甲烷/空气典型混合模式在约束空间内的燃烧特性研究

燃料质量浓度分布在一定程度上影响混合气体的燃烧效率,能使燃气充分混合的同轴射流、旋片同轴、轴切结合、切向旋流等典型混合模式在航空发动机、燃气轮机及火箭发动机等先进燃烧技术应用中较为常见。因此,设计了甲烷/空气部分预混的燃烧实验装置,较为系统地实验研究了旋流数和轴向流速对混合气体在约束空间燃烧室内燃烧特性的影响。结果表明：对于有中心射流的混合结构,燃气轴向流速较低时产生黄色火焰,增大轴向流速,黄色火焰转为蓝色湍流火焰,且温度分布趋于均匀;纯切向旋流燃烧器的掺混效果较好,受燃气轴向流速的影响小,火焰结构稳定,均为蓝色火焰,温度轴/径向分布均匀且趋势一致,同当量比下燃烧产物中的污染物体积分数最小。     

Thermal characteristics of a premixed impinging circular laminar-flame jet with induced swirl

A swirling flow has been induced in a premixed gas-fired impinging circular flame jet by adding two tangential air flows to the main axial air/fuel flow. The flame jet system was considered to be small-scale and operated under low-pressure, laminar flow conditions. The effects of Reynolds number of the air/butane mixture and nozzle-to-plate distance on the heating performance of the flame were studied and compared with the heat-flux distributions on an impingement plate under different operating conditions. The whole investigation was conducted under the stoichiometric air/fuel condition (i.e., equivalence ratio, Φ = 1) with the Reynolds number being varied from 800 to 1700, and nozzle-to-plate distance being selected between 1.5 and 4.0. The introduction of swirl to small-scale, low-pressure, laminar premixed gas-fired impinging circular flame jets is the method for enhancing their thermal performances. The heat-flux distribution on the impingement plate was more uniform and the flame temperatures essentially higher when compared with a similar flame jet system without induced swirl.     

Heat transfer from an impinging premixed butane/air slot flame jet

Experiments were performed to study the heat transfer characteristics of a premixed butane/air slot flame jet impinging normally on a horizontal rectangular plate. The effects of Reynolds number and the nozzle-to-plate distance on heat transfer were examined. The Reynolds number varied from 800 to 1700, while the nozzle-to-plate distance ranged from 2d_e to 12d_e. Comparisons were made between the heat transfer characteristics of slot jets and circular jets under the same experimental conditions. It was found that the slot flame jet produces more uniform heat flux profile and larger averaged heat fluxes than the circular flame jet.     

Multiscalar imaging in partially premixed jet flames with argon dilution

Simultaneous imaging of depolarized and polarized Rayleigh scattering combined with OH-LIF and two-photon CO-LIF provides two-dimensional measurements of mixture fraction, temperature, scalar dissipation rate, and the forward reaction rate of the reaction CO+OH=CO₂+H in turbulent partially premixed flames. The concept of the three-scalar technique for determining the mixture fraction using CO-LIF with depolarized and polarized Rayleigh signals was previously demonstrated in a partially premixed CH₄/air jet flame [J.H. Frank, S.A. Kaiser, M.B. Long, Proc. Combust. Inst. 29 (2002) 2687-2694]. In the experiments presented here, we consider a similar jet flame with a fuel-stream mixture that is better suited for the diagnostic technique. The contrast between the depolarized and the polarized Rayleigh signals in the fuel and air streams is improved by partially premixing with an argon/oxygen mixture that has the same oxygen content as air. The substitution of argon, which has a zero depolarization ratio, for the nitrogen in air decreases the depolarized Rayleigh signal in the fuel stream and thereby increases the contrast between the depolarized and the polarized Rayleigh signals. We present a collection of instantaneous 2-D measurements and examine conditional means of temperature, scalar dissipation, and reaction rates for two downstream locations. The emphasis is on the determination of the scalar dissipation rate from the mixture-fraction images. The axial and radial contributions to scalar dissipation are measured. The effects of noise on the scalar dissipation measurements are determined in a laminar flame, and a method for subtracting the noise contribution to the scalar dissipation rates is demonstrated.     

