The screen arrangement (green) and the intake channel to the hydroelectric station

Hydropower installations typically lead to changes in the natural water course of rivers and streams, such as damming to provide water storage. When hydroelectric power stations are constructed, it is important, despite these changes, to conserve the habitats of wildlife in and around the nearby waters. This is also true for some varieties of fish, because fisheries can be a major contributor to the local economy in certain areas.

At one such installation in the United Kingdom, the intake channel to the power station turbines is protected by a bar screen. The screen effectively prevents debris from entering the channel and does not significantly restrict the flow to the turbines. To prevent fish from entering the channel, engineers at Kvaerner Markham were summoned to help design a secondary screen system that can be used to augment the existing screens in the original installation.

Kvaerner Markham, based in Sheffield, England, is a well-established designer and manufacturer of special-purpose equipment. They recognized the potential benefit of applying CFD technology to this problem. CFD could help them assess designs based on a key design criterion for the new screens: that the flow velocity at the screen face be low enough to ensure that fish do not become trapped by excessive dynamic pressure in the moving water.

The first of the screen systems, a zig-zag screen, named for the shape of its cross-section, was designed to sit on top of the existing screens. The idea was to create a screen with a reduced bar spacing compared with that of the existing screens, while maintaining an equivalent or greater open area for the flow. It was hoped that, with these screens, a significant increase in the local flow velocity could be avoided.

The second screen system proposed consisted of a box-like construction that was mounted onto the existing screens. The panels of the box screen consisted of vertical bars, with a bar spacing less than that of the existing screens. As with the zig-zag screen, the aim of the box screen arrangement was to reduce the bar spacing while maintaining a high open area for flow.

Engineers at Fluent Europe Ltd. were asked to carry out a CFD analysis on behalf of Kvaerner Markham to determine the flow patterns and velocities near each screen face under normal operating conditions. A two-stage approach was adopted. First, 2D simulations of each screen configuration were carried out to determine their hydraulic characteristics. These characteristics were then incorporated into global 3D models of the lake and intake channel using the porous media model.

The box screen arrangement

The results of the study showed that the flow velocity near the face of the zig-zag screen was too high for immature fish to handle. The flow pattern also indicated that the fish could become trapped at the internal apex of the screen. The velocity magnitude in the region of the box screen was significantly better than that near the zig-zag screen. However, this configuration caused a local increase in velocity near the existing screens. Based on the results provided by the CFD study, engineers at Kvaerner Markham were able to understand the effects of the proposed screen systems on the flow pattern, and are currently in the process of refining a final screen design.

Velocity magnitude close to the zig-zag screen face

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