A reduction of CO2 emissions of aero engines can only be achieved by a reduction in the fuel consumption. For turbofan engines a major key for this is an increase in the engine bypass ratio to enhance the propulsive efficiency.

This leads to an increased mass flow in the bypass duct and hence higher contributions of the bypass duct system to the flow pressure losses and hence the engine fuel consumption. Additionally the turbofan core engine gets smaller in size and the overall diameter of the engine increases. This requires advanced engine mounting concepts, such as core mounted subsystems, which influence the arrangement and the mechanical and aerodynamic design of components in the bypass duct such as fan outlet guide vane, struts, fairings or bifurcations.

This requires an optimized design of the turbofan bypass system with regards to structural, aerodynamic and acoustic criteria. The topology of the bypass duct as well as the position and the design of the individual components have a significant impact on the weight, the efficiency and the noise radiation of the engine and hence need to be investigated in a systematic approach.

Such a design approach was developed in the R&T project OPAL, led by Rolls-Royce Deutschland and funded by the German Federal State of Brandenburg.

The design approach covers the following steps:

- Optimization of the bypass duct shape with regards to minimum pressure loss taking into consideration the mechanical requirements of an engine with core mounted subsystems.

- Optimization of strut, fairing and bifurcation shapes with regards to minimum pressure loss, robustness against flow deviations and minimized upstream flow effects taking into consideration structural and mechanical requirements by engine loads and subsystem routings.

- Optimization of fan outlet guide vane profile and plan form shape with regards to minimum pressure losses and maximum working range taking into consideration structural and acoustic constraints.

- Optimization of fan-outlet guide vane, strut and bifurcation interaction with regards to minimum pressure losses and maximized aerodynamic stability of the fan and outlet guide vanes.

The current paper will present the design approach, the optimization processes and the results of the optimization of the turbofan bypass duct system for the application on modern high-bypass ratio aero engines.

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