This paper presents the validation of a design method for aspirated compressor blades, combining a passive separation control by blade shaping with an active flow control by aspiration.
In a first part, a linear aspirated cascade designed according to this method was built and tested at low speed, without and with aspiration. The latter was only applied on the suction surfaces of the blades. Particle Image Velocimetry measurements performed at mid-span of the cascade, in the central passage, showed a complete reattachment of the separated boundary layer on the suction side of the blade. A flow deflection of approximately 65 degrees was achieved requiring an aspirated mass flow rate of 3.3%. However, boundary layer reattachment is effective in a zone centered at mid-span covering 30% of blade span. Flow visualization revealed large corner separation in the presence of aspiration. This is due to the re-establishment of strong pressure gradient on sidewalls of the cascade. No flow control was applied on these zones for optical access purpose. These secondary-flow regions reduced the diffusion occurring within the cascade by nearly 60% in comparison with the design intent. They also increased the expected level of total pressure losses measured by wake traverses downstream of the cascade.
In a second part, numerical simulations of the aforementioned experiment were carried out to help the understanding of the experimental results. The simulations were able to reproduce correctly the characteristic flow features, without and with aspiration, observed and measured during the experiment. Thus, they confirmed the potential of this design method developed for aspirated compressor blades, as well as CFD capabilities to simulate the influence of technological effects like suction slots. A uniform and a non-uniform aspiration distribution along the blade span direction were considered during simulations. Suction distribution was found to have a significant impact on the control by aspiration. This design feature, in addition to flow control on endwalls, has to be taken into account in the three-dimensional design of highly loaded aspirated compressor blades.
Measurements have been performed at low speed in the confluent region of a two dimensional wake and turbulent boundary layer. A tandem symmetrical arrangement was used, placed in a variable pressure gradient wind tunnel. Pressure and turbulent quantities were measured and current semi-empirical laws were examined in the light of the experimental results.
Experiments have been performed with two cascades of turbomachines. The first cascade is composed of highly loaded turbine blades, and has been used in the low subsonic Mach number range. The second cascade consists of highly loaded compressor blades, of the DCA type. The Mach number was then in the high subsonic range. The experimental results are presented in the form of mean values in the pitch direction. Detailed local values are also described. The growth of a passage vortex and a corner effect are presented in the compressor case. Their interactions with the whole flow are analyzed. In the turbine case, the passage vortex is found to be a dominant effect. Results obtained with a theoretical method of calculation of the flow in the blade passage are used to complete the analysis.
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