The long term existence of a low-head dam in the river channel significantly affects river geomorphology and river ecosystem. Because more and more low-head dam structures have deteriorated in recent years, the attent...The long term existence of a low-head dam in the river channel significantly affects river geomorphology and river ecosystem. Because more and more low-head dam structures have deteriorated in recent years, the attention for low-head dam removal is increasing as one of alternatives for river restoration. Thus, this study intends to investigate the impacts of low-head dam removal on river geomorphology and riparian vegetation with developing a quantitative method to predict the changes of river morphology as well as invasion, growth, expansion and destruction of riparian vegetation after a low-head dam removal. To verify the numerical simulation model, the low-head dam removal case in Gongreung River was employed with investigation of low-head dam removal responses on river geomorphology and riparian vegetation. Following the low-head dam removal, the results of monitoring and numerical simulation indicated that new sand bars has formed as well as increasing the extent of existing sand bars in the upstream of the low-head dam. The sand bars have been colonized in a year after the low-head dam removal by grass type plants. After a decade to several decades, the riparian vegetation in sand bars often developed to tree type plants in several low-head dam removal cases. As other cases, Gongreung River also showed the growth of tree type plants in 5 years after the removal.展开更多
A comprehensive 3D turbulent CFD study has been carried out to simulate a Low-Head(LH)vertical Direct Chill(DC)rolling ingot caster for the common magnesium alloy AZ31.The model used in this study takes into account t...A comprehensive 3D turbulent CFD study has been carried out to simulate a Low-Head(LH)vertical Direct Chill(DC)rolling ingot caster for the common magnesium alloy AZ31.The model used in this study takes into account the coupled laminar/turbulent melt flow and solidification aspects of the process and is based on the control-volume finite-difference approach.Following the aluminum/magnesium DC casting industrial practices,the LH mold is taken as 30 mm with a hot top of 60 mm.The previously verified in-house code has been modified to model the present casting process.Important quantitative results are obtained for four casting speeds,for three inlet melt pouring temperatures(superheats)and for three metal-mold contact heat transfer coefficients for the steady state operational phase of the caster.The variable cooling water temperatures reported by the industry are considered for the primary and secondary cooling zones during the simulations.Specifically,the temperature and velocity fields,sump depth and sump profiles,mushy region thickness,solid shell thickness at the exit of the mold and axial temperature profiles at the center and at three strategic locations at the surface of the slab are presented and discussed.展开更多
Outcomes of experimental researches of the low-pressure adiabatic flow of the boiling liquid through two-dimensional Laval nozzles in a vacuum atmosphere were adduced.Requirements of critical conditions of flow were d...Outcomes of experimental researches of the low-pressure adiabatic flow of the boiling liquid through two-dimensional Laval nozzles in a vacuum atmosphere were adduced.Requirements of critical conditions of flow were determined.Structural forms of a stream were investigated and their connection with crisis of flow was shown.It was established periodic non-stationary macrostructures of a stream which was stipulated by the rotational gear of origin of a vapor phase.展开更多
The long-term existence of dam structures significantly modified the river channel. In accordance with a drastic increase of low-head dams under consideration for removal in recent years, it is important to predict th...The long-term existence of dam structures significantly modified the river channel. In accordance with a drastic increase of low-head dams under consideration for removal in recent years, it is important to predict the effects of low-head dam removal from the modified river channel by the low-head dam construction. This study intends to investigate the long-term channel evolution process following low-head construction and removal and to find out the influential parameters (sediment diameter, river bed slope, dam height) for those channel evolution by two-dimensional numerical simulation model. Following the low-head dam construction, sediment deposition rates in upstream of the low-head dam are varied with the influential parameters. The sediment deposition rates and sandbar formation with riparian vegetation settlement on sandbars have significantly affected for channel evolution following low-head dam removal. Especially the knickpoint formation and the types of vegetation (grass type and tree type) on the sandbars are critical factors for channel evolution following low-head dam removal. Through the numerical simulation results of low-head dam construction (50 years) and low-head dam removal (50 years), it is identified that the modified river channel by low-head dam may not be easily restored to pre-dam conditions following its removal especially in river geomorphology and riparian vegetation. Consequently, this study found that the reversibility following low-head dam construction and removal depends on the sediment deposition rates in upstream of the low-head dam.展开更多
