Abstract:Chemical mechanical polishing (CMP), now regarded as the only method to obtain whole-wafer planarization and super-smooth surface without sub-surface defects, is usually employed as the final processing method for silicon carbide (SiC) wafer, but the processing efficiency of conventional SiC-CMP is too low to fulfill the current requirement. Aiming at surface quality and material removal rate (MRR) of polished single crystal SiC wafer, the current state of SiC-CMP research is discussed and categorized by CMP using different slurries, e.g. conventional alkaline colloidal silica base slurry, mixed abrasives slurry and slurries with strong oxidizers, respectively. Then from the viewpoint of improving the mechanical role, as well as the chemical one of CMP, the existing efficiency-enhancement methods of SiC-CMP, including the catalyst assisted polishing, electrochemical-mechanical polishing (ECMP), fixed abrasive polishing, and photo-catalyst assisted polishing are extensively reviewed and discussed. Analyses and discussions are conducted from the reviewed researches of SiC-CMP and its related efficiency-enhancement methods, the difficulties and future direction for further improvement on polishing MRR of single crystal SiC are proposed.

Abstract:To study the effect of microstructural components on the mechanical properties and deformation mechanism of polycrystalline copper during the nanoindentation process, a large-scale molecular dynamics simulation model of polycrystalline copper is structured by Poisson-Voronoi method and Monte Carlo method. Based on the microstructural components of the nanocrystalline copper, the polycrystalline copper nanoindentation simulation models with initial nanoindentation position at different microstructural components that contain grain cell, grain boundary, triple junction and vertex points are established, respectively. The nanoindentation process with the four different initial nanoindentation positions are simulated by molecular dynamics method, and the nanoindentation force and internal stress of the microstructural components are calculated. Centrally symmetric parameter method is used to analyze the dislocation nucleation and propagation process in the surface and subsurface of the polycrystalline copper with different initial nanoindentation positions. The results show that there is obvious regularity of the microstructural components during the nanoindentation process: the nanoindentation force rate, the difficulty of dislocation propagate to adjacent grains, the size expansion of dislocation distribution range on the polycrystalline surfaces, as well as the ability of the microstructural component low dimension accumulating atomic potential energy satisfy the descending relationship: grain cell, grain boundary, triple junction and vertex points. In addition, when the indentation position is at the high-dimensional microstructural component, the adjacent microstructural component exhibits tensile stress, while the indentation position is at the low-dimensional microstructural component, the adjacent microstructural component exhibits compressive stress. Therefore, during the nanoindentation process of polycrystalline copper, it is suggested to machine the microstructural components like grain cells of the polycrystalline material and to avoid the microstructural components like vertex points and triple junctions to reduce the number and energy accumulation of dislocations and the residual stress in the workpiece.

Abstract:A nondestructive detection method of subsurface damage depth was proposed, in which nano-fluorescent quantum dots were added as marker in grinding and polishing process, and the quantum dots were excited by light and produced fluorescence, then the slice images of the samples at different depths were obtained by laser confocal microscopy. When the scanning depth reached a certain value, the fluorescence intensity became weak and the subsurface damage depth was determined by fluorescence intensity change of the feature points. A nondestructive detection software of subsurface damage depth, which has functions of image threshold processing, bright spot automatic recognition, image display and curve characterization, is developed and it can realize the rapid detection of subsurface damage depth. The results of non-destructive detection and damage detection were compared, and it showed that the relative error between the two detection methods was less than 10%, which verified the effectiveness of the proposed method.

