Experimental and Numerical Study of a Subsonic Aspirated Cascade

Abstract

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.

Design strategies and numerical simulation of highly loaded aspirated compressor blades

“…One method with a curvature induced ''Shock'' is realized by multi-objective CFD optimization. 24 The other one is prescribed-curvature-distribution blading which is achieved by geometric method 25,26 The method proposed in this work was developed with the consideration of blade strength. (b) High loading.…”

“…Highly loaded aspirated compressor blade without effective suction at high incidence conditions will reduce the stall margin. Comparing with previous investigations, [24][25][26] the design strategies for improving the stall margin were investigated in this study, which will be illustrated in the Design for maximum stall margin of highly loaded aspirated blades section.…”

“…The design method of aspirated blades with effectively controlled flow separation was first introduced by Power et al 24 This method features a curvatureinduced ''Shock'' on the suction surface. Godard et al 25,26 also presented a design method of highly loaded aspirated compressor blades. Based on these studies, an improved design strategy for aspirated blades with controlled flow separation is presented in Figure 8.…”

“…However, most studies are concentrated on BLS of existing compressor blades. Only Merchant, 11 Power et al 24 and Godard et al 25,26 investigated the design method of highly loaded aspirated blades. On the basis of their investigations, the design method of highly loaded aspirated compressor blades was further developed in this study.…”

Active Flow Control in a Radial Vaned Diffuser for Surge Margin Improvement: A Multislot Suction Strategy

“…Given the importance of the flow separation in the surge inception process for centrifugal compressor stages enlightened by these studies, the use of an active control technique aiming at controlling the flow separation within the diffuser is expected to be an effective way for delaying the surge. In particular, the boundary-layer suction technique has proven its effectiveness in case of axial compressors in order to either delay the surge or increase the blade loading [16][17][18][19][20].…”