The Effect of Pressure Gradient on Boundary Layer Receptivity

This paper presents numerical results for the receptivity of three laminar boundary layers with zero (ZPG), adverse (APG) and favourable (FPG) pressure gradients. Each boundary layer is subjected to a series of simple freestream waveforms which can be considered as constituent parts of either an isotropic or a non-isotropic turbulent freestream. Each freestream waveform has a single frequency in each spatial direction and is divided into two mutually perpendicular components. The first component has a zero spanwise velocity and hence lies in the streamwise normal plane whereas the second component lies in a plane which is perpendicular both to this plane and the spatial frequency vector. High boundary layer receptivities are only obtained for a minority of these waveforms and so only the resulting flow structures for these waveforms are considered in detail. The dominant flow structures are identified as either Tollmien Schlichting (T-S) waves or streaky structures. The streaky structures can be induced by both freestream components, but the response to the second component, which results in streamwise vortices in the freestream, is considerably stronger and occurs over a much larger streamwise frequency range. The boundary layer is only receptive to a relatively narrow band of spanwise wavelengths ranging from approximately one to four times the local boundary layer thickness. The APG leads to receptivities which are more than double those for the FPG case. The ratio of the freestream fluctuation streamwise wavelength to the distance from the plate leading edge is identified as an important influential parameter for receptivity leading to streaks. Significant T-S activity is only observed for APG, but is also detected for ZPG.     

质子交换膜燃料电池扩散层电阻的实验研究

气体扩散层是质子交换膜燃料电池的重要组件,其与极板之间的接触电阻是燃料电池内阻的重要组成部分,也是导致燃料电池功率损失的重要因素。为了得到扩散层与极板间接触电阻以及扩散层本体电阻,首先在扩散层试样表面溅射厚度为50nm的金属铜,再在实验室搭建的燃料电池压力-位移测试平台上用四点法测得试样的总电阻,而后用斜率-截距法分离了气体扩散层本体电阻和其与极板的接触电阻,计算出了相应的电阻率,得到了压力和接触电阻之间的关系。结果表明,接触电阻随着接触面压强的增大而减小,接触电阻率和接触压强关系满足经验公式ρ=43.39P-0.725。     

Effect of Characteristic Time on Scaling of Breakthrough Time Distribution for Two-Phase Displacement in Percolation Porous Media

Transport in Porous Media - Determining the time of breakthrough of injected water is important when assessing waterflood in an oil reservoir. Breakthrough time distribution for a passive tracer...     

U71Mn钢轨气压焊焊接接头滚动磨损与损伤性能研究

研究了U71Mn钢轨气压焊焊接接头上各区域的组织与性能,并利用MMS-2A轮轨滚动磨损与接触疲劳试验机对焊接接头材料进行试验,分析了各区域的磨损与损伤特性. 结果表明:焊接接头组织为珠光体,但晶粒大小及渗碳体形态和大小存在差异. 焊缝中珠光体晶粒较小,渗碳体呈细小片层状及细小颗粒状,因此硬度高且塑性变形能力强. 在1×105和2×105循环次数时耐磨性优于母材,在3×105循环次数时,焊缝磨损量大于母材磨损量,且焊缝表面损伤较母材严重. 在焊缝两侧各有1个区域(软化区),组织为粒状珠光体和少量片层状珠光体,颗粒大小和片层厚度不均匀,硬度较小,磨损量较大,塑性变形层较厚,表面损伤最严重.      

扩散处理对氮离子注入纯铁摩擦学性能的影响

研究了Ｎ＋注入纯铁分别在１５０℃和２５０℃进行扩散退火后的摩擦磨损性能,并根据ＡＥＳ和ＳＥＭ分析结果对磨损机理进行了讨论．结果表明,经扩散处理后显微硬度提高,摩擦系数降低,耐磨性显著提高．其原因在于扩散处理后氮化物和碳化物析出,产生显著的弥散强化．     

Effect of Compressible Foam Properties on Pressure Amplification During Shock Wave Impact

A comprehensive study is made of the influence of the physical properties of compressible open-cell foam blocks exposed to shock-wave loading, and particularly on the pressure distribution on the shock tube walls. Seven different foams are used, with three different shock Mach numbers, and three different slab lengths. Foam properties examined include permeability, density, stiffness, tortuosity and cell characteristics. The investigations concentrate on both side-wall and back-wall pressures, and the peak pressures achieved, as well as collapse velocities of the front face and the strength and nature of the reflected shock wave. The consequences of deviations from one-dimensionality are identified; primarily those due to wall friction and side-wall leakage. The results presented are the most comprehensive and wide ranging series conducted in a single facility and are thus a significant resource for comparison with theoretical and numerical studies. The different foams show significant differences in behavior, both in terms of peak pressure and duration, depending primarily on their density and permeability.This paper was based on work presented at the 2nd International Symposium on Interdisciplinary Shock Wave Research, Sendai, Japan on March 1–3, 2005.     