Abstract:To explore the mechanism of ultrasonic vibration assisted polishing, the effects of different ultrasonic vibration parameters on the flow field parameters, e.g., absolute pressure, velocity and vapour volume fraction, have been analyzed by using FLUENT unsteady dynamic grid turbulence and discrete phase model combined with cavitation module of mixture. The inherent rules and relations of these parameters have been investigated to reveal the phenomenon of secondary cavitation. The simulation results show that the effect of ultrasonic cavitation is mainly concentrated on the small area just below the specimen, so there is an effective processing area in the flow field, the minimum film thickness required for polishing effectiveness of the flow field can be determined. There are two peaks of the absolute pressure, velocity, vapour volume fraction within a vibration cycle which is related to the vapour bubble grow and collapse, namely, the secondary flow field cavitation phenomenon, which enhances the erosion effect during polishing process. There are time-lags between these fluid parameters at same position and ultrasonic vibration parameters, as well as for the same flow parameter at different position in the flow domain. The ultrasonic vibration results in transverse wave in the fluid, after meeting the reflected one, mechanical wave interferes and half wave loss occurs, which will affect the effective processing area. When the standing wave appears at condition of large-enough film thickness, the vibration amplitude of the wafer surface is increased and polishing effectiveness can be greatly improved. So the wafer processing can be optimized by determining the effective processing area and utilizing standing wave phenomenon. The secondary cavitation and time-lag phenomena are helpful in revealing the mechanisms of ultrasonic-assisted polishing(UAP).

Abstract:According to the characteristic of elastic-plastic deformation in the forming process, a model of contact and heat transfer in the closed biomass thermal compression process is established on the basis of EDEM discrete element analysis software. A program is developed based on the interface design of API, and a simulation analysis of closed biomass thermal compression is carried out under conditions of different die diameter and dwell time parameters. The simulation results show that with the increase of compression, the heat transfer of forming process is accelerated, and the core temperature of the largest compression column of biomass molding fuel is higher compared with the smallest one at the same time. With the die diameter increases from 10 mm to 20 mm and 30 mm, the core temperature of the biomass molding fuel decreases although the contact area of die and biomass fuel increases at the same compression volume. The reasonable setting of the holding time is significant to the fuel forming quality and energy saving.

Abstract:A novel remaining useful life(RUL)prediction method based on unscented Kalman filter(UKF)is proposed for structure with fatigue crack in machinery systems, which mainly includes two parts: performance evaluation of fatigue crack and RUL prediction. In the first part, a discrete state-space model is established based on the Paris law. Then the UKF is applied to estimate the two unknown Paris' law constants C and m combining with the real-time information obtained by sensors, in order to alleviate the negative influence on prediction accuracy caused by the uncertainty of incompletion of status information, as well as environmental noise. In the second part, the RUL of fatigue structure is predicted based on the discrete crack growth model according to the estimated result obtained by the UKF. The numerical experiments indicate that the UKF accurately identified the unknown parameters, furthermore, better performance in RUL prediction is obtained by comparing with extended Kalman filter(EKF)method. The RUL prediction accuracy can be efficiently improved by combining the discrete Paris law with UKF.

Abstract:To realize nitric oxides(NO_x) emission regulation below 200mg/m³ (O₂ volume fraction at 3.5%) in heating furnace fired refining gas, a high efficiency low nitrogen reconstruction scheme by reburning technology was proposed, and it was confirmed by numerical calculation and industrial test. The software of FLUENT was applied to investigate the effects of reburn refining gas ratio and primary area stoichiometric ratio on the distribution of temperature and NO_x mass concentration at different sections, generation, reduction characteristics of NO_x and unburned rate of refining gas. According to the optimized numerical calculations, the reconstruction was implemented and industrial test was carried out for one week. The results showed that, with the reburn fuel ratio increasing, the proportion of high temperature space decreased, the amount of thermal NO_x reduced and reduction increased, the NO_x reduction efficiency by reburning impact firstly increased and then decreased. The highest NO_x reduction efficiency by reburning impact when the reburn fuel ratio was 0.20. Increasing the reburn fuel ratio and decreasing the primary area stoichiometric ratio could increase the NO_x reduction efficiency, but the unburned fuel ratio increased meanwhile. When the reburn fuel ratio between 0.15~0.20 and the primary area stoichiometric ratio between 1.15~0.20, the greatest and average values of NOx mass concentration were 160mg/m³ and 130mg/m³ (O₂ volume fraction at 3.5%) respectively.

