旋流预混燃烧室燃烧特性及排放特性的数值模拟研究

为了综合考察燃气轮机燃烧室在高稳定性、低排放以及燃料适应性等方面的新要求,基于旋流预混燃烧技术,通过三维数值模拟方法开展了甲烷/空气、丙烷/空气预混燃烧特性及排放特性研究。结果表明：在一定的预混气进气质量流量条件下,当量比增大易引发回火,燃烧温度更高,同时NO_x排放指数增大,增加预混气质量流量,可在一定程度上提高回/熄火极限;当量比固定,增加预混气进气质量流量可避免潜在的回火现象,且NO_x排放指数线性降低;旋流器的旋流数增大能形成强旋流,稳定火焰,降低NO_x排放指数,但过大的旋流强度会引发回火现象;相比于甲烷/空气预混燃烧,丙烷/空气预混燃烧温度偏高,NO_x排放指数较大,但回熄火边界更宽,对应更广阔的稳定燃烧区间。     

Experimental and numerical studies on the premixed syngas swirl flames in a model combustor

Experimental and numerical investigations were performed to study the combustion characteristics of synthesis gas (syngas) under premixed swirling flame mode. Four different type of syngases, ranging from low to high H₂ content were tested and simulated. The global flame structures and post emission results were obtained from experimental work, providing the basis of validation for simulations using flamelet generated manifold (FGM) modelling approach via a commercial computational fluid dynamic software. The FGM method was shown to provide reasonable agreement with experimental result, in particular the post-exhaust emissions and global flame shapes. Subsequently, the FGM method was adopted to model the flame structure and predict the radical species in the reaction zones. Simulation result shows that H₂-enriched syngas has lower peak flame temperature with lesser NO species formed in the reaction zone.     

Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging

Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner structures including the pulverized coal concentrator, flame stabilizing ring and baffle plate create an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to in-flame NO_X reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers.     

Numerical investigation on combustion characteristics of premixed hydrogen/air in a swirl micro combustor with twisted vanes

The combustion characteristics of the swirl micro combustor with twisted vanes (Swirl-MC-TV) and the conventional micro combustor (Conventional-MC) are investigated and compared under different inlet velocities (8–40 m/s), wall materials (quartz, steel, and SiC), and equivalence ratios (0.6–1.4). The results show that the larger area of recirculation zones and the stronger recirculation intensity are the key factors for Swirl-MC-TV to stable combustion. When the inlet velocity is 40 m/s, compared with the Conventional-MC, the wall heat loss of the Swirl-MC-TV is reduced by 15.9%, and the reaction heat and combustion efficiency of the Swirl-MC-TV are increased by 17.5% and 5.9%, respectively. When the wall materials of steel and SiC, combustors have a better preheating effect and higher combustion intensity. When the equivalence ratio is greater than 0.6, the wall heat loss of Swirl-MC-TV is larger but the combustion efficiency and the reaction intensity are still higher than Conventional-MC.     

Large eddy simulation and laser diagnostic studies on a low swirl stratified premixed flame

This paper presents numerical simulations and laser diagnostic experiments of a swirling lean premixed methane/air flame with an aim to compare different Large Eddy Simulations (LES) models for reactive flows. An atmospheric-pressure laboratory swirl burner has been developed wherein lean premixed methane/air is injected in an unconfined low-speed flow of air. The flame is stabilized above the burner rim in a moderate swirl flow, triggering weak vortex breakdown in the downstream direction. Both stereoscopic (3-component) PIV and 2-component PIV are used to investigate the flow. Filtered Rayleigh scattering is used to examine the temperature field in the leading flame front. Acetone-Planar Laser Induced Fluorescence (PLIF) is applied to examine the fuel distribution. The experimental data are used to assess two different LES models; one based on level-set G-equation and flamelet chemistry, and the other based on finite rate chemistry with reduced kinetics. The two LES models treat the chemistry differently, which results in different predictions of the flame dynamic behavior and statistics. Yet, great similarity of flame structures was predicted by both models. The LES and experimental data reveal several intrinsic features of the low swirl flame such as the W-shape at the leading front, the highly wrinkled fronts in the shear layers, and the existence of extinction holes in the trailing edge of the flame. The effect of combustion models, the numerical solvers and boundary conditions on the flame and flow predictions was systematically examined.     