Stability of the Nonstationary Swept Attachment-Line Boundary Layer with Time Variation of the Surface Temperature and Gas Suction Velocity

The possibility of controlling the stability of a nonstationary boundary layer on the attachment line of a high-aspect-ratio swept wing by means of periodic variations of the surface temperature or the gas suction velocity at sub- or supersonic free-stream velocities is considered. The characteristic time scale of the variations of the surface temperature or the gas suction velocity on the attachment line is assumed to be equal to the characteristic aerodynamic time. On this assumption the stability characteristics of quasisteady attachment-line boundary layer flows are studied, the minimum values of the critical Reynolds numbers Re^* of loss of stability are determined as functions of the temperature and the suction velocity, and examples of the periodic dependence of the surface temperature and the suction velocity for which, in the case of nonstationary flow, the time-average values of Re^* exceed the analogous values for the steady-state boundary layer are constructed.     

Time and Space Fractional Diffusion in Finite Systems

Spatiotemporal nonlocal diffusion in a bounded system is addressed by considering fractional diffusion in a linear, composite system. By considering limiting conditions, solutions for combinations of Neumann and Dirichlet boundary conditions (either zero or nonzero) at the ends of a finite system are derived in terms of Mittag–Leffler functions by the Laplace transformation. Computational viability is demonstrated by inverting the solutions numerically and comparing resulting calculations with asymptotic solutions. Time and space fractional derivatives, defined by variables \(\alpha \) and \(\beta \), respectively, are employed in the Caputo sense; a single-sided, asymmetric space derivative is used. Inspection of the asymptotic solutions leads to insights on the structure of the solutions that may not be available otherwise; the resulting deductions are verified through the numerical inversions. For pure superdiffusion, characteristics of some of the solutions presented here are very similar to those of classical diffusion but combined effects for the corresponding situation result in power-law behaviors. Incidentally, to our knowledge, the pressure distribution for space fractional diffusion at long enough times in a finite system is derived based on first principles for the first time.     

节流孔截面形状对静压干气密封稳态性能和压力波动特性影响

建立了不同形状节流孔出口截面静压干气密封的几何模型,采用湍流大涡模拟方法数值求解了小孔节流静压干气密封的瞬态流场和压力场,对比了节流孔出口开设圆角、倒角与经典小孔节流静压干气密封的开启力、泄漏率等稳态性能参数和节流孔出口附近的压力波动特性,以较大密封开启力和较小压力波动为目标,获得了节流孔出口倒角和圆角的优选值范围. 结果表明:相较于经典小孔节流静压干气密封,节流孔出口开设倒角或圆角能提高静压干气密封的开启力,显著降低节流孔出口压力波动,且当倒角为0.3~0.4 mm,圆角为0.1~0.3 mm时具有最佳效果.      

表面修饰SiO2纳米微粒对锂基脂抗磨性能影响的研究

合成了表面修饰SiO2纳米微粒,利用四球摩擦磨损试验机考察了SiO2纳米微粒作为锂基脂添加剂的摩擦磨损行为,用扫描电子显微镜、能量色散谱仪和X射线光电子能谱仪对钢球磨损表面进行了分析.结果表明:SiO2纳米微粒作为锂基脂添加剂具有良好的抗磨损性能,能够显著提高锂基脂的失效载荷.这是由于在摩擦过程中,SiO2纳米微粒富集在磨损表面并形成边界润滑膜,对磨损表面起到修复作用,从而使得锂基脂的抗磨和承载能力明显提高.     

摩擦温度对Ni—P基复合刷镀层摩擦学性能的影响

为了考察摩擦温度对Ni-P基复合镀层摩擦学性能的影响,作者按球-盘接触形式推导出了计算摩擦温度的数学表达式,其对Ni-P/WC和Ni-P/BN及Ni-P/MoS_2等几种复合镀层摩擦温度的计算结果与试样表面层的硬度分布显示了良好的对应性。磨损试验结果表明,特别在高PV值的情况下,Ni-P基复合镀层的摩擦学性能主要受摩擦温度的影响。这是因为Ni-P基复合镀层在镀态下呈非晶态组织,受热发生晶化反应后会使硬度上升,但温度过高时硬度却又下降。因此,镀层硬度是依摩擦温度的高低及其在镀层中的梯度分布而呈现出不同的分布特征,从而揭示了Ni-P/WC复合镀层在高PV值条件下显示良好固体润滑性能的原因。     