Abstract:On the impinging injector platform, the effects of jet asymmetry, airflow disturbances and metal particles on the atomization characteristics were studied by PIV (particle image velocimetry) technology. The experimental results indicate that when the two asymmetric jets impinges at low weber number, the chain sheet appears, and as the eccentricity increases, the link-length decreases. Fish bones regime can be found when the jet is with different length. Additional airflow disturbances can increase the atomization quality, because when the surface wave's intensity is increased, it will break up more easily, as a result, the width of fluid sheet and the SMD (sauter mean diameter) of droplets decrease. When power-law fluid contains aluminum particles, its strength is smaller and its max length decreases obviously. The droplets' SMD convergence does not change, as well, the SMD reaches convergence in shorter time. In conclusion, proper jet asymmetry, airflow disturbances and metal particles can improve impinging jets' atomization performance with power-law fluid.

Abstract:Dual-stage lean premixed combustion is proposed to improve the stability of premixed flame. It is organized by dividing premixed gas into radial stages with different equivalence ratio for the purpose of stable combustion under lower global equivalence ratio. The minimum equivalence ratio of primary gas in jet flame or dual-stage lean premixed flame is defined as extinction limit and measured experimentally, and the effect of flame manner and coflow parameters on extinction limit is investigated. The result shows that the outer coflow affects the distribution of concentration and velocity near flame root. The secondary premixed coflow in dual-stage lean premixed flame can reduce the dilution effect of outside atmosphere on the premixed gas, so the flame has a wider extinction limit. The secondary equivalence ratio is key parameter affecting extinction limit of dual-stage lean premixed flame. Comparing with uniform jet flame, the dual-stage lean premixed flame can combust under lower equivalence ratio, which is important for industrial controlling of NO_x emission.

Abstract:Firstly, the principle and structure of hydraulic energy regenerative damper is designed based on a brand rear damper in order to reduce energy consumption and realize energy recovery of suspension vibration. Based on the working principle, the theoretical model of the relation between flow rate and pressure drop of the hydraulic energy regenerative damper is established. Then, an equivalent parameterized simulation model is established in AMESim to analyze whether the external characteristics and energy recovery characteristics of the designed damper can meet actual requirements. Finally, the simulation model of the energy regenerative suspension is established and combined with the pavement time-domain input model built in Simulink in order to analyze the influence of the energy regenerative damper on the vehicle ride comfort and the energy recovery effect under actual road conditions. The results indicate that: the compression/extension damping force of the damper meets the national standard requirements; the performance characteristic curve is full without obvious distortion and presents good external characteristics; hydraulic energy regenerative damper meets the original idea of energy recovery only during compression strokes. The results also indicate that: the energy regenerative characteristic curve is affected by the high-frequency response characteristics and presents a significant peak performance; the suspension ride comfort meets the requirement and has certain energy recovery potential in actual driving. The hydraulic energy regenerative damper designed in this paper can meet the expected target, which has reference value for energy conservation and emission reduction.

Abstract:To explore the influence of spherical joint with clearances on the dynamic performance of spatial linkage mechanism, a kinematic model of the spherical joint with clearance is established based on the Cartesian coordinates, a spatial four-bar mechanism RSSR is used as an illustrative example and some numerical results are presented. The dynamics model of the RSSR mechanism with spherical joint clearance is also established based on the Lagrange-multiplier method. The dynamic properties of the RSSR mechanism with joint clearance are analyzed, which considered the effect of the size of joint clearance, crank driving speed and joint material. The results show that the difference contact force model of the joint will influence significantly the output result, the contact points of the ball and socket spread mostly in two regions on the clearance sphere, the clearance size of spherical joint affects the dynamic response of the mechanism dramatically. The influence of clearance size on the angel acceleration of the rocker and the contact force of the spherical joint with clearance is non-linearly and positively relation.

