Abstract: Polycyclic aromatic hydrocarbons (PAHs) are pollutants that have attracted great attention in recent years regarding the issue of atmospheric pollution. Being harmful to human health, PAHs also promote the growth of secondary particles. Consequently, several countries have begun to control the emission of PAHs in exhaust gases with different techniques, which is a hot topic in the atmospheric environment field. Adsorption is one of the most significant methods and has been accepted by industries, of which the adsorbent properties are key factors. Researchers have studied the adsorption equilibrium, kinetics, and desorption properties of PAHs as special low-volatile gases on conventional activated carbons and novel mesoporous adsorbents. They have also examined the optimal balance between adsorption and desorption based on the corresponding adsorbents. In this paper, these results and related applications are reviewed, and the advantages of mesoporous adsorbents over traditional adsorbents on purifying PAHs are specifically analyzed. This work can be a reference for future studies on the adsorption purification of PAHs and other low-volatile gases.
Abstract: For shale gas exploitation, it is scientifically essential to clarify the mechanism of gas flow in nanopore media. Shale is a kind of tight rock, whose pore size mainly ranges from several nanometers to dozens of nanometers. Since it is of the same magnitude as the gas molecular mean free path, the collision between the gas molecular and pore surface is not negligible for gas flow and results in the failure of Darcy's law when describing this low permeability reservoir gas flow mechanism. In order to clarify the micropore gas flow mechanism and the real extraction process of shale gas, a multiphysics governing equation was established, which combines slip flow, Knudsen diffusion, Langmuir isotherm adsorption, and pore compressibility. The effect of different flow regimes on slip flow was analyzed, and the threshold pore size was obtained with consideration of the slippage effect. Then, the contribution of desorption to gas production rate and gas output was clarified based on the organic content of disparate shale samples. The results indicate that the gas production rate is sensitive to pore compressibility, and it is more reasonable to simulate the gas flow process when considering pore compressibility.
Abstract: To study the dynamic damage characteristics of sandstone under different water bearing conditions, three different kinds of samples with different moisture states (dry, semi-saturated, and saturated) were prepared. The impact damage of the rock was tested using a split Hopkinson pressure bar (SHPB) with four values of low incident energy. The T2 spectrum curves, porosity, and pore image of the samples were obtained by scanning the samples by nuclear magnetic resonance test. The results show the following:(1) an increase in the impact energy led to increased average strain rate and strength; (2) after impact, the porosity and porosity variation rate of every sample under different water bearing conditions increase with different amounts; (3) compared with the test before impact, T2 spectrum curve has obvious right movement trend and large pore peaks increase. The greater the impact energy increase, more obvious the large pore peaks increased are; (4) nuclear magnetic resonance images show that the number and size of pores increase, which show the expansion and evolution of pores in the rocks.
Abstract: The sludge height and underflow rate are the two most important parameters for deep cone thickeners. Therefore, studying the law of change of the underflow concentration with sludge height is important. Discontinuous or continuous dynamic compaction processes of the tailings were investigated physically by using a small, self-made deep cone thickener. Using the power function relation between effective void ratio and mud pressure and combining with force analyses of the tailing particles, this study deduced a mathematical model of underflow concentration and sludge height. The results revealed the inner floc structure relation between underflow concentration and sludge height of the deep cone thickener. In addition, the law of change of this model was explained by applying a spatial structure theory of tailings particles. Based on the requirements for the underflow concentration of mine production, the mathematical model was applied, the reasonable range of sludge height was presented, and the reliability of this model was verified. Moreover, this mathematical model provides a theoretical basis for the design and operation of deep-cone thickeners.
Abstract: The porosity ration of weathered crust elution-deposited rare earth ores is the main factor that affects the leaching results and the stability of the ore body. To determine the strength characteristics of weathered crust elution-deposited rare earth ores under different porosity ratios, six groups of rare earth minerals were reconstituted, and direct shear tests were performed for different samples. The effect of pore evolution on the shear strength of the orebody was studied, and the mechanism of the influence of porosity ratio on cohesion and internal friction angle was revealed. The results show that the unsaturated rare earth ores with different porosity ratios correspond to different shear strengths. The relation between shear stress and shear displacement was summarized as parabolic type based on the experimental data, and the relation between the porosity ratio and the shear strength index was established. The mechanism analysis shows that with an increase in the porosity ratio, the effect of the bonded water film effect gradually weakens, the number of the contact points reduces, and the shear strength of the minerals reduces.
