采用多源涡技术强化燃料/空气小尺度混合研究

通过数值模拟,研究了在空气入口总温为294.28 K,燃料气体为乙烷(C₂H₆)时在轴向旋流器出口截面上安装多个小突片的多源涡(MSV)结构对燃烧室内燃料/空气掺混效果的影响.结果表明:在每个小突片下游产生的小尺度流向涡使得局部的燃料/空气掺混加强;装有多源涡结构的燃烧室内对应的C₂H₆质量分数在文氏管下游2 mm截面上的混合不均匀度由不带多源涡结构时的13.0%下降到小于7.4%.在该研究的范围内,当小突片总的阻塞比增大时,对应的混合不均匀度减小,掺混效果变好.      

空气节流对超燃发动机燃烧性能的影响

为优化隔离段及燃烧室内的流场结构,减小流动速度,提高流道压力,在凹槽稳焰器下游布置空气节流装置,向燃烧室内喷入高压空气,目的是在隔离段建立合适的激波串,以利于点火和火焰稳定.采用CFD方法,对比了无/有空气节流时的冷流流场、燃烧流场,结果表明:节流激波串扩大了低动量区域,激波引起的流动扭曲和较长的驻留时间使得分离流内的空气/燃料混合程度得到显著增强,混合效率由无节流时的40%提高到85%,燃烧效率由无节流时的10%提高到60%.对节流位置和节流量进行了参数化研究,以隔离段入口为坐标原点,分别考察了785,1000,1100mm三个流向节流位置,以及10%,15%,30%三种节流量的影响,结果表明:距离凹槽较近的节流位置对燃烧的影响更为显著,所需节流量更少.30%的节流量将使燃烧室背压急剧提高,引起流动壅塞.研究表明在785mm的流向节流位置采用15%的节流量可获得最好的燃烧性能.      

Experimental study on NOx emission correlation of fuel staged combustion in a LPP combustor at high pressure based on NO-chemiluminescence

Experimental investigations on NO_x emissions of a single-cup, Lean Premixed Prevaporized (LPP), module combustor were carried out at elevated inlet temperature and pressure up to 810 K and 2.0 MPa, close to the real operating conditions of aero-engine combustors. This LPP combustor adopts centrally staged fuel injections which could produce separated stratified swirling spray flame. In the NO_x emissions measurements, the ranges of dome equivalence ratio and fuel stage ratio were from 0.55 to 0.58 and 8% to 24%, respectively. The optical diagnosis on separated stratified swirling spray flame were carried out with fuel stage ratio changing from 15% to 30%. Therefore, NO* and OH* chemiluminescence images were obtained. The results show that NO_x emissions increase with the increase of the fuel stage ratio. And from the chemiluminescence images, the main flame and pilot flame are found weakly coupled. The pilot flame plays a significant role in NO_x emission production because of its higher adiabatic flame temperature. Based on the results of chemiluminescence optical tests, a new NO_x emission prediction model is proposed based on the Lefebvre’s single flame model. The estimate of local equivalence ratio of the pilot stage’s non-premixed flame is modified considering the characteristics of spray combustion, and a “PLUS” emission prediction model suitable for separated stratified swirling spray flame is obtained. Compared to the experimental data, the “PLUS” model exhibits a good prediction in a range of ±13% of deviation.     

Study of Dynamic Loads of Aircraft Bevel Gear Systems

This paper studies the dynamic loads of bevel gears with circular and straight teeth with the overlap factors less than two (ε_α < 2) and more than two (ε_α > 2). The advantages of the gears with straight teeth with ε_α > 2 are shown.     

SCC evaluation of a 2297 Al-Li alloy rolled plate using the slow-strain rate technique

