Volume 43,Issue 8,2020 Table of Contents

1 Solid mechanical simulation and pore-scale modelling of reconstructed carbon felt for vanadium redox flow battery

SUI Bangjie , Roswitha Zeis , LENG Wenliang , DUAN Kangjun , ZHU Lijun , LUO Maji , XIAO Liusheng

2020, 43(8):1-10. DOI: 10.11835/j.issn.1000-582X.2020.08.001

[Abstract](733) [HTML](1041) [PDF 6.60 M](1658)

Abstract:
X-ray computed tomography (XCT) reconstruction technique, finite element method (FEM)and pore scale modelling (PSM) were employed to investigate the displacement distribution of vanadium redox flow battery (VRFB) carbon felt and its effects on the transport properties under compression at pore scale. With contact friction, and extrusion bending between carbon fibers taken into consideration, the microstructure of a carbon felt was reconstructed by XCT first, then the 3D (X, Y and Z direction) displacement distribution of the microstructure with different compression ratio (CR) was investigated, and the relationship between the displacement and transport properties was quantified. The results show that the carbon-fiber displacement in the Z direction (through plane) under compression is more noticeable. As CR is increased to 30%, the displacement change in Z of domains A and B are -59~+5 μm and -145~+16 μm, respectively. The diffusion coefficient of the vanadium ion in XY direction is decreased by 15.4%, and in Z direction by 24.2%.The conductivity in XY direction is increased by 112.5%, and in Z direction by 113.3%. The simulation results are in good agreement with the experimental data, and the suggested domain size that can better simulate and predict the carbon felt conductivity is: X/Y=300~400 μm, and Z=200~400 μm.

2 Fault diagnosis of wind turbines gearbox bearings based on information fusion

WANG Yili , ZHU Caichao , LU Jiong

2020, 43(8):11-22. DOI: 10.11835/j.issn.1000-582X.2020.08.002

[Abstract](738) [HTML](662) [PDF 2.23 M](1300)

Abstract:
A fault diagnosis method for wind turbines gearbox bearings was proposed based on information fusion combining BP neural network and D-S evidence theory. Firstly, based on big data, the fault characteristics of vibration, temperature, current, torque and rotating speed signals related to the faults of wind turbines gearbox bearings in SCADA system were explored. Then, the fault feature quantity of each signal was used as the input of the neural network, and the outputs of the neural network were normalized as the Basic Probability Assignment (BPA value) of D-S evidence theory. In order to solve the conflict between evidences, a weighted-based improvement method was used to improve the evidence. Finally, the combination rules were used to fuse the improved evidences to obtain final diagnosis results. The study was based on actual operating data of a 2 MW wind turbine in a wind farm, and the results show that as the dimension of the fusion signal increases, the accuracy of the final diagnosis will gradually increase. The reliability of fusing multi-dimensional signals is significantly higher than that of single signals.

3 Optimal design of the structure of CFRP-TRB supper-hybrid composite B pillar

JING Min , HU Ning , NING Huiming , YANG Zhonglei

2020, 43(8):23-31. DOI: 10.11835/j.issn.1000-582X.2020.08.003

[Abstract](770) [HTML](738) [PDF 3.01 M](1207)

Abstract:
The B pillar is one of the key load-bearing structural members of vehicles. To improve the side crash performances of a vehicle and realize the lightweight design of the B pillar, a hybrid structure of B pillar made from carbon fiber reinforced composite (CFRP)-tailor rolling steel blank (TRB) was adopted based on structural and different material characteristics of the B pillar. The traditional reinforcing plate in the middle of the B pillar was removed, and the B pillar outer plate was designed as a TRB steel plate with the CFRP partially attached to the inner side of the B pillar outer plate for reinforcement. In addition, the thickness distribution of the inner and outer plates of the hybrid structure B pillar and the layup order and layup thickness of the reinforced composite were optimized based on the software of Optistruct. Finally, the side crash simulation results of the whole vehicle show that the optimized hybrid-structure B pillar achieves a weight reduction of 27.7% and the resistance to side impact is improved.