Abstract: Absorption refrigeration is an ideal way to utilize solar energy in summer. For a single-stage solar-driven refrigeration cycle based on the typical working pair of LiBr/H2O, the required generation temperature is up to 88.0℃, which is too high for the commonly used flat plate solar collectors or vacuum glass tube solar collectors. Thus far, this problem has been addressed via two methods:(1) using high-temperature solar collectors and (2) applying a two-stage absorption refrigeration cycle. In literature, a study pertaining to the former was conducted on the performance and economic feasibility of a 100 kW single-stage solar-driven absorption air conditioning system using heat pipe vacuum tube solar collectors, and another study pertaining to the latter was conducted on the performance and economic feasibility of a 100 kW two-stage solar energy absorption refrigeration air conditioning system using flat-plate solar collectors. Their results show that the system cost is high for the former and that the system cost is high and the coefficient of performance is low for the latter. In addition, the results show that the latter system is complicated. Thus, there are currently few commercial applications for the solar-driven absorption refrigeration system. In this study, a new method has been proposed that can address the existing problem, i.e., the required generation temperature is too high for flat plate solar collectors or vacuum glass tube solar collectors in a solar-driven single-stage refrigeration cycle, using a new working pair instead of LiBr/H2O. In this study, it is found that CaCl2/H2O has an absorption characteristic benefitting refrigeration, although its absorption ability is limited owing to its relatively low solubility. Based on CaCl2/H2O, a new working pair of CaCl2-LiBr(1.35:1)/H2O has been proposed. The crystallization temperature, saturated vapor pressure, density, and viscosity of this working pair were systematically measured, and the results show that the required solar collector temperature or the generation temperature of CaCl2-LiBr(1.35:1)/H2O for a single-stage absorption refrigeration cycle is 6.2℃ lower than that of LiBr/H2O under the same refrigeration conditions. In addition, the corrosion rates of the carbon steel, 316L stainless steel, and copper in CaCl2-LiBr(1.35:1)/H2O were measured with a weight loss method, and the results show that the corrosion rates of 316L and copper are sufficiently low for practical applications.
Abstract: The increasing demand for high-quality aluminum alloys in the automobile industry and other manufacturing fields has motivated manufacturers to produce cleaner aluminum alloys. However, conventional methods can barely meet the cleanliness requirements of many applications due to their low removal efficiencies. To develop an innovative and highly efficient method for separating inclusions from aluminum melt, this study investigated the separation behavior of silicon particles by super gravity under different gravity fields using the primary silicon particles of Al-17%Si-4.5%Cu melt to simulate the inclusions in molten metal. The experimental results show that primary silicon particles accumulate in the upper region of samples obtained by super gravity, while the area in which there are no primary particle appears in the sample. The accumulation effect of the silicon particles improves as the gravity coefficients increase. In addition, the purification efficiency of samples obtained by super gravity increases as the gravity coefficient increases. This paper found the purification efficiency of samples to reach 84.98% at a gravity coefficient of G=500. Using the discrete phase model (DPM), the paper also analyzed the forces acting on the particles in the melt and simulated the separation behavior of silicon particles in the melt under various gravity fields. The simulation results indicate that the movement of silicon particles along the direction of super gravity approximately obeys Stokes' law. There results demonstrate that inclusion particles in aluminum melt can be separated effectively by super gravity.
Abstract: Certain requirements regarding the chemical composition of slag have to be satisfied to perform a stable blast furnace operation. Particularly, the ratio of MgO to Al2O3 in the slag is observed to have a significant influence on the fluidity and desulfurization ability of the slag, along with the the stability of the blast furnace. However, the amount of MgO in the ore could not satisfy the smelting requirement of the blast furnace. Thus, Mg-containing flux has to be added into the ferrous burden. In the ferrous burden structure of the blast furnace that is observed in China, sinter generally accounts for more than 70%. Therefore, the effects of various concentrations of MgO on the quality of sinter are of great significance and must be investigated in further detail. In this study, the influence of MgO on the mineral structure and softening-melting property of Ti-containing sinter were investigated using scanning electronic microscopy (SEM)-energy dispersive spectrometer (EDS) and drop testing. The results depict that increasing the concentration of MgO from 2.04% to 3.96% results in a decrease in the contents of hematite and complex calcium ferrite in the sinter, whereas there is an increase in the mass of liquid phase, magnetite, and silicate. Further, the pore size also gradually increases. Additionally, the hematite content reduces from 13.57% to 9.99%, and the complex calcium ferrite in sinter reduces from 38.7% to 30.17%. Therefore, the increase of MgO in sinter is unfavorable for the development of a liquid phase, which depicts a negative effect on the tumbler strength and reduction index of sinter. Mg in sinter is mainly distributed in complex calcium ferrite. Further, an increase in the concentration of MgO causes a gradual increase in the concentration of magnetite. Additionally, it also causes a part of Mg to be dissolved in magnetite. An olivine solid solution, which contains a minor amount of Mg and Ti, can be observed in high-Mg sinter. Finally, by increasing the concentration of MgO in the sinter, the softening temperature can increase, which is observed to be greater than 1120℃. Further, the softening interval (ΔtA) of the sinter is observed to increase gradually with an increasing MgO content.
