Results 1 - 10 of 226
Results 1 - 10 of 226. Search took: 0.017 seconds
|Sort by: date | relevance|
[en] To investigate the behaviour of recycled aggregate concrete (RAC) under combined compression and shear stresses, 75 hollow cylinder specimens prepared with various replacement ratios of recycled coarse aggregate (RCA) were tested with a self-designed loading device. The results showed that the failure pattern was similar for RAC with different replacement ratios of RCA. The ultimate shear stress improved with an increasing axial compression ratio of less than 0.6 and declined after exceeding 0.6. A modified failure criterion for RAC with normal strength under combined compression and shear stresses was proposed. A new procedure to predict the shear strength for RAC beams without stirrups was developed based on the proposed failure criterion, showing a better correlation with the experimental results than the predictions calculated by GB50010, Eurocode 2, fib Model Code 2010 and ACI 318-11.
[es]En este estudio, se ensayaron 75 probetas cilíndricas huecas preparadas con distintos porcentajes de sustitución de árido grueso reciclado (RCA) con una máquina de ensayos auto-diseñada con el fin de investigar la resistencia del hormigón con árido reciclado (RAC) a la acción conjunta de los esfuerzos de compresión y de corte. Según los resultados obtenidos, el patrón de fractura del RAC era similar independientemente del porcentaje de sustitución. La resistencia a cortante aumentó hasta una relación de compresión axial de 0.6 y disminuyó a partir de ese valor. En el artículo se propone modificar el criterio de rotura del RAC de resistencia normal ante la acción conjunta de los esfuerzos antedichos. Se ha desarrollado un nuevo procedimiento para predecir la resistencia al corte de las vigas RAC sin estribos basándose en el criterio de rotura propuesto, consiguiéndose una mejor correlación con los resultados experimentales que en el caso de las predicciones calculadas mediante los métodos GB50010, Eurocode 2, fib Model Code 2010 y ACI 318-11
[en] Thermodynamic properties of the air-water mixture at elevated temperatures and pressures are of importance in the design and simulation of the advanced gas turbine systems with water addition. In this paper, comprehensive available experimental data and calculation methods for the air-water mixture were reviewed. It is found that the available experimental data are limited, and the determined temperature is within 75 deg. C. New experimental data are needed to supply in order to verify the model further. Three kinds of models (ideal model, ideal mixing model and real model) were used to calculate saturated vapor composition and enthalpy for the air-water mixture, and the calculated results of these models were compared with experimental data and each other. The comparison shows that for the calculation of saturated vapor composition, the reliable range of the ideal model and ideal mixing model is up to 10 bar. The real model is reliable over a wide temperature and pressure range, and the model proposed by Hyland and Wexler is the best one of today. However, the reliability of the Hyland and Wexler model approved by experimental data is only up to 75 deg. C and 50 bar, and it is necessary to propose a new predictive model based on the available experimental data to be used up to elevated temperatures and pressures. In the calculation of enthalpy, compared to the ideal model, the calculated results of the ideal mixing model are closer to those of real model
[en] A variable structure control (VSC) design for a class of uncertain chaotic systems subject to sector nonlinear inputs is considered in this paper. It is guaranteed that, under the proposed control law, chaotic systems can asymptotically drive the system orbits to arbitrarily desired trajectories even with both uncertainties and input nonlinearities. However, in the sliding mode, the investigated uncertain chaotic system with nonlinear input still possesses advantages of fast response, good transient performance and insensitive to the parameter uncertainties and external disturbances as the systems with linear input. A Duffing-Holmes system is used as an illustrative example to demonstrate the effectiveness of the proposed VSC design method
[en] Based on the concept of variable structure control, this paper investigates the controller design for suppressing hyperchaos of the Roessler system with H∞ norm constraint. A proportional-integral (PI) switching surface is newly adopted so that it becomes easy to solve the H∞ control problem for such chaotic systems in the sliding mode. Based on this PI switching surface, a variable structure controller (VSC) is derived to guarantee the occurrence of sliding mode motion in finite time, even when the system is undergoing both input nonlinearity and external disturbance. Furthermore, a modified continuous controller is proposed to reduce the chattering phenomenon due to the conventional switching type VSC. An illustrative example is given to show the applicability of the proposed VSC design approach
