Performance and preliminary design of combined cycle aero engines utilising supercritical carbon dioxide

Over past decades, the basic thermodynamic concept of the modern turbofan engine has remained largely untouched and its fuel consumption has been reduced through incremental design improvements. That evolutionary approach will show diminishing returns in the short-to medium-term and is expected to have exhausted its improvement potential by 2050. In the face of the predicted exponential growth in air traffic and growing concerns over aviation’s contribution to global warming, the development of more radical concepts is imperative. This PhD research provides an original contribution to knowledge through the investigation of a combined cycle turbofan using supercritical CO₂ as the working fluid in the bottoming cycle (CCTF2050).An additional combined cycle engine is investigated for the first time, which also features inter-turbine reheat between the two stages of the high-pressure turbine (CCTF2050_ITR). Both concepts are subsequently compared to a projected year 2050 reference engine (TF2050), representing the limit of the evolutionary engine development approach. Supercritical CO₂ shows strong real gas behaviour near its critical point, which makes many experience-based design approximation techniques for ideal-gas components infeasible. Several detailed component models were created specifically for this investigation to adequately determine the performance of the combined cycle engines, size them and asses their potential to reduce mission fuel burn. Specific fluid property models were created for CO₂, alongside scalable S-CO₂ turbomachinery maps and multiple heat exchanger preliminary design models, which allow for detailed assessments like the optimum external and internal tube geometry of the heat exchanger tubes. Many of these models are not only applicable to aero engines, but could also be extended to the design of S-CO₂ power cycles for marine or small-scale off-shore applications. Nevertheless, aero engines face some of the most stringent constraints when it comes to fuel consumption, size and weight. These constraints are addressed by integrating the bottoming cycle and the turbofan directly, and creating additional models to specifically capture the component inter-dependencies in an aero engine. For example, significant changes to the engine installation and their effects on fuel burn are addressed by incorporating diffuser models for the heat exchangers and geometry dependent loss models for the bypass flow that does not pass through the pre-cooler. Overall, the performance of the combined cycle aero engine is dominated by the compromise between the pre-cooler size and the choice of S-CO₂ compressor inlet temperature and pressure. Together they dictate the S-CO₂ mass flow and hence the power output of the bottoming cycle. The pre-cooler also directly affects the engine’s specific thrust and propulsive efficiency by changing the outlet temperature and pressure of part of the bypass flow, and it can potentially lead to significant pressure losses in the bypass flow not passing through it by constricting its flow area. Finally, the pre-cooler accounts for about 40% of the bottoming cycle’s weight. Extensive parametric studies were conducted to establish the possible fuel burn benefit the combined cycles might offer over the TF2050. The modelling approach also enables the quantification of the sensitivities the combined cycles exhibit to manufacturing limitations or technology assumptions. The CCTF2050 promises fuel burn savings up to 3.2 % relative to the TF2050, though with an approximate uncertainty of +0.3 to -4 %. The CCTF2050_ITR may only achieve a 3.3 % reduction in fuel burn, despite a demonstrated synergy between ITR and the bottoming cycle. Unfortunately, the higher number of assumptions has increased the approximate range of uncertainty to +0.5 to -4.8 %. Both combined cycles would reduce the fuel burn by about 47.6 % relative to an assumed year 2000 state of the art, if the original predictions are met. These improvements are equal to or lower than the benefits claimed for some competing and less complex engine concepts like intercooled-recuperated aero engines for example. However, the uncertainties around the respective claims of those concepts have not been made available publicly. Never the less, this might disqualify the combined cycle concept for aero application bar further substantial technological advances, such as new extremely compact yet light heat exchanger designs. However, such advances are not inconceivable given that the engines are considered for entry into service in 2050.