Abstract:To access the complex and unpredictable environment, a reconfigurable wheel-track robot with multiple locomotion modes is developed. It has the merits of both the wheel mechanism and the track mechanism. The robot consists of a control system unit, two symmetric reconfigurable wheels, rollover mechanism and robot body and has three locomotion modes-wheel mode, track mode and rollover mode. In the wheel locomotion mode, it is an omnidirectional two-wheel robot which can achieve high speed. In the track locomotion mode, the robot has a strong obstacle-climbing capability to complex terrain such as gravel road and grass. In the rollover locomotion mode, the robot rolls over to cross vertical obstacle. It can change its locomotion mode by the transformable parallel four-bar linkage according to the complex terrain. Based on the kinematic and mechanical models, optimization design for mechanism was subsequently carried out integrated with numerical simulations. The proposed mechanism can optimize the movement trajectory of transformation mechanism, reduce the performance requirements of motor and improve the motor service efficiency. Experiments show that the robot can efficiently traverse sands and climb vertical railings. It has prominent adaptability to environment and obstacle-surmounting ability. The results prove the design of the mobile mechanism system to be rational.

Abstract:To prevent ergonomic injuries, proper assessment of ergonomic risk is a key to identify risk factors and modify work practice in a timely manner, and a fuzzy logical approach to posture-based ergonomic evaluation tools is proposed. Rapid Upper Limb Assessment (RULA) is selected as a case study to describe the fuzzy logic modelling of RULA scoring systems. The trapezoidal functions are used to represent the angle ranges of the inputs as well as the force and muscle use imposed on the worker, and the triangular functions are used for intermediate variables which represent the RULA intermediate scores and the final RULA total score. The results of validation reveal that the proposed system produces high correlation coefficients with RULA (0.937, p < 0.01) and OWAS (0.725, p < 0.01). By taking the control console of a enterprise as a case, two participants completed six separate operation tasks, the model was used to analyze 12 tasks. The result indicated that the model had high reliability and could help to minimize human errors in observation for reliable on-site ergonomic assessment.

Abstract:Sliding friction experiments of wet multi-disc clutch with different frictional pairs under braking condition in the SAE#2 test bench are designed to investigate the friction torque attenuation phenomenon. The attenuation coefficient of friction torque of wet multi-disc clutch is proposed to describe the decrease degree of the tested friction torque. According to the test data of friction torque transferred by two and six frictional pairs respectively, the calculation formula of attenuation coefficient for two-six frictional pairs is fitted and the influences of relative rotational speed and average pressure on attenuation coefficient are analyzed. The results show that the attenuation coefficient is influenced by attenuation value of friction torque and calculated friction torque. Under the full oil film condition, the attenuation value of friction torque decreases with the rise of the relative rotational speed and increases while the average surface pressure increases. In general, the attenuation coefficient declines with the rise of the relative rotational speed, and when the average surface pressure increases, the coefficient increases first and then decreases.

Abstract:To achieve automatic shift function of a new type of modular transmission, a hydraulic shift system was designed for the prototype and the simulation model of shift process was built. A prototype with two speed was analyzed based on the principle of rigid body dynamics, a model including the driveline and hydraulic cylinder was established in MATLAB/Simulink. A simplified hydraulic shift system was designed with overflow pressure regulation. By using output torque change rate and input minimum speed as the evaluation index, the influence of parameters including filling flow and spring preload on the prototype's shift quality was studied. The test achieved the shift of prototype and verified the reliability of model. According to the simulation, the filling flow determined the velocity of part and the rising speed of inner pressure of cylinder, which affected the shift speed and braking. Increasing filling flow could effectively eliminate braking, but cause increase of shift impact; decreasing the preload of spring could reduce the shift impact.

Abstract:To explore the potential of remanufacturing industry and enhance its contribution to environmental protection without reducing commercial interests, energy saving is considered in the study of scheduling decisions for the remanufacturing system with parallel flow-shop-type reprocessing lines. An improved multi-objective artificial bee colony algorithm is proposed while turning off idle machine policy helps further cut down energy consumption. First, bi-objective mathematical model is established to minimize the makespan and energy consumption. On this basis, an original artificial bee colony algorithm was adapt to multi-objective algorithm with elite strategy and double neighborhood search to ensure the convergence of the algorithm, and local optimal escape operator was brought to improve the exploitation of the algorithm. Because of the component matching requirement of product assembly, the disassembly operation and reprocessing operation are not fixed, leaving machines lots of idle time to be decided if closing machine benefits saving energy, and the energy consumption subproblem is solved by a simulated annealing algorithm with elite strategy. Finally, numerical calculation and comparison with the existing typical algorithm demonstrate that the proposed algorithm is valid and feasible, and the energy can be saved substantially in desired makespan.

