Experimental investigation of a Kaplan runner under steady-state and transient operations

Abstract: Hydropower is a key part in electricity production nowadays. Hydropower electricity production rose to 3579.5 TWh in 2013, ranked as the second source of electricity production in the world after fossil fuels. It is the principle source of renewable electricity production, producing 16.2% of the electricity in 2013, accounting for 78% of the renewable electricity production in the world. Specifically in Sweden, hydropower is the main source of electricity production producing 47.5% of the required electricity. Nuclear, biomass, and wind placed in the following positions in the ranking in 2013 with 38.4%, 6.5%, and 4.3%, respectively.Besides meeting electricity demand with an environmental-friendly method, hydropower has a unique and important role which is grid regulation: balancing electricity production and consumption. Gas turbines and hydraulic turbines called “Primary reserves” are the only electricity production systems that can be used for fast regulations due to their short start-up time from 1 to 60 s. The obvious environmental problems, air pollution, and costs associated with gas turbines make hydropower a prime alternative whenever applicable. In Sweden, the share of fossil fuels in electricity production is small; 2.8% in 2012 with an average annual growth of -0.7% in the period 2002-2012. Hence, hydropower is practically the only available source used to regulate the grid fluctuations resulting from deregulated market and fast growth of intermittent power generation systems, i.e., solar and wind energy. Hydraulic turbines are subject to frequent off-design and transient operations because of their grid regulation responsibility. Such operating conditions decrease turbine’s efficiency and affect its lifetime significantly. Off-design and transient operation of hydraulic turbines may induce unpredicted pressure fluctuations on the stationary and rotating parts of the turbine. Special attention should be dedicated to the effects of such phenomena on the runner blades because of their importance on the efficiency of the turbine, and their vulnerability to the pressure fluctuations.This thesis presents an experimental investigation on the effects of off-design and transient operation of an axial hydraulic turbine on velocity fields and pressure fluctuations exerted on the runner and the draft tube of a turbine. The investigation was performed on a 1:3.1 scaled model of a Kaplan turbine known as Porjus U9. The main objective was to investigate the effect of operating point on pressure and velocity fluctuations in the runner and the draft tube. Another objective was to study the effect of transient operation on pressure fluctuations exerted on the runner and the draft tube, to investigate the formation and mitigation process of a rotating vortex rope (RVR) within the draft tube. Finally, the effect of the swirl leaving the runner and the draft tube bend on the performance of the turbine was investigated. The study involves pressure measurements on the runner blades and draft tube walls of the turbine, laser Doppler anemometry (LDA) measurements within and after the runner, and particle image velocimetry (PIV) measurements within the draft tube.The pressure and LDA results acquired during steady state operation of the turbine showed different sources of fluctuations on the runner at different operating points resulting in symmetric and asymmetric fluctuating forces on the runner. The pressure measurements during transient operating conditions exhibited pressure fluctuations exerted on the runner during load variations and elucidated some aspects of formation and mitigation process of RVR within the draft tube. The PIV measurements performed after the draft tube bend of the turbine focuses on the physical phenomena resulting in flow asymmetry after the draft tube bend of hydraulic turbines affecting their efficiency.