Investigation of the unsteady flow in a transonic axial compressor adopted in the compressed air energy storage system

Abstract

During operation, compressed air energy storage systems should respond rapidly to variations in power network demand, requiring that the compression system should always be in changeable off-design conditions. Compression systems with low flow rates confront difficulties such as diminished aerodynamic performance and increased flow losses. Given that the emergence of compressor instability is a process of the accumulation and intensification of flow unsteadiness. In this study, a numerical simulation was conducted for a transonic compressor to investigate the unsteady evolution characteristics of the tip leakage flow field, and it focused on the flow field of a condition where the flow unsteadiness was stimulated and further investigated the origin of flow unsteadiness, which was supposed to provide insights in developing flow control strategies and enable the application in a compressed air energy storage system under low mass flow rate conditions. The Fourier fast transformation (FFT) signal analysis showed that characteristic frequency of the unsteady tip flow field was 1863.3 Hz. Unsteady disturbances were predominantly distributed in the following three regions: the leakage vortex region of the blade suction surface, the location of the epitaxially detached shock wave at the entrance of passage, and the 5 %–30 % chord of the blade pressure surface. The high disturbance region of leakage vortex was generated by the periodic shift in leakage vortex intensity over time, the interaction between the main flow and various types of secondary flow due to leakage vortex breaking processes was the primary cause for the disturbance region of the epitaxially detached shock wave at the entrance of passage, and the high disturbance region of blade pressure surface was caused by the adverse pressure gradient effect and flow direction deflection of the airflow in the passage, as well as the impacted on the pressure surface of the adjacent blade. It was speculated that leakage vortex breakdown due to vortex wave interference was the key factor to induce the unsteadiness of the flow field. A new vortex structure was formed at the leading edge of adjacent blades after leakage vortex breakdown, which was defined as a “leading edge vortex”, and its series of evolutionary processes occurring timely were the main reasons for the unsteady flow in the tip flow field. This study laid a theoretical foundation for the research of the flow field control strategy of axial compressor operating in off-design conditions in a large-scale compressed-air energy storage system.