Experimental investigation of the stability and emission characteristics of premixed formic acid-methane-air flames in a swirl combustor

Formic acid (FA) is a potential hydrogen energy carrier and low-carbon fuel by reversing the decomposition products, CO₂ and H_2, back to restore FA without additional carbon release. However, FA-air mixtures feature high ignition energy and low flame speed; hence stabilizing FA-air flames in combustion devices is challenging. This study experimentally investigates the flame stability and emission of swirl flames fueled with pre-vaporized formic acid-methane blends over a wide range of formic acid fuel fractions. Results show that by using a swirl combustor, the premixed formic acid-methane-air flames could be stabilized over a wide range of FA fuel fractions, Reynolds numbers, and swirl numbers. The addition of formic acid increases the equivalence ratios at which the flashback and lean blowout occur. When Reynolds number increases, the equivalence ratio at the flashback limit increases, but that decreases at the lean blowout limit. Increasing the swirl number has a non-monotonic effect on stability limits variation because increasing the swirl number changes the axial velocity on the centerline of the burner throat non-monotonically. In addition, emission characteristics were investigated using a gas analyzer. The CO and NO concentrations were below 20 ppm for all tested conditions, which is comparable to that seen with traditional hydrocarbon fuels, which is in favor of future practical applications with formic acid.     

Development and characterization of a low-NOx partially premixed hydrogen burner using numerical simulation and flame diagnostics

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NO_x emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NO_x emissions for equivalence ratios below 0.85. The experimental investigations also show that NO_x emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.     

A combined computational and experimental characterization of lean premixed turbulent low swirl laboratory flames: I. Methane flames

This paper is the first in a series that presents a combined computational and experimental study to investigate and characterize the structure of premixed turbulent low swirl laboratory flames. The simulations discussed here are based on an adaptive solution of the low Mach number equations for turbulent reacting flow, and incorporate detailed models for transport and thermo-chemistry. Experimental diagnostics of the laboratory flame include PIV and OH-PLIF imaging, and are used to quantify the flow field, mean flame location, and local flame wrinkling characteristics. We present a framework for relating the simulation results to the flame measurements, and then use the simulation data to further probe the time-dependent, 3D structure of the flames as they interact with the turbulent flow. The present study is limited to lean methane–air flames over a range of flow conditions, and demonstrates that in the regime studied, local flame profiles are structurally very similar to the flat, unstrained steady (“laminar”) flame. The analysis here will serve as a framework for discussing a broader set of premixed flames in this same configuration. Papers II and III will discuss corresponding analysis for pure hydrogen–air and hydrogen–methane mixed fuels, respectively.     

Experimental investigation of stabilization and emission characteristics of ammonia/air premixed flames in a swirl combustor

Ammonia is a possible candidate for use as a hydrogen energy carrier as well as a carbon-free fuel. In this study, flame stability and emission characteristics of swirl stabilized ammonia/air premixed flames were experimentally investigated. Results showed that ammonia/air premixed flame could be stabilized for various equivalence ratios and inlet flow velocity conditions in a swirl burner without any additives to enhance the reaction of ammonia even though the laminar burning velocity of ammonia is very slow. The lean and rich blowoff limits were found to be close to the flammability limits of the ammonia flame. In addition, emission characteristics were investigated using an FTIR gas analyzer. The NO concentration decreased and ammonia concentration increased under rich conditions. Moreover, it was found that there is an equivalence ratio in rich condition in which NO and ammonia emission are in the same order.     

From Large-Eddy Simulation to Direct Numerical Simulation of a lean premixed swirl flame: Filtered laminar flame-PDF modeling

Large-Eddy Simulations (LES) and Direct Numerical Simulation (DNS) are applied to the analysis of a swirl burner operated with a lean methane–air mixture and experimentally studied by Meier et al. [19]. LES is performed for various mesh refinements, to study unsteady and coherent large-scale behavior and to validate the simulation tool from measurements, while DNS enables to gain insight into the flame structure and dynamics. The DNS features a 2.6 billion cells unstructured-mesh and a resolution of less than 100 microns, which is sufficient to capture all the turbulent scales and the major species of the flame brush; the unresolved species are taken into account thanks to a tabulated chemistry approach. In a second part of the paper, the DNS is filtered at several filter widths to estimate the prediction capabilities of modeling based on premixed flamelet and presumed probability density functions. The similarities and differences between spatially-filtered laminar and turbulent flames are discussed and a new sub-grid scale closure for premixed turbulent combustion is proposed, which preserves spectral properties of sub-filter flame length scales. All these simulations are performed with a solver specifically tailored for large-scale computations on massively parallel machines.     