Abstract: Based on the shortcomings of the traditional fracturing ball, the soluble magnesium alloy with excellent properties was prepared by casting. This study investigated the effects of aluminum elements content on the microstructure, solubility, and mechanical properties. The results reveal that the microstructure of soluble magnesium alloy comprises the following two phases:α-Mg and β-Mg17Al12. β-Mg17Al12 increases with an increase in aluminum content and continuously distributes in the α-Mg grain boundary. In addition, α-Mg becomes coarse. The soluble magnesium alloy could dissolve in the KCl solution, and the dissolution rate increases with an increase in KCl concentration until the rate reaches a maximum at 3% (mass fraction) KCl; the rate subsequently decreases after reaching this maximum. In addition, the dissolution rate increases with an increase in aluminum content until the dissolution rate reaches 7.42 mg·h-1·cm-2. The corrosion product of the soluble magnesium alloy is finer, and the median grain size is 38.691 μm, which ensures a smooth discharge of products, and the corrosion products are Mg17Al12 and Mg(OH)2. The compressive strength of the soluble magnesium alloy could reach 430 MPa and the material breaks when the deformation reaches 3.0%, which the plasticity of this material reduces with increased aluminum content.
Abstract: The conventional grain-oriented (CGO) silicon steel containing copper is a type of grain-oriented steel with low cost and high productivity. The processing parameters in CGO steels are quite different to those of low temperature heating and nitriding high permeability (HiB) grain oriented silicon steels, the fluctuation range of the magnetic properties in CGO is much higher than that of nitrided HiB steels. In this study, typical final sheets from production lines of CGO steels containing copper during early stage production were collected, and the structure, texture, and magnetic properties were determined and compared to establish a relationship between the structure of the steels and inhibitors and to elucidate the reasons for property fluctuation. The results indicate that the macrostructure in the final sheets has a relationship with inhibitors and that the grain sizes do not show monotonous behavior. In addition, the defective microstructure in hot-rolled bands and intermediately annealed sheets are investigated and a coarse grain region and a decarburized surface layer are observed. Such regions are considered to influence the inhibitor distribution and impact the fluctuation in structure and its properties.
Abstract: H11 steel with mass fraction of Al (0.77% and 0) was treated by different quenching and tempering processes, and the variation of hardness and impact energy were systematically investigated. Moreover, carbide extraction at annealed, 1060℃ quenched, 1060℃ quenched + 510℃ tempered, 1060℃ quenched + 560℃ tempered, and 1060℃ quenched + 600℃ tempered were conducted. Finally, the type and morphology of carbides were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The main conclusions are as follows:(1) Al can improve the impact toughness and tempering hardness of H11 steel; however, it reduces the hardness of quenching. (2) Al can promote the dissolution of carbides and the homogeneity of elements during the austenitizing process. (3) Al can prevent the precipitation and accumulation of carbides during the tempering process, which is more obvious under 560℃. (4) Al can prevent the accumulation of carbon and alloy elements, such that (Fe,Cr)2C, MoC and Cr7C3 become more stable and suppresses the precipitation of (Fe,Cr)3C, Mo2C, and Cr23C6 during the tempering process.
