为了解决MMC-HVDC(Modular Multilevel Converter Based on HVDC,MMC-HVDC)交流侧系统故障时的过流问题,以及增强MMC换流器的低压穿越能力,通过对换流器功率数学模型及控制方式进行分析,发现了换流器有功和无功功率解耦的PI控制方式。...为了解决MMC-HVDC(Modular Multilevel Converter Based on HVDC,MMC-HVDC)交流侧系统故障时的过流问题,以及增强MMC换流器的低压穿越能力,通过对换流器功率数学模型及控制方式进行分析,发现了换流器有功和无功功率解耦的PI控制方式。提出了当交流侧发生对称和不对称故障时,通过控制PI值限制功率输出,同时由交流电压偏差有效值生成正负序补偿电流的紧急功率支援控制策略。将这种控制策略添加到电磁暂态仿真系统当中,当系统网侧发生对称或不对称故障时,利用数值仿真技术分析了换流器阀侧的电能质量。仿真结果验证了所提出的控制方法对故障时过流抑制的有效性,同时增强了换流器的低压穿越能力。展开更多
In 2006, SOMAIR decided to increase the uranium production by 50% using heap leaching for the treatment of low grade ores. These ores, which come from different lodes with various properties, have been studied in four...In 2006, SOMAIR decided to increase the uranium production by 50% using heap leaching for the treatment of low grade ores. These ores, which come from different lodes with various properties, have been studied in four steps: ( 1 ) Lab tests: to compare the ores (characterization, acid consumptions, recovery...); (2) Column tests on an average sample: to define significant parameters for a feasibility study; (3) Column tests on specific samples: to optimize recovery for each ore and identify problems of percolation due to the clays; (4) Pilot tests in large boxes (stalls): to validate process parameters. Uranium production by heap leaching started in July 2009.展开更多
The over-exploitation of water resources in the Haihe River Basin (HRB) has now become a serious problem. This is clearly evidenced by the fact that many local rivers and lakes are drying up and the total amount of ...The over-exploitation of water resources in the Haihe River Basin (HRB) has now become a serious problem. This is clearly evidenced by the fact that many local rivers and lakes are drying up and the total amount of over-exploited groundwater has reached over 1000×10^8m^3. It is important to note that the exploitation of water resources in HRB was reasonable before 1979. After 1980, however, over-exploitation happened with an annual average amount of 40×10^8m^3. Both the dry season and rapid economic growth in HRB took place at the same time. Therefore, the over-exploitation of water in HRB was actually the negative result of the conjunction of a continuous dry season and rapid economic growth. So the over-exploitation would not be as serious as it is today if either of the above two stopped. After the first stage of south-to-north water transfer project, the water shortage problem in HRB could be eased for the following reasons: firstly, water transfer project will bring to the Basin 60x108m3 water resources; secondly, a wet season will come back eventually according to natural law of climate variability; finally, its agricultural and industrial use and total water consumption all have decreased from the peak value, so that the groundwater table will raise certainly and ecological water in rivers and lakes that were dried-up will be partly restored. In the future, the main problem of water resources security in HRB will include water pollution, operation risk of the south-to-north water transfer project, groundwater pollution and engineering geological hazards that may be brought by groundwater rise. The proposed countermeasures are as follows: keeping strengthening water demand management, raising water price as well as subsidies for the low- income family and improving other water related policies, preventing and dealing with water pollution seriously and getting fully prepared for the operation of south-to-north water transfer project.展开更多
Numerical simulations have been carried out to investigate the liquid atomization and spray process using the Discrete Phase Model of the commercial CFD code combined with the Wall-Film boundary conditions. The effect...Numerical simulations have been carried out to investigate the liquid atomization and spray process using the Discrete Phase Model of the commercial CFD code combined with the Wall-Film boundary conditions. The effects of spray parameters on droplets Santer mean diameter (SMD), droplet collision speed, the thickness of liquid-film, the surface temperature and its uniformity were analyzed in the present study. The simulation results and the experimental data obtained in the available literature agree within 13.8%, The computational results show that the spray pressure is the main factor to realize the atomization. Increasing the mass flux and the spray pressure, the droplet collision speed increases while the corresponding maximum film thickness on the heated surface declines. The surface temperature changes indistinctively with the increase of the spray distance, but the temperature distribution tends to be uniform.展开更多
In this paper,a two-level search method for searching transfer opportunities between interplanetary halo orbits,exploiting the invariant manifolds of the restricted three-body problem,is proposed.In the method,the fir...In this paper,a two-level search method for searching transfer opportunities between interplanetary halo orbits,exploiting the invariant manifolds of the restricted three-body problem,is proposed.In the method,the first-level search procedure is performed under the conditions of the initial time of escape manifold trajectory of the Sun-Earth halo orbit and the terminal time of capture manifold of the target planet fixed,by solving the optimal two-impulsive heliocentric trajectory to connect the two manifold trajectories.The contour map,helpful to the understanding of the global characteristics of the transfer opportunities,taking the initial time of escape manifold and the terminal time of capture manifold as variables,the optimal velocity increment of the first-level search as objective function,is used for the second-level search.Finally,taking the Earth-Mars and Earth-Venus halo to halo transfers for example,the transfer opportunities in 2015-2017 are searched.The results show the effectiveness of the proposed method and reveal the property of quasi-period of transfer opportunities between interplanetary halo orbits.展开更多
Fracture analysis of a plane crack problem under chemo-mechanical loading is presented based on a linear chemo-elasticity model.The flux conductivity is introduced to characterize the influence of the crack defect on ...Fracture analysis of a plane crack problem under chemo-mechanical loading is presented based on a linear chemo-elasticity model.The flux conductivity is introduced to characterize the influence of the crack defect on the diffusion process.Using Fourier transform and the dislocation density functions,the crack problem is reduced to a set of singular integral equations,which are solved numerically by the Lobatto-Chebyshev method.Parametric studies are conducted to reveal the effects of flux conductivity,geometric configuration,chemical and mechanical loads on the crack tip field.The numerical results show that the stress singularity at the crack tip is usually a mixture of mode Ⅰ and mode Ⅱ types.展开更多
文摘为了解决MMC-HVDC(Modular Multilevel Converter Based on HVDC,MMC-HVDC)交流侧系统故障时的过流问题,以及增强MMC换流器的低压穿越能力,通过对换流器功率数学模型及控制方式进行分析,发现了换流器有功和无功功率解耦的PI控制方式。提出了当交流侧发生对称和不对称故障时,通过控制PI值限制功率输出,同时由交流电压偏差有效值生成正负序补偿电流的紧急功率支援控制策略。将这种控制策略添加到电磁暂态仿真系统当中,当系统网侧发生对称或不对称故障时,利用数值仿真技术分析了换流器阀侧的电能质量。仿真结果验证了所提出的控制方法对故障时过流抑制的有效性,同时增强了换流器的低压穿越能力。
文摘In 2006, SOMAIR decided to increase the uranium production by 50% using heap leaching for the treatment of low grade ores. These ores, which come from different lodes with various properties, have been studied in four steps: ( 1 ) Lab tests: to compare the ores (characterization, acid consumptions, recovery...); (2) Column tests on an average sample: to define significant parameters for a feasibility study; (3) Column tests on specific samples: to optimize recovery for each ore and identify problems of percolation due to the clays; (4) Pilot tests in large boxes (stalls): to validate process parameters. Uranium production by heap leaching started in July 2009.