[en] Lignite, a kind of low rank coal, has the characteristics of high moisture, high volatile, high ash and low heat value. The low-temperature pyrolysis technology is potential to improve the utilization efficiency of lignite. Therefore, a lignite-based energy system integrated with pre-drying and low-temperature pyrolysis was proposed in this paper. To assess the influence of pre-drying process, theoretical models were developed based on thermodynamics, and a case analysis was then performed to get the quantitative effect of pre-drying on efficiency of energy utilization. Results show that pre-drying on PPPS theoretical model can significantly improve the utilization of lignite by 1.46%. (Author)
[en] Linear stability of creeping inverted-annular gas-liquid two-phase flow (liquid core and gaseous annular film) in microtubes, where buoyancy effect is suppressed by surface tension, is addressed. Using a long-wave linear stability solution [Physics of Fluids 14 (1971) 251], the flow field is shown to be linearly unstable for long-wavelength axisymmetric disturbances. The flow field approaches a neutrally stable state as the gas film thickness approaches zero, and instability is enhanced as the phasic velocities are reduced. A two-dimensional linear-stability analysis is also performed in order to examine the stability characteristics at the limit of zero phasic velocities. The analysis leads to a dispersion relation that results in an expression for the neutral wavelength that coincides with the prediction of the Kelvin-Helmholtz stability theory. The neutral and fastest-growing wavelengths are shown to be relatively insensitive to the liquid viscosity
[en] Oxygen plasma treatments have been performed prior to contact deposition on both n- and p-type GaN, and the effects of plasma pressure, rf power and treatment time on the contact characteristics are discussed. By exposing the surface of n-type GaN to an oxygen plasma prior to metal deposition, the as-deposited Ti/Al contacts change from rectifying to ohmic, and further improvements are observed after rapid thermal annealing (RTA). A specific contact resistivity better than 10-7 Ω cm2 was obtained using a plasma treatment of 20 s at 30 W and 0.2 mbar, followed by RTA at 500 deg. C in argon. The I-V characteristics of the Ti/Al contacts degraded when plasma treatments were performed for a longer time, at increased plasma pressure, or at higher rf power. However, unlike in the case for n-type GaN, oxygen plasma treatment prior to metal deposition deteriorated the electrical properties of the Ni/Au contacts to p-type GaN. X-ray photoelectron spectroscopy (XPS) was used in order to help elucidate the mechanism behind these effects
[en] Highlights: • A new approach (Li-doped Ca(OH)_2) is used to improve the heat storage process. • Air can seriously affect the heat release process in a fixed-bed test. • A lower initial sample temperature leads to a higher extent of the dehydration reaction. • A higher vapor pressure obviously improves the heat release capacity. - Abstract: The CaO/Ca(OH)_2 thermochemical heat storage system is a very promising method to realize solar thermal energy utilization and industrial waste heat recycling. In this study, a CaO/Ca(OH)_2 test-bed is setup to investigate the heat storage and release processes. In the heat storage process of pure Ca(OH)_2, a higher dehydration temperature results in a faster heat storage speed. In addition, the heat storage process of Li-doped Ca(OH)_2 can obtain a faster heat storage speed and produce more thermal energy than that of pure Ca(OH)_2. The results of the experiment show that the air can seriously affect the heat release process in a fixed-bed test. Therefore, the method of pumping out the air is used in this study to improve the heat release process. According to the results of the heat release experiment, a lower initial sample temperature can lead to a higher extent of the dehydration reaction. Furthermore, a higher vapor pressure obviously improves the heat release capacity.
[en] A real-space first-principles method based on density functional theory has been used to study the energetics, the electronic structure and the magnetic property of single 3d impurity in a GaAs host (with Ga substituted by Cr, Mn, Fe, Co, and Ni, respectively). It is found that Mn exhibits the largest exchange splitting in 3d orbitals, while Fe, Co, and Ni are almost nonmagnetic. The calculated substitutional formation energies are 3.87, 3.21, 1.36, 1.41, 0.37 eV, and the calculated local magnetic moments are 3.25, 4.26, -0.01, -0.03 and -0.01 μB, for Cr, Mn, Fe, Co and Ni, respectively. For a good understanding on the local magnetism of the 3d impurity, we analyzed the bonding characteristics and the charge transfer property in detail, and found that the hybridizations between 3d impurity and its neighboring As atoms play an important role
[en] Second sound in He II has been observed using a heat pulse method. At temperatures where well-developed second sound is observed, the entire pulse shape can be understood if heat sources and geometrical effects are properly taken into account. 4 figures