Abstract:
A numerical approach for modelling variable stator stagger in multistage stage
axial flow compressors is presented. The development of such an approach has been
motivated by the requirements of an optimisation methodology for stator vane setting
and active control of instability using controlled stator vane setting. The optimisation
methodology has been further developed but active control approaches are discussed
as future considerations.
Varying upstream stator vane stagger
.
changes the incident flow angle on
.
the
downstream rotor thus affecting the entire flow distribution within the compression
systems. The approach therefore begins by investigating the effect of a change in
stator stagger setting on stage performance. A meanline method was used for nu-
merical prediction of stage characteristics as it can simulate the effect of a change in
stagger settings and ( or) in rotational speeds. Overall compressor performance was
obtained by stacking the (experimental or predicted) stage characteristics and the
surge conditions predicted using a stage-by-stage dynamic compression model where
the compressibility was considered explicitly. This approach for variable stagger set-
ting was incorporated into a FORTRAN code and validated using the data from the
12-stage HP SPEY jTAY variable geometry compressor.
To optimise the setting, a direct search method incorporating a Sequential Weight
Increasing Factor Technique (SWIFT) algorithm was incorporated into the variable
stagger model. The objective function in this optimisation is penalised externally
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with an
updated factor which helped to accelerate convergence. The methodology has
been incorporated into a FORTRAN program and its validations were conducted
using the data from the 7-stage LP OLYMPUS and the 12-stage HP SPEY /TAY
compressors. Results have demonstrated that variable stagger setting is a powerful
method to rematch stages and which can be used to improve the desired overall
performance, and that the potential benefits of introducing additional rows of variable
setting vanes can be achieved.
Future work arising from the present study has been discussed and highlighted,
which involves the enhancement of the model capacity and development of active
control approaches.
In addition the thesis involves several reviews focusing on different topics. Most
reviews contain considerable information and it is expected that the information can
be of help for the interested readers to trace more relevant references. These reviews
consist of a general review in chapter 1; a brief review on stage characteristics modelling in chapter 2; a comparative review on incompressible and compressible surge
models in chapter 3; a review of various optimisation methods for practical problems,
especially for constrained non-smooth problems, in chapter 4; and a review of the
state-of-the-art active approaches in chapter 7. The suitability of various approaches
has been highlighted. Steinke's meanline method is suitable for investigating the in-
influence of stagger resetting on stage performance. To predict the surge conditions for
a (high-speed) multistage environment, the stage-by-stage compressible models are
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more promising. For constrained non-smoothed optimisation, the SWIFT algorithm
can be an alternative. The controlled stator vane regulated through nonlinear control
law will permit the robust control of compressor instabilities.