Experimental and computational study on partially premixed flames in a centerbody burner

The centerbody burner was designed with the objective of understanding the coupled processes of soot formation, growth, and burnout. Fuel that issues from the center of the burner establishes two flame zones – one associated with the recirculation zone (RZ) and the other, with the trailing jet. The sooting characteristics in these two flame zones can be quite different because of variations in residence time and transport of reactants and products. Calculations performed for this burner operating under a partially premixed fuel jet suggested that soot in the RZ decreases and that soot in the trailing jet flame increases with the amount of premixing. An experimental and numerical study is performed to aid the understanding of these differences. A time-dependent, axisymmetric, detailed-chemistry computational-fluid-dynamics (CFD) model known as Unsteady Ignition and Combustion using ReactioNs (UNICORN) is used for simulating flames under different equivalence-ratio conditions. Combustion and PAH formation are modeled using the Wang–Frenklach (99 species and 1066 reactions) mechanism, and soot is simulated using a two-equation model of Lindstedt. A Lagrangian-based particle-tracking model is used for understanding the evolution of soot-like particles. Flame and recirculation-zone structures and soot in the experiments are identified using direct photographs taken with and without Mie scattering from soot particles as well as laser-induced-incandescence (LII) measurements. Calculations predict the structures of the partially premixed centerbody flames for various equivalence ratios reasonably well. Experiments confirm the predicted soot suppression in the RZs and enhancement of soot in the trailing jet flame when air is added to the fuel jet. It is found that flame movement in the RZ increases soot-particle burnout and, thereby, reduces the amount of soot within the RZ. As the flame moves closer to the fuel jet, more soot becomes entrained into the inner vortex. Motion of soot-like particles explained the spiral rings observed in the experiment. Increased particle burnout with partial premixing leads to shrinkage of soot spirals.     

体积力场对预混火焰面形状的影响

介绍了一种在燃烧场中产生大于1g体积力的实验系统,分析了影响体积力场的因素．利用实时摄像的方法,研究了在燃烧过程中体积力对预混火焰面形状的影响．结果表明,高温烟气在体积力场中的浮力效应,使火焰面向与体积力相反的方向弯曲、偏转．体积力增加将导致火焰面变形程度增加,并导致燃烧过程不稳定,以致发生熄火．气流的射流角不同,体积力对火焰面的影响效果不同,火焰面会发生弯曲、拉伸或压缩．当射流角为负值时,火焰容易发生吹熄．     

Stabilization of air coflowed ammonia jet flame at elevated ambient temperatures

Ammonia is a promising carbon-free fuel, while, understanding of ammonia jet flame is still in lack. In this work, a novel facility was applied and air coflowed ammonia jet flames were achieved in an elevated ambient temperature range, 723–923K. Stabilization regimes and limits were investigated. Stable lifted flame with a classical triple structure was observed, and critical aerodynamic parameters were measured at three specific regimes, liftoff, reattachment and blowoff. Attached flame can only be retained under laminar conditions with flow Reynolds number <150. A linear correlation between velocities of fuel jet and coflow under critical conditions was uncovered, which is different from the literature research on methane flames. Effects of partially premixing and N₂ dilution were considered. Partially premixing was found harmful to stabilization at 823K, while this influence becomes unclear at 923K. Differently, a linearly adverse effect was observed under both N₂-diluted jet and coflow conditions at different temperatures.     

