Engine performance modelling is a major part of the engine design process, in whichspecialist solvers are employed to predict, understand and analyse the engine’s behaviourat various operating conditions. Sub-idle whole engine performance synthesis solvers arenot as reliable and accurate as design point solvers. Lack of knowledge and data result incomponent characteristics being reverse-engineered or extrapolated from above-idle data.More stringent requirements on groundstart and relight capabilities, has prompted theneed to advance the knowledge on low-speed engine performance, thereby requiring morerobust sub-idle performance synthesis solvers.The objective of this study, was to improve the accuracy and reliability of a currentaero gas turbine sub-idle performance solver by studying each component in isolationthrough numerical simulations. Areas researched were: low-speed and locked-rotor com-pressor characteristics, low-power combustion efficiency, air blast atomizer and combustorperformance at sub-idle, torque-based whole engine sub-idle performance synthesis, andmixer performance at far off-design conditions.The observations and results from the numerical simulations form the contribution toknowledge of this research. Numerical simulations of compressor blades under highlynegative incidence angles show the complex nature of the flow, with the results used todetermine a suitable flow deviation model, a method to extract blade aerodynamic char-acteristics in highly separated flows, and measure the blockage caused by highly separatedflow with operating condition and blade geometry. The study also concluded that the useof 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 itis suitable for low-power simulations with the advantage of having the potential to startengine simulations from static conditions.A study of air-blast atomization at windmilling relight conditions has shown that currentestablished correlations used to predict spray characteristics are not suitable for altituderelight studies, tending to overestimate the atomization quality. Also discovered is thehighly influential interaction of compressor wakes with the combustor and atomizer underaltitude relight conditions, resulting in more favourable lighting conditions than previousassumptions and models have shown. This is a completely new discovery which will resultin a change in the way combustors are designed and sized for relight conditions, and theway combustion rig tests are conducted.The study also has valuable industrial contributions. The locked-rotor numerical datawas 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 resultsvalidated against actual engine test data. The atomization studies at relight were used tofactor 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 combustionefficiency at low-power settings. The interaction of compressor wakes with the atomizershowed that atomizer performance at relight is underestimated, resulting in oversizedcombustors. By using the knowledge gained within this research, combustor size can bereduced, 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-idleperformance solvers, increasing their reliability and encouraging their use for future aerogas turbine engines.
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