Jet noise reduction of double-mixing exhaust system

A jet noise reduction technique by using the external chevron nozzle with lobed mixer in the double-mixing exhaust system is investigated under cold conditions. The computations of jet field and the experiments of noise field are conducted with scaled model of high-bypass-ratio turbofan engine mixing exhaust system composed of external chevron nozzle with lobed mixer. The computational results indicate that comparing with the baseline nozzle with lobed mixer, the external chevron nozzle with lobed mixer increases mixing of jet and ambient air near the nozzle exit. The experimental results show that the external chevron nozzle with lobed mixer has better jet noise reduction at low frequencies, and this reduction rises with the increase of chevron bend angle. The experimental results also show that the external chevron nozzle with lobed mixer has sound pressure level (SPL) increase which is not obvious at high frequencies. With chevron bend angle increasing, SPL has relatively marked increase at 60° (directivity angle measured from upstream jet axis) and little fluctuations at 90° and 150°. The external chevron nozzle with lobed mixer has overall sound pressure level (OASPL) reduction in varying degrees at 60° and 150°, but it has little OASPL increase at 90°.