Mechanisms to reduce the blade passing frequency tone for subsonic low-count OGV fans

Abstract

Conventionally, fan stages are designed with more stator vanes than rotor blades. These fan stages provide a tonal noise advantage as the blade passing frequency tone is cut-off. Contrary to conventional cut-off designs, low-count OGV (Outlet Guide Vane) designs are fan stages with fewer stator vanes than rotor blades. Low-count OGV designs benefit from lower broadband noise due to the reduced number of stator vanes. However, the blade passing frequency tone may no longer be cut-off. Therefore, we assess tonal noise reduction mechanisms for subsonic low-count OGV fan stages. We use an analytical and a numerical method to predict the rotor-stator interaction noise for different blade count pairings. The results indicate that the mode phase propagation angle and the cut-on factor are two acoustic parameters that dominantly influence the noise excitation. For certain blade count pairings an acoustic benefit exists if the mode phase propagation angle is congruent to the stator leading edge angle and / or if the mode is away from the cut-off limit. Moreover, reducing the rotor tip speed allows, for some configurations, a cut-off design even with fewer stator vanes than rotor blades (inverse cut-off). Overall, the study shows promising possibilities to reduce tonal interaction noise for low-count OGV fan stages.