4 High-temperature decomposition kinetics of large particle-size limestone

HU Bin , CHEN Kaifeng , XUE Zhengliang , WANG Liyou

2020, 43(8):32-46. DOI: 10.11835/j.issn.1000-582X.2020.08.004

[Abstract](611) [HTML](665) [PDF 5.19 M](1257)

Abstract:
The models of substituting limestone for lime in a converter in the slag-splashing furnace-protection stage and open blowing stage of steel making were established, and the high-temperature decomposition experiments of large particle-size limestone were carried out for different particle size under different heat transfer conditions. The decomposition conversion rate of limestone α was obtained by experiments, and through the analysis of suitable rate control steps, isothermal kinetic mechanism of large size limestone heating in the two stages were researched. The results show that the conversion of limestone decreases with the increase of particle size and increases with the rise of temperature. Interfacial chemical reaction controls the rate in slag-splashing furnace-protection stage, the decomposition model is random nucleation and subsequent growth pattern, and its mechanism is in line with the Avrami-Erofeev equation. The most probable integral fuction of limestone decomposition is G(α)=[-ln(1-α)]ⁿ, n=3/4. Product layer heat transfer and CO₂ diffusion control the rate in the open blowing stage. When the temperature is constant, there is a linear relationship between time t and (1-α)ln(1-α)+α.

5 Design of a wideband metamaterial absorber based on double-layer resistive film

LI Jianhui , MENG Jie , LYU Zhongliang , YANG Mengtao , LI Zelun , DENG Xianling

2020, 43(8):47-53. DOI: 10.11835/j.issn.1000-582X.2020.103

[Abstract](553) [HTML](1331) [PDF 2.88 M](1255)

Abstract:
A wideband, polarization-insensitive and wide-angle metamaterial absorber based on double-layer resistive film is proposed in this paper. A unit cell of the absorber consists of a ring resistive film, a substrate, a ring resistive film, a substrate layer and a metal backboard in turn. The simulations and analysis of electromagnetic absorbing properties of this absorber are performed by the finite-difference time-domain method. The simulated reflection and absorption indicate that the absorption of incoming electromagnetic waves by this absorber is higher than 90% in a frequency range of 11.5 GHz to 20.3 GHz. The simulated absorptions under different polarization conditions and incident angles show that this absorber is polarization-insensitive and of a wide incident angle. The simulated influence of structure parameters on the absorption indicates that the absorption of this absorber originates mainly from the absorbing mechanism of circuit resonance. The operation frequency range and band width can be adjusted by the design of the substrate thickness, the resistance and the width of the resistive film so as to achieve the strongest absorption with an ultra-wide band.

6 Properties and pore structure of foam concrete with different density

SUN Guoxing , ZHU Yu , WANG Shen , SUN Chao , ZHANG Yamei

2020, 43(8):54-63. DOI: 10.11835/j.issn.1000-582X.2020.102

[Abstract](929) [HTML](1120) [PDF 9.01 M](2328)

Abstract:
Density of foam concrete has a significant impact on its performance. In this study, foam concrete with dry density of 300 kg/m³ to 1 000 kg/m³ was prepared to test the slurry fluidity, compressive strength, water absorption, and drying shrinkage; and pore structures of the specimens were characterized by X-CT(X-ray computed tomography). The results showed that the fluidity of the slurry firstly increased and then decreased with the increase of the foam concrete density, and reached a maximum at 600 kg/m³. The compressive strength of the foam concrete increased gradually and there was an exponential relationship between it and the density grade. The drying shrinkage and water absorption were all gradually reduced with the increase of the density due to the decrease of internal porosity and the increase of closed pores. X-CT analysis showed that the pores in the foamed concrete with a density less than 600 kg/m³ were unevenly distributed, and there were more large pores on the surface of the specimen compared with the bulk. It is presumed that the volume ratio of the cement paste to the foam is too low for the paste to uniformly wrap the bubbles on the surface of the foam concrete.

7 Effect of grain control of Fe-Al-Cr alloy on its resistance to high temperature oxidation

WANG Yanhui , BO Xinwei , YAO Zhiyuan , LIU Shouping , WANG Xiaoyu , LIU Qi , WANG Jintao , HAN Xiaoyu

2020, 43(8):64-73. DOI: 10.11835/j.issn.1000-582X.2019.258

[Abstract](751) [HTML](578) [PDF 19.10 M](870)

