Surge Cycle Simulation and Validation at Different Rotational Speeds in a Two-stage Fan

Aerodynamic instabilities such as stall and surge may occur in fans and compressors, potentially leading to severe flow disruption, blade structural damage, and ultimately engine shutdown. To elucidate the transient inter-stage aerodynamics and underlying flow mechanisms during deep surge, this paper employs full-annulus unsteady Reynolds-averaged Navier-Stokes (URANS) simulations to model the complete surge cycles in a high-speed two-stage fan, thereby predicting unsteady blade loadings. Results from simulation show that the time history of casing static pressure above the first rotor (R1) tip during deep surge cycle is in substantial agreement with experimental measurements. Both the single-passage and the full-annulus models yield similar predictions regarding the shape of the surge cycle. A notable distinction, however, lies in the capability of the full-annulus model to capture the formation, development and clearing of rotating stall, which leads to a longer surge inception period than that predicted by the single-passage model. At 100% rotational speed, spike-type stall inceptions emerge initially in the second stage, whereas at 80% rotational speed, the first stage exhibits stall inceptions prior to the second stage. This is primarily attributed to differences in the range of incidence angle variations across different blade rows under varying rotational speeds. Analysis of surge loading indicates that the S2 blade row experiences the most significant variation in axial force across all rotational speeds, and should therefore be prioritized in the structural design process.