Institute Of Mechanics,Chinese Academy of Sciences
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Effect of coupled vibration modes of dual-strip pan head on pantograph-catenary interaction
No full textThe consistency of the dynamic behavior of the mechanical equivalent model of high-speed pantographs with that of actual high-speed pantographs under service conditions is crucial for the correctness and accuracy of the numerical simulation results of the pantograph-catenary interaction. Firstly, based on experimental data and the finite element method, models of a mass-point frame and reduced pan head were established, which can simulate the rolling and pitching motion characteristics of the dual-strip pan head. The correctness of the finite element numerical simulation of the pantograph-catenary system based on the model was verified through an industry standard and line tests. Then, the variation law of the standard deviation of the contact force (SDCF) in the speed range of 240-450 km/h was studied, and the mechanism of large fluctuation in SDCF was explained from the perspective of pantograph-catenary resonance. Finally, the influence of pan head degree of freedom and low-pass filtering frequency of the contact force time-domain signal on SDCF was studied, and the applicable speed level of the traditional three-mass model and 20 Hz filtering were provided
Dynamic interaction patterns of oblique detonation waves with boundary layers in hypersonic reactive flows
No full textDue to their high thermal cycle efficiency and compact combustor, oblique detonation engines hold great promise for hypersonic propulsion. Previous numerical simulations of oblique detonation waves have predominantly solved the Euler equations, disregarding the influence of viscosity and boundary layers. This work aims to study how the interaction between the oblique detonation wave and the boundary layer influences the detonation wave structures in confined spaces. Two-dimensional numerical simulations considering detailed chemistry are performed in a stoichiometric H2/air mixture. The results indicate that the wedge-induced oblique detonation wave generates a strong adverse pressure gradient upon impacting the upper wall, leading to boundary layer separation. The separation zone subsequently induces an oblique shock wave near the upper wall, and an increase in separation angle will cause the transition from an oblique shock wave to an oblique detonation wave. The formation of the separation zone reduces the actual flow area and may even lead to flow choking; its obstructive effect is similar to that of the Mach stem in inviscid flow. To establish a connection between the viscous recirculation zone and the inviscid Mach stem, we introduce a dimensionless parameter, eta , based on the inviscid assumption. It is defined as the ratio of the inviscid Mach stem height to the channel entrance height. This parameter can be used to identify three wave systems in a viscous flow field: separation shock-dominated wave systems, separation detonation-dominated wave systems, and unstable Mach stem- dominated wave systems. Among these, the appearance of detonation Mach stems leads to flow choking, and the shock-detonation wave system continually moves upstream, ultimately causing the failure of the oblique detonation combustion. The findings of this study provide new insights into the investigation of the influence of viscosity on the flow structure of oblique detonation waves
Effect of hydrogen ion irradiation on the mechanical properties of thermally aged Z3CN20.09M duplex stainless steel
No full textIn order to investigate the synergistic effect of thermal aging and hydrogen, Z3CN20.09M duplex stainless steels (DSS) were thermal aged at 400 C-degrees and then the irradiated by hydrogen ions. Due to the segregation of the Fe and Cr elements induced by spinodal decomposition, the nanohardness of the ferrite phase increased after thermal aging process. The irradiation effects by hydrogen ion could further increase the nanohardness of the damage region. But the {011} slip system structures formed by the irradiation significantly suppressed the hardening effects of the ions irradiation induced dislocations and spinodal decomposition induced element segregation. During the in-situ transmission electron microscopy (TEM) tensile process, severe deformation occurred and extended along a slip band that ran diagonally across the entire sample region. At last, a cracking formed at the deformation center area and propagated along the crack tip towards the irradiation damage side, ultimately leading to the sample fracture. This study serves as a valuable reference for improving the properties of Z3CN20.09M DSS utilized in nuclear plants
Dynamics and heat transfer characteristics on tube side and shell side of micro-tube compact air-to-air heat exchanger
