Ce17Fe78-xB6Mx (M=Cu, Al, Ga; x=0-1.0)快淬合金永磁性能研究

本文基于熔体快淬技术,研究了低熔点元素Cu、Al和Ga添加对Ce₁₇Fe₇₈B₆合金磁性能的影响。三类低熔点元素的加入,均降低了合金的磁化强度,而矫顽力有一定程度的提升。其中,Cu和Ga元素添加可优化晶粒尺寸分布,且Ga添加对Ce₁₇Fe₇₈B₆合金矫顽力的提升最为有效。研究发现,Ce₁₇Fe₇₈B₆合金回复曲线轻微开口;当Ga添加量为0.75 at.%时,合金具有较优异的综合磁性能,回复曲线完全闭合。适量Ga元素添加明显增强了Ce-Fe-B基合金晶间短程交换耦合作用,减小了合金平均回复磁导率,有效降低了Ce-Fe-B基合金在周期性反向磁场中的能量损失。     

A Numerical Study on Natural Convention Heat Transfer From a Horizontal Isothermal Cylinder Located Underneath an Adiabatic Ceiling

A numerical analysis is performed for steady-state and two-dimensional natural convection heat transfer from a horizontal isothermal cylinder located underneath a horizontal adiabatic ceiling. The finite-volume method based on the SIMPLE algorithm and a nonorthogonal grid discretization scheme are used to solve the continuity, momentum, and energy equations for the Rayleigh numbers in the range from 10^?1 to 10⁴. The Poisson equations are solved to find the grid points, which are distributed in a nonuniform manner with higher concentration close to the solid regions. In addition, the HYBRID differencing scheme is used for the approximation of the convective terms in the curvilinear coordinate. The effects of the Rayleigh numbers and cylinder spacing from the adiabatic ceiling on both the local and average Nusselt numbers around the cylinder are investigated. Numerical results are performed for the plate-to-cylinder spacing ranging from 0.1 to 1.4.     

Spectrum Correlated Criteria and Their Impacts on High Altitude Platform Station (HAPS) and Fixed Satellite Service (FSS) Coexistence in Frequency Range 5,850–7,075 MHz

High altitude platform station (HAPS) is an innovative technology which delivers some unique features, contrary to conventional communications networks, such as fixed satellite service (FSS). The absence of confirmed spectrum emission mask (SEM) of HAPS and its diversity to work within FSS networks are significant issues in evaluating the coexistence of HAPS and FSS. At this juncture, a practical SEM for HAPS gateway links is proposed which will endeavor to assess its functionality and its ability to coexist with FSS. HAPS SEM’s impact on coexistence issues are exposed upon consideration of criteria such as MD, NFD and ACIR. These facets are well-described and their measured amounts for specific applicable SEMs are proposed. Therefore, reckoning process regarding these factors is ascertained. It must be said that the remarked criteria and their amounts for HAPS gateway links are unprecedented. The simulation parameters are well organized based upon International Telecommunication Union and World Radiocommunications Conferences periodicals. The aim of this article is to shed more light on the associated facets of the HAPS network spectrum and their impressions on HAPS and FSS networks coexistence. The assessments have been performed for HAPS gateway links in the 5,850–7,075 MHz frequency band where FSS uplink frequency band in C-band (5,925–6,725 MHz) partially intrude with HAPS gateway links. Therefore, HAPS gateway links channelization regarding FSS uplink channel is appraised and light is shed on the impact of HAPS and FSS coexisting in the same frequency band.     

Systems dynamic model to forecast salinity load to the Colorado River due to urbanization within the Las Vegas Valley

This study evaluates the impact of urban growth in the Las Vegas Valley (LVV), Nevada, USA on salinity of the Colorado River. In the past thirty eight years the LVV population has grown from 273,288 (1970) to 1,986,146 (2008). The wastewater effluents and runoff from the valley are diverted back to the Colorado River through the Las Vegas Wash (LVW). With the growth of the valley, the salinity released from urban areas has increased the level of TDS in the wastewater effluents, ultimately increasing the TDS in the Colorado River. The increased usage of water softeners in residential and commercial locations is a major contributor of TDS in the wastewater effluents.Controlling TDS release to the Colorado River is important because of the 1944 Treaty signed between the USA and Mexico. In addition, the agriculture salinity damage cost for the Colorado River has been estimated to be more than $306 a million per year using 2004 salinity levels. With the expected growth of LVV in coming years the TDS release into Lake Mead will increase over time. For this purpose, it is important to investigate future TDS release into the Colorado in anticipation of potential TDS reducing measures to be adopted. In this research, a dynamic simulation model was developed using system dynamics modeling to carry out water and TDS mass balances over the entire LVV. The dynamic model output agreed with historic data with an average error of 2%. Forecasts revealed that conservation efforts can reduce TDS load by 16% in the year 2035 when compared to the current trend. If total population using water softeners can be limited to 10% in the year 2035, from the current 30% usage, TDS load in the LVW can be reduced by 7%.     

