Microstructure and mechanical properties of fine grained uranium prepared by ECAP and subsequent intermediate heat treatment

The microstructure and mechanical properties of fine grained uranium prepared by equal channel angular pressing (ECAP) and subsequent intermediate heat treatment were investigated systematically by the confocal laser scanning microscope (CLSM), electron backscatter diffraction (EBSD) and split Hopkinson pressure bar (SHPB). The results show that the initial coarse grained uranium was refined from about 1000 to 6.5 μm prepared by ECAP at 3 passes and subsequent heat treatment, and the corresponding dynamic yield strength increased from 135 to 390 MPa. For the ECAPed uranium samples, the relationship between grain size and yield strength could be described by classical Hall–Petch relationship, and the fitting Hall–Petch relationship for the fine grained uranium samples prepared by ECAP was drawn.     

Numerical investigation of liquid water dynamics in wave‐like gas channels of PEMFCs

For the air feed in proton exchange membrane fuel cells (PEMFCs), the wave‐like gas channel (GC) shows obvious advantages over the straight GC because the former enhances collision of secondary flow and diffusion in the gas diffusion layer (GDL). However, it is prone to water flooding, which brings greater pressure drop, larger pressure oscillation, and blocking of reaction area. In the present study, numerical models of the water dynamic processes, including water droplets emerging from micropores on the GDL surface and removing through the GC, are established based on the volume of fluid (VOF) method. Water coverage ratio and pressure drop are calculated to evaluate the water flooding. The effects of the dimensional parameters of wave‐like GC and contact angle of channel walls on the water accumulation are studied. The emergence and removal of liquid water is a quasiperiodic and oscillating process. Multicycle simulations show that channel pressure drop increases linearly with greater growth rate than channel length. The equilibrium position of water droplet is strongly dependent on the relative wettability of the GDL and bipolar plate (BPP) surfaces. And the geometric parameters of GC have a significant impact on the pressure, water removal behavior and detachment time. Smaller bent angle brings bigger pressure drop, and larger cycle length is helpful for relieving the oscillation of pressure.     

Mixed convective peristaltic flow of Eyring‐Prandtl fluid with chemical reaction and variable electrical conductivity in a tapered asymmetric channel

The influence of inconstant electrical conductivity and chemical reaction on the peristaltic motion of non‐Newtonian Eyring‐Prandtl fluid inside a tapered asymmetric channel is investigated. The system is concerned by a uniform external magnetic field. The heat and mass transfer are considered. The problem is controlled mathematically by a system of nonlinear partial differential equations which describe the velocity, temperature, and nanoparticle concentration of the fluid. By means of long wavelength and low Reynolds numbers, our system is simplified. It is explained by using the multi‐step differential transform method as a semi‐analytical technique. The distributions of velocity, temperature, nanoparticle concentration, as well as pressure gradient and pressure rise are obtained as a function of the physical parameters of the problem. The effects of these parameters on these distributions are deliberated numerically and illustrated graphically through a set of figures. The results indicate that the parameters play a significant role in controlling the velocity, temperature, nanoparticle concentration, pressure gradient, and pressure rise.     

Study on downward flame spread over extruded polystyrene foam in a vertical channel: Influence of opening area

Experimental methods and theoretical analysis are employed to investigate effects of channel opening area on downward flame spread characteristics of extruded polystyrene (XPS) thermal insulation materials on building facade. The average flame height first drops and then rises as dimensionless opening area (the ratio of sidewall opening area to sidewall area, ie, S^*) increases. As S^* rises, both the average and maximum temperature of the curtain wall decrease, and the decreasing of the average temperature is linear. XPS surface temperature history can be divided into four stages, ie, inapparent rising stage (preheating), significant rising stage (melting), dropping stage (pyrolysis), and rerising stage (ignition). The preheating length first rises and then drops as S^* increases. The XPS flame spreads steadily at the early period while acceleration occurs at the later period. For different opening areas, the difference in spread distance history is not apparent in the early stage while this difference is significant in the later stage. The flame spread rate (V_f) first increases and then decreases as S^* rises. A downward flame spread model for XPS in vertical channel with openings is built. The varied trend of V_f predicted using this model corresponds to the experimental result.     

输水状态下挖方渠道衬砌干地修复渗控方案

南水北调中线工程运行以来,发挥了巨大社会效益,沿途及京津冀地区对其调水需求日增,渠道停水检修将带来较大风险,因此面临输水状态下渠道衬砌板破坏抢修难题。部分挖方渠道衬砌板下有复杂的渗控措施,输水状态下采用围堰干地抢修渠底和较深部位的渠坡衬砌板时,需解决围堰基坑内外水位差可能带来的基坑内渠坡抗浮和渗透破坏问题。鉴于此,在渠道渗控设计及围堰布置基础上,根据“前堵后排”思想,提出一种围堰基坑“外、中、内”三防线渗控方案,即围堰基坑外围渠基透水管封堵,基坑周边安装水泵抽排。三维渗流场模拟结果表明,该方案可显著降低基坑周边衬砌板下砂垫层内水平渗透比降和基坑涌水量,能保证基坑渗流安全。研究成果可为南水北调中线输水状态下挖方渠道围堰干地抢修提供理论依据和技术支持。     