Abstract:
Fe-Al-Cr alloy has wide application prospects with its good resistance to high temperature oxidation, but it usually has the problem of coarse grains. In this research, by adding austenitizing elements such as nickel, carbon and copper to the Fe-Al-Cr alloy, the FeCr phase was formed during the cooling process, which hindered grain growth and the grain size could be controlled within 100 μm. The formation mechanism of oxide film and the effect of grain control on the high temperature oxidation resistance of Fe-Al-Cr alloy were discussed. The Fe-Al-Cr alloy oxidized for different oxidation time at high temperature was observed by metallography and it was found that the alumina film preferentially formed at the grain boundary, and the increase in the number of grain boundaries was favorable for the formation and growth of Al₂O₃. Electron microscopy scan and energy spectrum analysis revealed that (Al_0.9Cr_0.1)₂O₃ and Al₂O₃ formed a multilayer oxide. Combined with the first principle calculation, it is concluded that the multilayer oxide is formed on the surface of the alloy due to the diffusion of Cr, which is helpful to improving the high temperature oxidation resistance of the Fe-Al-Cr alloy. It is confirmed that the Fe-Al-Cr alloy with a smaller grain size has better resistance to high temperature oxidation.

8 Research progress of surface-recrystallization control technology of Ni-based single-crystal superalloy

DU Juan , JIANG Wenjuan , CHEN Suling

2020, 43(8):74-79. DOI: 10.11835/j.issn.1000-582X.2020.08.008

[Abstract](662) [HTML](844) [PDF 1.64 M](1104)

Abstract:
In this paper, the research progress of the surface-recrystallization control technology of Ni-based single-crystal superalloy is reviewed and analyzed from several aspects including the mechanism and surface effects of surface recrystallization, the component design for recrystallization prevention, and recrystallization control technology during thermal forming or cold working progress. The focus is on discussing recrystallization control technology in different countries. Meanwhile, the gap between China and some other countries in this respect is pointed out, and the future research scope is also discussed.

9 Persistent currents in toroidal carbon nanoscrolls

WANG Zhao

2020, 43(8):80-86. DOI: 10.11835/j.issn.1000-582X.2020.104

[Abstract](407) [HTML](592) [PDF 3.05 M](834)

Abstract:
Based on the toroidal carbon nanoscrolls tight-binding model, the expressions of persistent currents are derived, and the effects of geometric structure, separation energy level, temperature and Zeeman effect on the persistent current are systematically studied. The results show that the magnitude of the persistent current in the toroidal carbon nanoscrolls is only 10^-7 A, and as the circumference of the coil increases, the persistent current will gradually decrease. The states near the Fermi level carries almost all the persistent current, while the far-off states contribute very little. As the temperature rises, the persistent current decreases sharply; the larger the coil, the more obvious the influence of temperature on the persistent current. With the Zeeman effect taken into account, the persistent current changes with a new decrease and the periodic structure is destroyed for a stronger magnetic field.

10 Spatial distribution characteristics and influencing factors of landslide disasters in mountain cities

CHEN Jie , YU Xin , NIE Wen , XIE Wei , TENG Degui , WANG Xinsheng

2020, 43(8):87-96. DOI: 10.11835/j.issn.1000-582X.2020.08.010

[Abstract](672) [HTML](1863) [PDF 11.36 M](918)

Abstract:
In order to investigate the uncertainty of landslide disasters affected areas in mountain cities, the typical landslides in the central urban area of Chongqing were selected as the research objects, and the spatial distribution characteristics of historical landslide disaster points were analyzed by the nearest neighbor index, spatial hotspot detection and kernel density estimation methods. A landslide factor database was established with 12 influencing factors including elevation, slope, aspect, landform type, geological lithology, soil type, soil erosion, rainfall, water system, land use, normalized difference vegetation index (NDVI), and population density. A neural network model was used to analyze driving factors of spatial distribution characteristics of landslide disasters and quantitatively calculate the contribution weight of each influencing factor. The accuracy of the model was evaluated with the receiver operating characteristic (ROC) curve. The results obtained from the nearest neighbor index analysis show that the historical landslide disaster points in the study area are clustered, and the spatial hotspot detection and kernel density estimation indicate that Yuzhong District, Shapingba District, and northern Banan District are the areas where the landslides are most concentrated. Among all the factors, population density, land use and rainfall occupy the highest weight, while the weight of aspect and road are the lowest. The area value under the ROC curve (AUC) is 0.917, indicating that the model can accurately reflect the impact of landslide influencing factors in the area.

