Discharges
measured with the ASFM instrument at this particular power plant have
shown significant systematic errors. Theoretical and CFD studies by
ASL AQFlow have shown that the bias occuring in the ASFM is mainly due
to the presence of strong wakes produced by the large vertical members
of the trash rack at a short distance upstream of the plane of measurement.
In an attempt to gain a better understanding on the flow features affecting
the ASFM, a three dimensional numerical analysis in Stave Falls power
plant intake was carried out.
The top and middle-left Figures show the configuration of the intake
with trash rack, circular pipes supporting the ASFM frame, and grid
used at selected planes. The middle-right and bottom-left Figure show
the distribution of the mean horizontal velocity in a cross and lateral
section. The large wakes produced by the horizontal and vertical beams
of the trash rack are clearly demonstrated. The comparison of the CFD
data with the ASFM measurements has shown a discrepancy which can not
be explained by a lack of accuracy in the ASFM data. This discrepancy
can, however, be explained by the presence of a blockage at the bottom
of the trash rack (this has been confirmed later by the plant operator).
The effect of a blockage at the bottom of the trash rack was then studied
by assuming different blockage elevation and porosity values. The comparison
of the velocity profiles with the ASFM measurements is shown in the
last Figure.
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