Abstract:
The
performance requirements of turbofan engines demands a
stability and transient capability beyond that associated with
the
past generations of gas turbine engines. The axial flow fan
unit is most vulnerable to
loading limitations due to the
primary problems associated with the compression process, its
sensitivity to inlet distortion and the
difficulty to design
for an overall
optimum blade duty in a machine of wide
radial
blade
loading distribution. The development of mathematical
models with some
capability of predicting the stable operating
range of an axial flow fan has to overcome the
difficulties
associated with the
modelling of the radially distinct
flow
regions and their dynamic interaction. '
The current
investigation combined the available knowledge of
one-dimensional models (based on the
principles of conservation
of mass, linear momentum and
energy) with the assumptions of the
parallel compressor theory, in order to develop a linearized
system of equations for stability analysis (surge prediction).
The
stability conditions which emerged from this approach were
applied on the experimentally derived characteristics of a low
hub to
tip ratio split flow fan in a manner which involved the
modelling of the dynamic interaction of the inner and outer flow
region of the fan.
The
development of the governing equations was achieved by
applying one-dimensional flow analysis to the inner and outer
section of the fan. Their interaction was modelled on the
experimentally obtained radial movement of the splitter
streamline and the
discharge ,static pressure 'radial
distribution. The inner and outer region were treated as a
lumped volume element search operating on a local masflow
averaged total pressure rise characteristic and alternatively
acting in conjunction with a common nozzle and separate nozzles.
The experimental investigation was carried out on a low hub
totipratio two-stage split flow fan(with the facility of
independent bypass and core throttles)in order to examine the
localised and overall performance of such a fan(and the
staling processes involved)and to enable the application of
the stability analysis. The influence of reducing the distance
between the fan flow spliter and the last bladerowasal so
investigated,
«The mathematical mode1s predicted the point of dynamic
instability within 4.52 of the experimental observed mas flow
rate and pressure is value.