11 Study on the release fluxes of carbon source gases under different operating conditions in the middle reach of the main stream of the Three Gorges Reservoir

ZHENG Wang , ZHANG Yuyang , JIANG Chengyong , QIN Yu

2020, 43(8):97-106. DOI: 10.11835/j.issn.1000-582X.2019.263

[Abstract](551) [HTML](543) [PDF 5.77 M](1081)

Abstract:
The input of large quantities of organic carbon sources from the land, and the discharge of carbon sources such as CO₂ and CH₄ caused by the operation and damming in the Three Gorges Reservoir Area have become the hot spots of recent research. In order to reveal the variation characteristics of CO₂ and CH₄ concentrations in different flow levels in the Three Gorges Reservoir Area, this study took the middle reach of the main stream of the Three Gorges Reservoir as the research objects. Samples were taken from Zhongxian County (ZX), Wanzhou (WZ), Fuling (FL)monthly from August 2017 to November 2018 to analyze and study the characteristics of CO₂ and CH₄ fluxes at the water-air interface under different operating conditions. Results showed that during the research period, CO₂concentration of the ZX, WZ and FL was between 0.008 6 mmol/L and 0.115 3 mmol/L, 0.003 5 mmol/L and 0.116 8 mmol/L, and 0.006 3 mmol/L and 0.098 6 mmol/L, respectively. The concentration of CO₂ at these three sampling points tended to be consistent and reached its peak during the drain period from April to May and its lowest point in the low water level operation period from June to September. CH₄ flux exhibited the feature of a sink during the operation at a low water level, but at FL it reached the maximum value as a source in the discharge period. The CO₂ fluxes at the three locations showed the feature of a source during the discharge period, but during low water level operation period, CO₂ flux indicated the switch between a source and a sink. CO₂ flux was at a medium level and CH₄ flux was at a lower-middle level compared with those of other reservoirs in the world.

12 A feedback cognition method of insulator state based on attention mechanism

ZHANG Haitai , YANG Jianping , LI Jing , SU Xinyan

2020, 43(8):107-116. DOI: 10.11835/j.issn.1000-582X.2019.280

[Abstract](608) [HTML](614) [PDF 5.72 M](1007)

Abstract:
To overcome the drawbacks of the existing insulator state recognition models, open-loop cognitive mode and insufficient generalization ability of loss function for detailed recognition deep network, in this paper a feedback cognition method of insulator state is proposed based on attention mechanism in imitation of human inspection mode, i.e. real-time evaluation of reliability of cognitive results and self-optimizing and regulation of the multi-scale image knowledge space. Firstly, for the pre-processed insulator image, a stacked convolutional neural network with adaptive scale architecture is designed, which enables the network input to be scaled from the overall image to the detailed area. Each scaled network shares the same architecture with different parameters to ensure the discriminative ability of different resolution inputs and generate a detailed attention area for the next scale. Secondly, for multiple scale features, stochastic configuration network (SCN) builds the classification criterion of the insulator states with universal approximation ability. Thirdly, an inter-class classification loss function and an intra-class ranking loss function are constructed to optimize the attention network, which generates a higher confidence score ranking than the previous prediction. Finally, learning from closed-loop control idea, the generalized error entropy performance index is defined to evaluate the reliability of the uncertain cognition results of insulator states in real time.The network scale level is dynamically regulated to realize the self-optimizing regulation of the feature space and the reconstruction of the classification criteria based on the constraint of the uncertain detection results, which enables insulator states to be re-recognized with feedback mechanism. Experimental results show that compared with other network architectures, the proposed method enhances the generalization ability and improves the cognition accuracy of the model.

13 Flexible data-access control based on trust and reputation of power terminals

YANG Yun , FU Hong , WANG Lingyu

2020, 43(8):117-126. DOI: 10.11835/j.issn.1000-582X.2020.013

[Abstract](408) [HTML](802) [PDF 2.01 M](1324)

Abstract:
In order to solve the problems of password sniffing, replay attack and unauthorized operation in the current power payment terminal identity authentication and access control, in this paper a flexible data-access control scheme based on trust and reputation is proposed, which is applied to the power-terminal equipment data-access control in combination with cloud computing technology. The scheme controls the data access of the power terminal jointly by using attribute-based encryption and proxy re-encryption, the trust level evaluated by the terminal device and the user reputation generated by multiple reputation centers, and integrates the concept of user trust level and reputation evaluation into the encryption to support various control schemes and access strategies. Through the security and performance analysis of the proposed scheme, the fine-grained access control is proved, the data confidentiality is good, the communication overhead is flexible and controllable, the computational complexity is low, and the burden of the power terminal equipment is reduced.

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