Abstract:To meet the requirements of large deployment ratio and high precision for deployable membrane mechanism in space missions, a deployable membrane mechanism based on Miura elastic creases was proposed and subjected to model, analyze and develop prototype. According to the crease distribution law and geometric relations, Miura-ori geometric model was established to investigate the influence law of the crease parameters on the deployment ratio and creases total length, and to calculate and optimize the crease parameters. In ABAQUS/Explicit, the numerical simulation models of the four-creases basic unit with θ= 90° and θ<90° were established respectively to analyze the mechanical behavior of the key membrane creases, and the feasibility of two-dimensional elastic crease was preliminarily proved. The elastoplasticity of the triangular membrane of Miura-ori was further studied, and the change curve of stress with folding process at the intersection of creases was plotted and the peak stress of which was within the range of material elasticity. And the space deployable membrane mechanism prototype was developed to conduct validation and analysis. The results show that the mechanism configuration design scheme is reasonable and a membrane folding scheme based on Miura-ori with large deployment ratio and small creases total length could be obtained by optimizing the crease parameters, and the high surface flatness of the deployed membrane proves the feasibility and superiority of Miura elastic creases.

Abstract:To clarify the relationship between microscopic pore structure and coke distribution during the coke removal and regeneration process of porous catalytic particles, the lattice Boltzmann method coupled with the solid-phase renewal method is developed and the pore-scale model of carbon removal is established to investigate the dynamic evolution of pore structures owing to carbon removal behavior at the pore-scale. Meanwhile, the effects of pore size and coke removal rate on the coke removal reaction characteristics and mass transfer properties are evaluated. The results reveal that the coke removal reaction leads to the axial expanding pore structure and the contribution of viscous flow becomes more significant. The coke removal process weakens the gas rarefaction effect and reduces the mass transfer rate and reaction rate. The coke in the large pore and small pore is easier for the uniform elimination. In contrast, the entrance effect is more significant in the medium pore. Under the condition of high rarefaction effect and high reaction rate, the coke removal reaction impact on viscous flow is more significant in contrast to the Knudsen diffusion. The entrance effect also exists for complex random pore structure and coke removal enhances the uniformity of pore size distribution.

Abstract:The metastable zone width of ammonium bicarbonate is an important factor affecting its crystallization process. To further analyze its specific regulation mechanism, this paper uses dynamic method to explore the influence mechanism of key parameters such as temperature, magnetic field, stirring rate and solvent titration rate on the metastable zone of ammonium bicarbonate and optimize the operation conditions. The results showed that with the increase of temperature, Molar fraction of ethanol increased, the nuclear energy barrier of ammonium bicarbonate decreased, and the width of metastable zone became narrower. At the same temperature, under the condition of magnetic field, the super solubility curve showed a downward trend and the metastable zone narrowed, but the trend weakened with the increase of Molar fraction of ethanol. When the stirring rate increased from 200 r/min to 600 r/min, the metastable zone width decreased first and then increased. The width of metastable zone increased with the increase of the droplet acceleration rate of solvent. When the droplet acceleration rate increased to 8 mL/min, the change of metastable zone tended to be flat. Increasing the temperature, increasing the magnetic field, increasing the stirring rate within a certain range, and taking the appropriate drop acceleration rate can promote the crystallization process.

Abstract:Considering that material parameters of U-shaped electrothermal actuators are affected by the temperature nonlinearity and the discontinuous boundary problem in the simplified model, the transient displacement characteristics were studied. By introducing temperature-related material renewal functions, based on thermodynamic theories such as energy conservation equations and material mechanics theories such as virtual work principles, the electro-thermal-mechanical coupling model of U-shaped electrothermal actuators was established. The improved Chebyshev spectrum method was used to solve the constructed coupling model, and the expression of temperature and transient displacement of U-shaped electrothermal actuator was obtained. The results of finite element simulation and theoretical analysis were basically the same, verifying the correctness of the model. An experimental platform for the transient displacement characteristics of the electrothermal driver was built, and the displacement response experimental results of the U-shaped electrothermal driver under constant voltage excitation were compared with the theoretical and simulation results. The transient displacement characteristics of the driver under the action of a periodic sinusoidal voltage were analyzed. The test and analysis results show that the displacement change trend of the U-shaped micro-electric thermal driver showed a sinusoidal change after a period of time, and the change period was equal to the voltage period; the displacement change range varied positively with the peak-to-peak voltage, and negatively with the change in voltage frequency.

