Installation effects for ultra-high bypass engines
Paper i proceeding, 2017
In the pursuit of ever more fuel-efficient engines, the fan diameter and bypass ratio are increasing rapidly. Although it allows the engine to perform more efficiently, it penalizes the aircraft performance with bigger nacelles, meaning more weight and bigger wetted drag-generating area. The nacelle design of such high bypass ratio engine is a problem, as the efficiency gain from the engine is counterbalanced, and sometimes eliminated by the nacelle weight and drag penalty.
The paper presents a method that allows for a parametric design of two-dimensional axisymmetric nacelle geometry based on the shape function approach. An automated process is created that generates nacelle designs based on only a few parameters, meshes, and numerically computes the flow around the designed nacelles. A drag bookkeeping system is defined, and the nacelle drag is extracted from the simulation and analysed.
The computed drag of nacelles, ranging from conventional length and thickness nacelles, through short/thin nacelles and ultrashort fan shrouds is analysed. Contrary to the first belief, ultrashort nacelles generate more drag than conventional length when the after-body drag-generating surfaces are considered. Furthermore, the study shows that shorter nacelle will greatly increase the flow velocity around the nacelle cowl and create a shock that induces wave drag.
A boundary layer ingesting propulsor provides an alternative way to increase propulsive efficiency. The same automated design method was used to generate two differently sized propulsors mounted behind a fuselage, and the flow was numerically computed. In this case the flow around the nacelle cowl was found to be subsonic without shocks. The value of boundary pressure loss and ingested drag was shown to be predictable from the total pressure in the boundary layer on a similar fuselage without propulsor.