The stress corrosion cracking (SCC) susceptibility of 2297 Al-Li alloy in 1 M NaCl + 0.01 M H₂O₂ solution (CP solution) and 1 M NaCl + 0.01 M H₂O₂ + 0.6 M Na₂SO₄ solution (CPS solution) was investigated by slow-strain rate tests at various strain rates ranging from 10⁻⁵ s⁻¹ to 10⁻⁷ s⁻¹. The roles of H₂O₂ and SO₄²⁻ in the corrosion process were estimated by potentiodynamic polarization and electrochemical impedance spectroscopy. 2297 Al-Li alloy does not fracture ascribed to SCC in CP solution, while it undergoes SCC in CPS solution. In CPS solution, with a decreasing strain rate from 10⁻⁵ s⁻¹ to 10⁻⁷ s⁻¹, the SCC susceptibility firstly rises and then declines exhibiting a peak value at a strain rate of 10⁻⁶ s⁻¹. H₂O₂ promotes the active dissolution while SO₄²⁻ lowers the corrosion rate. The SCC fracture is associated with a decline in the dissolution rate of the crack tip by SO₄²⁻, which leads to stress concentration. In CPS solution, a reduction in the local dissolution rate of the crack tip leads to stress concentration, resulting in SCC fracture. As the preferred initiation site for a crack, pits also show a noteworthy effect on SCC of 2297 Al-Li alloy.     

RANS simulations on combustion and emission characteristics of a premixed NH3/H2 swirling flame with reduced chemical kinetic model

Ammonia (NH₃) is considered as a potential alternative carbon free fuel to reduce greenhouse gas emission to meet the increasingly stringent emission requirements. Co-burning NH₃ and H₂ is an effective way to overcome ammonia’s relative low burning velocity. In this work, 3D Reynolds Averaged Navier-Stokes (RANS) numerical simulations are conducted on a premixed NH₃/H₂ swirling flame with reduced chemical kinetic mechanism. The effects of (A) overall equivalence ratio Φ and (B) hydrogen blended molar fraction X_H2 on combustion and emission characteristics are examined. The present results show that when 100%NH₃-0%H₂-air are burnt, the NO emission and unburned NH₃ of at the swirling combustor outlet has the opposite varying trends. With the increase of Φ, NO emission is found to be decreased, while the unburnt ammonia emission is increased. NH₂ → HNO, NH → HNO and HNO → NO sub-paths are found to play a critical role in NO formation. Normalized reaction rate of all these three sub-paths is shown to be decreased with increased Φ. Hydrogen addition is shown to significantly increase the laminar burning velocity of the mixed fuel. However, adding H₂ does not affect the critical equivalence ratio corresponding to the maximum burning velocity. The emission trend of NO and unburnt NH₃ with increased Φ is unchanged by blending H₂. NO emission with increased X_H2 is increased slightly less at a larger Φ than that at a smaller Φ. In addition, reaction rates of NH₂ → HNO and HNO → NO sub-paths are decreased with increased X_H2, when Φ is larger. Under all tested cases, blending H₂ with NH₃ reduces the unburned NH₃ emission, especially for rich combustion conditions. In summary, the present work provides research finding on supporting applying ammonia with hydrogen blended in low-emission gas turbine engines.     

Presupernova evolution of the most massive stars

Taking as example a 60M _⊙ star of solar metallicity, the state of the art of model calculations for very massive, from the main sequence to the supernova stage, is reviewed. It is argued that — due to the simple internal structure of Wolf-Rayet stars — the post main sequence evolutionary phases are currently those which are better understood. A brief discussion of the supernova outcome from very massive stars is given. Then, the more uncertain main sequence evolution is discussed. A first attempt to incorporate results about pulsational instabilities of very massive stars in stellar evolutionary calculations is performed. On its basis, a new type of evolutionary sequence for very massive stars is obtained, namely O-star → Of-star → H-rich WNL → LBV → H-poor WNL → WNE → WC → SN. This scenario is shown to correspond better to many observed properties of very massive stars than the standard one. It includes a model for the prototype LBV P Cygni.     

Supersonic expansion on non-equilibrium plasmas

In the supersonic expansion of an ionized gas, the dominant factor in describing the atomic processes is the recombination rate constant K_R. Several models describing the recombination process have been reviewed in some detail. It has been found that, depending on the adopted definition, different models will yield different values of K_R for the same electron temperature and number density. A comparison of experimentally and theoretically derived values for K_R has to be done with great care, as in the majority of the experiments K_R is determined from the measured rate of disappearance of free electrons. These measurements give the correct “decay coefficient”, but only in certain circumstances will it reduce to the correct recombination rate. In the light of the important role that K_R plays in any numerical solution of nonequilibrium expansion flow of plasmas, details of experiments on a 15-degree corner expansion flow of ionized argon are given. In these experiments the plasma flow which was generated by driving strong normal shock waves into quiescent argon was studied mainly by optical diagnostics. Using a dual-frequency laser interferometer, the plasma properties around a corner expansion were recorded. The analysis of the interferograms has yielded values for the recombination rate constant as a function of the plasma macroscopic properties. The range of shock Mach number, electron number density, temperature and initial channel pressure and temperature were as follows:

$\text{13 < M, < 19; 10}{}^{16}\text{< n, < 1.5 × 10}{}^{17}\text{cm}{}^{\mathrm{?3}}\text{; 9000°}\text{K}\text{< T < 13,000°}\text{K}\text{; 2.2 < p}{}_1\text{< 10}\text{torr}\text{; T}{}_1\text{? 300°}\text{K}\text{.}$

It was found that the theoretically predicted values for the three-body, electron-ion-electron collisional recombination rate are in good agreement with those measured gasdynamically in a well-defined flow. The measured flow quantities substantiate a previous analysis based on the method of characteristics.     

直混燃烧与LPP组合燃烧室数值研究

设计了直混燃烧与贫油预混预蒸发(LPP)组合的单管燃烧室.燃烧室头部采用同轴同旋向主模旋流器和副模旋流器结构,主、副油路分别采用直射式喷嘴和压力雾化喷嘴,可以在单管燃烧室上掌握和实现低污染燃烧排放控制技术,并采用Fluent软件对设计的单管燃烧室模型进行数值模拟.计算结果表明:主副模燃烧区相互独立;副模是直混燃烧,主要作用稳定火焰.主模是预混燃烧,燃烧区温度分布均匀,从而实现低NO_x排放.解决了大工况下低NO_x排放与慢车贫油熄火之间的矛盾.      

富氢/富氧燃气同轴剪切气-气喷嘴性能仿真分析

采用正交试验设计方法对富氢/富氧燃气同轴剪切气-气喷嘴设计参数氧压降比、燃氧速度比、氧喷嘴出口壁厚进行组合,数值模拟单喷嘴燃烧室燃烧流场,研究喷嘴设计参数及参数之间的交互作用对燃烧性能和燃烧室热载性能的影响,评价指标为燃烧长度、燃烧室壁面平均燃气温度和喷注面板平均燃气温度.仿真结果表明,燃氧速度比对燃烧与热载有显著影响,氧压降比与氢氧燃氧速度比的交互作用影响明显,分析结果对气-气喷嘴试验设计有重要指导意义.      

High-energy gamma-astronomy with large underground detectors

Large underground (underwater) detectors (such as ‘Baikal’ and DUMAND) are discussed for their possible use as gamma-ray telescopes. The signal is caused by high-energy muons (E _μ ? 30–100 TeV) produced by the primary gamma-radiation in the Earth's atmosphere. The production of gamma-rays in the source through the reaction p + p → π ⁰ + X is calculated for a low density target of arbitrary thickness taking the electromagnetic cascade into account. The muon production by gamma-rays in the Earth's atmosphere is calculated using three processes: (i) photoproduction of π- and K-mesons followed by decay to muons, (ii) the direct production of μ⁺ μ^?-pairs: γ + Z → Z + μ ⁺ + μ^?, and (iii) photoproduction of charmed particles. It is shown that for thin sources with a flat spectrum (integral exponent γ = 1.1) a large (S = 0.1 km²) underground detector can detect both neutrinos and gamma-quanta generated by the source. Finally we compare the performances of underground detectors with S = 0.1 km² for the search of gamma sources at E = 100 TeV with those of the previously proposed 1 km² EAS array.     