Abstract:The purpose of this paper is to tackle the problem of cost and efficiency brought by DC/DC to the fuel cell vehicle drive system. The energy transfer between dual power sources and the independent control of the corresponding torque are also expected to be realized. The operating mode, control system, and output performance of the dual-power source motor drive system were analyzed. Firstly, the models of the dual-power source electric motor, the fuel cell, and the power battery were developed. The multiple operating modes of the dual-power source electric motor were discussed. Then, considering the coupling effect between voltage of the two sets of windings in dual-power source motor, the feed-forward compensation combined zero direct-axis current strategy was adopted to control the winding current and corresponding torque individually. Finally, bench tests under laboratory-designed operating condition and basic urban drive cycle were conducted respectively. The output torque of the dual-power source electric motor drive system could quickly respond to the torque demand. The fuel cell and power battery winding currents in different operating modes were consistent with their corresponding torque waveforms. The bench test results showed that the multiple operating modes of the dual-power source electric motor drive system could realize the starting, acceleration, downhill, and braking energy recovery conditions of the fuel cell vehicle. The adopted control strategy could realize the independent control of the output torque corresponding to the fuel cell and power battery, and the energy transfer between the fuel cell and power battery.

Abstract:To investigate the effect of liquid phase convective on the mass burning rate of medium scale pool fires, the liquid phase of pool fires was studied. A 3-D numerical model based on gas-liquid two-way coupling was used to model the pool fire. The gas phase and liquid phase were solved by large eddy simulation and direct numerical simulation respectively, taking into account buoyancy and Marangoni effects in the liquid phase. The heat and mass transfer between the two phases were calculated using a conjugate heat transfer method and an evaporation model. The proposed model was then validated by three pool fire experiments with different fuel sizes, fuel thicknesses and fuel types. The research results showed that the proposed model could accurately predict the mass burning rate of medium scale pool fires with a prediction error of less than 3%; in the development stage of pool fires, ignoring the Marangoni effect and buoyancy effect led to the maximum liquid velocity increased by 34.3%, the liquid surface temperature difference increased by 70.1% and the mass burning rate prediction error increased by 11.2%. In the stable combustion stage of pool fires, the buoyancy effect and the Marangoni effect had little influence on the instantaneous mass burning rate. With the increase of the diameter and the decrease of the depth of pools, the effect of the buoyancy effect on the combustion rate gradually decreased; Considering the descending process of the liquid level of thin-layer pool could reduce the prediction error of the combustion rate by 19.2%. In the numerical simulation, considering the liquid phase convective motion and the liquid surface drop process contributed to improve the prediction accuracy of the mass burning rate of medium scale pool fires.

Abstract:To describe the nonlinear magneto-mechanical coupling effect of materials more accurately, a coupled magneto-elastic model and a variable stiffness model were proposed based on nonlinear magneto-strictive strain equation, effective field theory, and energy balance equation. The magneto-mechanical effects and variable stiffness effects of ferromagnetic materials were studied, and the theoretical results of the nonlinear magnetization model were coupled with the simulation process using numerical analysis software. The results showed that the defect leakage field distribution obtained by the simulation was consistent with the existing research results, which verified the feasibility and accuracy of the proposed model and simulation method. The effects of stress, defect size, and defect location on the surface magnetic field were also analyzed. The results showed that under the action of tensile load, the normal magnetic field signal on the surface of the sample was like an S-shaped curve, and the tangential signal was like a conical curve, and its extreme values first decreased and then increased with the increase in the load. When there was a defect in the sample, the signals obtained on different acquisition paths were very different, and the peak value of the leakage magnetic field on the defect edge path was negatively correlated with the defect length, but the peak distance and span were opposite. On the collection route far from the defect, the peak value and span of the leakage magnetic field signal were positively correlated with the defect length.