Abstract:To evaluate the interaction characteristics between the soil and the Active Lugged Wheel (ALW) developed in our lab, an ALW-soil interaction testbed has been developed. The drawbar pull and the vertical force generated by the ALW mechanism in various trajectories are measured and compared with forces generated by a smooth wheel and a single lug, respectively, to identify the interference between the wheel rim and lugs. In addition, based on the experimental results, the effects of lug inclination angle and lug sinkage length on soil reaction forces are analyzed, and the relationship between the lug sinkage length and the soil reaction forces is established by the least square method. It has been confirmed that the ALW generates a larger soil reaction forces than the sums of the respective forces generated by the smooth wheel and the single lug as the wheel rotates within a certain range, thus the ALW can exploit the deformation property of the sandy soil to gain larger soil reaction forces due to the interference between the wheel rim and the lugs. Moreover, it has been found that the lug inclination angle has no regular influence on the soil reaction force, while both drawbar pull and vertical force are quadratic functions of lug sinkage. Thus, it is possible to improve the traveling performance of the ALW mechanism by adjusting the lug sinkage length.

Abstract:To solve the problems that the construction method of assessment system was unscientific, the weight calculation of evaluation indexes and the distribution of subjective and objective weight were irrational, a new method of constructing evaluation system of drivability for vehicles and determining indexes weight for drivability evaluation was proposed. Considering the connotation of vehicle drivability in creep condition, the drivability evaluation system of creep condition was designed based on SMART principle, and the evaluation system was established from three dimensions of longitudinal response, longitudinal smooth and longitudinal steady state. The analytic hierarchy process and combined entropy was also proposed to calculate the subjective and objective weights respectively, and a comprehensive weight optimization model was designed based on network hierarchy and combined entropy to calculate the weight of evaluation indexes in the evaluation system to balance the subjectivity and objectivity. Many vehicles were evaluated under creep condition and the results show that the evaluation results obtained by the proposed method are more accurate than those by the analytic hierarchy process and the entropy weight method. The construction method of the evaluation system is more scientific and the weight calculation method is more effective.

Abstract:To study the failure propagation mechanism of CNC lathe, i.e., to identify the critical nodes and critical failure propagation path of system, by applying the digraph theory, integrating DEMATEL with ISM method, the hierarchy failure propagation digraph model of CNC lathe is constructed based on failure data. A failure propagation strength is defined based on failure propagation probability and edge betweenness to characterize failure propagation behavior, and then a failure propagation mechanism analysis method for CNC lathe according to the hierarchy failure propagation digraph model and the failure propagation strength is proposed. The analysis result shows that the system hierarchy failure propagation digraph model achieves the system hierarchy decomposition, and simplifies the failure propagation analysis process, which lays a foundation for the critical path identification. Regarding the failure propagation strength as criterion of critical path identification avoids the deviation arose from the traditional analysis method based on single index to describe the failure propagation behavior of CNC lathe, and realizes precise failure source and system critical path identification. This helps to the system failure diagnosis and maintenance.

Abstract:To accurately predict the springback of high strength steel and ultra high strength steel in cold roll forming, two mechanically-measured tests, uniaxial and loading-unloading-loading cycle tests, were introduced to determine the elastic modulus degradation of DP980 with the increase of plastic strain. Considering yield surface and elastic degradation surface, the chord modulus was developed in addition to elastic and plastic strain. The 3D finite element analysis models with eight stands were carried out by the professional COPPA RF and MSC MARC software. The simulation results achieved good agreement with experimental results observed in roll forming for thin cross-section automotive products. It is found that, with the increase of the plastic strain, the value of Young's modulus firstly reduces rapidly, and then slowly, finally tends to a steady value. Degradation of the Young's modulus is found to be 25% from the initial Young's modulus for the saturated value. Compared to the constant elastic modulus, the springback prediction result by using the variable elastic modulus is more accurate.

