CFD validation of pressure fluctuations in a pump turbine

Abstract: With growing environmental awareness, power producers turn their attention
to renewable energy sources. Favorable government support has in many
countries lead to a dramatic increase of wind power development. This has in
turn raised demand for hydro pumped storage plants to guarantee the
stability of the power grids. To allow for increased reliability and
operating range, manufacturers focus on learning more about the dynamic
behavior of pump turbines.

The Hydrodyna research project is a collaboration project between turbine
manufacturers and Ecole Polytechnique Federale de Lausanne, Switzerland,
which aims to improve the understanding of high head pump turbine dynamics.
It is not unusual that modern pump turbines operate at heads around 500 -
700 m. For such high heads, rotor stator interaction is known to be of
greater significance due to higher water velocities and thicker geometries
in stay and guide vanes. The Hydrodyna research project aims to break
through a technology gap in the understanding of pressure fluctuations due
to rotor stator interaction.

This work describes the results from simulations of the turbine operating
at full load and its comparison to laboratory measurements. The
commercial software Ansys CFX 5 has been used to perform transient
simulations with second order backward Euler transient scheme and the
k-epsilon turbulence model on structured, high density meshes. The
simulations
have been run on a twelve CPU cluster with 0.2 degree runner
rotation in each time step. The dynamic behavior of rotor stator interaction
is visualized and examined at key frequencies on the pressure and
suction side of a runner blade. In order to find the best setup, different
simulations have been run on guide vanes and runner with combinations
including the stay vanes and draft tube.

The results are validated against laboratory pressure measurements
on a runner blade. The transient pressure readings, from transducers
with measurement error below 0.001 bar, make it possible to determine
the accuracy of the CFD code. A comparison is also made with Laser
Doppler Velocimetry (LDV) measurements of flow velocities in a guide
vane channel.