Abstract:To further improve the cooling performance of the combustor flame tube, a binaural hole film cooling structure with higher cooling performance is proposed. The flow, heat transfer and cooling characteristics of traditional cylindrical hole, diffuser hole, convergent hole and binaural hole with blowing ratio from 0.67 to 2.01 were analyzed by numerical simulation. The results show that, compared with the other three film hole shapes, the aspect ratio of the cooling wall is in the range from 0 to 40, the kidney-shaped vortices formed by the cooling air flow at outlet of the binaural hole under the action of high temperature main flow are smaller in size, weaker in strength, larger in distance between the centers of the vortices, wider in transverse distribution of the cooling air flow, and lower in heat transfer coefficient on the wall, the film cooling performance is improved. When the blowing ratio is 2.01, compared with cylindrical hole, the flow coefficient of diffuser hole is increased by 13.7% , the wall heat transfer coefficient ratio is decreased by 1.5% , the flow coefficient of the convergent hole is unchanged, the heat transfer coefficient ratio is decreased by 2.7%. However, the flow coefficient of the binaural aperture decreased by 3.1% and that in the aspect of heat transfer coefficient ratio decreased by 11.25%. When the blowing ratio is 1.33, compared with diffusion hole and convergent hole, the flow coefficient of binaural hole is lower. When the slenderness ratio less than 40, the heat transfer coefficient ratio of binaural hole is the lowest and the cooling effect is the best.

Abstract:To solve the problem that the single-vortex gripper is easy to cause the rotation of workpiece due to the internal rotating flow field, resulting in the failure of non-contact handling, a concentric double-vortex non-contact vacuum gripper scheme was proposed. The structures of the inner and outer vortex chambers are concentric and the flow directions are opposite. The inner vortex generates a vacuum to provide suction force, and the reverse airflow of the outer vortex is used to balance the friction torque of the inner vortex on the workpiece. The effects of the working parameters of the gripper on the friction torque and suction force on the workpiece were studied. Simulation results show that the friction torque generated by the outer vortex on the workpiece is slightly greater than that of the inner vortex under the same air supply pressure, suggesting that the rotation prevention can be achieved by appropriately reducing the air supply pressure of the outer vortex chamber. Under the same air supply flow, the suction force of double-vortex gripper is slightly less than that of single-vortex gripper. The influence of the structural parameters of the double vortex gripper on the suction force was analyzed, and the orthogonal experiment was carried out to obtain the optimal structural parameter. The prototype of double-vortex gripper was tested, the results show that under the same conditions, the suction force of concentric double-vortex non-contact vacuum gripper is basically equivalent to that of single-vortex gripper, and the torque of the workpiece is reduced by about 90%, that improves the stability of the workpiece and the reliability of handling.

Abstract:The prestress of piezoelectric transducer assembly directly affects the product performance. The large-diameter piezoelectric transducer has higher requirements for prestress control and stress distribution uniformity of piezoelectric ceramics. For the high pre-tightening force assembly of large-diameter piezoelectric transducer, the structure design, assembly method design, and radial stress homogenization of large-diameter piezoelectric transducer were studied. First, a piezoelectric transducer was designed based on the transmission line method and modal analysis. The Morris method was used to analyze the sensitivity of size parameters to the longitudinal vibration frequency, amplitude ratio, and pitch position deviation of the piezoelectric transducer, and piezoelectric transducer with expected performance was designed. Then, a hydraulic tension assembly method was proposed to realize high pre-tightening force assembly of piezoelectric transducer. In order to address the issue of uneven radial stress distribution of piezoelectric ceramics in the process of tensile assembly, the static stress of two key states was analyzed. Finally, a tensile assembly device was designed, and the assembly test of piezoelectric transducer was completed using the device. Results showed that the tensile assembly method could realize the expected performance of piezoelectric transducer assembly. After assembly, the series resonant frequency and parallel resonant frequency of the piezoelectric transducer were stable at 18.05 kHz and 19.57 kHz, the series resonant impedance was within 10 Ω, and the electromechanical coupling coefficient was between 0.34 and 0.36. The research results provide a new method for controlling the prestress of piezoelectric transducer assembly, and offer a solution for the design of large-diameter piezoelectric transducers with uniform ceramic stress.

