Design Optimization of a Boundary Layer Ingestion Propulsor Using a Coupled Aeropropulsive Model

Abstract

Within a few years of the first jet engine powered aircraft entering military service, engineers proposed a tightly coupled aeropropulsive concept called boundary layer ingestion (BLI) that could offer reduced aircraft fuel burn. The central idea was that the jet engine, essentially a massive air pump, would ingest the boundary layer air and thereby reduce aircraft drag and improve propulsion system efficiency at the same time. Although a promising idea, BLI failed to catch on at the time due to a combination of computational limitations and and the availability of easier to achieve performance gains. More recently though, BLI concepts have seen renewed interest. Due to the increased computational power and developments in the field of multidisciplinary design optimization RANS CFD is now a viable design tool for early stage aircraft design and which has now opened the door to the design of BLI propulsion systems.

This thesis presents a detailed aeropropulsive study of the aft-mounted BLI propulsor for NASA's turboelectric STARC-ABL aircraft. The multidisciplinary modeling was performed using a fully coupled model built with RANS CFD and newly developed 1-D thermodynamic propulsion analysis. First an aeropropulsive study is presented that investigates the fundamental interactions between aerodynamics and propulsion and provides a quantitative analysis of the relative contributions of each to the overall BLI gain. The results show that both contribute equally to the overall BLI effect and that in order to accurately capture each contribution fully coupled aeropropulsive models are required. Next the results of a performance and sizing study for the aft-mounted BLI propulsor are presented. These results were generated with a simplified 2-D aerodynamic model of the STARC-ABL configuration. The study was performed using efficient gradient based optimization with analytic derivatives in order to enable investigation of the large design multidisciplinary space. The sizing study shows that STARC-ABL could use between 1% and 4.6% less energy at cruise compared to a non BLI aircraft, depending on assumptions made about the efficiency of the turboelectric power transmission system. Last a more detailed study of the aft-mounted BLI propulsor was performed using a 3-D aerodynamic model that included the wings, vertical tail, and full fuselage. The 3-D aerodynamic effects create inlet distortion, which was mitigated using design optimization. The results compare the BLI efficiency with and without the presence of distortion, demonstrating that while it is possible to reduce the distortion doing so comes at the cost of reduced BLI efficiency.

Collectively the results in this thesis represent the first aeropropulsive design studies of the STARC-ABL performed with fully coupled models. The studies were made possible through the development of new design methods that leveraged gradient based optimization with analytic derivatives. The efficiency of that approach enabled multiple optimizations to be run to conduct the design studies presented here. The results of those studies have demonstrated the potential for BLI to offer significant improvements to aircraft performance and motivate continued future work.