Elements of Cycle Design for Parallel Hybrid Mixed Flow Turbofans

Select Trade-Offs in Performance and Energy-Efficiency

Abstract

In the last decade, parallel and serial hybrid combining gas turbine propulsion with electric power systems have been investigated. For the commercial aviation sector, they provide fuel burn and emission reduction potential and have been explored to some extent. Depending on the chosen concept of operation, propulsion cycle, and reference basis, minimal to mild double digit fuel consumption reductions are anticipated. A key challenge remains the low specific energy of current battery technology, leading to a high penalty on electrical energy use. High-speed civil propulsion has gained interest in recent years due to advances in material capabilities and technology enhancements, especially for what concerns aircraft and sonic boom noise reductions. As a result, there is now a renewed and stronger focus on developing supersonic transport aircraft that are environmentally sustainable, technologically feasible, and economically competitive with the existing civil subsonic aviation market sector. The propulsion system for supersonic civil aircraft must satisfy more stringent requirements and more limiting constraints compared to a subsonic application, due to the more severe operating conditions. In this context, the publicly available literature is more oriented towards aircraft rather than propulsion system design. For high-speed propulsion and supersonic transport, and the corresponding mixed-flow turbofan cycles, very limited research propulsion system-centred is available, both for the conventional design and for hybrid electric integration. For these cycles, both emission reductions, noise requirement compliance, and performance improvement potential are of substantial interest, thus widening the metrics of interest. In this work, a hybrid mixed-flow turbofan configuration is investigated and modelled, and cycle design trade-offs are identified and selected. A non-hybridised baseline configuration targeting low bypass ratios based on performance data obtained from the open literature is considered for comparison. A multipoint synthesis scheme for the gas turbine is combined with a hybrid concept of operations through physics-based cycle modelling and previously published polytropic efficiency corrections. Trade studies are conducted introducing hybrid configuration, analysis on key performance and fuel consumption metrics are presented and recommendations on hybrid potential exploitation are provided.