超低水头轴流式液力透平模型试验发现,设计工况下转速波动较大,功率输出不稳定。为分析其原因,该研究以DN860超低水头轴流式液力透平为对象,采用ANSYS Fluent2020R2中动网格SDOF(six degree of freedom)求解器,分析负载扭矩、流量和含...超低水头轴流式液力透平模型试验发现,设计工况下转速波动较大,功率输出不稳定。为分析其原因,该研究以DN860超低水头轴流式液力透平为对象,采用ANSYS Fluent2020R2中动网格SDOF(six degree of freedom)求解器,分析负载扭矩、流量和含气率变化对其转速特性的影响。结果表明:液力透平启动时,转速急剧升高后经短时小幅波动再缓慢升高直至稳定,转速稳定后,由于内部沿程摩擦损失,透平模拟转速随扭矩增大非线性减小,且模拟转速均低于理论转速,扭矩越小,二者差值越大。流量无论线性增加还是周期性正弦变化,液力透平的转速变化与流量变化基本一致,呈正相关。含气率对液力透平转速和效率影响较大,当含气率从0增加到30%时,透平的转速和功率系数分别降低16.2%和16.4%。研究结果可为同类型超低水头轴流式液力透平转速特性及其控制研究提供参考。展开更多
Despite significant Brazilian social programs to expand energy access,approximately one million people in rural Amazonia still lack electricity.Moreover,the existing rural electricity grid in the region is inadequate ...Despite significant Brazilian social programs to expand energy access,approximately one million people in rural Amazonia still lack electricity.Moreover,the existing rural electricity grid in the region is inadequate for supporting efficient small-scale production systems due to both the poor quality and high cost of supplied energy.In parallel,traditional wooden bridges in the Amazon have been progressively replaced by more durable concrete structures in recent years.In this context,this study explores the application of very low-head hydropower installations in the Amazon,focusing on integrating axial-flow turbines beneath small concrete bridges.The methodology involves analyzing flow duration curves and key hydraulic parameters upstream and downstream to design a micro-hydropower system comprising two or more units,thereby maximizing annual energy generation.Additionally,the study assesses the applicability of the minimum pressure coefficient criterion to improve the efficiency of very low-head turbines.This design criterion is first validated against an existing experimental study featuring a propeller turbine with a simple curved plate blade.The proposed methodology is then applied to a case study involving a small dam-bridge with a head variation from 1.4 to 2.4 m and a flow rate ranging from 0.23 to 0.92 m^(3)/s.The optimal configuration achieves an energy output of 26.2 MWh/year with an efficiency of up to 86%.展开更多
文摘The long term existence of a low-head dam in the river channel significantly affects river geomorphology and river ecosystem. Because more and more low-head dam structures have deteriorated in recent years, the attention for low-head dam removal is increasing as one of alternatives for river restoration. Thus, this study intends to investigate the impacts of low-head dam removal on river geomorphology and riparian vegetation with developing a quantitative method to predict the changes of river morphology as well as invasion, growth, expansion and destruction of riparian vegetation after a low-head dam removal. To verify the numerical simulation model, the low-head dam removal case in Gongreung River was employed with investigation of low-head dam removal responses on river geomorphology and riparian vegetation. Following the low-head dam removal, the results of monitoring and numerical simulation indicated that new sand bars has formed as well as increasing the extent of existing sand bars in the upstream of the low-head dam. The sand bars have been colonized in a year after the low-head dam removal by grass type plants. After a decade to several decades, the riparian vegetation in sand bars often developed to tree type plants in several low-head dam removal cases. As other cases, Gongreung River also showed the growth of tree type plants in 5 years after the removal.
文摘A comprehensive 3D turbulent CFD study has been carried out to simulate a Low-Head(LH)vertical Direct Chill(DC)rolling ingot caster for the common magnesium alloy AZ31.The model used in this study takes into account the coupled laminar/turbulent melt flow and solidification aspects of the process and is based on the control-volume finite-difference approach.Following the aluminum/magnesium DC casting industrial practices,the LH mold is taken as 30 mm with a hot top of 60 mm.The previously verified in-house code has been modified to model the present casting process.Important quantitative results are obtained for four casting speeds,for three inlet melt pouring temperatures(superheats)and for three metal-mold contact heat transfer coefficients for the steady state operational phase of the caster.The variable cooling water temperatures reported by the industry are considered for the primary and secondary cooling zones during the simulations.Specifically,the temperature and velocity fields,sump depth and sump profiles,mushy region thickness,solid shell thickness at the exit of the mold and axial temperature profiles at the center and at three strategic locations at the surface of the slab are presented and discussed.
