Time dependent pressure phenomena in hydropower applications

Abstract: Time resolved pressure measurements in hydropower applications are of great interest. Different parts of the machine experience highly transient flows that influence the function and efficiency of the turbine. This thesis addresses different time dependent pressure phenomena. Assessment of the efficiency of a hydropower plant requires accurate flow measurements. Gibson's method is a pressure time based method to measure the flow rate. To improve the method outside its standard range an experimental investigation is performed in a laboratory setup in parallel with numerical solutions of the governing equations. The results indicate that it is possible to correct the flow measurements outside the limitations of the standard. A draft tube is an integrated part of a hydropower plant with a reaction turbine where the remaining kinetic energy of the flow after the turbine is converted into pressure. An experimental investigation is performed on a model hydropower draft tube at Älvkarleby to establish the detailsof the pressure recovery in the early part of the draft tube. The objective is to increase the understanding of the pressure behaviour and to contribute with data for CFD (Computational Fluid Dynamics) validations. The results show a high damping of the oscillating parts of the pressure in the axial direction. From earlier investigations done as part of the Turbine-99 workshops, it has been observed that the radial pressure distribution just under the turbine runner show a marked discrepancy between experiments and CFD. The flow in the region is highly time dependent so the behaviour of the Pitot tube used for the pressure measurements is investigated for oscillating flow in a lab setup. A method to derive more accurate data is proposed.