旋流火焰稳定性的影响,分析了不同旋流数、叶片数、当量比以及预混气总流量条件下,旋流火焰形态变化。测定并分析了不同参数对旋流火焰燃烧极限范围的影响。结果表明,随掺氢比的增大,火焰逐渐由“V”型转化为稳定的“M”型,燃烧反应愈发充分;高旋流数(1.27)或低叶片数(6片)相比低旋流数(0.42)或高叶片数(8片)更有利于旋流火焰的稳定和燃烧的充分进行;相比富燃,贫燃有利于形成稳定的旋流火焰;预混气总流量较大时,火焰高度较高.对于燃烧极限,掺氢比越高,极限范围越大;总流量的变化对贫燃极限影响较小,对富燃极限影响较大;高旋流数(1.27)条件下,燃烧极限范围较大。     

Effects of residence time on the NOx emissions of premixed ammonia-methane-air swirling flames at elevated pressure

In this study, a bespoke single-stage swirl burner was used to experimentally investigate the effects of residence time on emissions from premixed ammonia-methane-air flames. The residence time was altered in two ways: by modifying the combustion chamber's length or by modifying the swirl number. Exhaust emissions of O₂, CO₂, CO, NO, NO₂, and N₂O were measured at an absolute pressure of 2 bar for equivalence ratios between 0.50 and 0.95 and ammonia fractions in the fuel blend between 0 and 100%. Spatial distributions of NO and OH radicals were also imaged using PLIF inside the combustion chamber at different heights above the nozzle. Data shows that increasing residence time can further advance chemical reactions, as evidenced by a reduction in O₂ concentration in the exhaust. Increasing the swirl number reduces emissions of NO, NO₂, and N₂O more efficiently than tripling the chamber's length. However, a decrease in the combustion efficiency may be responsible for a fraction of this NOx reduction when the swirl number is increased for some equivalence ratios. NO emissions are not modified when the chamber's length is increased, which is consistent with the fact that the NO-LIF signal does not decay when the distance from the nozzle increases. Therefore, NO formation is somehow restricted to within the main reaction zone of the swirling flame, that is, the zone whose height does not exceed 60 mm for this burner. Conversely, tripling the chamber's length reduces the concentrations of NO₂ and N₂O. This reduction is not reflected in a measurable increase in NO concentration because NO is present in much larger quantities than NO₂ and N₂O in flames examined here. Consistent with the fact that OH promotes NO formation via fuel-NOx pathways, a positive correlation is found between NO- and OH-LIF intensities.     

Three-dimensional numerical simulations of cellular jet diffusion flames

Recent experimental investigations have demonstrated that the appearance of particular cellular states in circular non-premixed jet flames significantly depends on a number of parameters, including the initial mixture strength, reactant Lewis numbers, and proximity to the extinction limit (Damköhler number). For CO₂-diluted H₂/O₂ jet diffusion flames, these studies have shown that a variety of different cellular patterns or states can form. For given fuel and oxidizer compositions, several preferred states were found to co-exist, and the particular state realized was determined by the initial conditions. To elucidate the dynamics of cellular instabilities, circular non-premixed jet flames are modeled with a combination of three-dimensional numerical simulation and linear stability analysis (LSA). In both formulations, chemistry is described by a single-step, finite-rate reaction, and different reactant Lewis numbers and molecular weights are specified. The three-dimensional numerical simulations show that different cellular flames can be obtained close to extinction and that different states co-exist for the same parameter values. Similar to the experiments, the behavior of the cell structures is sensitive to (numerical) noise. During the transient blow-off process, the flame undergoes transitions to structures with different number of cells, while the flame edge close to the nozzle oscillates in the streamwise direction. For conditions similar to the experiments discussed, the LSA results reveal various cellular instabilities, typically with azimuthal wavenumber m = 1–6. Consistent with previous theoretical work, the propensity for the cellular instabilities is shown to increase with decreasing reactant Lewis number and Damköhler number.     

Control of oscillating combustion and noise based on local flame structure

To control combustion oscillations, the characteristics of an oscillating swirl injection premixed flame have been investigated, and control of oscillating combustion and noise based on local flame structure has been conducted. The r.m.s. value of pressure fluctuations and noise level show significantly large values between = 0.8 and 1.1. The beating of pressure fluctuations is observed for the large oscillating flame conditions in this combustor. Relationship between beating of pressure fluctuations and local flame structure was observed by the simultaneous measurement of CH/OH planar laser induced fluorescence and pressure fluctuations. The local flame structure and beating of pressure fluctuations are related and the most complicated flame is formed in the middle pressure fluctuating region of beating. The beating of pressure fluctuations, which plays important roles in noise generation and nitric oxide emission in this combustor, could be controlled by injecting secondary fuel into the recirculating region of oscillating flames. Injecting secondary fuel prevented lean blowout, and low NO_x combustion was also achieved even for the case of pure methane injection as a secondary fuel. By injecting secondary fuel into the recirculating region near the swirl injector, the flame lifted from the swirl injector and its reaction region became uniform and widespread, hence resulting in low nitric oxide emission. Secondary mixture injection, fuel diluted with air, is not effective for control of combustion oscillations suppression and lean blowout prevention.     

Numerical investigation of flame–vortex interactions in laminar cross-flow non-premixed flames in the presence of bluff bodies

Flame stabilisation in a combustor having vortices generated by flame holding devices constitutes an interesting fundamental problem. The presence of vortices in many practical combustors ranging from industrial burners to high speed propulsion systems induces vortex–flame interactions and complex stabilisation conditions. The scenario becomes more complex if the flame sustains after separating itself from the flame holder. In a recent study [P.K. Shijin, S.S. Sundaram, V. Raghavan, and V. Babu, Numerical investigation of laminar cross-flow non-premixed flames in the presence of a bluff-body, Combust. Theory Model. 18, 2014, pp. 692–710], the authors reported details of the regimes of flame stabilisation of non-premixed laminar flames established in a cross-flow combustor in the presence of a square cylinder. In that, the separated flame has been shown to be three dimensional and highly unsteady. Such separated flames are investigated further in the present study. Flame–vortex interactions in separated methane–air cross flow flames established behind three bluff bodies, namely a square cylinder, an isosceles triangular cylinder and a half V-gutter, have been analysed in detail. The mixing process in the reactive flow has been explained using streamlines of species velocities of CH₄ and O₂. The time histories of z-vorticity, net heat release rate and temperature are analysed to reveal the close relationship between z-vorticity and net heat release rate spectra. Two distinct fluctuating layers are visible in the proper orthogonal decomposition and discrete Fourier transform of OH mass fraction data. The upper fluctuating layer observed in the OH field correlates well with that of temperature. A detailed investigation of the characteristics of OH transport has also been carried out to show the interactions between factors affecting fluid dynamics and chemical kinetics that cause multiple fluctuating layers in the OH.