Abstract:To study the load distribution mechanism of power-split transmission and improve the carrying capacity of the gear box for herringbone star gear transmission system, a dynamics model for the geared turbofan(GTF) engine with a flexible pin was established based on finite element method. The dynamic load sharing characteristic of the system was studied in equivalent meshing error, and the influence of the error incentives, structure of pins, and working condition on load sharing characteristic was analyzed. The results show that the vibration track of the center gears deviates from the origin resulted by the eccentricity error, which is the primary cause for the inequality of load distribution. The load sharing coefficient of the system increases linearly with the error, and the influence of manufacturing errors is larger than assembling errors. The flexible pin can significantly improve the load sharing performance, and the flexible pin improved by Montestruc has best load sharing performance among the four kinds of pins. The load sharing coefficient declines with the increase of input speed, the resonance would lead to a sharp drop of loading sharing performance. The load sharing performance is improved with the increase of the input power, and the advantage of flexible pin is more prominent when the input power becomes lower.

Abstract:To evaluate the assembly sequence reasonably and ensure the validity of assembly sequence selection results, a fuzzy comprehensive evaluation method for assembly sequence was proposed. Firstly, the multi-objective evaluation index system of assembly sequence was constructed by survey and expert knowledge, and the quantification methods of each evaluation index were defined. Then the fuzzy analytic hierarchy process method was used to evaluate the relative relation among the indexes so as to obtain more accurate weight of each evaluation index of the assembly sequence. Finally, the initial value of each index of the feasible assembly sequence was calculated according to the quantization method of the evaluation index. Combining the relative weights of indexes, the advantages and disadvantages of the different assembly sequences were evaluated by means of elimination and choice translating reality, and according to the result of priority and inferiority, the best assembly sequence was selected. Taking an assembly sequence optimization process of X axis of a machine tool transmission system in an enterprise as an example, the feasibility of the proposed approach is illustrated. This result shows that the comprehensive evaluation result of feasible assembly sequence is reasonable, and the optimized scheme has better precision and efficiency. The multi-objective comprehensive evaluation method of assembly sequence is simple and practical, and the evaluation result is reasonable.

Abstract:To predict gear wear rapidly and accurately, a new wear numerical simulation model is established considering distributed load between double tooth meshing area based on Kriging method. The distributed pressure and meshing speed, which should be obtained before calculating wear depth, are obtained based on the Winkler surface model and gear meshing theory. The Load to determine distributed pressure is distributed dynamically considering the effect of clearance caused by wear. Based on Archard's wear model, the calculation wear model of spur gear is derived, and the wear depth of each meshing points on tooth profile under the different wear cycles are obtained. A surrogate model which can describe the relation between wear and gear parameters is constructed based on the Kriging and artificial neural network method, the approximation level and goodness of fit among different Kriging models with different samples are studied. As shown in a numerical example, the wear depth tends to accumulate as wear cycles and are varying over the teeth flanks with minimum wear at the pitch and increase as the meshing point moves towards the root. By comparison of three Kriging models with different original sample numbers, the minimum sample number is 100 if the approximation level and goodness of fit meet the demands. The Kriging model has high computational efficiency and accuracy and can overcome time-consuming defect.

Abstract:To provide a theoretical basis for accelerated life test and support for product optimization design, the tooth profile equation of the flexible wheel and rigid wheel is derived based on envelope theory. The load condition, curvature radius and volume absorption speed of the meshing area are analyzed. Considering the macroscopic geometry of contact area and the real surface roughness of meshing area, a mixed lubrication model of the flexible wheel and rigid wheel was established. The influence of speed and temperature on lubrication performance was studied by analyzing the film thickness ratio, friction coefficient and other related affecting factors. The results show that, when the speed is higher, the average film thickness and film thickness ratio are greater, the contact load ratio and the contact area ratio are smaller and the lubrication performance is better. When the speed is higher than 2 200 r/min, the meshing zone turns from boundary lubrication to mixed lubrication, the contact load ratio and the contact area ratio are reduced by more than 90%, the friction coefficient is reduced by more than half compared to 50 r/min. Controlling the speed over 2 200 r/min is helpful to improve the lubrication condition. When the temperature of the meshing zone increases, the average film thickness and film thickness ratio decrease and the contact load ratio and the contact area ratio increase, the lubrication condition deteriorates. When the temperature is at 60 ℃, the friction coefficient, contact load ratio and contact area ratio are more than doubled compared to that at 10 ℃. The harmonic reducer's working temperature should be controlled below 60 ℃ strictly.

