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Intake Flow Simulation and Optimisation for Hydropower

Final Report Summary - IFLOW (Intake Flow Simulation and Optimisation for Hydropower)

The aim of the project was focused on the increasing demand of intake efficiency in hydropower. In this project, the whole hydro-mechanical system of the power plant and the site specific interaction of the approaching flow, intake structure and the runner respectively were taken into consideration. The hydraulic losses due to flow separation and swirling as well as the vibration stimulation were investigated. A low head run-off-river power plant must generally be adapted to the local topography and environmental condition, and the Intake structure should be designed to bring the flow field uniformly distributed and free from disturbances to the turbine. Due to the low head, even very low hydraulic losses are responsible for relative high financial losses in the total annual energy production. Since the losses depend up on the local conditions, the general solutions are not available for all cases. Moreover, the solution obtained from one project cannot be directly replicated to other project unless the local conditions are matched. Hence, there is a need to develop the methods of investigation and optimization of intake structures for low head hydropower so the IFLOW project was executed. There were two methods of investigation concerned in foreground of the project:
- CFD (computational fluid dynamics) represented by the commercial code ANSYS CFX,
- Physical modeling methods and PIV (Particle Image Velocimetry) as an enhanced measurement technique used within the hydraulic laboratory experiments.

The PIV, often used in experimental hydromechanics, thermodynamics etc. provides the velocity data at sufficient resolution which are comparable with them from numerical modeling (CFD). The measured flow is being illuminated by a laser light sheet and one or two side viewed cameras capture images of small reflecting particles which are seeded in the water. The local particle displacements and the corresponding vector field on the illumination plane can be analyzed from a pair of time shifted images. The PIV represents disturbance free measuring technique due to its optical principal, which is still rarely implemented in hydraulic engineering research. Over the numerical modeling with CFX, the proper utilization of a stereo-PIV-system for the intake flow measurement were a big challenge within the IFLOW project.
In the initial phase of the IFLOW, a numerical study of flow condition of a semi-spiral Kaplan turbine was carried out using CFX and U-RANS (Unsteady- Reynolds Averaged Navier-Stokes) modeling. The aim was to examine the influence of artificial flow distribution disturbances at intake section on the flow field in front of the runner. The numerical results have shown that the continuous flow acceleration governed by the narrowing semi-spiral casing eliminates the flow disturbances very quickly and the flow becomes rather independent on the intake condition. Since only the flow distribution non-uniformity was modeled, the next investigation concerned the swirl phenomenon which was found to play an important role by the turbine operation problems. Although the flow acceleration may help to adjust the flow distribution, the swirl becomes stronger due to acceleration and is drawn deep inside the turbine passage. Pressure fluctuations, turbine cavitation, reduction of discharge and hydraulic losses may certainly be the main consequences of it.