Pulsed Air Injection System Characterization for Stall Margin Improvement in an Axial Compressor
Creators: name:  G. MARGALIDA 
name:   P. JOSEPH 
name:   A. DAZIN 
name:   O. ROUSSETTE 
name:   J. DELVA 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLOW CONTROL
AEROSPACE

description: Active flow control is an aerodynamic improvement method that could be applied to turbomachinery to improve their performance. In particular, it could be used to mitigate the long-lasting issue of rotating stall and surge instabilities occurring at some points on the performance curve of axial compressors [1]. Practically speaking, active flow control could be used here to stabilize the flow at high pressure ratios, allowing to achieve better performance, or using less compressor stages [2]. In any cases, interesting gains could be achieved in both fuel consumption and pollutant emission. This paper presents the early phase of the deployment of such an active flow control system in a semi-industrial axial compressor. The machine is the CME2 compressor operated in the Lille Laboratory of Fluid Mechanics (LMFL), and consists in a single stage, 550 mm diameter, axial compressor driven by a 200 kW DC motor. The paper describes first the design, the implementation, and the characterization of the control system itself. As in any flow control study, the parametric space is large, and if there is a near consensus for some parameters in the considered application, there is still some debates about the influence of many of them [3]. Consequently, present control system is intended to be modular, consisting in 20 blocks of two injectors flush mounted on the compressor casing. Each injector consists in 3D printed parts designed to exploit Coand? effect to direct airflow right into the tip gap region at the top of the blades. Various injector shapes can be tested, and yaw angle injection can be investigated as the injectors are able to rotate by 15 steps around their axis. These injectors are feed by magnetic valves, reaching an actuation frequency up to 500Hz, allowing studying both continuous and pulsed jets actuations. When performing flow control experiments, a good knowledge of the control system is needed to estimate its effect on the flow. Second part of this paper present then a detailed characterization of the actuator behavior. First, some results achieved in quiescent air with an isolated actuator are presented. These results have been obtained thank to a dedicated test bench at ONERA Lille, using hot wire anemometry. Some interesting physics link to the coupling between pulsed actuation and Coand? effect have been observed in transient regime, but also an impressive vectorization of the jet along the casing wall. These detailed tests have been completed by an in situ calibration of the whole system with the 40 injection ports, allowing a global verification of their homogeneity. Finally, this paper is concluded by the presentation of some of the results achieved with this system on the compressor in terms of stall margin improvement. The effect of several parameters is presented, with, in particular, an interesting demonstration of the duty cycle as a command parameter allowing a precise control of the surge margin. In addition, the role of the time without blowing occurring with pulsed actuation is also investigated. To conclude this paper, some original physics are presented concerning the stalling pattern of the compressor with and without control, as some clues seem to indicate a shift from a spike type stall inception to a modal one. References 1. Day, I.J. Stall, Surge, and 75 Years of Research. J. Turbomach. 2015, 138, 011001. 2. Tahara, N.; Kurosaki, M.; Ohta, Y.; Outa, E.; Nakajima, T.; Nakakita, T. Early Stall Warning Technique for Axial-Flow Compressors. J. Turbomach. 2007, 129, 448456. 3. Li, J.; Du, J.; Nie, C.; Zhang, H. Review of tip air injection to improve stall margin in axial compressors. Prog. Aerosp. Sci. 2019, 106, 1531.