Abstract:For the simulation of micro-gravity environment, in order to reduce the vortex interference torque, an aerostatic rotary table system based on porous bronze was designed. First, in the selection of thrust air bearing, the local porous restrictor and full porous restrictor were compared. It was found that at gas film thickness of 20 μm, the load capacity of full porous restrictor was 40% higher than that of local porous restrictor, and thus the full porous restrictor was selected for thrust bearing design. Then, the load capacity and stiffness characteristics of thrust bearing and journal bearing of aerostatic rotary table under different air supply pressure and different material permeability were analyzed. Results showed that when the material permeability was smaller, the gas film stiffness was higher, which provides basis for the structural design of the aerostatic rotary table. Finally, the open-type thrust bearing was adopted in the aerostatic rotary table, and the restrictor was made of self-developed porous bronze material. By precision machining, the permeability was remained approximately unchanged. The rotary accuracy of the trial produced aerostatic rotary table was less than 0.8 μm. There was no self-excited vibration under 150 kg load. The maximum disturbance torque of the developed aerostatic rotary table system was 9×10^-4 Nm. The proposed aerostatic rotary table can be used not only in the field of micro-gravity tests, but also in the fields such as ultra-precision machining.

Abstract:To study the damage evolution and ductile fracture behavior of high-strength steel sheet under cold forming, we proposed a semi-coupled ductile fracture criterion. The DF2012 ductile fracture criterion was modified as the initial damage criterion. On the basis of the continuum damage mechanics (CDM) theory, the damage variable D was introduced to measure the cumulative damage of the material, and was coupled to the plasticity model. Taking TRIP780 high-strength steel as research object, six types of specimens were designed for different stress states. The resistance damage measurement and DIC digital image method were utilized to test the mechanical properties and initial damage displacement of specimens. The Drucker anisotropic yield function with non-associated flow rule (NAFR) and the Swift-Voce hardening model were used to accurately describe the plastic deformation process of the material. The initial damage model and the damage evolution parameters were calibrated by the hybrid test-simulation method and the inverse engineering method respectively. The constructed constitutive model and damage evolution model were compiled into the VUMAT subroutine using Fortran and implemented in the ABAQUS/Explicit module for fracture prediction, which were then compared with uncoupled damage model. Results show that the constructed semi-coupled ductile fracture criterion could accurately predict the damage-induced softening of TRIP780 sheets under different stress states. The maximum relative error of predicted damage displacement was only 1.31%. Compared with the commonly used uncoupled fracture model, the proposed model showed obvious superiority for high-strength steel with softening effect, which is suitable for fracture prediction in cold stamping forming of high-strength steel sheets.

