为探明拦污栅、隔流墩、底坎等局部结构对进水口流态及水头损失的影响规律,以某低水头大流量水电站为例,建立了由库区至水轮机蜗壳的取水流道三维水流数学模型,分析了不同机组引用流量以及排沙底孔运行条件下的进水口水力特性。研究发现,由于左右进水孔下部设置排沙底孔,水流从拦污栅底部与进水口底坎之间的进流通道斜向流入进水口,栅前过流面积较大,左右进水孔拦污栅栅前流速分布与中孔不同,最大流速比中孔偏小30%,平均流速偏小26%。排沙底孔运行致其对应孔的栅前过流面积减小,流速分布更加均匀,较排沙底孔不运行平均流速增大了21%。栏污栅横梁、中隔、栅条尾部以及各隔流墩墩头转角处、进口底坎处存在低压、高紊动能区,是进水口水头损失产生的主要区域,其中拦污栅段水头损失约占进水口总水头损失的50%。排沙底孔不运行时,左、中、右孔拦污栅水头损失系数分别为2.1、0.9、1.7;排沙底孔运行后,栅前断面流速分布更加均匀,较排沙底孔运行左孔拦污栅水头损失系数减小43%左右。因此,进水口优化应重点关注进口段及拦污栅段的水头损失控制,尤其是横梁、中隔、栅条尾部及隔流墩墩头转角等局部结构的流线型优化。

[Objective] The power generation efficiency of low-head, large-discharge hydropower stations is highly sensitive to head loss. Reducing the head loss at the intake of such stations is essential to improve their hydraulic energy utilization. This study aims to 1) investigate the distribution of flow velocity, pressure, and turbulent kinetic energy in low-head riverbed-type intakes, 2) examine the sources of head loss at the intake, 3) quantify head losses in different sections of the intake and the head loss coefficients of the trash racks, and 4) finally provide guidance for the design and optimization of similar intakes. [Methods] We established a three-dimensional model encompassing the intake (including the trash rack), the flow passage from the intake to the turbine spiral case, and the upstream intake channel. Hydraulic simulations of these regions were performed using the k-ε turbulence model in ANSYS Fluent. A hydrostatic pressure boundary condition was applied at the inlet, while a flow velocity boundary condition was specified at the outlet of the turbine spiral case. [Results] (1) When the bottom sediment outlets were not in operation, the flow velocity before the trash rack at the middle orifice was the highest (ranging from 1.4 m/s to 2.7 m/s), while the velocities before the trash rack at the left and right orifices were lower (ranging from 0.5 m/s to 1.9 m/s). After the left bottom sediment outlet was activated, the flow area in front of the trash rack at the left orifice decreased, with the maximum flow velocity increasing to approximately 2 m/s. (2) The overall pressure was slightly higher before the trash rack and slightly lower after it. The maximum pressure differences across the beams, intermediate support, and rack bars were 0.3 m, 0.3 m, and 0.08 m, respectively. (3) Across the operating conditions, the head losses at the intake were 0.121 m, 0.229 m, 0.385 m, and 0.376 m, respectively. The corresponding average head losses for the trash rack sections were 0.078 m, 0.120 m, 0.172 m, and 0.174 m. (4) When the bottom sediment outlets were not in operation, the head loss coefficients for the trash racks of the left, middle, and right orifices were 2.1, 0.9, and 1.7, respectively. After the bottom sediment outlet was activated, the head loss coefficient for the trash rack at the corresponding orifice decreased to 1.2. [Conclusion] The operation of a bottom sediment outlet reduces the flow area before the trash rack of the corresponding orifice, making the velocity distribution more uniform and increasing the average flow velocity. Low-pressure zones and high turbulent kinetic energy zones occur at the tails of beams, intermediate support, and rack bars, as well as at the turning corners of the separation pier heads for each orifice and at the intake sill. These regions are the primary locations of head loss in the intake and trash rack. As a result, head losses in the intake entrance section and trash rack section are significant, with the trash rack section accounting for approximately 50% of the total head loss at the intake. When the bottom sediment outlets are not in operation, the head loss coefficient for the left orifice's trash rack is the highest, followed by the right orifice, with the middle orifice being the smallest. After the bottom sediment outlet is activated, the flow velocity distribution at the pre-rack cross-section becomes more uniform, resulting in an approximately 43% reduction in the head loss coefficient for the trash rack at the corresponding orifice.