Research On Jet Noise Reduction For Turbo-fan Engine With Beveled Nozzle

The jet noise is one of the main noise emissions of the turbofan engine. In order to adapt the strict standard of noise level which is made by International Civil Aviation Organization, one of the factors that should be taken into account is the jet noise reduction in civil turbofan engine system design. Aiming at this objective, a numerical investigation was made on the jet noise reduction with bevel exhaust nozzle by using the large eddy simulation method(LES) and aero-acoustic model in Actran software. The effects of boundary condition, bevel angle and beveled location on the internal aerodynamic and acoustic performance were analyzed, and far-field noise and near-field noise were compared as well. The results are summarized as the following:1. For the separate flow nozzle, both the boundary condition and beveled angle are tightly associated with the reduction of the jet noise. The beveled location influences the distribution of the acoustic field. The beveled nozzle enhances the mixing process between the jet and outer ambient surrounding, which is benefit for shortening the length of the jet turbulent core zone. As a result, the radiated noise is effectively transferred from low-frequency to high-frequency. There is a noise reduction of 2 to 5dB as a whole in the core-beveled nozzle. When the bypass nozzle is beveled, the jet has an obvious deflection. Compared with the conventional nozzle, the noise level is reduced 1~6d B in most detective directions while enhanced a little in some directions. By comparison, the core-beveled scheme seems to be better than bypass-beveled scheme.2. For the mixed flow exhaust, the beveled nozzle make the exhaust deflect obviously. As the beveled angle increases, the length of the jet turbulent core zone is shortened. The distribution of the acoustic field has an obvious asymmetric figure. The overall sound pressure level(OSPL) detected from the top direction is larger than that from the bottom. The noise level of the beveled nozzle is reduced 2~6dB in comparison with the conventional nozzle, realizing the action of noise reduction to the ground. The magnitude of noise reduction increases with increasing bevel angle.3. By comparing the far-field and near-field noise features, the noise has a distinct decay for the far distance of the observation point. The far-field noise feature reflects the acoustic directive property obviously.