Aeropropulsive Design Optimization of a High-Bypass Turbofan Engine

Abstract

The design of modern aircraft propulsion systems is an inherently multidisciplinary process. However, current state-of-the-art design methods are disconnected across company silos with limited communication between disciplines. This fragmentation drives sequential optimization processes that rely on manual or gradient-free methods, yielding suboptimal designs. Propulsion original equipment manufacturers (OEM) typically develop optimized thermodynamic cycle decks and geometries independently before transferring them to airframe OEM. The airframe OEM then attempts to minimize airframe-propulsion interference drag with a fixed propulsion system. This disconnected approach lacks coupled multidisciplinary effects and compromises system-level performance.

Aeropropulsive design optimization considers the interactions between aerodynamics, thermodynamics, and geometry simultaneously. High-fidelity simulation of these coupled physics presents significant computational challenges, particularly for complete flow path analysis. Mixed-fidelity aeropropulsive methods utilize a range of modeling fidelities to represent the multidisciplinary system. In our case, we use a mixed-fidelity method to couple a high-fidelity aerodynamic solver with a zero-dimensional thermodynamic cycle model. Combined with gradient-based optimization, mixed-fidelity approaches are computationally efficient and can accommodate design spaces with many variables and constraints.

Connecting aerodynamic and propulsion disciplines requires careful treatment of the passing of information between models. In this dissertation, we develop a novel hybrid aeropropulsive coupling method for turbofan engines with approaches for both the fan and core propulsion models. We then apply this coupling method to a single-point optimization of a high-bypass turbofan engine with 40 design variables and 135 constraints. The optimizer converges below the target optimality and feasibility tolerances in 128 iterations, with a wall time of 3 hours and 38 minutes on 160 cores. We optimize designs at 25 fan pressure and bypass ratio combinations to demonstrate efficiency, robustness, and optimal mixed-fidelity design trends. All cases converge in under 3 hours and the optimal trends adhere to expected results from literature and zero-dimensional cycle models.

The ability to converge and validate 25 design points is a milestone for the robustness and utility of our mixed-fidelity aeropropulsive optimization approach. The validation study shows that the method over-predicts core thrust by 20% due to thermodynamic equation of state differences between the propulsion and aerodynamic models. We demonstrate that this discrepancy is less that 1% when the underlying thermodynamic models are consistent. The new coupling methods are a significant advancement in aeropropulsive optimization capability and enable the efficient design of tightly integrated propulsion systems.

The next step is to extend the single-point optimization to a multipoint optimization problem. The multipoint problem considers the engine performance at multiple operating conditions, such as sea-level-static, rolling take off, top-of-climb, and cruise. This is a first of its kind application of mixed-fidelity aeropropulsive optimization to a multipoint problem for a turbofan engine. The results of this study show the potential for future multipoint optimization studies and the importance of considering the full operating envelope of the engine.