Research focus with the background of numerical and analytical calculation of hydraulic machine components and the transient behavior of hydropower plants.

Project: Numerical simulation of the vibration excitation of underflow weirs by unsteady flow phenomena

Overview of projects

In contrast to run-of-river power plants, where the flow rates cannot be freely selected, the flow rate can be adjusted to the fluctuations in electricity demand with storage power plants. Pelton turbines are particularly suitable for the electrification of high-altitude water storage reservoirs. The potential energy is converted into kinetic energy at the nozzles by producing high-speed water jets. The buckets, that are mounted on an impeller, absorb the water jet impulse. Thereby, the impeller is set in rotation and this rotational energy is converted into electrical power by the generator. This impulse turbine concept enables energy conversion with high efficiency over a wide operating range. Therefore, Pelton turbines are ideal for efficiently stabilising the power grid with a wide range of fluctuation amplitudes in electricity demand.

The shape of the Pelton turbine bucket is crucial to reach high efficiencies, which should deflect the water jet by 180º. In the design of the bucket shape, a circular water jet shape with a certain diameter is assumed. The deviation of the actual from the ideal water jet shape causes losses in energy conversion at the bucket and is therefore quantified as water jet quality. Feed lines to the Pelton turbine nozzles often consist of elbows and branches. Flow structures are created there, which subsequently deform the water jet. With large water jet deformations and deviations, the efficiency can decrease by 0.5% to 2%. It is therefore essential to simulate the flows in feed lines of Pelton turbines using numerical methods and to increase the water jet quality through feed line optimisation.

Publications in this context

Sluice gates are hydraulic structures that regulate water mass flows to protect a downstream object. The flow under the sluice gate develops unsteady phenomena, which under certain circumstances can induce oscillations and vibrations. These vibrations can damage the structure and thus reduce the life time of weir systems. Failure by fatigue can cause flooding. It is therefore important to keep the flow induced vibrations to a minimum.

Powerful vibrations arise due to the coupling of forces generated by unsteady flow processes and the vibration of the sluice gate. The periodic shedding of flow vortices forces the displacement of the sluice gate, which reacts actively as a mass-spring-damper system. The movement of the sluice gate in turn creates shedding vortices. When the frequency of vortex shedding is close to the natural frequency of the sluice gate suspension, resonance occurs.

In order to shift the natural frequency of the sluice gate suspension from the frequency range of the hydrodynamic excitation (e.g. by attaching additional masses), this must be determined. Just as the prediction of the natural frequency of real weir systems is challenging, the reliable determination of the vortex shedding frequency at the gate is a complex task.
In the present investigation, fluid mechanical simulations are carried out in order to characterise the phenomenon of vortex shedding precisely. The study of the flow behaviour serves in particular to understand the excitation process of the oscillation by the flow separation and to determine the frequency range.

This answers the following questions in this project:

1) Which flow phenomenon causes the oscillations of the sluice gate?
2) In which frequency range are these vibrations excited?
3) Can this flow phenomenon be prevented or its impact mitigated?

The basic research question of the FFG project AxFeeder is to find out how distributor systems of Pelton turbines have to be designed in order to have similar flow conditions in the feed line and the injectors in all operating conditions and to achieve the best possible jet quality.

For this purpose, a comprehensive parameter study based on numerical flow simulations of potentially suitable hydraulics of distributor systems with axial inflow was conducted in a first project step. The findings of this parameter study will now be used to perform a detailed flow/vibration and structural mechanics analysis of the overall system. For this purpose, the project team will perform scale-resolving simulations on the Vienna Scientific Cluster (VSC) and, in parallel, design a laboratory test rig on which experimental investigations will be carried out.

Poster EN [PDF Download], opens an external URL in a new window

The figure shows the contours of the normalised velocity amplitude in the mid plane of a distributor system. In addition, contour plots of the normalised secondary velocity for six evaluation stations of the branch line are shown.

[1] F. J. J. Hahn, A. Maly, B. Semlitsch, and C. Bauer. Numerical Investigation of Pelton Turbine Distributor Systems with Axial Inflow, opens an external URL in a new window. Energies, 16(6), 2023. ©

 

Contact:

Senior Scientist Dipl.-Ing. Dr.techn. Anton MALY, BSc
Phone: +43158801302413
E-mail: anton.maly@tuwien.ac.at

Publication record Anton Maly, opens an external URL in a new window

Especially when operating in pump-mode, part load flow in pump-turbines is still a challenging task since crucial flow regimes become apparent in the entire flow
domain. In deep part load, the occurrence of flow instabilities is evident. Flow phenomena like rotating stall in the guide vane or pre-rotating flow at the runner
inlet are mainly responsible for an improper blade load. Another consequence is oscillating flow upstream of the runner, which results in the so called "off-design"
operating conditions, see Figure. Existing modeling approaches and further developments of linear eddy-viscosity RANS models considering rotation effects are implemented into the CFD soft-ware and validated by means of generic flow problems. The two-equation turbulence model SST k-ω is primarily employed for sensitizing methods used for transient incompressible single- and two-phase flow simulation. Simulation results of pump-turbine flow are directly compared with experiments making the interpretation of results more reliable.
A practical two-phase study considers the performance break-up due to leading edge cavitation of the pump-turbine runner during low partial flow rates, as well as a direct comparison with test-rig measurements.

Within the last decade the rising amount of volatile sources of energy, such as solar and wind power, led to a falling price on the energy spot market. As a result , there are higher requirements in terms of grid stabilization efforts. Therefore operators of hydropower plants are using their machines to provide balancing energy to the electrical grid. However, this leads to significant worse operating conditions in off-design points as well as to a higher number of start-ups and shut-downs. Thereby, flow phenomena and mechanical structure resonance determine the runner lifetime. Based on the results of the previous project PSP-LowLoad, a further development of the fatigue analysis including prototype site measurements and numerical simulations was aspired. Furthermore, the detection of critical operating points by means of the monitoring systems was investigated. The results of the project have/will been published in several papers and presented at scientific conferences.