Intermittent renewable energy sources have become a significant part of the electric grid in the last few decades. With their implementation into the energy system and varying electricity demands from the market, certain challenges in maintaining a stable electric grid have arisen. Hydropower here finds an important role in the stabilisation of the grid with its possibilities to regulate frequency and power. However, this causes hydropower to operate in transient modes more frequently. Consequentially, more studies are necessary in order to safely operate the turbines during transients, to plan maintenance, and to predict the lifetime of the hydropower plants and the costs associated with new operating circumstances. There has been an extensive series of studies on transient operation of Francis turbines and pump turbines in recent years. However, transient operation of Kaplan turbines needs more in-depth studies. Therefore, the present work is focusing on the formation of oscillating flow structures and the evolution of the resulting flow-induced forces during load reduction of the U9-400 Kaplan turbine model. The study of the flow field is performed using the OpenFOAM open-source CFD code. The flow-induced horizontal and axial forces, and consequential bending moments and torque acting on the runner are analysed together with the flow structures forming in the draft tube at the best efficiency point (BEP) and at part load (PL).

Today’s electric energy system includes more renewable sources than ever before, with a large increase in wind and solar power. The intermittency of wind and solar power brings certain challenges in maintaining the balance of the electric grid. Hydropower has shown great potential in solving the grid balancing problem. However, historically hydropower has had a completely different role, covering only the base load. The water turbines were designed to operate at the best efficiency point for most of their lifetime, and other kinds of operation may cause unpredictable shortening of the lifetime of the machines. Safe operation and planned maintenance are of great importance for both continuous energy distribution and human safety. In order to use the potential of hydropower to operate with highly variable loads, more comprehensive studies need to be made on the effects of transient operation of water turbines. An extensive series of studies have been made in recent years on the transient operation of Francis turbines and pump turbines. However, transient operations of Kaplan turbines require more in-depth studies. The present work analyses the flow-induced forces and resulting torques and bending moments on the U9-400 Kaplan turbine model runner blades during a load reduction sequence. New implementations in the OpenFOAM open-source CFD code have been developed to extract the forces, torques and bending moments on individual blades with respect to coordinate systems rotating with each individual blade.

A novel numerical framework in OpenFOAM is proposed in this work, to simulate transient operation of Kaplan hydraulic turbines. Such transient operations involve a variation of both runner blade and guide vane angles, which also gives rise to a flow rate variation. A numerical simulation of such a process is very challenging, since it requires a deformation of both guide vane and runner meshes, with mesh slip conditions at arbitrarily shaped surfaces, at the same time that the runner mesh is rotating around the turbine axis. The currently available mesh morphing methodologies in OpenFOAM are not able to properly accomplish this. Thus a novel framework for OpenFOAM, including dynamic mesh solvers and boundary conditions, is developed to tackle this problem.

The new framework is utilized to simulate the flow during transient operation of the U9-400 Kaplan turbine model. The guide vanes and runner blades are rotated individually around their own axes with a constant rotational speed, while the runner is rotating, and the flow rate is linearly changed with the guide vane angle. It is shown that the novel numerical framework can successfully be utilized to simulate the load change of Kaplan turbines.

Renewable sources of energy are on the rise and will continue to increase the coming decades [1]. A common problem with the renewable energy sources is that they rely on effects which cannot be controlled, for instance the strength of the wind or the intensity of the sunlight. The ALPHEUS Horizon 2020 EU project has the aim to develop a low-head hydraulic pump-turbine which can work as a grid stabilising unit. This work presents numerical results of an initial hub-driven counter-rotating pump-turbine design within ALPHEUS. Computational fluid dynamics simulations are carried out in both prototype and model scale, for pump and turbine modes, and under steady-state and unsteady conditions. The results indicate that the initial design have a hydraulic efficiency of roughly 90 % in both modes and for a wide range of operating conditions. The unsteady simulations reveal a complex flow pattern downstream the two runners and frequency analysis show that the dominating pressure pulsations originates from the rotor dynamics. Given the promising high efficiency, this initial design makes an ideal platform to continue the work to optimise efficiency and transient operations further.

Power generation from intermittent renewable energy resources (e.g. wind, solar) requires regulation of the electric grid. Although most hydraulic turbines are designed to work in their best efficiency points, nowadays they are being used more often under varying operating conditions to stabilize the electric grid. Unstable and varying conditions of fluid flow in hydraulic turbines during transient operation cause significant pressure fluctuations and load variations that could negatively affect the turbine lifetime. Therefore, the development of high-fidelity numerical tools for hydraulic turbine flow during transient operation, i.e. changing from one condition to another or during start-up and shut-down, is of great importance for the lifetime prediction of the machines.

In the present work, we are investigating the capabilities of the OpenFOAM open-source CFD tool to predict such phenomena. The transient operation of hydraulic turbines most of the time involves changing the guide vane angles while the runner is rotating, which must thus also be allowed by the employed numerical techniques. The high-head Francis-99 turbine is used as a test case, due to the availability of the geometry and rich experimental data. The turbulence resolving computations are performed using the SAS turbulence model. The numerical results are validated against the experimental data and compared with each other in terms of accuracy and usability. The results are also used for describing the flow behaviors during the shutdown.