Effect of high hydrogen enrichment on the outer-shear-layer flame of confined lean premixed CH4/H2/air swirl flames

In this study, we investigated the H₂-induced transition of confined swirl flames from the “V” to “M” shape. H₂-enriched lean premixed CH₄/H₂/air flames with H₂ fractions up to 80% were conducted. The flame structure was obtained with Planar Laser-Induced Fluorescence (PLIF) of the OH radical. Flow fields were measured with Particle Image Velocimetry (PIV). It was observed that the flame tip in the outer shear layer gradually propagated upstream and finally anchored to the injector with the hydrogen fractions increase, yielding the transition from the “V” to “M” flame. We examined the flame structures and the flame flow dynamics during the transition. The shape transition was directly related to the evolution of the corner flame along the outer shear layer. With H₂ addition, the outer recirculation zone first appeared downstream where the corner flame started to propagate upstream; then, the recirculation zone expanded upward to form a stable “M” flame gradually. The flow straining was observed to influence the stabilization of the outer shear layer flame significantly. This study can be useful for the understanding of recirculation-stabilized swirling flames with strong confinement. The flame structure and the flow characteristics of flames with a high H₂ content are also valuable for model validation.     

Combustion characteristics of a slotted swirl combustor: An experimental test and numerical validation

Combustion performance of a non-premixed combustor with a slotted swirler was experimentally and numerically investigated. The velocity and temperature profiles in the downstream of the swirler were measured in both cold-flow and combustion experiments. Concentration of CO₂ emission was measured by utilizing a flue gas analyzer and the combustion efficiency was accordingly determined. The same experiments with a non-slotted swirler were repeated for comparison. The results show that, compared to the non-slotted swirler, the slotted swirler results in a larger recirculation zone, in which the flow velocity is smaller and the temperature is higher, implying the enhanced combustion performance. The combustion efficiency of the slotted swirler based on the measured CO₂ emission is 75%, which is higher than 60% of the non-slotted swirler. These results suggest that the slotted swirler holds potential to enhance the combustion performance of gas turbine combustor and merits further, comprehensive studies.     

Modelling and simulation of lean premixed turbulent methane/hydrogen/air flames with an effective Lewis number approach

Recent work on reaction modelling of turbulent lean premixed combustion has shown a significant influence of the Lewis number even at high turbulence intensities, if different fuels and varied pressure is regarded. This was unexpected, as the Lewis number is based on molecular transport quantities (ratio of molecular thermal diffusivity to mass diffusivity), while highly turbulent flames are thought to be dominated from turbulent mixing and not from molecular transport. A simple physical picture allows an explanation, assuming that essentially the leading part of the wrinkled flame front determines the flame propagation and the average reaction rate, while the rear part of the flame is of reduced importance here (determining possibly the burnout process and the flame brush thickness but not the flame propagation). Following this argumentation, mostly positively curved flame elements determine the flame propagation and the average reaction rate, where the influence of the preferential molecular diffusion and the Lewis number can easily seen to be important. Additionally, an extension of this picture allows a simple derivation of an effective Lewis number relation for lean hydrogen/methane mixtures. The applicability and the limit of this concept is investigated for two sets of flames: turbulent pressurized Bunsen flames, where hydrogen content and pressure is varied (from CNRS Orléans), and highly turbulent pressurized dump combustor flames where the hydrogen content is varied (from PSI Baden). For RANS simulations, comparison of flame length data between experiment and an effective Lewis number model shows a very good agreement for all these flames with hydrogen content of the fuel up to 20 vol.%, and even rather good agreement for 30% and 40% hydrogen.     

Combustion characteristics and radiation performance of premixed hydrogen/air combustion in a mesoscale divergent porous media combustor

To improve flammability and radiation efficiency, a divergent porous media combustor is proposed and numerically studied. The local thermal non-equilibrium model is used to consider the temperature difference between gas and solid matrix. Effects of equivalence ratio, the wall thermal conductivity, solid matrix thermal conductivity, and divergent ratio on combustion characteristics, radiation efficiency, and flammability limits are studied. The results show that the divergent channel extends the blowout limit by 186% and obtains a maximum radiation efficiency of 29.3%, increased by 70% compared with the straight channel. A smaller wall thermal conductivity is recommended considering the flammability range and radiation efficiency. A careful choice of solid matrix thermal conductivity and the divergent ratio is suggested to balance their opposing effects on the radiation efficiency and the flammability.     

