Tunnel
Design
DG
Phillips, PJ Richards and RGJ Flay demonstrate various stages of the
diffuser, using CFD for a diffuser augmented wind turbine
ADiffuser Augmented Wind Turbine (DAWT) has a duct that surrounds the
wind turbine blades and increases in cross-sectional area in the stream
wise direction. The resulting sub-atmospheric pressure within the diffuser
draws more air through the blade plane, and hence more power can be
generated compared to a ‘bare turbine’ of the same rotor blade
diameter. Several researchers have examined the benefits and economics
of placing a diffuser around a wind turbine. An extensive programme
of experimental work performed at the Grumman Aerospace Corporation
in the 1970’s and early 1980’s identified the use of external
air jets to prevent separation within the diffuser. The high speed jet
flow reenergised the boundary layer within the diffuser enabling a short
length-to-diameter diffuser with a large outlet-to-inlet area ratio
to be developed. The economical benefits derived from this design were
not seen until 1997. It was the use of High Tensile Reinforced Fibrous
Ferrocement (HT Ferro) that enabled Vortec Energy to secure the rights
to the design and the production of the first full-scale DAWT. The Vortec
7, a technology demonstration unit built by Vortec Energy has a rotor
blade diameter of 7.3m and is situated near the Franklin west coast,
120 km south of Auckland, New Zealand. During the initial testing of
the Vortec 7 it became apparent that the power generated did not match
that predicted by Foreman and his co-workers. Since altering the configuration
of full-scale wind turbine is both difficult and expensive, Vortec Energy
enlisted the help of staff and post-graduate students at the University
of Auckland to carry out research into the cost-effective alterations
that could be made to the design. This research primarily involved the
use of PHOENICS to test out the possible gains from a variety of alterations.
Following on from this modelling, PHOENICS has also been used to investigate
a number of new diffuser designs.
Modelling techniques
The initial study investigated the effects of the
diffuser shape and boundary layer control slots of the as-built Vortec
7 and subsequent modifications. The model is asymmetric with specified
inlet conditions and reference length making the model dimensionless.
A body-fitted grid is used to reproduce the complex geometry of the
DAWT with the turbine modelled as a flow resistance. The turbine has
a specified thrust coefficient that produces a pressure drop across
the blade plane proportional to the local dynamic pressure. This is
analogous to the use of a gauze screen in wind tunnel testing. Features
such as tip vortices and flow swirl have been omitted in order to reduce
the computation time. In this regard it should be noted that the focus
of these studies is on the diffuser design and not on the design of
the motor itself. Hence it is considered that a simple turbine model
is sufficient in this situation. These assumptions allowed the use of
the CFD modelling for rapid development of diffuser designs. The inlet
truss of the Vortec 7 was modelled by shear stress acting in both the
axial and radial directions over the cells in the inlet boundary layer
control slot. The k-e turbulence model has been used with uniform inlet
boundary conditions specified for k and e. The values of k and e were
calculated for the hub height and terrain in which a turbine would be
situated. Modelling of later diffuser designs has generally used similar
techniques.
....CONTD