Role of Buoyancy on Instabilities and Structure of Transitional Gas Jet Diffusion Flames

Transitional jet diffusion flames provide the link between dynamics of laminar and turbulent flames. In this study, instabilities and their interaction with the flow structure are explored in a transitional jet diffusion flame, with focus on isolating buoyancy effects. Experiments are conducted in hydrogen flames with fuel jet Reynolds number of up to 2,200 and average jet velocity of up to 54 m/s. Since the fuel jet is laminar at the injector exit, the transition from laminar to turbulent flame occurs by the hydrodynamic instabilities in the shear layer of fuel jet. The instabilities and the flow structures are visualized and quantified by the rainbow schlieren deflectometry technique coupled with a high-speed imaging system. The schlieren images acquired at 2,000 frames per second allowed exposure time of 23 μs with spatial resolution of 0.4 mm. Results identify a hitherto unknown secondary instability in the flame surface, provide explanation for the observed intermittency in the breakpoint length, show coherent vortical structures downstream of the flame breakpoint, and illustrate gradual breakdown of coherent structures into small-scale random structures in the far field turbulent region.     

醇和羧酸添加剂对菜籽油抗磨与极压性能的影响

在四球摩擦磨损试验机上考察了醇－菜籽油及羧酸－菜籽油对钢－钢摩擦副抗磨与极压性能的影响,并分析了其润滑机制。结果表明,醇不能改善菜籽油的抗磨性能及承载能力,羧酸能明显改善菜籽油的抗磨性能,但却降低其承载能力,这与菜籽油本身的特性及三者的极性强弱有关。钢球磨损表面ＸＰＳ分析表明：２种润滑剂体系在摩擦过程中均形成了复杂的表面保护膜。２种润滑剂体系在钢球表面形成的保护膜的特性不同,这决定了它们个有不同的     

压力驱动微通道流中尺度与扩散关系的研究

对压力驱动微通道流中的扩散进行了数值模拟,说明微尺度效应使流体扩散的不均匀性趋于明显;分析了在各种W/D情况下,距离管道壁面各个层面上的扩散尺度变化,说明了微管道中的对流和壁面效应对于扩散尺度的影响。本文结论对于对微通道的优化设计有参考价值。     

Stochastic Aspects of Mass Transport in Gas Diffusion Layers

The relationship between the 3D morphology of gas-diffusion layers (GDL) of HT-PEFCs and their functionality is analyzed. A stochastic model describing the microstructure of paper-type GDL is combined with the Lattice-Boltzmann method (LBM) to simulate gas transport within the GDL microstructure. Virtual 3D microstructures representing paper-type GDL are generated by a stochastic model, where the binder morphology is systematically modified. On these structures, single phase single component gas flow is computed by the LBM. Quality criteria evaluating the spatial homogeneity of gas supply are introduced and related to the binder morphology. The spatial homogeneity of the gas supply is analyzed by a parametrized stochastic model describing the gas flow at the exit of the GDL. This approach gives insight into the spatial structure of the gas flow at the GDL exit. The quality of gas supply is quantified by characterizing size and arrangement of regions with high gas supply. This stochastic gas flow model predicts the quality of gas supply for further binder morphologies. Analyzing the quality criteria and the stochastic evaluation of the spatial structure of the gas flow field at the GDL exit, it is found that the binder morphology has an essential influence on the gas supply.     

The Effect of Preferential Diffusion on the Soot Initiation Process in Ethylene Diffusion Flames

The influence of differential diffusion of chemical species in the soot initiation process in turbulent flows is investigated through Direct Numerical Simulations coupled to a compact global chemical mechanisms for ethylene (C₂H₄) flame combustion (Løvås et al., Combust Sci Tech 182(11):1945?C1960, 2010) featuring the important reaction steps for acetylene production. Our focus is on the formation of acetylene (C₂H₂) which is one of the most important species indicative of soot formation layers, especially in relation to the location of the H and H₂ layers. The effect of preferential diffusion is assessed by comparison of results from unity and non-unity Lewis number simulations. The results indicate that under moderate turbulent conditions, where preferential diffusion effects become prominent, and with the global scheme used preferential diffusion greatly enhances the spread of the radical H whose peak value in mass fraction is reduced by a factor of about two; the spread of H₂ is also enhanced though to a lesser extent. Importantly, the H and H₂ spread into a range of mixture fraction Z between 0.2 and 0.3 which contains the soot formation range, supporting the hypothesis that soot formation is enhanced by preferential diffusion. Nevertheless, the acetylene formation layers themselves show little adjustment in the presence of non-unity Lewis numbers suggesting that the acetylene formation is dominated under the current conditions by the direct thermal decomposition of ethylene to acetylene in the global chemistry used. The specific F _i factors that appear in flamelet models are explicitly computed; only F _H, F _H2 and F _CO show appreciable differences on the fuel lean range of mixture fraction due to non-unity Lewis numbers, suggesting that the effects of non-unity Lewis numbers could be incorporated by a selective inclusion of only a few of the F _i factors in order to save computational time.     

