Abstract:
An
investigation was carried out to study the process of
rotating stall inception in a low
hub-tip ratio fan. Such
fans are
expected, based on an elementary analysis, to
stall from the root.
However, experimental evidence had
led to the belief that the fans stalled from the
tip. The
effects of streamtube contraction were first studied and
this was followed
by an experimental investigation on an
isolated
rotor, with successive build modifications to
increase the likelihood of
rotating stall inception
occurring at the root.
A
computer based streamline curvature method was used to
study the effects of streamtube contraction and streamtube
diffusion that
commonly occur when a fan is operated at
flows below its'
design flow rate. The results indicated a
reduced
expectation for the root to stall first when
compared to a simple 2-D flow analysis.
Experimental measurements were then carried out to
determine how the
experimental local characteristics
differed from the
predicted characteristics. It was
apparent that real fluid effects tended to
steepen the
root
characteristic, thus enhancing the stability of the
root. The
tip characteristics tended to droop and become
less stable.
The enhancement of the root
stability was also seen in the
profiles of deviation angle. The axial Velocity contours
at the rotor exit supported the conclusion that the root
stability enhancement was caused by "centrifuging".
To determine the actual radial location of rotating stall
inception, an array of hot wires was used to record events
during the inception transient. Inception was first
detectable at the tip. This tip stalling behaviour
persisted for all the build modifications.
Measurements of unsteady pressure were also made to study
the movement of the overall
operating point since it was
felt that this could continuously alternate between a pair
of
closely spaced characteristics. The results indicated
that the fan operated along a unique characteristic.
The overall conclusion was that a low hub-tip ratio fan
shows a
strong reluctance to stall at the root due to
"centrifuging" of the blade boundary layer. The inception
process appears to be dominated by events in the tip
region.