基金Supported by the Federal Target Programm “the Scientific and Scientific and Pedagogical Staff of Innovative Russia”on 2009-2013(State Contract No.P1514from 03.09.09)
文摘Outcomes of experimental researches of the low-pressure adiabatic flow of the boiling liquid through two-dimensional Laval nozzles in a vacuum atmosphere were adduced.Requirements of critical conditions of flow were determined.Structural forms of a stream were investigated and their connection with crisis of flow was shown.It was established periodic non-stationary macrostructures of a stream which was stipulated by the rotational gear of origin of a vapor phase.
文摘The long-term existence of dam structures significantly modified the river channel. In accordance with a drastic increase of low-head dams under consideration for removal in recent years, it is important to predict the effects of low-head dam removal from the modified river channel by the low-head dam construction. This study intends to investigate the long-term channel evolution process following low-head construction and removal and to find out the influential parameters (sediment diameter, river bed slope, dam height) for those channel evolution by two-dimensional numerical simulation model. Following the low-head dam construction, sediment deposition rates in upstream of the low-head dam are varied with the influential parameters. The sediment deposition rates and sandbar formation with riparian vegetation settlement on sandbars have significantly affected for channel evolution following low-head dam removal. Especially the knickpoint formation and the types of vegetation (grass type and tree type) on the sandbars are critical factors for channel evolution following low-head dam removal. Through the numerical simulation results of low-head dam construction (50 years) and low-head dam removal (50 years), it is identified that the modified river channel by low-head dam may not be easily restored to pre-dam conditions following its removal especially in river geomorphology and riparian vegetation. Consequently, this study found that the reversibility following low-head dam construction and removal depends on the sediment deposition rates in upstream of the low-head dam.
文摘超低水头轴流式液力透平模型试验发现,设计工况下转速波动较大,功率输出不稳定。为分析其原因,该研究以DN860超低水头轴流式液力透平为对象,采用ANSYS Fluent2020R2中动网格SDOF(six degree of freedom)求解器,分析负载扭矩、流量和含气率变化对其转速特性的影响。结果表明:液力透平启动时,转速急剧升高后经短时小幅波动再缓慢升高直至稳定,转速稳定后,由于内部沿程摩擦损失,透平模拟转速随扭矩增大非线性减小,且模拟转速均低于理论转速,扭矩越小,二者差值越大。流量无论线性增加还是周期性正弦变化,液力透平的转速变化与流量变化基本一致,呈正相关。含气率对液力透平转速和效率影响较大,当含气率从0增加到30%时,透平的转速和功率系数分别降低16.2%和16.4%。研究结果可为同类型超低水头轴流式液力透平转速特性及其控制研究提供参考。
基金funded by the Conselho Nacional de Desenvolvimento Científico e Tecnológico(CNPq),project number 408034/2022−0.
文摘Despite significant Brazilian social programs to expand energy access,approximately one million people in rural Amazonia still lack electricity.Moreover,the existing rural electricity grid in the region is inadequate for supporting efficient small-scale production systems due to both the poor quality and high cost of supplied energy.In parallel,traditional wooden bridges in the Amazon have been progressively replaced by more durable concrete structures in recent years.In this context,this study explores the application of very low-head hydropower installations in the Amazon,focusing on integrating axial-flow turbines beneath small concrete bridges.The methodology involves analyzing flow duration curves and key hydraulic parameters upstream and downstream to design a micro-hydropower system comprising two or more units,thereby maximizing annual energy generation.Additionally,the study assesses the applicability of the minimum pressure coefficient criterion to improve the efficiency of very low-head turbines.This design criterion is first validated against an existing experimental study featuring a propeller turbine with a simple curved plate blade.The proposed methodology is then applied to a case study involving a small dam-bridge with a head variation from 1.4 to 2.4 m and a flow rate ranging from 0.23 to 0.92 m^(3)/s.The optimal configuration achieves an energy output of 26.2 MWh/year with an efficiency of up to 86%.
