Optimized design of mixed flow compressors for an active high-lift system

Abstract

To support an active high-lift system for future civil aircraft small, electrically powered compressors are required, which exceed the current state of the art concerning stage loading and compactness. To satisfy the requirements of these compressors concerning high pressure ratio, small size, low mass, and high efficiency (high power density) an automated design process for single stage, transonic compressors is developed. The resulting compressor designs cover the required operating range with total pressure ratios larger than 2.3 at a corrected mass flow of 1.1 kg/s and high total efficiencies above 85 to 90%. The designs resemble mixed flow compressors but have typical characteristics of axial flow compressors like axial in- and outflow. Having very high stage loading at low specific speed, the designs significantly exceed the conventional design space of mixed flow compressors. A design guideline was developed to make use of this design space extension for future design processes of similar compressors. For the developed compressor type a new, characteristic flow effect is described which is defined by a flat, oval, „bubble“ shaped region of low flow momentum in the tip-trailing edge region of the rotor passage. It is shown that the origin of this effect is inviscid but that its final shape is largely determined by viscous effects and the interaction with the tip clearance flow. The flow effect is inherent to the design of highly loaded compressors with high rotor trailing edge loading and high shroud slope. It does not cause significant losses and therefore does not cause a considerable performance penalty.