Aspirated Compressors

The performance of compressors can be improved in two ways by judicious removal from the flow path of the viscous flow in the boundary layers. First, removal of the boundary layer fluid just prior to or in a region of rapid pressure rise, either at shock incidence or more generally at the point of rapid pressure rise on the suction surface of the airfoil, can enable significant increases in the diffusion, hence in the work done by a stage for any given blade speed. Second, removal of the high entropy fluid in the boundary layer minimizes the required compression work in subsequent stages of compression, thereby raising the compression efficiency. Analysis has shown that the latter effect can result in approximately one half point increase in efficiency for each percent of (high entropy) fluid removal. Design studies have been carried out for two different stages to assess the increase in pressure ratio that may be achieved. One stage that has been designed would produce a pressure ratio of 2 at a tip speed of 1000 ft/sec, and may be very attractive for the fan stage of high-bypass turbofan engines. The other stage would produce a pressure ratio of 3 at a tip speed of 1500 ft/sec, and should be attractive as the first stage of a core compressor or the fan stage of a low-bypass ratio engine. An experiment has been completed, to examine the effect of boundary layer removal just prior to shock impingement on the suction surface of blades in a transonic rotor. The suction was implemented on 5 of the 23 blades of the rotor, providing a direct comparison of the flow behavior with and without suction. Analysis of the data has shown that the blades with suction have increased mass flow and that the flow more closely followed the suction surface near the trailing edge. The differences between aspirated and normal blades were most pronounced when the rotor was very close to stall. The third and fourth of the blades in the group with suction appear to be representative of the behavior to be expected of a rotor with suction on all blades. They exhibited improved efficiency and increased mass flow. The rotor as a whole with suction showed different stall behavior than its counterpart without boundary layer control. Future plans include the fabrication and experimental evaluation in the MIT Blowdown Compressor, of one of the two stages discussed above. In this experiment suction will be applied to all the blades in both rotor and stator, so that the increased work enabled by suction can be realized. This research was supported by AFOSR, Dr. James McMichael, and by AlliedSignal Aircraft Engines, Dr. Arun Sehra.