•The frequency characteristics of SRF-PLL, DSOGI-FLL and DDSRF-PLL are modeled respectively.•The impedances of the inverter are modeled with the PLLs displayed, and the influences of different PLLs ...on the inverter impedance are compared.•The effects of the PLLs on the stability of the system are studied to discuss the stability boundary conditions of the system.•Based on the equivalent impedance model of the PLL, the optimized design method of the PLL is proposed to improve the system stability.
For grid-connected inverters, phase-locked loop (PLL) is an indispensable part for grid currents to track grid voltages. Hence, PLL will have a nonnegligible effect on the stability of the interconnected system involving a grid-connected inverter and a power grid. Therefore, it is necessary to model the frequency characteristic of PLL and analyze its effect on the system stability. In this paper, three kinds of the commonly used PLL circuits are firstly employed to model and compare their frequency characteristics. Second, the impedances of the inverter are modeled with the equivalent impedance model of PLL displayed, and the influences of different PLLs on the impedance characteristics of the inverter are compared. Then, the effects of PLLs on the stability of the interconnected system are studied to discuss the stability boundary conditions of the system. Based on the equivalent impedance model of the PLL, the optimized design method of the PLL is proposed from the perspective of optimizing the inverter impedance characteristics to improve the system stability. Finally, the simulation and experimental results are presented to verify the correctness of the PLL frequency characteristics and the inverter impedance models, and confirm the effectiveness of the optimized design method.
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•The tunnel slag waste is recycled to produce concrete material.•Appropriately adding TSP is beneficial to improve the performance of tunnel slag aggregate-based concrete.•Recycling ...of high-volume slag waste is beneficial to the sustainability of concrete.
In order to promote the resource utilization of tunnel slag and investigate the influence of tunnel slag stone powder (TSP) on the uniaxial compression performance and microstructure of concrete, the tunnel slag aggregate-based concrete with TSP content of 0%, 4%, 8% and 12% was tested and compared with natural aggregate-based concrete (NABC). The influence of TSP on workability and uniaxial compression characteristic parameters of tunnel slag aggregate-based concrete was analyzed. The results show that adding an appropriate amount of TSP (4%) can increase the slump and compressive strength of concrete. The shape of stress–strain curve of tunnel slag aggregate-based concrete is similar to that of NABC. Adding appropriate amount of TSP increases the peak stress and peak strain of concrete, and improves the toughness of concrete. TSP has weak reactivity and can react with tricalcium aluminate to produce hydrated calcium carboaluminate. The porosity of tunnel slag aggregate-based concrete is lower than that of NABC, and a small amount of TSP is added to further optimize the pore structure. TSP improves the performance of the interfacial transition zone (ITZ) of slag aggregate-based concrete by increasing the density of hydration products at the interface. This study provides a theoretical basis for the in-situ disposal of tunnel slag waste and the development of full-volume slag aggregate-based concrete.
The structure and thermoacoustic instability (TI) of premixed CH4/H2/air swirling flames were experimentally investigated for a range of hydrogen fraction (ηH2) up to 80% under different equivalence ...ratio (Φ) and swirl number (S) conditions. It is shown that the onset of TI is enhanced when increasing either ηH2, S, or Φ. The dominant frequency of TI increases dramatically with ηH2. The higher dominant frequency in the hydrogen-enriched flames can be attributed to a shorter flame length which results in a reduced flame convection time. It is observed that the unstable flames are always accompanied by the appearance of outer recirculation zone (ORZ) flame. Therefore, the flame kernel residing in the ORZ can be an indicator of the occurrence of TI. The flame front of thermoacoustic unstable flames was observed to be more wrinkled, e.g., with larger mean absolute curvature (κabs) and local flame surface area ratio (δΣmax). Importantly, the phase-locked analysis shows that κabs and δΣmax can be modified at different oscillation phases, and their maximum and minimum values are simultaneously achieved at phase angles θ of about 0° and 180°, respectively. Variations of κabs and δΣmax are in phase with the heat release rate, indicating a strong correlation between the TI and flame structure modification; however, they show a phase lag of about 72° behind the pressure in this work. These results are vital when understanding and predicting the TI based on the flame structure, especially when adopting a visual detection method of the instability.
