Two oxide dispersion strengthened (ODS) steels with compositions of 9Cr-ODS (9Cr-ODS) and 9Cr-3Al-ODS (9Cr-ODS-Al) were fabricated by hot isostatic pressing (HIP). The microstructures and precipitates of the ODS samples were characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The results showed that the addition of Al could reduce the precipitation of coarse M23C6 and optimized the size of carbides in ODS steels. Furthermore, it was found that the mean size of nanoparticles in 9Cr-ODS-Al is larger than that in 9Cr-ODS due to the formation of large-size Y–Al–O nanoparticles with Al addition. The tensile test results indicated that the addition of Al slightly decreased the strength of ODS steels, but substantially increased the ductility.
Abstract Heat-resistant poly( m -phenylene isophthalamide) (PMIA) has attracted considerable attention as a novel separator for application in lithium-ion batteries (LIBs); however, its mechanical strength and electrolyte wettability are not ideal. Herein, a nano-silica-decorated poly( m -phenylene isophthalamide) (PMIA@SiO 2 ) separator was fabricated with SiO 2 nanoparticles uniformly attached to the pores and pore walls of the PMIA separator. The as-prepared PMIA@SiO 2 separator has good mechanical strength (a 16% improvement compared with pristine PMIA) and wettability toward the electrolyte (the contact angle decreases from 34.0° to 23.1°). The PMIA@SiO 2 separator also had a high ionic conductivity (0.75 mS/cm) and low interfacial electric resistance (75 Ω). The assembled LiCoO 2 /PMIA@SiO 2 -liquid electrolyte/Li cell displays good cycle performance with a capacity retention of 88.1% after 50 cycles. Furthermore, the cycling performance and rate capacity rarely changed after high-temperature treatment. Therefore, the nano-silica-decorated PMIA separator is a potential candidate for application in LIBs with high safety.
In this paper, the numerical simulation errors of the non-hydrostatic version GRAPES-Meso (Mesoscale of the Global and Regional Assimilation and Prediction System) at the resolution of 0.18?for a torrential rain case, which happened in May 31st to June 1st 2005 over Hunan province, are diagnosed and investigated by using the radiosondes, intensive surface observation and the operational global analysis data, and the sensitivity experimental results as well. It is shown that the GRAPES-Meso could reproduce quite well the main features of large-scale circulations, especially the 500-hpa circulation patterns and their evolutions, while the distribution of the accumulated 24h precipitation and the key locations of the torrential rainfall are captured reasonably well by the model. Seeing from the viewpoint of the synoptic scale, the model could provide valuable numerical guidance for short-range weather forecasting. However, errors exist in the simulation of the mesoscale features of the torrential rain and details of the relevant systems, for example, the simulated rainfall that is too earlier in model integration and remarkable underprediction of the peak value of rainfall rates over the heaviest rainfall region, the weakness of the upper jet simulation and the overprediction of the south-west wind in the lower troposphere etc. The investigation reveals that the sources of the simulation errors are different. The erroneous model rainfall in the earlier integration stage over the heaviest rainfall region is induced by the model initial condition errors of the wind field at about 925hPa over the torrential rainfall region, where the errors grow rapidly and spread upward to about 600hPa level within the few hours into the integration and result in abnormal convergence of the wind and moisture, and thus the unreal rainfall over that region. The large bias on the simulated rainfall intensity over the heaviest rainfall region might be imputed to the following combined factors of (1) the simulation errors on the strength and detailed structures of the upper-level jet core which bring about significant underpredictions of the dynamic conditions (including upper-level divergence and the upward motion) for heavy rainfall due to unfavorable mesoscale vertical coupling between the strong upper-level divergence and lower-level convergence; and (2) the inefficient coupling of the cumulous parameterization scheme and the explicit moisture in the integration, which causes the failure of the explicit moisture scheme in generating grid-scale rainfall in a certain extent through inadequate convective adjustment and feedback to the grid-scale. In addition, the interaction of the combined two factors could form a negative feedback to the rainfall intensity simulation, and eventually lead to the obvious underprediction of the rainfall rate.
The software, Fluent, was used to analyze the cavitation behavior of the submerged water jets generated by contraction nozzle, angle nozzle and organ pipe nozzle. Angle nozzle was chosen for cavitation peening based on the results of numerical simulation. The cavitation peening experiments for aluminum alloy 2A12 were conducted with the different processing parameters. The surface roughness, residual stress distribution and morphology of the treated sample surface were investigated. Results show that the distribution of strengthening area is consistent with that of simulated cavitation bubbles cluster. Using the optimized parameters, the surface residual compressive stress and its depth reach the maximum values of 320MPa and 390μm, respectively, which were increased nearly 2.7 times and 5.5 times than those of the original sample, respectively, while the corresponding surface roughness was only 1.29μm, which was much smaller than that of conventional shot peening.
The present work studies the effect of cylindrical discrete roughness elements on the multi-scale turbulent motions in a canonical turbulent boundary layer. Flow fields with considerably large field of view in streamwise—wall-normal planes were measured by two-dimensional particle image velocimetry. It is found that large-scale motions and very-large-scale motions in the outer layer of the middle-to-far-wake regime are enhanced due to the bottom-up process of roughness-introduced disturbance, in which vortical structures shedding from the roughness element lift up to higher flow layer as they convect downstream, thus promoting the cross-layer interaction. The modulation effect is found to extend to the lower bound of the outer region and persist longer than traditionally believed. An interesting finding is that in the near-wall region of the far-wake regime, small-to-moderate-scale components of Reynolds shear stress is suppressed. The reason might be attributed to the inhibition of the generation of near-wall prograde vortical structures. This provides a new perspective for turbulent control by passive discrete element.
The present work investigates the effect of discrete surface roughness, which falls in the transitionally rough regime, on the near-wall velocity fields of a smooth-wall turbulent boundary layer with zero pressure gradient. A spanwise array of cylindrical elements, whose diameters were and heights were , were attached onto the wall with spanwise spacing of . Three roughness Reynolds numbers , 1000, and 1500 were studied, and the ratio between the roughness height and the boundary-layer thickness was . Two-dimensional velocity fields in multiple wall-parallel planes at were measured by two-dimensional particle image velocimetry. For each case, it is found that roughness elements always introduce -shaped low-speed regions in the near-wall mean velocity fields by direct wake interaction, in the downstream of which a phase shift occurs to form one high-speed mean streak with reduced streamwise velocity fluctuation intensity behind each roughness element, marking the onset of the modulation effect on the mean velocity fields. Further analysis on the swirling strength fields suggests that such a modulation effect can be attributed to the roughness-introduced streamwise vortex pairs, which compete with hairpin vortices shedding from the top of the roughness elements and promote the phase shift in a bottom-up way.
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