The aim of this study is to develop a reliable numerical model that provides additional information to experimental measurements and contributes to a better exploitation of hydraulic turbines during transient operation. The paper presents a numerical analysis of the flow inside a Kaplan turbine model operated at a fixed runner blade angle during load variation from the best efficiency point (BEP) to part load (PL) operation. A mesh displacement is defined in order to model the closure of the guide vanes. Two different types of inlet boundary conditions are tested for the transient numerical simulations: linear flow rate variation (InletFlow) and constant total pressure (InletTotalPressure). A time step analysis is performed and the influence of the time discretization over the fluctuating quantities is discussed. Velocity measurements at the corresponding operating points are available to validate the simulation. Spectrogram plots of the pressure signals show the times of appearance of the plunging and rotating modes of the rotating vortex rope (RVR) and the stagnation region developed around the centerline of the draft tube is captured.

Hydropower plants are currently being intensively employed for electrical grid regulation. As a consequence, the frequency of start/stops and load variations is considerably increasing, leading to the operation of hydraulic turbines under improper conditions. During the last years, studies have focused on Francis turbines. The present paper aims to investigate a Kaplan turbine model. The flow through the turbine is modelled during transient operation, from the best efficiency point to a part load operating point, using a moving mesh for the guide vane displacement. The simulations are validated against experimental velocity profiles. A time step sensitivity analysis is performed in order to determine the optimum discretization time. The possibility of using large time steps is explored. The numerically simulated unsteady pressure pulsations on the runner blades are analysed. The influence of the inlet boundary conditions on the accuracy of numerical simulations is studied. The results show that a linear flow rate variation defined during the guide vane closure leads to an overestimation of the turbine head compared to the experimental value due to an overestimation of losses. The second type of boundary conditions, a constant total pressure, results in an underestimation of the flow rate compared to the experimental value due again to an overestimation of the losses.

Hydropower plants often work in off-design conditions to regulate the power grid frequency. Frequent transient operation of hydraulic turbines leads to premature failure, fatigue and damage to the turbine components. The speed-no-load (SNL) operating condition is the last part of the start-up cycle and one of the most damaging operation conditions of hydraulic turbines. Hydraulic instabilities and high-stress pressure fluctuations occur due to the low flow rate and unsteady load on the runner blades. Numerical simulations can provide useful insight concerning the complex flow structures that develop inside hydraulic turbines during SNL operation. Together with experimental investigations, the numerical simulations can help diagnose failures and optimize the exploitation of hydraulic turbines. This paper introduces the numerical model of a full-scale 10 MW Kaplan turbine prototype operated at SNL. The geometry was obtained by scaling the geometry of the corresponding model turbine as the model and prototype are geometrically similar. The numerical model is simplified and designed to optimize the numerical precision and computational costs. The guide vane and runner domains are asymmetrical, the epoxy layer applied to two runner blades during the experimental measurements is not modelled and a constant runner blade clearance is employed. The unsteady simulation was performed using the SAS–SST turbulence model. The numerical results were validated with torque and pressure experimental data. The mean quantities obtained from the numerical simulation were in good agreement with the experiment. The mean pressure values were better captured on the pressure side of the runner blade compared to the suction side. However, the amplitude of the pressure fluctuations was more accurately predicted on the suction side of the runner blade. The amplitude of the torque fluctuations was considerably underestimated.

The role of hydropower has become increasingly essential following the introduction of intermittent renewable energies. Quickly regulating power is needed, and the transient operations of hydropower plants have consequently become more frequent. Large pressure fluctuations occur during transient operations, leading to the premature fatigue and wear of hydraulic turbines. Investigations of the transient flow phenomena developed in small-scale turbine models are useful and accessible but limited. On the other hand, experimental and numerical studies of full-scale large turbines are challenging due to production losses, large scales, high Reynolds numbers, and computational demands. In the present work, the operation of a 10 MW Kaplan prototype turbine was modelled for two operating points on a propeller curve corresponding to the best efficiency point and part-load conditions. First, an analysis of the possible means of reducing the model complexity is presented. The influence of the boundary conditions, runner blade clearance, blade geometry and mesh size on the numerical results is discussed. Secondly, the results of the numerical simulations are presented and compared to experimental measurements performed on the prototype in order to validate the numerical model. The mean torque and pressure values were reasonably predicted at both operating points with the simplified model. An analysis of the pressure fluctuations at part load demonstrated that the numerical simulation captured the rotating vortex rope developed in the draft tube. The frequencies of the rotating and plunging components of the rotating vortex were accurately captured, but the amplitudes were underestimated compared to the experimental data.