THE DEVELOPMENT AND APPLICATION OF METHODS FOR PREDICTING LIFT FAN INSTALLED PERFORMANCE

摘要

Short Takeoff and Vertical Landing (STOVL) operation requires an engine-based source of vertical thrust. One way of generating this thrust is to use a lift fan, driven by a shaft from the aircraft's main engine or engines; another is to use a separate lift engine. Air enters the lift fan through a dorsal inlet just aft of the cockpit and exits through a vectoring ventral nozzle. While the aircraft is hovering, the inlet and exit flows remain reasonably uniform, but during transition between engine-borne and wing-borne flight the air is obliged to make one 90 degree turn at the fan inlet and another one at the exit. The resulting static pressure distortions at fan entry and exit can be significant and extend over the entire annulus. This is the non-uniform environment in which the fan must operate. Previous programmes using lift fans and lift engines, particularly in the 1960's, used testing, often at large scale, in dedicated wind tunnels to evaluate these effects. It is now possible to consider CFD as a tool to provide an early pre-test assessment and to optimise test requirements. In order to correctly predict the effect of the distorted flow on the turbomachinery the computational domain must contain the whole fan installation, including inlet ducting, the structural vanes supporting the fan body, and a vectoring exhaust nozzle. Sheer size would make such a computation costly and time-consuming if every blade passage were resolved individually, but the problem may be made more manageable by replacing each turbomachinery bladerow by an actuator disc: a two-dimensional model of a three-dimensional component. An existing CFD package has been modified to accommodate both an actuator disc model and the unusual geometry of a lift fan installation. This modified code was used to compute flows through a two-stage lift fan geometry provided by Rolls-Royce (MAEL) pic. Calculations show the extent of performance reduction when operating in crossflow and the attenuation of the distortion as it passes through the machine. As an example of the use of the methods, further calculations investigate the effect of modifying the geometry, within the limits of the available space, to reduce the distortion at the lift fan outlet.