Design and Performance of a Low Fan-Pressure-Ratio Propulsion System

Abstract

A propulsion system, comprising of a low pressure ratio fan stage with bypass and core section and an engine representative, short intake and nacelle was designed and its aerodynamic performance was assessed over a wide range of engine and aircraft operating conditions. Since the detailed fan and intake design was carried out separately and its design intent is known and understood, an assessment of how the individual component performance was influenced by the respective coupling was possible. The basis for all design efforts was a thermodynamic cycle derived from Single-Aisle short-medium range airliner thrust requirements over the entire flight regime and major certification points. A bypass ratio of 16 was targeted and 2025+ technology assumptions were made. This specification defined all fan operating conditions and a detailed and engine representative fan design was carried out to meet those requirements. All major design choices will be justified based on existing design expertise for aero engine fans over a wide range of application and data from open literature. Concurrently to the fan design, an ultra-short intake was designed to accommodate the large fan diameters as expected for future UHBR engines, targeting at meeting requirements such as little cross wind and angle of attack sensitivity as well as a high flow capacity. The entire propulsion system was then assessed by means of steady and unsteady CFD. Here the focus will be on typical angle of attack conditions as experienced during Take-Off at high fan power settings. The resulting unsteady interaction between the fan and the non-axisymmetric inflow, stemming from both simple kinematics due to induced swirl, as well as additional acceleration and diffusion near the upper and lower intake lip, will be discussed in detail. Furthermore, the influence of the angle of attack on maximum flow capacity of the fan will be discussed, along with an analysis of the fan presence effect on the occurrence of separation within the intake for very high values thereof. All results shown in the paper will focus on both analysing the flow physics as well as better understanding how the fan and intake design is influenced by the close coupling between a short intake and a low fan pressure ratio fan.