No full textApplied to the design and selection of compact micro-tube heat exchangers for aviation, an experimental study on the flow and heat transfer performance on both the tube side and shell side of micro-tube heat exchangers was conducted based on the Wilson plot method. The results demonstrated that, although the fluid on the tube side entered the hydraulically smooth region earlier at Reh = 3000, the enhancement in the heat transfer performance and drag reduction characteristics was primarily manifested on the shell side. Compared with traditional empirical correlations, the shell-side Nusselt number showed a maximum enhancement of 47.99 %, and the flow resistance was reduced by a maximum of 59.84 %. Furthermore, a new evaluation index was employed to verify the performance of the heat exchanger. The results indicated that the heat exchanger exhibited advantages in designs requiring low flow resistance, compactness, and lightweight compactness
Ni transformation and hydrochar properties during hydrothermal carbonization of cellulose
No full textThe harm of heavy metals to the environment and human health has become a major concern due to their high toxicity, ease of accumulation in the human body, and resistance to degradation. In particular, Ni is widely used in various industrial and consumer products, which is a toxic pollutant posing great harm to humanity and the environment. Hydrothermal carbonization has broad prospects for reducing the ecological toxicity of heavy metals. However, the effects and mechanisms of hydrothermal carbonization conditions on the stabilization of heavy metals still need to be further explored. This research aimed to explore Ni migration and transformation within cellulose throughout the hydrothermal carbonization process. The results indicated that hydrothermal carbonization facilitated the immobilization of heavy metals due to the generation of hydrochars with complex surface structures. In addition, the hydrothermal carbonization process significantly decreased the weakly bound parts of Ni, thereby reducing the environmental risk of Ni. The optimal conditions for the hydrothermal carbonization process of cellulose added with Ni were 250 degrees C and 90 min. However, further increasing the reaction temperature or retention time resulted in negligible or even negative effects on Ni immobilization. In general, this study proposed possible mechanisms for the effects of hydrothermal carbonization on the migration and immobilization of heavy metals, which may provide insights into handling heavy metals in biomass
Influence of atomic coordination on the activity of lattice oxygen and catalytic oxidation of toluene over regular Cu2O crystalline
No full textVOCs oxidation over transition metal catalyst is commonly understood via the Mars-van Krevelen mechanism involving the crucial role of lattice oxygen (OL) activity, however, how it is influenced by atomic coordination is still unclear. Herein, we use model catalysts of Cu2O-cub, Cu2O-oct and Cu2O-dod with crystal planes of (100), (111) and (110), respectively, to investigate the OL activity and catalytic oxidation of toluene. The activity of Cu2O-oct is found to be the highest, followed by Cu2O-cub and Cu2O-dod. Experiments results combined with density functional theory show that, although low di-coordinated O atoms leads to the lowest surface oxygen vacancy formation energy (2.47 eV) and the highest surface OL activity of Cu2O-cub, it cannot determine the activity. The lowest bulk oxygen vacancy formation energy (3.16 eV) in Cu2O-oct terminated with tricoordinated O atoms and open surface can accelerate the migration and replenishment of OL, thereby promoting the catalytic activity
Industrial-scale sustainable rare earth mining enabled by electrokinetics
No full textOwing to their irreplaceable role in several essential technologies, rare earth elements (REEs) are critical raw materials for the global economy. However, the supply of REEs raises serious sustainability concerns due to the large environmental footprint of conventional mining processes. We previously proposed an electrokinetic mining (EKM) technique that could enable green and selective extraction of REEs from ores. Here we further develop this technique to industrial scale by addressing challenges related to electrode reliability and flow leakage and evaluate its mining efficiency, environmental footprint and economic performance. Moreover, a voltage gradient barrier strategy based on electroosmosis is developed to facilitate electrokinetic REEs mining. As a result, we successfully achieved a high REE recovery efficiency of 95% on a 5,000-ton REEs ore. A rigorous environmental risk assessment revealed a 95% reduction of ammonia emissions, indicating a notably reduced environmental footprint. A comparative technoeconomic analysis between the conventional and the EKM techniques demonstrates the economic viability of the EKM technique. This work validates a new sustainable path for REEs mining, paving the way to a greener resources supply
