Towards Full Predictions of the Unsteady Incompressible Flow in Rotating Machines, Using OpenFOAM
Doctoral thesis, 2012
The main objective of the present work is to validate
methodologies for accurate numerical predictions of the incompressible flow of
water in the U9 Kaplan turbine model. The term “prediction” implies that the use
of detailed experimental data for boundary conditions should be avoided, and
that all important features of the studied case should be included. That
includes specifying boundary conditions at locations where the flow conditions
can be easily estimated, and where reasonable variations in those estimations do
not significantly affect the flow prediction. As an example, the U9 Kaplan
turbine model has a sharply bent inlet pipe, and it is here argued that the
secondary flow from that should be taken into account by including the pipe in
the simulation. In the case of rotating machines, such as the U9 Kaplan turbine
model, the interaction between rotating and stationary components (rotor-stator
interaction) is a feature that must be included in the simulations.
Three highly relevant well-documented cases have been used in the present work,
the ERCOFTAC Centrifugal Pump, the Timisoara Swirl Generator and the U9 Kaplan
turbine model. All three cases include rotor-stator interaction. The latter,
being the main goal of the studies, has just recently been studied
experimentally and is a computationally demanding case. Thus, the former two
cases were used while validating the new implementations and evaluating the
numerical settings, until the results were reliable and efficient. Two rotor-stator
interaction methods were investigated, the steady-state frozen-rotor approach,
and the unsteady sliding grid approach, and the results from four turbulence
models were compared.
The results show that both approaches can
be used to couple the rotating and stationary parts of the domain. However, the
frozen rotor yields an unphysical advection of the runner wakes, and such
results should only be used for a first estimation or as initial conditions for
full unsteady sliding grid simulations. The predictions compare very well with
the experimental results, and the main differences can be explained by the
geometrical simplifications that were made. The four turbulence models behave
similarly, with a minor preference for different models in the different cases.
The present work is done using the OpenFOAM OpenSource CFD toolbox. The code is
chosen to facilitate an OpenSource distribution of the developments, to be
shared in the scientific community, and to be directly useful in industry. It was not possible to achieve the results presented
here with OpenFOAM before the start of the present work,
which has significantly contributed to the validation of, and trust in, the new
implementations.
Turbulent Flow
Sliding Grid
Hydro Power
Kaplan Turbine
OpenFOAM
CFD