Rehabilitation and modernisation of old constructions are important for a
contemporary energy market. Among the renewable energy sources, hydropower
has an eminent potential for further improvements since a great number of
the hydropower plants are ageing and are as well often run at off-design
conditions. An important part of a hydropower plant (low and medium headed)
is the hydraulic turbine draft tube that contributes to a large portion of
the hydraulic losses. The purpose of the draft tube, often being a curved
diffuser connecting the runner to the outlet, is to recover kinetic energy
and thus creating an artificial head. Traditionally the design has been
based on model tests and simplified analytic methods. Today and in the
future Computational Fluid Dynamics (CFD) in combination with computer
optimization will be used more frequently as a design tool. The numerical
prediction of the flow field in the draft tube is however challenging,
caused by its complex flow features e.g. unsteadiness, swirl, separation
etc. Therefore several numerical difficulties have to be solved before it
can be applied routinely in product development. One of the key issues in
this context is the turbulence modelling.
Here the flow field is analyzed and validated with previously performed
measurements on two draft tube geometries, a sharp-heel draft tube and a
modification of it (where the sharp heel is smoothed). Both steady and
unsteady simulations are performed, with the standard k-epsilon turbulence
model as well as the SST turbulence model. The focus is set on the
alteration in the pressure recovery factor and the overall flow field as a
function of the shape of the draft tube and the implemented turbulence
model.
The steady and unsteady CFD simulations performed with the standard k-
epsilon turbulence model yield about the same result. To exemplify, the
difference in the pressure recovery factor between these simulations is
much less than 0.001%. The main difference is that the unsteady simulation
required less CPU-time as compared to the steady ones. The improvement in
the pressure recovery between the original and the modified geometry is
also small, about 0.006%. This can be compared to the experiments where the
efficiency of the system improved with about 0.5%, indicating that the
pressure recovery, as defined, should increase even more. The CFD
simulations with...