The structure of partially premixed methane flames in high-intensity turbulent flows

Direct numerical simulations (DNS) are conducted to study the structure of partially premixed and non-premixed methane flames in high-intensity two-dimensional isotropic turbulent flows. The results obtained via “flame normal analysis” show local extinction and reignition for both non-premixed and partially premixed flames. Dynamical analysis of the flame with a Lagrangian method indicates that the time integrated strain rate characterizes the finite-rate chemistry effects and the flame extinction better than the strain rate. It is observed that the flame behavior is affected by the “pressure-dilatation” and “viscous-dissipation” in addition to strain rate. Consistent with previous studies, high vorticity values are detected close to the reaction zone, where the vorticity generation by the “baroclinic torque” was found to be significant. The influences of (initial) Reynolds and Damköhler numbers, and various air-fuel premixing levels on flame and turbulence variables are also studied. It is observed that the flame extinction occurs similarly in flames with different fuel-air premixing. Our simulations also indicate that the CO emission increases as the partial premixing of the fuel with air increases. Higher values of the temperature, the OH mass fraction and the CO mass fraction are observed within the flame zone at higher Reynolds numbers.     

Parametric and statistical investigation of the behavior of a lifted flame over a turbulent free-jet structure

Partially premixed combustion is involved in many practical applications, due to partial premixing of combustible and oxidant gases before ignition, or due to local extinctions, which lead to mixing of reactants and burned gases. To investigate some features of flames in stratified flows, the stabilization processes of lifted turbulent jet flames are studied. This work offers a large database of liftoff locations of flames stabilized on turbulence-free jets for different fuels and nozzle diameters studied over their flame stability domains. Methane, propane, and ethylene flames are investigated for nozzle diameters of 2, 3, 4, and 5 mm. Blowout velocities are measured and compared with an approach based on large-scale structures of the jet. The axial and radial locations of the flame base are measured by planar laser-induced fluorescence (PLIF) of the OH radical through high sampling (at least 5000 points). From this large database the average locations of the flame base are analyzed for the fuels investigated. The pdfs exhibit an evolution of their shapes according to the region of the turbulent jet where the flame stabilizes (potential core, transition to turbulence, or fully developed turbulence regions). This dependence is probably due to the interaction of the flame with the jet structures. This is confirmed by the comparison between the amplitude of the height fluctuations and the local size of the large-scale structures deduced from particle image velocimetry measurements and self-similarity laws for velocity. The results show the flame can be carried over a distance equal to the local diameter of the jet within the region of fully developed turbulence for propane and ethylene, and over a slightly larger distance for methane.     

Predicting the main geometrical features of horizontal rectangular source fuel jet fires

The main geometrical features of horizontal jet fire with rectangular source fuel have seldom been revealed in the past, especially the rectangular orifice with same area but different aspect ratios. In order to better understand the rectangular jet fire, a set of numerical simulations were carried out by rectangular source fuel with same rectangular orifice area S (4 cm²) but different aspect ratios (orifice length to orifice width: L/W = 1, 2, 4) to investigate the flame shape, flame length and flame width. The simulated flame lengths and flame widths were compared with previous experimental data and calculated values using the Thornton model. The non-dimensional flame length and flame width were defined, in which the flame geometrical features were found in relation to the orifice aspect ratio and fuel jet velocity. Results show that the flame length and flame width increases with fuel jet velocity, while the flame length decreases with aspect ratio n for same orifice area, but the flame width increases simultaneously. The simulated data agree well with previous experimental data, but the predictions by Thornton model are larger than simulated and previous experimental values. The modified Thornton model is proposed considering both orifice shape and aspect ratio to apply to rectangular jet fire.     

Control of flames by tangential jet actuators in oxy-fuel burners

The active control of oxy-fuel flames from burners with separated jets is investigated. The control system consists of four small jet actuators, placed tangential to the exit of the main jets to generate a swirling flow. These actuators are able to modify the flow structure and to act on mixing between the reactants and consequently on the flame behavior. The burner (25 kW) is composed of separated jets, one jet of natural gas and one or two jets of pure oxygen. Experiments are conducted with three burner configurations, according to the number of jets, the jet exit velocities, and the separation distance between the jets. OH chemiluminescence measurements, particle image velocimetry, and measurements of NO_x emissions are used to characterize the flow and the flame structure. Results show that the small jet actuators have a significant influence on the behavior of jets and the flame characteristics, particularly in the stabilization zone. It is shown that the control leads to a decrease in lift-off heights and to better stability of the flame. The use of jet actuators induces high jet spreading and an increase in turbulence intensity, which improves the mixing between the reactants and the surrounding fluid. Pollutant measurements show important results in terms of NO_x reductions (up to 60%), in particular for low swirl intensity. The burner parameters, such as the number of jets and the spacing between the jets, also impact the flame behavior and NO_x formation.     

