Variable Inlet and Outlet Combined With Meredith Effect: Investigations for Drag Minimization and Optimal Design of Ram Air Duct for Electric Propulsion

Abstract Hybrid-electric or full electric propulsion systems are considered as potential solutions for sustainable aviation, providing reduced emissions and improved efficiency. However, among the various challenges associated with thermal management in electric propulsion systems, the design and integration of heat exchangers remain particularly critical. Heat exchangers with large frontal areas can significantly increase aerodynamic drag, thereby reducing overall system efficiency and aircraft performance. Therefore, innovative approaches are required to mitigate or overcome these drawbacks. The aerodynamic drag can be mitigated partially or even completely by using ducted heat exchangers. In this regard, this paper investigates a variable duct geometry at both the inlet and outlet, exploring the impact of varying diffuser and nozzle geometries on realizing the benefits of Meredith effect and drag minimization. A thermodynamic efficiency relation for the duct process is derived as a function of geometrical and operating conditions. CFD simulations for a scoop inlet are carried out using a pressure-based coupled solver to accurately assess the aerodynamic behavior. The results demonstrate that a fixed-geometry duct cannot satisfy the conflicting requirements of different phases of the flight envelope. Parametric analysis identified that a diffuser with 4° half angle and nozzle with 13° half angle showed the best performance for cruise phase, moderately reducing the cooling drag to offset the high heat loads. A 4° diffuser and 15° nozzle is found to be suitable during take-off to ensure aerodynamic stability at low speeds, while a more open geometry proved effective for drag maximization during descent.