Design of a multi-spool, high-speed, counter-rotating aspirated compressor

Abstract

The current motivators in gas turbine aircraft engine design are improved efficiency and reduced engine weight. The drive for improved efficiency leads to high pressure ratio compressors with multiple stages, as thermodynamic efficiency increases with pressure ratio. These compressors are both heavy and expensive. Recent work by Kerrebrock, Merchant, and Schuler, has led to the possibility of achieving high pressure ratios with a reduction in the number of stages. These compressors use aspiration, or suction on the surface of the blades and end-walls, to keep the boundary layer attached under higher loading conditions, reducing losses. The work done by a rotor is related to the change in swirl across the blade and the rotational velocity of the blades. Counter-rotation provides for the largest change in swirl across a series of rotors, facilitating the largest pressure ratio. A three-stage, counter-rotating, compressor was designed to take advantage of these benefits. The tip speeds were kept as high as possible to increase the work done on the flow. The final design has a three-stage pressure ratio of 27, much higher than is currently available in three stages. The blade aspect ratios are smaller than on current compressors because of the large swirl change across each blade row. The flow passage has large contractions across each blade row, causing sharper turns in the flow path than is currently observed. The design consists of the flow passage, blade numbers and locations, rotor speeds, and desired swirl changes through each blade row. These data will allow later efforts to design the blade shapes. A mean line analysis was conducted to design a three stage turbine to drive the compressor. The benefits of counter-rotation are apparent in the turbine, as the turbine is designed without inter-rotor nozzles. Reduced engine weight and improved efficiency are available through the turbine section with the elimination of these nozzles.