D-BAUG - Departement Bau, Umwelt und Geomatik - ETH Zürich

Highlights ▪ Infrastructure Systems
Intake structure of the Handeck 2
power plant
Enhancement of the Handeck 2 power plant's
efficiency by increasing the discharge and
reducing head losses.
The Kraftwerke Oberhasli AG (KWO), based in Innertkirchen
(BE), plans to enhance the Handeck 2 power plant's efficiency
by increasing the discharge from 42.5 to 65.5 m 3 /s
and reducing head losses in the hydraulic system.This upgrade
is part of the project “KWO plus”.The project design
uses the existing intake structure in the Räterichsboden
reservoir. In addition to the existing head race tunnel, a parallel
tunnel shall be constructed and connected by a junction
to the existing tunnel. The upper edge of the intake
cross-section is located about 9.5 m below the existent
minimum operating water level.
VAW was commissioned in April 2009 with the construction
of a physical model at a scale of 1:35 in order to investigate
and analyse the problems related to the vortex formation
due to the increased flow velocity at the intake (Fig. 1). In an
additional detail model the intake structure was reproduced
in a channel at a scale of 1:25. This model should give information
about scale effects related to vortex formation.
The model tests show that the formation of vortices with
risk of air entrainment would lead to an increase of about
13 m of the minimum operating level.This would negatively
affect the efficiency of the hydropower plant. At the moment,
anti-vortex devices are tested in order to allow a further
lowering of the reservoir water level.
Because of the large number of upgrading projects of existing
power plants and the related problem of tightening
flow conditions, VAW launched a research project which
should investigate the air entrainment rate due to vortices
(Fig. 2).With this information it is possible to estimate
the consequences of air in the pressure system and, if applicable,
to accept air entrainment.The latter would require
counter-measures such as de-aeration devices in the system.
For a precise design of such de-aeration systems the
air entrainment rate as input parameter has to be known.
58 ▪ D-BAUG Annual Report 2009
Cracking due to hygric or thermal
shocks
Concrete surfaces, such as floors and pave-
ments, develop micro- and macrocracks
when exposed to dry air. How can this
phenomenon be explained?
by G. Möller, M. Pinotti, A. Lais / VAW by J. Bisschop, F.K.Wittel / IfB
The life-time of a concrete floor is strongly affected by the
amount of surface damage. Pre-existing surface cracks accelerate
deterioration of concrete by traffic loading and by the
ingress of aggressive fluids. Drying shrinkage has long been
recognized as an important cause for cracking of concrete.
However,explaining the geometry of drying shrinkage crackpatterns
in concrete is a difficult task because the shrinking
material is constrained in multiple ways.
In this project we explain a particular type of shrinkage
cracks, namely those formed by pure contraction gradients.
These form in unconfined samples of hardened cement
paste suddenly exposed to severe drying conditions.
The surface layers immediately contract, while the inner
parts do not, since it takes time for moisture to diffuse out
of the material.This problem is equivalent to the formation
of surface cracks in certain materials under thermal shock.
What has been theoretically predicted, also is confirmed
experimentally in this project. The deeper surface cracks
propagate into the material driven by the evolution of the
contraction gradient, the more cracks are able to interact
as explained by a hierarchical 2D model (Fig. 3). From some
critical point onwards only every second crack can propagate,
shielding the intermediate ones. We showed that
this crack spacing doubling can occur once or twice, depending
on the drying rate and sample thickness.