| Summary: | Engine performance modelling is a major part of the engine design process, in which
specialist solvers are employed to predict, understand and analyse the engine’s behaviour
at various operating conditions. Sub-idle whole engine performance synthesis solvers are
not as reliable and accurate as design point solvers. Lack of knowledge and data result in
component characteristics being reverse-engineered or extrapolated from above-idle data.
More stringent requirements on groundstart and relight capabilities, has prompted the
need to advance the knowledge on low-speed engine performance, thereby requiring more
robust sub-idle performance synthesis solvers.
The objective of this study, was to improve the accuracy and reliability of a current
aero gas turbine sub-idle performance solver by studying each component in isolation
through numerical simulations. Areas researched were: low-speed and locked-rotor com-
pressor characteristics, low-power combustion efficiency, air blast atomizer and combustor
performance at sub-idle, torque-based whole engine sub-idle performance synthesis, and
mixer performance at far off-design conditions.
The observations and results from the numerical simulations form the contribution to
knowledge of this research. Numerical simulations of compressor blades under highly
negative incidence angles show the complex nature of the flow, with the results used to
determine a suitable flow deviation model, a method to extract blade aerodynamic char-
acteristics in highly separated flows, and measure the blockage caused by highly separated
flow with operating condition and blade geometry. The study also concluded that the use
of Blade Element Theory is not accurate enough to be used at such far off-design con-
ditions. The linearised parameter-based whole engine performance solver was converted to used torque-based parameters, which validated against engine test data, shows that it
is suitable for low-power simulations with the advantage of having the potential to start
engine simulations from static conditions.
A study of air-blast atomization at windmilling relight conditions has shown that current
established correlations used to predict spray characteristics are not suitable for altitude
relight studies, tending to overestimate the atomization quality. Also discovered is the
highly influential interaction of compressor wakes with the combustor and atomizer under
altitude relight conditions, resulting in more favourable lighting conditions than previous
assumptions and models have shown. This is a completely new discovery which will result
in a change in the way combustors are designed and sized for relight conditions, and the
way combustion rig tests are conducted.
The study also has valuable industrial contributions. The locked-rotor numerical data
was used within a stage-stacking compressible flow code to estimate the compressor sub-
idle map, of which results were used within a whole engine performance solver and results
validated against actual engine test data. The atomization studies at relight were used to
factor in the insensitivity of current spray correlations, which together with a newly de-
veloped sub-idle combustion efficiency sub-routine, are used to determine the combustion
efficiency at low-power settings. The interaction of compressor wakes with the atomizer
showed that atomizer performance at relight is underestimated, resulting in oversized
combustors. By using the knowledge gained within this research, combustor size can be
reduced, resulting in lower NOx at take-off and a smaller and lighter core, with a com-
bustor requiring less cooling air.
The component research has advanced the knowledge and modelling capability of sub-idle
performance solvers, increasing their reliability and encouraging their use for future aero
gas turbine engines.
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