基金supported by the National Natural Sciences Fund of China (40971298)
文摘The over-exploitation of water resources in the Haihe River Basin (HRB) has now become a serious problem. This is clearly evidenced by the fact that many local rivers and lakes are drying up and the total amount of over-exploited groundwater has reached over 1000×10^8m^3. It is important to note that the exploitation of water resources in HRB was reasonable before 1979. After 1980, however, over-exploitation happened with an annual average amount of 40×10^8m^3. Both the dry season and rapid economic growth in HRB took place at the same time. Therefore, the over-exploitation of water in HRB was actually the negative result of the conjunction of a continuous dry season and rapid economic growth. So the over-exploitation would not be as serious as it is today if either of the above two stopped. After the first stage of south-to-north water transfer project, the water shortage problem in HRB could be eased for the following reasons: firstly, water transfer project will bring to the Basin 60x108m3 water resources; secondly, a wet season will come back eventually according to natural law of climate variability; finally, its agricultural and industrial use and total water consumption all have decreased from the peak value, so that the groundwater table will raise certainly and ecological water in rivers and lakes that were dried-up will be partly restored. In the future, the main problem of water resources security in HRB will include water pollution, operation risk of the south-to-north water transfer project, groundwater pollution and engineering geological hazards that may be brought by groundwater rise. The proposed countermeasures are as follows: keeping strengthening water demand management, raising water price as well as subsidies for the low- income family and improving other water related policies, preventing and dealing with water pollution seriously and getting fully prepared for the operation of south-to-north water transfer project.
基金supported by National Natural Science Foundation of China (No.50776087).
文摘Numerical simulations have been carried out to investigate the liquid atomization and spray process using the Discrete Phase Model of the commercial CFD code combined with the Wall-Film boundary conditions. The effects of spray parameters on droplets Santer mean diameter (SMD), droplet collision speed, the thickness of liquid-film, the surface temperature and its uniformity were analyzed in the present study. The simulation results and the experimental data obtained in the available literature agree within 13.8%, The computational results show that the spray pressure is the main factor to realize the atomization. Increasing the mass flux and the spray pressure, the droplet collision speed increases while the corresponding maximum film thickness on the heated surface declines. The surface temperature changes indistinctively with the increase of the spray distance, but the temperature distribution tends to be uniform.
基金supported by the National Basic Research Program of China ("973" Program) (Grant No. 2012CB720000)the National Natural Science Foundation of China (Grant Nos. 10832004 and 11102020)
文摘In this paper,a two-level search method for searching transfer opportunities between interplanetary halo orbits,exploiting the invariant manifolds of the restricted three-body problem,is proposed.In the method,the first-level search procedure is performed under the conditions of the initial time of escape manifold trajectory of the Sun-Earth halo orbit and the terminal time of capture manifold of the target planet fixed,by solving the optimal two-impulsive heliocentric trajectory to connect the two manifold trajectories.The contour map,helpful to the understanding of the global characteristics of the transfer opportunities,taking the initial time of escape manifold and the terminal time of capture manifold as variables,the optimal velocity increment of the first-level search as objective function,is used for the second-level search.Finally,taking the Earth-Mars and Earth-Venus halo to halo transfers for example,the transfer opportunities in 2015-2017 are searched.The results show the effectiveness of the proposed method and reveal the property of quasi-period of transfer opportunities between interplanetary halo orbits.
基金supported by the National Natural Science Foundation of China(Grant Nos.11932005 and 11772106).
文摘Fracture analysis of a plane crack problem under chemo-mechanical loading is presented based on a linear chemo-elasticity model.The flux conductivity is introduced to characterize the influence of the crack defect on the diffusion process.Using Fourier transform and the dislocation density functions,the crack problem is reduced to a set of singular integral equations,which are solved numerically by the Lobatto-Chebyshev method.Parametric studies are conducted to reveal the effects of flux conductivity,geometric configuration,chemical and mechanical loads on the crack tip field.The numerical results show that the stress singularity at the crack tip is usually a mixture of mode Ⅰ and mode Ⅱ types.