Geochemical Reservoirs and Timing of Sulfur Cycling on Mars

Sulfate-dominated sedimentary deposits are widespread on the surface of Mars, which contrasts with the rarity of carbonate deposits, and indicates surface waters with chemical features drastically different from those on Earth. While the Earth’s surface chemistry and climate are intimately tied to the carbon cycle, it is the sulfur cycle that most strongly influences the Martian geosystems. The presence of sulfate minerals observed from orbit and in-situ via surface exploration within sedimentary rocks and unconsolidated regolith traces a history of post-Noachian aqueous processes mediated by sulfur. These materials likely formed in water-limited aqueous conditions compared to environments indicated by clay minerals and localized carbonates that formed in surface and subsurface settings on early Mars. Constraining the timing of sulfur delivery to the Martian exosphere, as well as volcanogenic H₂O is therefore central, as it combines with volcanogenic sulfur to produce acidic fluids and ice. Here, we reassess and review the Martian geochemical reservoirs of sulfur from the innermost core, to the mantle, crust, and surficial sediments. The recognized occurrences and the mineralogical features of sedimentary sulfate deposits are synthesized and summarized. Existing models of formation of sedimentary sulfate are discussed and related to weathering processes and chemical conditions of surface waters. We also review existing models of sulfur content in the Martian mantle and analyze how volcanic activities may have transferred igneous sulfur into the exosphere and evaluate the mass transfers and speciation relationships between volcanic sulfur and sedimentary sulfates. The sedimentary clay-sulfate succession can be reconciled with a continuous volcanic eruption rate throughout the Noachian-Hesperian, but a process occurring around the mid-Noachian must have profoundly changed the composition of volcanic degassing. A hypothetical increase in the oxidation state or in water content of Martian lavas or a decrease in atmospheric pressure is necessary to account for such a change in composition of volcanic gases. This would allow the pre mid-Noachian volcanic gases to be dominated by water and carbon-species but late Noachian and Hesperian volcanic gases to be sulfur-rich and characterized by high SO₂ content. Interruption of early dynamo and impact ejection of the atmosphere may have decreased the atmospheric pressure during the early Noachian whereas it remains unclear how the redox state or water content of lavas could have changed. Nevertheless, volcanic emission of SO₂ rich gases since the late Noachian can explain many features of Martian sulfate-rich regolith, including the mass of sulfate and the particular chemical features (i.e. acidity) of surface waters accompanying these deposits. How SO₂ impacted on Mars’s climate, with possible short time scale global warming and long time scale cooling effects, remains controversial. However, the ancient wet and warm era on Mars seems incompatible with elevated atmospheric sulfur dioxide because conditions favorable to volcanic SO₂ degassing were most likely not in place at this time.     

Temperature measurements by coherent anti-Stokes Raman spectroscopy in hydrogen-fuelled scramjet combustor

Instantaneous temperature measurements in two dimensional stepped dual-mode hydrogen-fuelled scramjet combustor were performed by the broad-band CARS technique. The experiment consisted of a direct-connect test of a Mach 3 combustor with three fin fuel injectors. The freestream flow was combustion heated to total temperature T_t=1600–1700 K. Temperature measurements with repetition rate of 10 Hz during run duration of 10 s and hydrogen combustion duration of 5 s were carried out at one spatial point located downstream of a rearward-facing step. For scramjet mode of operation (combustion in supersonic flow) the temperatures derived from single shot measurements were in the range 600–900 K. For ramjet mode (combustion in subsonic flow in pseudo-shock) the temperatures increased to 1200–1600 K. The temperature values in the last case were grouped around both 600 K and 1500 K, making the shape of the temperature probability density function bimodal. The nature of such a probability distribution is discussed.     

WZ5发动机改用中热值气体燃料

为了WZ5航空发动机改用小化肥厂的废气(含氢约50％、甲烷约20％,其余为氮气,热值约3000kcal/m~2),重新设计了燃烧室喷咀。试验证明,改型后的燃烧室点火、火焰稳定性、燃烧效率、燃烧室总压损失、出口温度分布、冒烟和壁温等均达到发动机基本指标要求。     

试验研究旋流数对燃烧室气动性能的影响

为了深入了解双级轴向旋流器的旋流数变化对模型环形燃烧室内气流结构与气动性能的影响,采用粒子图像速度仪(PIV)对燃烧室内冷态和液雾燃烧流场进行测量,试验研究在冷、热态情况下不同旋流数对模型燃烧室内回流区的尺寸、平均速度(u,v)、脉动速度(U<,rms>,V<,rms>)以及雷诺剪切应力u'v'分布的影响.研究结果表明...     

Competing effects of surface catalysis and ablation in hypersonic reentry aerothermodynamic environment

Under hypersonic flow conditions, the complicated gas-graphene interactions including surface catalysis and surface ablation would occur concurrently and intervene together with the thermodynamic response induced by spacecraft reentry. In this work, the competing effects of surface heterogeneous catalytic recombination and ablation characteristics at elevated temperatures are investigated using the Reactive Molecular Dynamics(RMD) simulation method. A GasSurface Interaction(GSI) model is establi...     