Abstract: Due to the rapid development of naval aviation equipment, naval aircraft will encounter complicated atmospheric corrosion problems resulting from exposure to the oceanic atmosphere. The corrosion of the fasteners in the aircraft, especially due to the galvanic corrosion between fasteners, seriously compromises the safety of a particular component in an aircraft. When an aircraft is used for a long duration in an oceanic atmosphere containing high humidity and salinity, a layer of liquid film having a thickness of less than 1 μm will form on the surface of the structure, which causes electrical conduction between different structures, thereby increasing the risk of galvanic corrosion. Currently, many studies mainly focus on investigating the stress corrosion cracking (SCC) of bolt or galvanic corrosion between the bolt and aluminum alloy plates. However, only a few studies have investigated the galvanic corrosion of bolt/nut in assembly. Therefore, three types of nuts (Cd-plated 30CrMnSiA, Zn-plated 30CrMnSiA, and passivated 0Cr16Ni6) were utilized in this study, which were assembled to a Cd-plated 30CrMnSiA bolt. The salt-spray corrosion simulation, observations from scanning electron microscope (SEM), and electrochemical measurements (open circuit potential, potentiodynamic polarization, and galvanic corrosion current tests) were used to investigate the effect of the galvanic corrosion between the bolt/nut couples on the corrosion behaviors of bolts and nuts. The results depict that the highest potential difference and galvanic current are observed between the Cd-plated 30CrMnSiA bolts and passivated 0Cr16Ni6 nuts, which indicates the most significant galvanic effects. Further, the galvanic corrosion sensitivity rating reaches an E level, which significantly promotes the propagation of the pitting corrosion of the bolt. Additionally, the acceleration factor (AF) becomes 3.4. For the couple including the Cd-plated 30CrMnSiA bolt and Zn-plated 30CrMnSiA nut, the nut acts as an anode. Further, the corrosion rate of the nut increases, and the AF approximately becomes 1.2. Compared to the aforementioned two couples, the Cd-plated 30CrMnSiA bolt and 30CrMnSiA nut exhibit the weakest galvanic effect, and the galvanic corrosion sensitivity rating is observed to be at an A level.
Abstract: The hot deformation behavior of TA2, which is commercially pure titanium, was investigated using a Gleeble-3800 simulator over temperature and strain ranges of 750-1000℃ and 0.01-10 s-1, respectively. The results show that during hot compressive deformation, work hardening, dynamic recovery, and dynamic recrystallization occur. The flow stress increases as temperature decreases and strain rate increases. To accurately predict the flow behavior for the alloy, constitutive equations considering the effect of strain were derived based on the obtained experimental data and a hyperbolic sine Arrhenius-type model. The material constants α, n, Q, and lnA were found to be functions of strain and could be fitted by employing a sixth-order polynomial. Subsequently, the developed constitutive model can be employed to describe the deformation behavior of commercially pure titanium TA2.
Abstract: Local deformation of workpieces is a main process characteristic of cross wedge rolling; thus, it is difficult to cross wedge rolling the workpiece with a small area reduction owing to the weak ability to flow in the axial direction and the significant deformation difference between the surface and interior. Besides the central damage of workpiece, a spiral microstructure defect in a certain depth of surface layer reduces the mechanical strength of products manufactured by cross wedge rolling. A rolling experiment is performed to reveal that the spiral microstructure defect appears macroscopically as a bright band along the stretching spiral line in a certain depth of the surface layer after machining and microscopically as a strip microstructure distribution that extends from the surface fold to the interior of the workpiece. The cause of the microstructure defect is investigated by a combination of rolling experiments with finite element analysis, and the metal in the forming zone flows along the negative stretching direction, resulting in the surface fold being distributed along the stretching spiral line and the small axial strain band on the formed surface. Meanwhile, large radial compression near the spiral band causes the directional distribution of the microstructure along the fold cracks from the surface to the interior of the workpiece. In addition, the fillet of the wedge tip is adopted to improve the local metal axial flow in the negative stretching direction, which can eliminate the spiral microstructure defects near the surface and avoid the risk of central damage to ensure that the forming quality in the center of workpiece meets the requirements. Finally, the optimal value of fillet on the wedge tip is determined via experimental verification.
Abstract: This study investigated the cooperative optimal preview tracking problem of discrete-time multi-agent systems under the assumption of the communication topology containing a directed spanning tree. First, a state augmentation technique was used to convert the cooperative preview tracking problem into a global optimal regulation problem of an augmented system. Second, by resorting to relative conclusions about the discrete-time linear quadratic regulation theory, an optimal controller was obtained, which can guarantee the asymptotic stability of the closed-loop augmented system. Moreover, a global optimal preview controller was provided in order for the original system to achieve the tracking consensus. Finally, the simulation results not only verified the effectiveness of the designed controller, but also indicated that it is critical for accurately tracking the reference signal to moderately increase the preview length.
Monthly, started in 1955 Supervising institution：Ministry of Education Sponsoring Institution：University of Science and Technology Beijing Editorial office：Editorial Department of Chinese Journal of Engineering Publisher：Science Press Chairperson：Ren-shu Yang Editor-in-Chief：Ai-xiang Wu ISSN 2095-9389CN 2095-9389
