鱼类通过混流式水轮机转轮时受压强及剪切损伤的概率分析

Probability evaluation of pressure and shear damage for fish passing through francis turbine runner

鱼类在通过混流式水轮机流道时会遭遇最低压强及压强梯度引发的压强损伤以及剪切应力引发的剪切损伤。为了辨识混流式转轮中压强损伤和剪切损伤的主次关系,进而有针对性的开展混流式转轮的鱼类生态友好性能优化,该文基于计算流体动力学分析方法研究了不同水头条件下混流式转轮内超出压强以及剪切损伤阈值的体积分布规律,并获得了不同工况下混流式转轮内压强、压强梯度以及剪切应力引发鱼类受损的概率。结果表明:鱼类受压强及剪切损伤的概率与流量成正相关关系,对于该文分析的混流式转轮,鱼类受最低压强损伤的概率在最小水头的最大流量工况下达到最大值9.1%,剪切损伤及高压强梯度损伤的概率在额定工况下达到最大,分别为0.823%和8.31%。在相同工况下进行3种损伤概率的对比后发现,在大流量工况下,鱼类受最低压强和高压强梯度损伤的概率更大,在小流量工况下则是压强梯度损伤概率相对较高,所以综合分析结果可知最低压强和高压强梯度是开展亲鱼型混流式转轮优化时需要考虑的主要因素,而剪切应变率则是次要因素。

Hydropower is a major source of renewable,noncarbon-based electrical energy.Although hydropower has many environmental advantages,hydropower dams alter the natural ecohydrological conditions of the rivers and cause significant ecological impact,especially for fish that live in or migrate through impounded river systems.Injury and mortality of fish that pass through hydraulic turbines and other downstream passage routes can result from several mechanisms,such as rapid and extreme pressure changes,shear stress,strike,cavitation,and grinding.For example,a large or fast pressure drop can lead to internal bleeding of fish,rupture of the swim bladder or vapor bubbles in eyes,which will result in direct mortality and reduces the ability to escape predators in the tailrace.Shear stress can causes fish scales flake,muscle tissue tearing,bruising,and even the fish body are cut off.So understanding the biological responses of fish to the conditions of hydraulic turbine is important for designing advanced fish-friendly turbines.Since the injury of fish may be caused by a combination of multiple damage mechanisms,it is necessary to identify primary and secondary damage mechanisms by research.In this paper,the computational fluid dynamic analyze method was adopted to simulate the three dimensional turbulent flow in an francis turbine.The simulation was conducted at different discharge conditions of maximum,rated and minimum head.The rated head of the turbine Hr is 106 m,the maximum head Hmax is 120 m and the minimum head Hmin is 73 m.The whole flow passage of the turbine was discretized by hexahedron structured mesh,and the SST k-ωturbulence model was used in the simulations.Then,the fish friendly threshold for pressure,pressure change rate and shear strain rate were used to analyze the volume size and distribution that may lead to the damage of fish.The ratio of the volume exceeding the fish friendly threshold to the total volume of the runner channel was defined as the fish damage probability.Finally,according to the calculation results,the main and secondary mechanisms of fish damage under different conditions were identified.Meanwhile,the law between the probability of fish injury caused by these mechanisms and the working conditions was further analyzed.From the results it can be seen that the volume which the pressure beyond the threshold in runner is mainly distributed at the outlet of the suction side of the runner blade,and the volume which the pressure change rate beyond the threshold is distributed at the leading and trailing edge of the runner blade.Besides,the volume which the shear strain rate beyond the threshold is distributed near the wall of the crown,band and runner blade.The fish damage probability caused by pressure,shear stress and pressure change rate were defined as P(A),P(B)and P(C)respectively in this paper.Based on the results of this paper,the probability P(A)reaches the maximum value at the condition L1.And the probability P(B)and P(C)reach the maximum value at the rated condition R1.The maximum value of P(A),P(B)and P(C)are 9.1%,0.823%and 8.31%respectively.By comparing the fish damage probability of pressure,pressure change rate and shear stress under different discharge conditions at the same head,it can be concluded that the minimum pressure and the pressure change rate are the two important factors to prevent fish damage.The shear stress is less important than that of them.Therefore,in the process of designing fish friendly francis turbine runner,the pressure in the runner must be raised as much as possible.Meanwhile,the pressure change rate in runner must also be decreased.