低排放燃烧室燃料空气预混均匀性研究

为了解决燃料与空气均匀预混的问题,以气态燃料-低排放塔式同轴分级燃烧室为研究对象,采用数值模拟与实验设计数理统计方法相结合,研究了主燃级叶片角、叶片数、燃料速度和单面孔数对燃料和空气预混特性的影响,筛选出对燃料空气预混均匀性影响的显著项,并拟合得出均匀性的预测公式。结果表明：单面孔数及其平方项对预混均匀性影响最大;燃料速度与叶片角、燃料速度与单面孔数的交互效应其次;叶片数、叶片角、燃料速度及其平方项和交互项对均匀性的影响最小。     

Plasma assisted flame ignition of supersonic flows over a flat wall

A nanosecond pulsed plasma discharge located between two fuel jets is used to ignite and hold jet flames in supersonic crossflows, without the use of additional devices (e.g., cavities or backsteps) for flame holding. The fuel injection nozzles and discharge electrodes are mounted flush with the surface of the flat wall adjacent to the freestream flow. The nonequilibrium plasma is produced by repetitive pulses of 15 kV peak voltage, 10 ns pulse width and 50 kHz repetition rate. Sonic or subsonic fuel jets (hydrogen and ethylene) are injected into a pure oxygen freestream of Mach numbers Ma = 1.7-2.4. The shockwave/flow structures induced by the fuel jets and the OH radical distribution resulting from combustion are characterized by Schlieren photography and planar laser induced fluorescence imaging, respectively. A configuration combining an upstream subsonic oblique jet and a downstream sonic transverse jet is shown to provide adequate flow conditions for jet flame ignition assisted by the plasma discharge. The experimental results are interpreted using a simple model by which the pulsed plasma serves as a source of reactive radicals added to a flammable gas mixture.     

Investigation on combustion mode and heat release in a model scramjet engine affected by shocks

A model scramjet engine in which the 1.0 Ma hydrogen jet mixes and reacts with the 2.0 Ma surrounding airstream is investigated using large eddy simulation. The flame structure is analyzed with a focus on the relationship between premixed/diffusion combustion mode and heat release in the supersonic reacting flow. The flame filter is used to evaluate the contributions to heat release rate by different combustion modes qualitatively and quantitatively. Results show that the heat is released from a combination of premixed combustion mode and diffusion combustion mode even when the fuel and airstream are injected into the combustor separately. Local mode-transition occurs as the supersonic jet flame propagates and interacts with shocks. The diffusion combustion mode dominates during the ignition stage and the premixed combustion becomes dominant during the intensive combustion region. When the shock wave impinges on the flame, the combustion area decreases a little due to the compression effects of the shock. However, the heat release rate is significantly improved in the interaction region since the shock could increase the air entrainment rate by directing the airflow toward the fuel jet and enhance the mixing rate by inducing vorticity due to baroclinic effects, which is good for flame stabilization in the supersonic flow. For the present case, 33.3% of the heat is released by diffusion combustion and 66.7% of the heat is released by premixed combustion. Thus the premixed combustion mode is dominant in terms of its contributions to heat release in the model scramjet engine.     

