Abstract

Low pressure ratio fans constitute one of the dominant components on several evolving aero-engine configurations like boundary layer ingesting fans or distributed propulsion or, even, ultra-high bypass ratio engines. The well unchoked fan nozzle during take-off and initial climb imposes significant operability and efficiency issues. This paper studies a set of low pressure ratio aft-fans, quantifying the effect of variable geometry concepts – variable area nozzle and variable pitch fan- at engine and aircraft level, in terms of surge margin and efficiency improvement.
Performance assessments are carried out for the aft-fan of a hybrid-electric configuration, which comprises a single-aisle aircraft for short range applications with 150-passenger capacity, integrated with two underwing year-2035 entry into service geared turbofan engines and one fuselage mounted boundary layer ingesting electric fan (or aft-fan). Power demands for the electrically-driven aft-fan are extracted from the low-speed shaft of the boosted geared turbofan engines.
For the performance of the integrated airframe-engine system, an in-house engine conceptual design framework has been utilised. Variable area nozzle has been implemented within the tool and is validated against numerical simulations for next generation low specific thrust turbofan engines. Variable pitch fan concept has been modelled using correction deltas for mass flow and efficiency, thereby re-locating the map characteristics according to the user input blade pitch angle, which in turn is validated against established performance prediction software. An enhanced streamline curvature method with an embedded and pre-optimized airfoil profile data base is deployed to generate accurate off-design aft-fan performance maps, alleviating any scaling issues from down-scaled higher pressure ratio fan maps. An enhanced method for fan stage modelling has also been implemented, by decoupling fan rotor and fan outlet guide vane performance maps, being able to interpret challenging conditions, e.g. mid-altitude descent, where pressure losses from outlet guide vanes can be higher than pressure rise from fan rotor.
Evaluations of performance are carried out at engine and aircraft level for different sets of aft-fan pressure ratio and power extraction, demonstrating a design space of feasible aft-fan designs along with restricting bounds. Specific operating points like top of climb, end of runway take-off, cruise and descent are examined, with remarkable focus on take-off conditions where operability and efficiency issues become dominant. Take-off working line, from sea level static to end of runway conditions, with gradual change in Mach number and thrust requirements is simulated, showcasing the crucial importance of variable geometry on surge margin improvement under conditions where aft-fan operation is restricted by the surge effect and certification procedures for temperature limits.
Conducting an operability analysis of a set of low pressure ratio aft-fans using variable geometry gives insight and limits the design space of feasible rear-fuselage mounted electric aft-fans for hybrid-electric applications, while examines the cases where variable geometry is essential for a safe aft-fan operation considering certification issues during take-off.