A novel and exact analytical model for determination of critical depth in trapezoidal open channels

Critical depth is a significant parameter in the designing and management of open channels and related hydraulic structures, understanding the flow characteristics and calculations of varied flows (gradually, spatially, etc.). The trapezoidal cross sections are the most commonly used geometric sections in the network of water transmission and distribution channels, thus discussing its geometrical and hydraulic parameters is inevitable. The used nonlinear and mathematical relationships governing the critical depth problem in the trapezoidal channels are implicit and complex, hence the methods of trial and error, graphical and numerical are used to calculate it. In the present study, new explicit equations are presented based on mathematical analysis of the critical depth problem in the trapezoidal channel. Mathematical analysis has led to completely mathematical and analytical solutions having a definite physical (hydraulic) concept. Having the explicit equations provided by the ease of calculation process with no limitation of the application range and high accuracy are the advantages of this analysis. The accuracy of the presented equations is desired and determined according to the required accuracy. The data used for verification of the results are based on the critical flow condition (Froude number equal to 1), which has been generated in a wide and practical range of the geometric and hydraulic characteristics of the channel. Also, the calculated values are compared with the real values of the considered parameter.     

Hydraulics of free overfall in steeply sloping rough rectangular channel: A general computational approach

All supercritical free overfalls were almost considered as frictionless flow or pseudo-uniform flow before, the effect of friction had not been paid due attention. Currently, a new mathematical model is proposed to analyze supercritical rectangular free overfall by Boussinesq-type energy equation accounting for frictional effect. A computational approach of loop iteration algorithm is utilized to simultaneously calculate the implicit water surface profile by Runge-Kutta method, end-depth-ratio, and discharge. Subsequently, the dependency of discharge on end depth and Nikuradse equivalent sand roughness for different slope is illustrated. Comparison of computed results with experimental observations reveals satisfactory agreement. Additionally, the present energy approach leads to more accurate results than the momentum method for high roughness. Finally, to aid application for engineers, an explicit solution of discharge for practical application in the commonly using range is also proposed. The most important purpose of this study is to present a general computational approach, which could be used for the rough supercritical free overfalls of any other cross-sections.     

Premixed hydrogen-air flame front dynamics in channels with central and peripheral ignition

Experimental and analytical study of burning hydrogen-air mixtures with 12, 13, and 15 vol% hydrogen concentrations in channels with central and peripheral ignition was performed. Flame propagation speeds were determined by shadow and infrared high-speed imaging in the transverse and longitudinal directions, respectively. It was found that the increase in the flame front speed during the peripheral ignition reaches up to 1.7 times compared to the central ignition depending on mixture content. The pressure growth rate was examined in a closed channel. It was estimated that the time to reach a maximum pressure is 1.1 times less in the case or peripheral ignition than the central one. An analytical model was formed to describe the dynamics of the flame front in both cases. The model of a “reversed finger-flame” generated by a peripheral ignition was presented. The obtained results could be used in designing hydrogen-fueled combustible engines with the reduced knock-effect.     

Grain refinement in low SFE and particle-containing nickel aluminium bronze during severe plastic deformation at elevated temperatures

The influence of particle size and morphology on grain refinement in low stacking fault energy(SFE)alloys was studied by comparing the grain structures in single-and multi-phase Al-bronze(AB)alloys following equal channel angular pressing(ECAP)between 350 and 500℃.In particular,nickel aluminium bronze(NAB)was chosen as it contained both coarse and fine rounded particles,as well as a lamellar phase which evolved during ECAP.Grain refinement in the single-phase alloy was achieved through dynamic recrystallisation initiated at deformed twin boundaries.By contrast,different mechanisms were observed in the particle-containing NAB.Recrystallisation around the coarse κⅡ particles(～5 μm)was promoted through particle stimulated nucleation(PSN),whereas recrystallisation in the region of the fine κⅣ(～0.4μm)was delayed due to the activation of secondary slip.Grain refinement in areas of the lamellar κⅢ showed significant variation,depending on the lamellar orientation relative to the shear plane of ECAP.As the lamellae deformed,numerous high angle grain boundaries were generated between fragments and served as nucleation sites for recrystallisation,while PSN occurred around spheroidised lamellae.The spreading of the κⅢ particles by ECAP then enhanced the total area of recrystallised grains.     

认知双向中继网络在κ-μ衰落信道下的中断性能分析

针对认知双向中继网络在进行数据传输时面临的复杂无线信道场景问题,采用广义κ-μ分布构建认知双向中继网络中的视距和非视距无线传输信道,推导次网络在κ-μ衰落信道下的统一中断概率,并分析次网络在多种单一和混合衰落信道情况下的中断性能。仿真结果表明,无线信道的衰落程度对次网络的中断概率影响显著,依据衰落信道类型合理设置网络参数将有助于提升次网络中断性能。