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•Thermoacoustic instability of swirling lean premixed CH4/H2/air flames is studied.•The dominant frequency increases significantly with hydrogen fraction.•Appearance of the ORZ flame is highly related to the instability.•The flame front is more wrinkled in unstable flames.•Flame curvature and surface area oscillate in phase with heat release rate.
The transport of chloride in cement-based materials is influenced by various factors, and the chloride diffusion coefficient in numerical models should be the steady-state value under specific ...conditions, which is difficult to obtain through traditional experiments. In this article, based on an improved non-contact resistivity instrument, the steady-state chloride diffusion coefficient and porosity of cement-based materials are characterized. Considering the acceleration effect of moisture convection and electric field on chloride transport, a numerical model for moisture and chloride transport coupling diffusion, convection, and electromigration is established. Based on the measured steady-state chloride diffusion coefficient and porosity values, the influence of interface transition zone (ITZ), electric field type, chloride binding effect, initial conditions, and boundary conditions on moisture and chloride transport is numerically analyzed. It is found that the increase of ITZ thickness, electric field strength, boundary concentration, and initial concentration can accelerate chloride transport. The chloride concentration distribution curve exhibits a “shoulder peak” caused by chloride binding effect, and its peak height and width are positively correlated with the chloride binding coefficient. In conclusion, this study aims to better understand the ion transport behavior inside cement-based materials under coupling of convection, diffusion, and electromigration, and identify the key factors influencing this behavior, which provide a feasible scheme for durable design of concrete construction in environments rich in erosive media.
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•The chloride diffusion coefficient and porosity of cement-based materials were measured using the improved NC-ERM method.•A numerical model was established for cement-based materials, incorporating convection, diffusion, and electromigration.•Accelerated chloride transport was observed with ITZ thickness, electric potential difference, and concentration.
Inverse microemulsion copolymerization of acrylamide (Am), acrylic acid (AA), and sodium 4-styrenesulfonate (SSS) initiated by redox initiators composed of ammonium peroxodisulphate (APS) and sodium ...bisulfite, and stabilized by the mixed emulsifier system sorbitan monooleate (Span-80) and polyoxyethylene sorbitan monooleate (Tween 80) were examined as a function of the combination of hydrophilic (Tween 80) and hydrophobic (Span 80) emulsifiers, reaction temperature, AM/AA mass ratio, SSS concentration, and initiator concentration. The physicochemical and thermal properties and the structure of this copolymer were also determined and discussed. The reaction rates for all runs of the experiments exhibited two intervals, which were typical of microemulsion polymerization. The copolymer had only one glass transition temperature of 115.5 °C, indicating a random structure.
A new method is proposed for solving box constrained global optimization problems. The basic idea of the method is described as follows: Constructing a so-called cut-peak function and a choice ...function for each present minimizer, the original problem of finding a global solution is converted into an auxiliary minimization problem of finding local minimizers of the choice function, whose objective function values are smaller than the previous ones. For a local minimum solution of auxiliary problems this procedure is repeated until no new minimizer with a smaller objective function value could be found for the last minimizer. Construction of auxiliary problems and choice of parameters are relatively simple, so the algorithm is relatively easy to implement, and the results of the numerical tests are satisfactory compared to other methods.
Surface protective coatings are prone to cracking and failure due to the deformation of concrete structure itself and harsh service environment factors such as UV, ion erosion, and large temperature ...differences during the period of service. It is urgently required to develop new types of self-healing coatings to provide long-lasting protection for concrete structures and extend their service life. In this investigation, a lignin-based self-healing agent was prepared using aniline and lignin as raw materials. Its characterization was analyzed by SEM and FT-IR. By introducing the lignin-based healing agent and dynamic disulfide bonds, a new kind of self-healing polyurethane coating was prepared, and its self-healing ability was qualitatively and quantitatively evaluated, indicating that it had excellent self-healing performance. After self-healing process, the crack could withstand bending force. Moreover, with the increased temperature, the self-healing efficiency of tensile strength gradually increased, reaching a maximum of 86.42% at 60 ℃. Meanwhile, the self-healing efficiency of its elongation at break firstly increased and then gradually decreased, reaching a maximum of 93.77% at 40 ℃. This self-healing polyurethane coating had high hydrophobicity (139.4°) and excellent abrasion resistance. After 40 friction cycles, its hydrophobicity only decreased by 10.9% to 124.2°. In addition, this self-healing protective coating also showed excellent resistance to chloride ions, effectively protecting cementitious materials with the damaged surface. This work provides a new method for long-term protection of concrete structures.
