Design of Sand Traps in Hydropower Plants

Abstract

This PhD thesis focuses on sand traps in hydropower tunnels. The research includes a literature review of previous studies on this topic, as well as physical model tests carried out at NTNU and TU Graz to investigate the behavior of sediments in these systems. The literature review shows that the main requirement for efficient particle extraction is to prevent turbulence from re-entrain the sediments. Many sand traps have been built as a conventional tunnel with an enlarged cross-section area, either constant or increasing along the sand trap length. This is not a recommended design. Turbulence-free sedimentation is best achieved with a kind of physical separation between the sediments and the flow, for example concrete ribs or steel plates with a certain spacing. The model tests indicated that retrofitting of old sand traps with ribs in a small portion of the length was beneficial to the sand trap efficiency, but the design of the models have some weaknesses related to the surface smoothness, which causes the turbulence in the models and prototype to develop differently. The physical model at NTNU was also equipped with ribs and measurements with Particle Image Velocimetry (PIV) were done. The measurements were used in a computer program developed for simulation of particle trajectories in a sand trap equipped with ribs. The particle simulator computes particle trajectories in 2D and time. All relevant forces are modelled, but drag and gravity are the dominating forces influencing the particle acceleration. The computer program can be a helpful tool for improving the efficiency of sand traps, and the flow data used as input for the program can come either from physical measurements or CFD.