Abstract:The large deviation of adjacent concave and convex angles in multi-pass roll forming of P-shaped special-shaped tubes for automobile columns cannot meet the high-precision requirements. On the basis of the large elastic-plastic deformation theory, a 15-pass roll forming finite element model of P-shaped special-shaped tube was established by using the professional design software COPRA. Combined with the actual production process, the finite element simulation of the complete forming process was carried out, and it was found that the insufficient supply of the perimeter of the welded pipe and the uneven metal flow of the corners of the P-shaped special-shaped tube in the roll pass caused the large deviation of the adjacent concave and convex angles. Thus, a precise control method of adjacent angles was proposed based on simulation and production practice, which combines selecting the appropriate compression coefficient, assigning the appropriate section deformation, and correcting the roll shape. The finite element simulation of roll forming of the P-shaped special-shaped tube was carried out by the proposed method. Results showed that the cross-section of the simulated final product was basically the same as the design cross-section, the concave and convex angles were 90.5° and 89.9°, and the angle deviation was within the error range of ±1°. The metal flow was in place, so that the corner metal filled the roll pass, which improved the forming quality of the adjacent concave and convex angles of the P-shaped special-shaped tube. The proposed method achieved an increase of 33.84% and 36.70% in the accuracy of the adjacent concave and convex angles of the P-shaped special-shaped tube through industrial application, which can effectively improve the forming quality of the P-shaped special-shaped tube and provide basis for production practice.

Abstract:The rolling bearing of gearbox of electric multiple-unit (EMU) is in a variable speed condition with high temperature and heavy load during operation, which is easy to induce faults such as cracks and pitting corrosion that are difficult to be detected. In order to diagnose the faults of rolling bearing in gearbox of EMU in time and ensure the safe operation of EMU, a rolling bearing fault diagnosis method under variable speed condition was proposed. First, a fusion time-frequency analysis algorithm was proposed, combining the characteristics of no interference term of short-time Fourier transform (STFT) and high time-frequency resolution of Wigner-Ville distribution (WVD), which can improve the time-frequency matrix accuracy of variable speed signal analysis. Then, the dynamic path planning method was improved considering the limitation that this method cannot deal with the normalized time-frequency matrix, and the speed curves in the fused time-frequency matrix were extracted. Furthermore, an order analysis method of interpolation resampling was proposed. The interpolation resampling of the original signal was performed according to the speed. The signal was reconstructed in the angular domain, and the corresponding order spectrum was obtained to realize the fault diagnosis of rolling bearing. Finally, the proposed fault rolling bearing diagnosis method was verified on test bench, and results showed that the proposed method could effectively extract the variable speed curves of the rolling bearing when the speed of the EMU changed, and accurately identify the fault types of the rolling bearing in the gearbox.

Abstract:The icing of transmission lines has seriously affected the safe operation of the power grid. Existing transmission line icing models mostly ignore the axial icing difference and consider the key icing parameters as time-invariant (single-step) conditions, while there are few reports on three-dimensional ice accretion model with time-varying (multi-step) parameters. Based on lubrication theory and line icing mechanism, this paper proposes a transmission line icing model considering the influence of time-varying icing parameters. By adopting the ANSYS-Fluent ICING module, the icing calculation for time-varying parameters was carried out on the three-dimensional line model. The validity of the calculation method was verified by using actual transmission line icing test data, and the calculation results were in good agreement with the test results. On this basis, the single-step and multi-step ice accretion calculation methods were compared. The influence of the transmission line inclination angle and line diameter on the shape and mass of ice accretion was analyzed. Results show that the accuracy of multi-step icing calculation method was about 8% higher than the single-step icing calculation method. Under the condition of dry ice coating, with the increase in the inclination angle of the transmission line, the shape and mass of ice accretion on the transmission line had no obvious change. Under wet icing conditions, the inclination angle of the transmission line had a significant effect on the ice accretion. As the inclination angle increased from 0° to 60°, the ice coverage area decreased and the shape of ice accretion gradually became smoother, but the ice accretion mass decreased by about 21%. The calculation results of ice accretion on transmission lines with large and small diameters were compared, and the amount of ice accretion on lines with large diameters was significantly higher than that on lines with small diameters.