Effect of Confining Wall on Properties of Gas Flow Through Metal Foam: An Experimental Study

While the Darcy and Forchheimer relations are widely applied to determine the permeability and the form drag coefficient of open-cell metal foam, they both assume that the porous medium is infinite in all directions, i.e., large enough so that the effect of any confining walls is negligible. Many researchers, however, pay little or no attention to the size of metal foam samples in pressure-drop studies. The size of a foam sample perpendicular to the flow direction may be small enough such that wall effects are significant. This article experimentally investigates the wall effect on the permeability and form drag coefficient for two types of open-cell aluminum foam subjected to airflow entering the foam in the Forchheimer regime. The Forchheimer equation was recast in two different manners, which resulted in new non-dimensional numbers that correlated very well with the diameter of the foam samples measured in cells. The correlations are valid for a confining-tube-diameter-based Reynolds number ranging from approximately 13,000 to 105,000, and for diameters ranging from 12 to 36 cells and 24 to 60 cells for 10- and 20-pore per inch foam, respectively. The obtained correlations allow for determining pressure drop given only the velocity and the diameter of an aluminum foam sample.     

Footing Settlement on a Consolidating Soil Layer with Stochastic Properties

Geotechnical engineering applications are characterized by various sources of uncertainties, most of them attributed to the stochastic nature of soil parameters and their properties. In particular, soil’s inherent random heterogeneity, inexact measurements and insufficient data necessitate numerical methods that incorporate the stochastic soil properties for a realistic representation of the soil behavior. In this paper, the process of consolidation of saturated soils is examined on the basis of the coupled u–p finite element formulation. A generalized Newmark implicit time integration scheme is implemented to treat the time integration of the coupled consolidation equations. A benchmark geotechnical engineering problem of a strip footing resting on a saturated soil layer is analyzed. The soil permeability coefficient k, as well as the elastic modulus E, are treated as lognormal random fields in two dimensions. The investigation of the effect of the spatial variability of the soil properties on the response of a footing–soil system is examined by means of the direct Monte Carlo simulation. The influence of the coefficient of variation and correlation length of the stochastic fields is quantified in terms of footing settlements, as well as excess soil water pore pressure. The effects of spatial variability of the permeability coefficient k and the elastic modulus E on the system response are demonstrated. It is shown that the footing differential settlement, along with generated excess pore pressures, is highly affected by the variation of the soil properties considered, as well as the correlation length of the underlying random fields.     

Correction to: Time and Space Fractional Diffusion in Finite Systems

Additive manufacturing technology, or 3D printing, with silica sand has enabled the manufacture of porous rock analogues for the use in experimental studies of geomechanical properties of reservoir rocks. The accurate modelling of the fluid flow phenomena within a reservoir and improving the performance of hydrocarbon recovery require an understanding of physical and chemical interactions of the reservoir fluids and the rock matrix. Therefore, for the 3D printed samples to serve as rock analogues, flow properties have to be equivalent to the petrophysical properties of their natural counterparts, such as Berea sandstone. In this study, sandstones that were 3D printed with silica sand and Poly-Furfuryl alcohol (PFA) binder, were used to investigate interactions between porous media with different fluids. Wettability preference of 3D printed samples was characterized through contact angle measurements, as well as co-current and counter-current spontaneous imbibition experiments. Results indicated that 3D printed sandstones had mixed-wet characteristics due to the high preference of silica grains for polar fluids and the affinity PFA binder to the oleic phase. Printing configurations including binder saturation were found to greatly influence the wettability preference of the 3D printed analogue rocks as higher PFA concentrations resulted in more strongly oil-wet preferences. Efforts to optimize the printing process and challenges to control the wettability preferences of the 3D printed samples are also highlighted.

     

纯铜表面双层辉光离子渗钛合金层的摩擦磨损性能

采用双层辉光离子渗金属技术，在纯铜表面形成含钛的合金渗层；采用扫描电子显微镜观察分析磨损表面形貌，并分析了磨损机理．结果表明：纯铜表面经离子渗钛后形成了以金属问化合物Cu4Ti和含钛的α固溶体为主的合金渗层，从而使表面强度提高，磨损性能明显改善；纯铜磨损表面呈撕裂特征，而离子渗钛试样仅发生轻微擦伤．