Effect of process conditions on Fischer–Tropsch synthesis product selectivity over an industrial iron-based catalyst in slurry reactor

The authors present the application of the statistical model in CO hydrogenation to CH₄, C₂-C₄ and C₅₊ over industrial iron-based catalyst (100 Fe/5 Cu/4.2 K/25 SiO₂) in a 1-L stirred tank slurry reactor. The effect of different reaction conditions, including temperatures (T = 493, 513 and 533 K), pressures (P = 0.8, 1.5, 2.25, and 2.5 MPa), synthesis gas feed molar ratios (H₂/CO = 0.67 and 2), and gas space velocity (GSV) from 0.52 to 23.5 Ndm³/g-Fe/h on selectivity investigated via a statistical models. The proposed selectivity model is very useful in the oil, gas, and petrochemical industries and can be used for interpretation of experimental data, comparison of performance of different reactor conditions, and reactor modeling and simulation studies. Furthermore, interaction between operating parameters such as pressure, temperature, H₂/CO, and GSV was investigated in selectivity models. A CUBIC polynomial was successfully fitted to the experimental data. It was concluded that C₅₊ selectivity shifts to higher with increasing total pressure (H₂/CO) ratio and decreasing temperature. Decreased H₂/CO ratio and temperature and increasing in the reactor pressure cause CH₄ formation decrease. Temperature and pressure fluctuations vary product distribution. As it is observed, the insignificant term in C₂-C₄ selectivity is pressure. With the models obtained from regression we can reach to the optimum condition for favorite products such as C₂-C₄ or C₅₊. So that optimization must be done to illustrate the optimum conditions. It was obtained that the maximum amount of C₅₊ and C₂-C₄ and minimum amount of methane achieved in T = 528.97 K, P = 1.23 bar, H₂/CO = 2, and GSV = 23.49 Ndm³/g-Fe/h.     

Synthesis and characterization of polyethylene glycol mediated silver nanoparticles by the green method

The roles of green chemistry in nanotechnology and nanoscience fields are very significant in the synthesis of diverse nanomaterials. Herein, we report a green chemistry method for synthesized colloidal silver nanoparticles (Ag NPs) in polymeric media. The colloidal Ag NPs were synthesized in an aqueous solution using silver nitrate, polyethylene glycol (PEG), and β-D-glucose as a silver precursor, stabilizer, and reducing agent, respectively. The properties of synthesized colloidal Ag NPs were studied at different reaction times. The ultraviolet-visible spectra were in excellent agreement with the obtained nanostructure studies performed by transmission electron microscopy (TEM) and their size distributions. The Ag NPs were characterized by utilizing X-ray diffraction (XRD), zeta potential measurements and Fourier transform infrared (FT-IR). The use of green chemistry reagents, such as glucose, provides green and economic features to this work.     

Density Variation in the Ultrathin Liquid Perfluoropolyether Films on Solid Surfaces

Variation of density in the ultrathin liquid perfluoropolyether films on solid surfaces has been studied by Monte Carlo simulations of Lennard-Jones systems. The liquid film is 8.75 nm thin film consisting of polymer molecules assuming the structure of perfluoropolyether Z having molecular weights of 3840, 2500 and 1700 g/mol and interacting among themselves by Lennard-Jones potential. The substrate is assumed to be continuous without atomic structure and exerting Lennard-Jones potential on liquid molecules in the ultrathin film. The system temperature is considered to be 25 °C and the liquid molecules also have the gravitational potential. It is found that the bead density decreases towards the surface in a thinner sublayer in the ultrathin liquid film above the surface and the thickness of this sublayer just above the surface may increase with the increase of molecular weight of the of polymers in the film. Repulsive potential of the surface further decreases the bead density near the surface. The results are compared with the experimental results of the pefluoropolyether lurbricants by X-ray reflectivity (Shouji, et al., 1998).     

An Experimental Investigation of Silica Nanoparticles Effect on the Rheological Behavior of Polyacrylamide Solution to Enhance Heavy Oil Recovery

The use of polymer flooding as one of enhanced oil recovery methods has recently increased. The occurrence of high shear rates in reservoir and near well bore through perforation nozzles during polymer flooding cause shear degradation of polymers and therefore polymer viscosity has decreased. Rheological behavior of polymer solution in different conditions of oil reservoir is one of the key factors to develop use of polymer solutions. A few researches are available regarding improving rheological behavior of polymeric solution. In this study, to investigate the effect of nanoparticles on rheological behavior of polymer solutions two samples were prepared: polyacrylamide solution in water and suspension of silica nanoparticles in polyacrylamide solution. The sample viscosities in different shear rates were measured. The best rheology models were developed to state rheological behavior of prepared samples and the measured data were compared to power law model. An increase in the viscosity of the suspension solution with respect to polymer solution in different shear rates was observed. Rheological analysis showed that power law model is a good rheology model to demonstrate rheological behavior of suspension in low and medium shear rates and is an acceptable model for polymer solution in low shear rates. Two types of flooding test were performed in a glass micromodel: flooding by polyacrylamide solution and suspension of silica nanoparticles in polyacrylamide solution. The results of flooding test showed a 10% increase in oil recovery for nanosuspension solution in comparison with polymer solution after one pore volume fluid injection.     

Modeling of CO2 capture from gas stream emissions of petrochemical industries by membrane contactor

Abstract

Process optimization of CO₂ removal from natural gas by a polyvinylidene fluoride hollow-fiber membrane contactor is a major goal of many computational fluid dynamics (CFD) simulations in this area. In this study, a 2D CFD model based on mass transfer equation inside the tube, the membrane, and the shell section of a HMFC at steady state and laminar conditions is developed and solved by COMSOL Multiphysics with finite element approach. Simulation results show an excellent agreement with experimental data. The model predicts that higher liquid velocity and membrane porosity results in higher CO₂ removal, because of enhancement of effective diffusion coefficient. Also, taller fiber length results in higher contact area and higher mass transfer of CO₂ from natural gas into distilled water. Although higher temperature will decrease the CO₂ removal.