Effects of Defect, Mean Stress and Lower Loading on High Cycle and Very High Cycle Fatigue Behavior of Ti-6Al-4V Alloy
No full textIn this study, the effects of defect, mean stress and lower loading are investigated for high cycle (HCF) and very high cycle fatigue (VHCF) behavior of Ti-6Al-4V alloy. It indicates that the S-N curve of Ti-6Al-4V alloy exhibits a linear decreasing trend or a plateau characteristic in HCF and VHCF regimes, which depends on the defect size and stress ratio. VHCF strength decreases with increasing the defect size, and it is irrespective of stress ratios. The fatigue crack initiates from specimen surface at R = -1 in both HCF and VHCF regimes. While the fatigue crack initiates from the subsurface or the interior of the specimen at R = 0.1 in VHCF regime. A sequence of lower stress amplitude below the fatigue strength at 109 cyc has no or negligible influence on the fatigue life of 105-109 cyc. The lower stress amplitude in variable amplitude loadings does not affect the failure mechanism. The residual compressive stress relaxation is not observed after a large number of lower loadings under ultrasonic frequency fatigue test. Gerber formula and Goodman formula give dangerous predictions of VHCF strength for both smooth specimens and specimens with defects
A constitutive model for amorphous solids considering intrinsic entangling of shear and dilatation, with application to studying shear-banding
No full textIn amorphous solids, shear transformations, as elementary rearrangement events operating in local regions, are intrinsically entangled with dilatation deformation, which results in the physical process of the shear band being complex. To capture such entanglement, we propose a finite- deformation continuum framework for amorphous solids by incorporating nonequilibrium thermodynamics. Within this framework, we develop a constitutive model where the thermodynamic glass is divided into the kinetic and configurational subsystems. In the model, the dilatation is attributed to an athermal expansion of configuration. As a result, the effect of shear transformation on dilatation can be considered by generating plastic cold work to change the freedom degrees of the configurational subsystem. The effect of dilatation on shear transformation can be realized through the enthalpy change of the configurational subsystem that gives rise to physical aging. Based on the proposed model, we discuss the entangling mechanism of shear and dilatation, and predict the shear-banding behaviors of metallic glasses during tensile and compressive deformations at room temperature. We reveal that due to the shear-dilatation entanglement, the elastic deformations significantly influence the evolution of configurational temperature, which plays a pivotal role in controlling the degree of strain softening and the shear-banding mode
Controls of the Nucleation Rate and Advection Rate on Barite Precipitation in Fractured Porous Media
No full textMineral precipitation is ubiquitous in natural and engineered environments, such as carbon mineralization, contaminant remediation, and oil recovery in unconventional reservoirs. The precipitation process continuously alters the medium permeability, thereby influencing fluid transport and subsequent reaction kinetics. The diversity of preferential precipitation zones controls flow and transport efficiency as well as the capacity of mineral sequestration and immobilization. Taking barite precipitation as an example, previous studies have examined this process in porous and/or fractured media, but pore-scale mechanisms under varying flowing and geochemical conditions remain unexplored. In this study, we conducted real-rock microfluidic experiments to investigate the precipitation dynamics within a fractured porous system. Direct observations of the evolution of the porous structure and flow channel and quantifications of barite precipitation dynamics using X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), revealed two distinct precipitation regimes: precipitation on the fracture surface (regime I) and precipitation in the alteration zone (regime II). Through theoretical analysis of the rate of advection and nucleation, we defined a dimensionless number Da above which regime I occurs and regime II prevails otherwise. At the large Da number, when the precipitation rate is large compared with the flow rate, precipitation on the fracture surface is favored. As the precipitation regimes are expected to impact differently the permeability of the fractured porous media, the mass transfer across matrix and fractures, and the spatial distributions of coprecipitated contaminants, our work sheds light on accurately modeling reactive transport in fractured porous media across diverse applications