Fan Stage Design and Performance Optimization for Low Specific Thrust Turbofans

In modern turbofan engines, the bypass section of the fan stage alone provides the majority of the total thrust required in cruise, and the size of the fan has a considerable effect on the overall engine weight and nacelle drag. Thrust requirements in different parts of the flight envelope must also be satisfied together with sufficient margins towards stalling. An accurate description of the interdependencies between the relevant performance and design attributes of the fan stage alone—such as efficiency, surge margin, fan-face Mach number, stage loading, flow coefficient, and aspect ratio—are therefore necessary to estimate system-level objectives such as mission fuel burn and the direct operating cost with enough confidence during the conceptual design phase. The contribution of this study is to apply a parametric optimization approach to the conceptual design of fan stages for low specific thrust turbofans based on the streamline curvature method. Trade-offs between fan stage attributes for Pareto-optimal solutions are modeled by training Kriging surrogate models on the results from the parametric optimization. A case study is provided in the end to demonstrate the potential implications of including a higher level of fan-stage parameter interdependency in an engine systems model. Results implied that being able to predict the rotor solidity required to maintain a given average blade loading—in addition to stage efficiency—is of significant importance when it comes to evaluating the trade-off between engine weight and thrust-specific fuel consumption.