Coupled Aeropropulsive Design Optimization of a Boundary Layer Ingestion Propulsor

Airframe-propulsion integration concepts that use boundary layer ingestion have the potential to reduce aircraft fuel burn. One concept that has been recently explored is NASA’s STARCABL aircraft configuration, which offers the potential for fuel burn reduction by using a turboelectric propulsion system with an aft-mounted electrically driven boundary layer ingestion (BLI) propulsor. So far, attempts to quantify this potential fuel burn reduction have not considered the full coupling between the aerodynamic and propulsive performance. To address the need for a more careful quantification of the aeropropulsive benefit of the STARC-ABL concept, we run a series of design optimizations based on a fully coupled aeropropulsive model. A 1-D thermodynamic cycle analysis is coupled to a Reynolds-averaged Navier-Stokes simulation to model the aft propulsor at a cruise condition and the effects variation in propulsor design on overall performance. A series of design optimization studies are performed to minimize the required cruise power, assuming different relative sizes of the BLI propulsor. The design variables consist of the fan pressure ratio, static pressure at the fan face, and 311 variables that control the shape of both the nacelle and the fuselage. The power required by the BLI propulsor is compared with a podded configuration. The results show that the BLI configuration offers 6% to 9% reduction in required power at cruise, depending on assumptions made about the efficiency of power transmission system between the under-wing engines and the aft propulsor. Additionally, the results indicate that the power transmission efficiency directly effects the relative size of the under-wing engines and the aft propulsor. This design optimization, based on computational fluid dynamics, is shown to be essential to evaluate current BLI concepts and provides a powerful tool for the design of future concepts. Received 20/11/2017. 2 The Aeronautical Journal NOMENCLATURE Aref wing reference area BLI boundary layer ingestion CD drag from the lifting surfaces of the aircraft CFx net force coefficient in the axial direction on the fuselage and aft propulsor CFpod net force coefficient in the axial direction on an isolated propulsor FFD free form deformation IDF individual design feasible optimization architecture ṁ mass flow rate ps static pressure pt total pressure Pwrshaft shaft power delivered to a propulsor Tt total temperature V∞ freestream velocity ()FE flow quantities at the fan exit ()FF flow quantities at the fan face ()∗ target design values used by the IDF optimization formulation ()′ quantities computed on the podded reference configuration