  Citation: Rosell, D.; Grönstedt, T. Design Considerations of Low Bypass Ratio Mixed Flow Turbofan Engines with Large Power Extraction. Fluids 2022, 7, 21. https://doi.org/ 10.3390/fluids7010021 Academic Editor: Nikolaos Aretakis Received: 12 November 2021 Accepted: 30 December 2021 Published: 1 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). fluids Article Design Considerations of Low Bypass Ratio Mixed Flow Turbofan Engines with Large Power Extraction Daniel Rosell 1, * and Tomas Grönstedt 2 1 Saab Aeronautics, Bröderna Ugglas Gata, 581 88 Linköping, Sweden 2 Division of Fluid Dynamics, Chalmers University of Technology, Mechanics and Maritime Sciences, 412 96 Göteborg, Sweden; tomas.gronstedt@chalmers.se * Correspondence: daniel.rosell@saabgroup.com Abstract: The possibility of extracting large amounts of electrical power from turbofan engines is becoming increasingly desirable from an aircraft perspective. The power consumption of a future fighter aircraft is expected to be much higher than today’s fighter aircraft. Previous work in this area has concentrated on the study of power extraction for high bypass ratio engines. This motivates a thorough investigation of the potential and limitations with regards to performance of a low bypass ratio mixed flow turbofan engine. A low bypass ratio mixed flow turbofan engine was modeled, and key parts of a fighter mission were simulated. The investigation shows how power extraction from the high-pressure turbine affects performance of a military engine in different parts of a mission within the flight envelope. An important conclusion from the analysis is that large amounts of power can be extracted from the turbofan engine at high power settings without causing too much penalty on thrust and specific fuel consumption, if specific operating conditions are fulfilled. If the engine is operating (i) at, or near its maximum overall pressure ratio but (ii) further away from its maximum turbine inlet temperature limit, the detrimental effect of power extraction on engine thrust and thrust specific fuel consumption will be limited. On the other hand, if the engine is already operating at its maximum turbine inlet temperature, power extraction from the high-pressure shaft will result in a considerable thrust reduction. The results presented will support the analysis and interpretation of fighter mission optimization and cycle design for future fighter engines aimed for large power extraction. The results are also important with regards to aircraft design, or more specifically, in deciding on the best energy source for power consumers of the aircraft. Keywords: low bypass ratio mixed flow turbofan; power extraction; fighter; engine performance 1. Introduction Air vehicle design is moving towards increased electrification [1]. This holds for the military sector as well [2]. An increased amount of electrical equipment, with higher power demands, is contributing to this development. Hydraulic systems and components are being replaced by electrical systems and components [3]. Increasing aircraft power consumption will not only put new requirements on the aircraft engine with regards to engine performance, stability, and operability, but increased power will create excessive heat, which must be taken care of by the aircraft [4]. Efficient energy management of the aircraft will be crucial to handle this excessive heat [2]. In turbofan engines, electrical power is extracted by a generator connected to one of the rotating shafts, usually via an accessory gearbox [5], but future engines might have generators directly mounted to the shaft [6]. Typical power consumers are pumps, aircraft hydraulic systems, pneumatic systems and electrical systems such as control systems [7,8]. The extracted power is a small fraction of the total useful power generated by the engine and the main power output is in the form of high velocity jets and thrust generation. Civil aircraft typically use high bypass ratio turbofan engines with separate exhausts, as they increase propulsive efficiency [7]. The search for improved efficiency to achieve Fluids 2022, 7, 21. https://doi.org/10.3390/fluids7010021 https://www.mdpi.com/journal/fluids