Numerical investigation of premixed hydrogen/air combustion at lean to ultra-lean conditions and catalytic approach to enhance stability

Premixed combustion of hydrogen/air over a platinum (Pt) catalyst is numerically investigated in a planar channel burner with the aim of stabilising the flame at lean to ultra-lean conditions. A steady laminar species transport model is examined in conjunction with elementary heterogeneous and homogeneous chemical reaction schemes and validated against experimental results. A stability map is obtained in a non-catalytic burner for the equivalence ratios (φ) of 0.15–0.20, which serves as the basis for the catalytic flame analysis. Over the Reynolds numbers (Re) investigated in the non-catalytic burner, no flame is observed for φ ≤ 0.16, and flame extinction occurs at Re < 571 and Re < 381 for φ = 0.18 and 0.20, respectively. Moreover, a significant amount of unburned H₂ exits the burner in all cases. With the Pt catalyst coated on the walls, complete H₂ combustion is attained for 0.10 ≤ φ ≤ 0.20 where the contribution of gas phase (homogeneous) reaction increases with Re. Furthermore, radiation on the wall and at the inlet affects the combustion kinetics and flame temperature. Finally, NO_x emission is investigated under the same conditions and found to increase with equivalence ratio but has a negligible effect with the inflow Reynolds number.     

Laminar burning velocities and flame stability analysis of hydrogen/air premixed flames at low pressure

An experimental and numerical study on laminar burning velocities of hydrogen/air flames was performed at low pressure, room temperature, and different equivalence ratios. Flames were generated using a small contoured slot-type nozzle burner (5 mm × 13.8  mm). Measurements of laminar burning velocity were conducted using the angle method combined with Schlieren photography. Numerical calculations were also conducted using existing detailed reaction mechanisms and transport properties. Additionally, an analysis of the intrinsic flame instabilities of hydrogen/air flames at low pressure was performed. Results show that the behavior of the laminar burning velocity is not regular when decreasing pressure and that it depends on the equivalence ratio range. The behavior of the laminar burning velocity with decreasing pressure can be reasonably predicted using existing reaction mechanisms; however changes in the magnitude of the laminar burning velocity are underestimated. Finally, it has been found experimentally and proved analytically that the intrinsic flame instabilities are reduced when decreasing the pressure at sub-atmospheric conditions.     

Large eddy simulations and rotational CARS/PIV/PLIF measurements of a lean premixed low swirl stabilized flame

This paper reports on numerical and experimental studies of a lean premixed low swirl stabilized methane/air flame. The burner is made up of a central perforated plate and an annular swirler. A premixed methane/air mixture at an equivalence ratio of 0.62 is injected to an ambient co-flow of air through the burner under atmospheric pressure and room temperature condition with a Reynolds number of 30,000. Stereoscopic Particle Image Velocimetry (PIV) and simultaneous OH/acetone Planar Laser Induced Fluorescence (PLIF) are used to characterize the flame front and the turbulence field downstream of the burner. The flame is stabilized in the low speed central region and in the inner shear-layer vortices, where ambient air dilution to the flame is found to eventually quench the reactions downstream. Rotational Coherent Anti-Stokes Raman Spectroscopy (RCARS) measurements are carried out to characterize the temperature field and the relative oxygen mole fraction field, which enables quantification of the air dilution to the flame. The experimental data provides a challenging test case for numerical simulation models owing to the stratification of the mixture and quenching of the flame. Large eddy simulations are carried out using a three-scalar level-set G-equation flamelet model, which is shown to capture the basic flame characteristics and quenching at the trailing edge of the flame.     