Prediction of emissions in turbojet engines exhausts: relationship between nitrogen oxides emission index (EINOx) and the operational parameters

The prediction of nitrogen oxides emissions in turbojet engines exhausts remains a field of immense importance given the increasing coercive environmental requirements in relation to the emissions from turbojet engines. Emissions of nitrogen oxides (NOx) are subjected to limits fixed by legislators in a lot of countries because of their toxic character beyond specific concentrations in the air and their effects on climate change. However, analytical methods used to evaluate the quantities of the emissions are not efficiently established because of a lack of complete understanding of the phenomena governing the formation of the NOx in turbojet engines. It is a combination of chemical, thermal, and fluid dynamic processes. In this paper, a more accurate empirical correlation is determined for the prediction of nitrogen oxides emission index (EI_NOx) in turbojet engines exhausts using the main combustion operational parameters. The relationship between EI_NOx and the following parameters: fuel flow rate, output power, pressure ratio, efficiency, flame temperature and combustor inlet temperature is analysed with 227 ICAO certification data measured on turbojets engines from manufacturers such as Pratt and Whitney, General Electric, CFM International and Rolls-Royce. A set of 556 test point data used to show flame and combustor inlet temperature relationship with NOx emission index is from the work of G.F. Pearce et al. on twelve gas turbine engines. From this analysis it can be noticed that the relationship between EI_NOx and flame temperature is strong but not as the relationship between EI_NOx and the combustor inlet temperature. Since the combustor inlet temperature is easier to measure, EI_NOx value can be deduce from it. By predicting NOx emissions from combustor inlet temperature we can avoid exhausts gas measurements and analysis, saving on time and money.     

A theoretical and 1-D numerical investigation on a valve/valveless air-breathing pulse detonation engine

The important operating characteristics of pulsed Pressure Gain Combustion (PGC) propulsion are the pressure gain of the combustor component and the propulsive performance gain of the engine. A ramjet-type valve/valveless air-breathing pulsed detonation engine with a supersonic internal compression inlet is investigated. Based on an ideal thermal cycle, the ideal equivalent pressure ratios (π_cb) of the Pulsed Detonation Combustor (PDC) are obtained theoretically which are directly related with the propulsive performance of the engine. By introducing an orifice loss model into the cycles, the critical pressure drop ratios through the orifice for the PDC achieving pressure gain and the engine achieving thrust gain are studied. More influencing factors are investigated by the use of a one-dimensional (1-D) numerical simulation model. The operating characteristics of the pulse detonation engine are investigated with changes of the valve type, the inlet/outlet area ratio of the PDC, the nozzle area ratio, and flight conditions. All these factors can affect π_cb of the PDC, and π_cb can be optimized by changing the geometry of the engine. The most important influence parameter is the valve type. When using an orifice-type aerodynamic valve, simulation results show that the PDC cannot achieve the pressure gain characteristics. When a supersonic internal compression inlet is introduced to the engine, whether the Pulse Detonation Engine (PDE) can achieve thrust gain comparable with that of an ideal Brayton cycle engine not only is related to the pressure gain of the combustor, but also needs to optimize the engine structure to reduce the total pressure loss.     

Quantification of parametric uncertainty in γ-Reθ model for typical flat plate and airfoil transitional flows

Owing to the lack of physical knowledge of boundary layer transition, the γ-Re_θ transition model introduces closure parameters, which increase the uncertainty of transition prediction. The objective of this work is to quantify the uncertainties of closure parameters in the quantities of interests and identify the key parameters. The six closure parameters in the uncertainty intervals are used as input variables, and the uncertainties of the output results are propagated by a stochastic expansion based on the point-collocation nonintrusive polynomial chaos method. The relative contribution of each parameter to uncertainty is evaluated by the Sobol index. The computational cases include natural and bypass transitional flows on zero-pressure-gradient flat plates, and subsonic and transonic flows around airfoils. For most cases, c_e2, c_a2, and c_a1 dominate the uncertainty, and the influence of σ_θt is also significant when the history effects of flow are evident. The contribution of parameters in airfoils is more complex than that in flat plates. The transonic airfoil case shows that flow separation dramatically changes the distribution of Sobol indices, which poses a challenge to the accurate prediction of transition. Generally, c_e2 and c_a2 are the key parameters of the γ-Re_θ model.