| dc.description |
Summary
The present report is a continuation of an interim
report dated 8th August, 1951. The latter- report dealt with the
flow in a model casing whereas the present reports deals with a
similar investigation on the ‘Reavell’ compressor inlet casing
which is to be part of the 9in x 9in supersonic wind tunnel installation,
In the basic condition the flow had a circumferential
instability* caused by the splitting of the inlet flow around the
central shaft fairing. In addition the main flow through the
casing was under-turned by approximately 200, Consequently the
flow at the exit annulas was unsteady and changes of 300 in the
exit flow direction were found. It was also found that at least
four guide vanes were stalled in any one flow configuration.
The instability was prevented by a flat dividing plate
placed behind the central shaft fairing, but smooth flow at exit,
in the vicinity of the plate, was only possible when curved fairings
were added to both sides of the plate. A gauze screen, of
30 wires per inch, placed at entry, partly reduced the unsteadiness
at exit but it had little effect on the flow direction and
did not unstall the guide vanes. A perforated plate, having 15.
inch diameter holes and 50 per cent blockage, similarly placed,
had little or no effect.
The guide vanes were unstalled by adding curved extensions
to their leading edges. The large changes in the flow
direction across the annulus at exit were, however, only reduced
by modifying the internal contour of the casing. With these
modifications flow angle variations of between 5° and 10° were
obtained at exit. The velocity at exit was not uniform over the
exit passages between the guide vanes, but the variations, which
were not unduly large, were caused by secondary flows and the
wakes from the vanes.
This investigation has clearly shown that in the basic
condition the unsteady, non-uniform flow at the exit from the
inlet casing, which is immediately upstream of the first stage of
the compressor, will tend to cause periodic stalling of the rotating
blades in that stage. Since the relative velocities are
high the danger of running into stn3ling flutter cannot be overlooked,
especially as the cascade effect may reduce the critical
flutter speed of the isolated blade. Stalling flutter, which
usually takes place in the fundamental torsional mode, may be
aggravated by a high order resonance occuring at the given rotational
speed of the compressor.
The suggested' modifications to the design of the inlet
casing should prevent stalling of the first stage blades 'of the
compressor caused by upstream, unsteady, non-uniform flow. |
|