Experimental investigation of the two-phase theory in a fluidized-bed combustor

Though the two-phase theory of fluidization is well-accepted, no direct experimental measurements of the different gas concentrations predicted to occur in bubble and particulate phases could be found in the literature. For the first time, theoretical predictions of these different gas concentrations have been validated experimentally, using a combined oxygen/bubble probe. Based on the two-phase theory, a mathematical model was developed for the combustion of a batch of char particles in a fluidized-bed combustor. The experimental oxygen concentration in the particulate phase as a function of time was well predicted by the model. Slight discrepancies for the bubble phase values were eliminated when low-oxygen-concentration bubbles were excluded from the data, attributed to some char combustion occurring in bubbles being contrary to the model assumption. The temperature difference between char and bed particles (ΔT) was the only adjustable parameter in the model. A value of 20°C fitted the burnoff times measured by visual observation of the top of the bed, for both 5 and 10 g char batch masses. Model predictions of the oxygen concentrations were not sensitive to ΔT during the first half of burnoff, when mass transfer controlled the combustion rate, so the mass transfer processes were predicted correctly by the model effectively with no adjustable parameter. The ΔT value of 20°C was significantly lower than experimental measurements of maximum burning char particle temperatures, reported to be 70°C for the small-diameter bed particles used in this work. The discrepancy was attributed to two factors: (i) the decrease in char particle temperature towards the end of the burnoff, when kinetics significantly affected the combustion rate; and (ii) a lower char particle temperature in the particulate phase than in the bubble phase, with experimental char particle temperature measurements biased towards the higher bubble phase values. It was inferred: (i) that the maximum values of ΔT measured experimentally are too high for calculation of the char particle combustion rate during the kinetic-controlled latter stage of burnoff and (ii) that reported values of the heat transfer coefficient from burning char particles to the particulate phase deduced from these particle temperature measurements may have been underestimated.     

Enhancement of thermal performance by converging-diverging channel in a micro tube combustor fueled by premixed hydrogen/air

As the core component of the micro thermophotovoltaic (MTPV) system, the micro combustor with a high and uniform wall temperature distribution is beneficial to improve the energy conversion efficiency. In this paper, a micro tube combustor with converging-diverging channel is proposed and the thermal performance is numerically investigated, compared with that of the micro combustor with cylindrical channel. The effects of inlet velocity of H₂/air mixture, dimensionless position and diameter of throat, and solid material on the thermal performance are widely analyzed. Results show that the outer wall temperature and emitter efficiency of the micro combustor with converging-diverging channel are higher than that of the micro combustor with cylindrical channel, and the converging-diverging channel has more uniform temperature distribution. The converging-diverging micro combustor with dimensionless throat position l = 0.375 and dimensionless throat diameter β = 0.4 is more suitable for the application of MTPV system. When H₂/air inlet velocity is 11 m/s and H₂/air equivalence ratio is 1.0, the mean wall temperature is increased by 82.39 K and the emitter efficiency is increased by 6.59%, while the normalized temperature standard deviation is reduced by 65.85%. Additionally, the use of SiC as wall material can improve the thermal performance of the micro combustor. It is worth noting that this work will offer us significant guidelines for the optimized work of micro tube combustor.     

Heat release rate variations in high hydrogen content premixed syngas flames at elevated pressures: Effect of equivalence ratio

Three-dimensional direct numerical simulations with detailed chemistry were performed to investigate the effect of equivalence ratio on spatial variations of the heat release rate and flame markers of hydrogen/carbon monoxide syngas expanding spherical premixed flames under turbulent conditions at elevated pressures. The flame structures and the heat release rate were analysed and compared between fuel-lean, stoichiometric and fuel-rich centrally ignited spherical flames. The equivalence ratio changes the balance among thermo-diffusive effects, Darrieus–Landau instability and turbulence, leading to different flame dynamics and the heat release rate distribution, despite exhibiting similar cellular and wrinkling flames. The Darrieus–Landau instability is relatively insensitive to the equivalence ratio while the thermo-diffusive process is strongly affected by the equivalence ratio. As the thermo-diffusive effect increases as the equivalence ratio decreases, the fuel-lean flame is more unstable than the fuel-rich flame with the stoichiometric flame in between, under the joint effects of the thermo-diffusive instability and the Darrieus–Landau instability. The local heat release rate and curvature display a positive correlation for the lean flame, no correlation for the stoichiometric flame, and negative correlation for the rich flame. Furthermore, for the fuel-lean flame, the low and high heat release rate values are found in the negative and positive curvature zones, respectively, while for the fuel-rich flame, the opposite trends are found. It is found that heat release rate markers based on species concentrations vary strongly with changing equivalence ratio. The results suggest that the HCO, HO₂ concentrations and product of OH and CH₂O concentrations show good correlation with the local heat release rate for H₂/CO premixed syngas-air stoichiometric flame under turbulent conditions at elevated pressures.