Effects of Fan-Root Clearance on Leakage-Flow Dynamics and Transition-Duct Aerodynamics in an Open Fan

The open fan is regarded as a promising alternative to high-bypass-ratio turbofan engines due to its high propulsive efficiency. However, the variable-pitch configuration at the rotor root increases the complexity of the root flow and introduces pronounced inlet distortion and unsteadiness to the downstream transition duct. To clarify the coupled inner–outer flow mechanisms at the fan root, numerical simulations of an integrated open-fan and transition duct are conducted in this study. The mechanisms of variable-pitch-induced complex flow phenomena near the open-fan root are investigated for different root clearance heights. The impact of root flow on the downstream transition duct is investigated in terms of inflow distortion and loss characteristics. RANS and URANS simulations are performed in a coupled inner–outer computational domain. The RANS parametric study identifies root clearance height as the primary factor governing transition-duct inlet distortion, motivating URANS simulations of three representative clearance heights—no clearance, 0.2% span, and 1.27% span—to resolve the unsteady rotor-root flow and its interaction with the transition-duct IGV.,The results further indicate that the root spindle configuration exerts a negligible influence on the overall aerodynamic performance of the open fan, with thrust variations within 0.2%, whereas the clearance height governs the evolution of root flow structures. Building on this understanding, the mechanisms underlying inlet distortion in the transition duct are elucidated, with leakage flow identified as the primary driver of the distortion pattern. URANS simulations are subsequently performed for three representative clearance heights to characterize the unsteady flow behavior. The vortex structures at the rotor root are first analyzed, revealing coupled interactions between leakage flow and inherent secondary flow structures. The resulting distortion and unsteadiness at the transition-duct inlet are then quantified. Finally, the influence of clearance height on the aerodynamic performance of the transition-duct IGV is assessed.,The present study clarifies the underlying mechanisms by which rotor-root secondary-flow structures modulate the transition-duct inflow, providing physical insights for the coupled aerodynamic design of open-fan rotor root and downstream core-engine components.