The first step towards this understanding was the development
of a low order model for air injection control, the starting point of
which was the Moore and Greitzer model for axial flow compressors. The
Moore and Greitzer model was extended to include the effects of air
injection and bifurcation analysis was performed to determine how the closed
loop system dynamics are different from those of the open loop system.
This low order model was then used to determine the optimal placement of
the air injection actuators.
Experimental work focused on verifying that the low order model,
developed for air injection actuation, qualitatively captured the
behavior of the Caltech compressor rig. Open loop tests were performed
to determine how the placement of the air injectors on the rig
affected the performance of the compressor. The positioning of the air
injectors that provided the greatest control authority were used in
the development of air injection controllers for rotating stall. The
controllers resulted in complete elimination of the hysteresis
associated with rotating stall. The use of a throttle actuator for the
control of the surge dynamics was investigated, and then combined with
an air injection controller for rotating stall; the resulting
controller performed quite well in throttle disturbance rejection
tests.
A higher order model was developed to qualitatively match the
experimental results with a simulation. The results of this modeling
effort compared quite well with the experimental results for the open
loop behavior of the Caltech rig. The details of how the air injection
actuators affect the compressor flow were included in this model, and
the simulation predicted the same optimal controller that was
developed through experimentation.
The development of the higher order model also included the
investigation of systematic methods for determining the simulation parameters.
Based on experimental measurements of compression system transients, the
open loop simulation parameters were identified, including values for
the compressor performance characteristic in regions where direct
measurements were not possible. These methods also provided information on
parameters used in the modeling of the pressure rise delivered by the
compressor under unsteady flow conditions.
CDS Technical Report
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