Abstract:There are a large number of new and old concrete interfaces in prefabricated concrete structures. The template effect causes the enlargement of cement mortar porosity in the interfacial zones, which weakens their mechanical properties and durability. In order to quantitatively describe the porosity distribution characteristics of cement mortar in interfacial zones, new and old concrete specimens with smooth vertical interfaces and different water cement ratios were prefabricated. Scanning electron microscopy (SEM) was used to obtain the gray images of each specimen at different positions from the interface. Digital image processing (DIP) tools were used for image information enhancement and binarization. Thus, the ratio of pore pixels to total pixels was obtained, namely nominal porosity. With test results, the distribution characteristics of nominal porosity in the interfacial zones of new and old concrete with smooth vertical interfaces were analyzed. On the basis of the stable relationship between nominal porosity and real porosity, a porosity distribution model was established for the interfacial zones of new and old concrete with smooth vertical interfaces. Furthermore, considering the continuous variation of new and old concrete contents in the chiseled section, the porosity distribution model of chiseled interfacial zone was established. Results show that the nominal porosity reached the maximum at the interface, then decreased gradually towards the interior of concrete, and finally tended to be stable. The overall variation trend could be characterized by Gaussian function. With the increase in water cement ratio, the nominal porosity of each position from interface to interior concrete presented a relatively increasing trend, but the relative nominal porosity from the interfacial zone to the interior stable zone was nearly the same for the concrete with different water cement ratios.

Abstract:Under the combined actions of wind, wave, and current, the floating wind turbine is prone to creep in mooring, which can accelerate corrosion, increase the failure probability, and affect the stability of the platform. In order to ensure the safe operation of floating wind turbine and achieve early warning in the early stage of mooring creep, this paper proposes a mooring fault diagnosis method for floating wind turbine based on multifractal analysis. First, the mooring fault nonlinear information was extracted by variational mode decomposition (VMD) method, and the impact of mooring creep and mooring failure at different locations on the stability of floating wind turbine was analyzed. Then, considering the multi-metric characteristics of the nonlinear signal, multiple fractal detrended fluctuation analysis was used to extract the fault signal characteristics, and it was estimated whether mooring creep occurred and the locations of mooring creep. Finally, the platform response data under mooring creep at different locations were analyzed. Results show that when the original signal was processed by VMD and the fractal box dimension was used to filter the feature signal, the noise was effectively filtered and more representative nonlinear features were extracted. The mooring fault signal had multiple fractal features, and the mooring creep and its location were effectively estimated by the singularity index α₀. The nonlinear features extracted by VMD could be estimated according to the data complexity by the multiple fractal detrended fluctuation analysis. The state of the mooring could be determined based on the complexity of the data. The research results can provide theoretical methods for information extraction and fault determination of offshore floating wind turbine moorings.

Abstract:To further improve the polishing efficiency and quality of force rheological polishing, this paper proposes a vibration-assisted force rheological polishing (VFRP) method. The material removal process and the influence of different process parameters on polishing characteristics during the VFRP of stainless steel were studied. On the basis of the principle and tests of VFRP, the material removal rate (MRR) and surface roughness were used as evaluation conditions to analyze the effects of three key parameters (polishing speed, vibration frequency, and amplitude) on the polishing characteristics of stainless steel. The test was designed based on the Taguchi method. The signal-to-noise ratio was used to evaluate the test results, and the optimized process parameters were obtained. The weight of each factor was obtained by variance analysis method. Results show that the polishing speed had the greatest influence on MRR, followed by amplitude and vibration frequency. The polishing speed had the greatest impact on surface roughness, followed by vibration frequency and amplitude. Under the optimized combination of polishing parameters (polishing speed 40 r·min^-1, amplitude 0.35 mm, and vibration frequency 80 Hz), the surface roughness decreased from (80±10) nm to (7.1±0.9) nm and MRR reached 68 nm·min^-1 after processing for 30 min. There was a relative phase difference between the particles in the vibrating polishing fluid and a certain shear rate was formed, which caused the rheological effect of the polishing fluid and the free abrasives were held. The material could be removed in a plastic way by applying pressure and shear force on the workpiece surface under the action of relative motion. The scratches on the stainless steel surface could be effectively removed and the surface quality could be improved under the optimized process parameters.