With the continuous increase of aeroengine flight ceiling (>20 km), the thin atmosphere at high altitudes and the size effect all cause the compressor component inlet Reynolds number to decrease rapidly to a critical value (approximately 2.0 × 10⁵), and the significant transition process on the blade/endwall surface leads to the sharp degradation of compressor performance, which seriously affects the engine fuel consumption and working stability at high altitudes. In this paper, the research progress on the internal flow mechanism and flow control methods of axial compressors at low Reynolds numbers is reviewed from the aspects of quantification and prediction of performance variation, flow loss mechanism related to separation and transition, efficient transition control and flow field organization. The development trend of the low-Reynolds-number effect of axial flow compressors is noted, and the difficulties and application prospects of aerodynamic design and efficient flow control methods for compressors under low Reynolds numbers at high altitudes are discussed.

The transition process within a Laminar Separation Bubble (LSB) that formed on a compressor blade surface was investigated using Large Eddy Simulations (LESs) at a Reynolds number of 1.5 × 10⁵ and incidence angles of 0°, +3°, and +5°. The vortex dynamics in the separated shear layers were compared at various incidence angles and its effects on the loss generation were clarified through entropy analysis. Results showed that transition onset, which was accurately identified by the Linear Stability Theory (LST), was significantly promoted at the increased incidence angle. As such, the development of LSB was suppressed and the relative role of viscous instability played in the transition process was weakened. At the incidence angle of 0°, two-dimensional spanwise vortices detached from the blade surface and rolled up periodically, which were further stretched and eventually evolved into large-scale hairpin vortices. As time passed, the fully developed hairpin vortices broke down into small-scale eddies. Meanwhile, the flow near the wall reversely ejected into the outer separated shear layers and a sweeping process happened subsequently, forcing the separated shear layers to reattach and accelerating the generation of turbulent fluctuations. By comparison, the strength of vortex rolling-up was weakened at higher incidence angles, and the vortex pairing and breakdown of large-scale vortices were less pronounced. Therefore, the level of turbulent fluctuations that generated in the separated shear layers was reduced. Detailed entropy analysis showed that the turbulent dissipation effect related to the Reynolds shear stresses determined the largest amount of positive entropy generation, which declined to a lower level as the incidence angle increased from 0° to +5°. Correspondingly, the profile loss was reduced by 50.4%.