•Self-healing polyurethane coating had excellent self-healing performance.•Self-healing polyurethane coating had high hydrophobicity and good abrasion resistance.•Self-healing polyurethane coating had positive protective effects on cement-based materials.
The present study involves the application of the non-contact electrical resistivity measurement (NC-ERM) as a non-destructive method to continuously and in-situ measure the resistivity changes of ...cement-based materials. The NC-ERM was enhanced and utilized to evaluate the resistivity of the materials, and the steady-state chloride diffusion coefficients of various types of cement-based materials were experimentally determined. The results revealed that the highest resistivity was observed in mortar with a sand-to-cement ratio of 1.0 and concrete with an aggregate-to-cement ratio of 0.6, and the steady-state chloride diffusion coefficient increased with the concentration of the pore solution. Furthermore, a novel multi-level series-parallel model was developed to predict the resistivity and steady-state chloride diffusion coefficient of cement-based materials, with calculated values differing by 5% or less. The calculated chloride diffusion coefficient closely matched the value obtained from NC-ERM, indicating the satisfactory accuracy of the non-destructive experiment and theoretical calculation model.
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•A non-contact method for chloride diffusion coefficients is provided.•A theoretical model for chloride diffusion coefficient prediction is provided.•1.0 Sand-binder and 0.6 aggregate-binder ratio show highest electrical resistance.
Brain age has become an important analysis object in the diagnosis and mechanism research of neurodegenerative diseases. There is no consistent conclusion on whether major depression increases the ...brain age of patients, and few studies in this direction have been conducted in the Chinese population. In this paper, a REST-meta-MDD (resting-state functional magnetic resonance imaging dataset of major depressive disorder) dataset collected from 25 hospitals in China was used to construct a convolutional neural network model based on high-resolution T1-weighted three-dimensional magnetic resonance images of brain structures to predict the brain age of patients and calculate the difference from the actual age. The mean absolute error and correlation coefficients of the final results were 3.16 and 0.93, and the mean brain age of the patients with major depression increased by 3.94 years compared with the healthy group, further confirming that major depression accelerates brain aging, and the severity of the disease
The freeze–thaw damage of cement-based materials is a widely considered durability problem, which can be effectively inhibited by keeping its temperature over freezing point. In this paper, a methyl ...laurate/diatomite microcapsule phase change material (CPCM) was fabricated by porous adsorption and microencapsulation. It is found that an ethyl cellulose film is suitable as a packaging film of CPCM, and it can effectively prevent the leakage of methyl laurate during the phase change process. Based on the BET, XRD, FT-IR and TG-DSC tests, it was proven that the methyl laurate/diatomite CPCM has good chemical stability and thermal stability. The phase change temperature of the CPCM ranges from −1.75 °C to −5.51 °C, and the latent heat value is 65.70 J/g. The methyl laurate/diatomite CPCM was incorporated into cement paste at 10 %, 15 % and 20 % mass fractions, and the mass loss of the cement paste in freeze–thaw cycles can be reduced by up to 57 %; however, the mechanical properties of the cement paste decrease with increasing mass fraction of CPCM. Finally, a thermal transfer numerical model of the cement-based material with CPCM was established, based on which the simulation results agree well with the experimental values. The results show that the existence of CPCM could maintain the temperature of the specimen above zero for 4 h at an ambient temperature of −10 °C. The CPCM prepared in this study can significantly enhance the frost resistance of cement paste.
•An ethyl cellulose film can effectively prevent the leakage of methyl laurate during the phase change process.•The phase change temperature of the CPCM ranges from −1.75 °C to −5.51 °C, and the latent heat value is 65.70 J/g.•A thermal transfer numerical model of the cement-based material with CPCM was established.
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