NUMERICAL INVESTIGATIONS OF HEAT TRANSFER OVER A MOVING SURFACE DUE TO IMPINGING KNIFE-JETS

Turbulent flow field and heat transfer from an array of impinging horizontal knife jets on a moving surface have been investigated using large eddy simulation (LES) with a dynamic subgrid stress model. The surface velocity directed perpendicular to the jet plane is varied up to two times the jet velocity at the nozzle exit. Performance of a horizontal knife jet with an exit angle of 60° is compared with the standard axial jet. It has been observed that increasing surface motion reduces heat transfer for both types of jets. However, the amount of heat transfer from the knife jets is more than that from the axial jets when the surface velocity is within the order of half the jet velocity at the nozzle exit. For further increase in surface velocity, heat transfer from the knife jets is, however, less than that in the case of axial jets if the Reynolds number (Re) is low. For higher Re and higher surface velocity, the heat transfer from either type of jets is of comparable magnitude.     

零碳燃料对燃气锅炉燃烧过程调控作用

利用小型化模拟炉膛开展了零碳燃料氢气对燃气锅炉燃烧过程调控作用实验研究,研究了掺氢比对炉膛内部预混火焰宏观形态、炉膛温度均匀性、炉膛污染物排放规律的影响,并总结了CO及NO_x的排放规律。实验结果表明：随着预混当量比增加,纯甲烷火焰长度逐渐缩短;对于20%掺氢火焰,随着预混程度的提高,火焰长度降低明显;不同火焰条件下,炉膛温度只由燃烧功率控制;改变燃烧条件时,处于壁面附近位置的温度变化较为平稳,而靠近火焰处温度变化较大;天然气中掺入氢气,燃烧时可以有效降低未燃CO排放;在相同预混程度下,全局当量比减小导致未燃空气增加,热量被稀释,火焰温度降低,热力型NO_x的生成降低;随着掺氢比的增加,燃烧时火焰温度升高,导致热力型NO_x排放增加。     

Heat transfer and wall pressure characteristics of a twin premixed butane/air flame jets

Experimental studies were carried out to investigate the flame shape and the heat transfer and wall pressure characteristics of a pair of laminar premixed butane/air flame jets impinging vertically upon a horizontal water-cooled flat plate at jet Reynolds numbers of 800, 1000 and 1200, respectively. Equivalence ratio of the butane/air mixture was maintained constantly at unity. The flame shape, the pressure distribution on the impingement plate and the heat transfer from the flame to the plate were greatly influenced by the interference occurred between the two flame jets. This interference caused a sharp pressure peak at the between-jet midpoint and the positive pressures at the between-jet area, which led to the separation of the wall jet from the impingement plate after collision. Such interference became more significant when the non-dimensional jet-to-jet spacing (S/d) and the nozzle-to-plate distance (H/d) were reduced. Heat transfer in the interaction zone between the jets was at the lowest rate due to this interference at the smallest S/d ratio of 2.6, resulting from the separation of the high-temperature inner reaction zone of the flame from the impingement plate. On the other hand, the interference enhanced the heat transfer in the interaction zone between the jets when the S/d ratio was greater than 5, by enhancing the heat transfer coefficient. The average heat flux of the impingement plate was found to increase significantly with the increasing H/d ratio until H/d=6. The present study provided detailed information on flame shape and the heat transfer and wall pressure characteristics of a twin laminar pre-mixed impinging circular flame jets, which has rarely been reported in previous studies.     

Effect of free stream angle on mixing performance of hydrogen multi-jets in supersonic combustion chamber

The fuel mixing process within the combustion chamber is a critical procedure for advance of scramjet engine. In current study, the influence of free stream angle on the fuel mixing process of multi jets was thoroughly investigated. This research attempts to applied computational technique to disclose the structure of multi-fuel jets when the supersonic air stream is not normal to the jet direction. The effect of both positive and negative supersonic free stream on the diffusion and penetration of multi-hydrogen jets is fully described. The attention of this research is the flow structure of the multi jets and their interaction in the presence of different free stream angle. Our results indicate that the positive free stream angle expands the mixing zone in the downstream while the negative angle of free stream limited jet distribution inside the combustor. Our results show that mixing efficiency increase approximately 50% when the angle of free stream augments from +20° to −20°. According to our findings, mixing efficiency surges up to 17% when the jet spaces are increased from 4Dj to 10 Dj.