Abstract:To study the influencing factors on the thermal characteristics of Gas-insulated Transmission Lines (GIL) in pipe gallery, considering the inductance effect of the enclosure and the temperature effect of the impedance, including external air domain, a three-dimensional finite element method of coupling fluid field and thermal field was established. With this model, aiming at the characteristics of Sutong GIL integrated corridor project, the effects of five factors, such as air flow rate, current, ambient temperature, the pressure of insulation gas and emissivity, on the thermal characteristics of the GIL were studied. The simulation results show that increasing the air speed at an air flow rate below 10 m/s can effectively reduce the temperature of GIL, but with the air flow rate continues to increase the cooling performance is poor. With the increasing of current, the temperature of GIL will rise sharply, the temperature rise of the conductor is much higher than that of the enclosure, and the temperature difference between the conductor and the enclosure will increase. The temperature of GIL is linearly proportional to ambient temperature, and the temperature difference between enclosure and conductor becomes smaller. When the pressure of insulating gas is within the range of 0.5 MPa, increasing the gas pressure facilitates the heat dissipation of conductor, and the temperature changing of enclosure is less than 0.5 ℃. With the increasing of emissivity of the inner surface of enclosure or the outer surface of conductor, the temperature of conductor will reduce while the temperature of enclosure is constant, and the temperature of conductor is more sensitive to the changes of the emissivity of conductor surface.

Abstract:To obtain the droplet behavior in spray cooling, a CFD model based on volume of fluid (VOF) approach is built to study the droplet collisions in the high-speed gas flow. The changes of droplet shape and gap pressure during the collision process are determined. Four different conditions are considered including the collisions between two droplets of same or different size, the center collision and eccentric collision. The impact of initial conditions on droplet morphology and the subsequent droplets size are discussed under various collision conditions. The results show that the breakup form mainly depends on the velocity of gas flow when droplets collide in high-speed gas flow. Collision parameter B mainly determines the direction of the elongated ligament. A gas gap is formed before droplets merge, and the pressure in the gap increases at the beginning of the collision and then decreases to gas pressure. Since the merged droplet can rotate during the eccentric collision process, pressure in the gas gap can release faster compared with center collision process. The droplet detachment is the poorest in the collisions between droplets of same size because the elongated ligament is in the direction of gas flow. Under this condition, there is no breakup of the liquid film, and therefore the droplet evaporation is worst.

Abstract:To establish a high performance supercritical carbon dioxide (SCO₂) Brayton cycle system, a two-stage SCO₂ axial turbine was designed for the 5MW supercritical carbon dioxide thermal power platform by using the self-compiled one-dimensional turbine design program based on the Denton loss model, AXIAL software and AXCENT software. RANS equations and SST Turbulence Model were chosen for numerical study on the aerothermodynamics performance of SCO₂ turbines in the design working condition and variable operating conditions using commercial software CFX and the real physical properties of SCO₂ from the NIST software. Simulation results indicated that the two stage axial flow straight cascade turbine scheme was selected in considering the realizability of the auxiliary system of the SCO₂ turbine with high design parameters. After the blade optimization, the total pressure loss of two rows of turbine stator blades is about 0.042, and the relative total pressure loss of the first and second rotor cascade were 0.050 and 0.064, respectively. The Mach number distribution of the root, middle and top flow field of turbine blades is reasonable. Considering the leakage and mixing loss of the stator and rotor blades, the isentropic efficiency of the SCO₂ high pressure turbine with the good variable operating performance can reach 84.88% and the shaft power of the turbine is 3 251 kW at the design point, which can meet the design requirements.