Thermo-Fluid Modelling for Gas Turbines - Part II: Impact on Performance Calculations and Emissions Predictions at Aircraft System Level
Paper i proceeding, 2009
In this two-part publication, various aspects of thermo-fluid modelling for gas turbines are described and their impact on performance calculations and emissions predictions at aircraft system level is assessed. Accurate and reliable fluid modelling is essential for any gas turbine performance simulation software as it provides a robust foundation for building advanced multi-disciplinary modelling capabilities. Caloric properties for generic and semi-generic gas turbine performance simulation codes can be calculated at various levels of fidelity; selection of the fidelity level is dependent upon the objectives of the simulation and execution time constraints. However, rigorous fluid modelling may not necessarily improve performance simulation accuracy unless all modelling assumptions and sources of uncertainty are aligned to the same level. Certain modelling aspects such as the introduction of chemical kinetics, and dissociation effects, may reduce computational speed and this is of significant importance for radical space exploration and novel propulsion cycle assessment. This paper describes and compares fluid models, based on different levels of fidelity, which have been developed for an industry standard gas turbine performance simulation code and an environmental assessment tool for novel propulsion cycles. The latter comprises the following modules: engine performance, aircraft performance, emissions prediction, and environmental impact. The work presented aims to fill the current literature gap by: (i) investigating the common assumptions made in thermo-fluid modelling for gas turbines and their effect on caloric properties and (ii) assessing the impact of uncertainties on performance calculations and emissions predictions at aircraft system level. In Part II of this two-part publication, the uncertainty induced in performance calculations by common technical models, used for calculating caloric properties, is discussed at engine level. The errors induced by ignoring dissociation are examined at 3 different levels: i) component level, ii) engine level, and iii) aircraft system level. Essentially, an attempt is made to shed light on the trade-off between improving the accuracy of a fluid model and the accuracy of a multi-disciplinary simulation at aircraft system level, against computational time penalties. The results obtained demonstrate that accurate modelling of the working fluid is not always essential; the accuracy/uncertainty for an overall engine model will always be better than the mean accuracy/uncertainty of the individual component estimates as long as systematic errors are carefully examined and reduced to acceptable levels to ensure error propagation does not cause significant discrepancies. Computational time penalties induced by improving the accuracy of the fluid model as well as the validity of the ideal gas assumption for future turbofan engines and novel propulsion cycles are discussed.