The influence of booster design features on aicraft engine fan tonal noise

Fan and booster tonal noise makes significant contribution to the overall noise level of an aero engine, particularly in subsonic modes of operation. This noise is caused by the interaction not only of the fan rotor and stator, but also by the interaction of elements of the fan and booster. Tonal components at these frequencies are caused by nonlinear interaction of rotating and stationary blade rows. Therefore, fan noise spectra contain not only common fan tonal components at the blade passing frequency and its harmonics, but sum and difference tones of interaction between the fan and booster elements. In testing acoustic characteristics of a bypass wide-chord fan it was revealed that the sum and difference components of the fan and booster interaction noise appeared in subsonic operating modes and propagated basically in the forward hemisphere. Labyrinth seal is one of the features of the actual fan design. These seals are opened and let some airflow pass in low operating modes of the fan. As a result the flow pattern upstream the fan rotor is distorted. It was experimentally determined that the seal significantly affects the generation of the fan tonal noise in subsonic modes. So far, the impact of this fan design feature is not taken into account in fan noise prediction methods. However, we experimentally determined strong influence of labyrinth seals on the noise of the aero engine simulator. Experimental assessment of the influence of the labyrinth seal operating in off-design modes on UPS noise spectra is the purpose of this work.