Turbofans Engine Exhaust Noise Reduction Technology Development

Pratt & Whitney Canada (P&WC) led a project aimed at understanding and further reducing exhaust noise from turbofan engines featuring an internal forced mixer. The project was supported by the Green Aviation... [ view full abstract ]

Pratt & Whitney Canada (P&WC) led a project aimed at understanding and further reducing exhaust noise from turbofan engines featuring an internal forced mixer. The project was supported by the Green Aviation Research and Development Network (GARDN) with grants from the Canadian Federal Government BL-NCE (Business Led - Networks of Centres of Excellence) covering the fiscal years 2009-2013. GARDN projects focus on improving the environmental performance of Canadian civil aviation products. More specifically, this project was strategically aimed at developing and leveraging understanding of key technologies to further mitigate noise from turbofan engine exhaust system to meet future aircraft noise targets.

Many turbofan engine exhaust designs feature internal forced mixers to rapidly mix the hot core flow with the cold bypass flow before the nozzle exit, primarily to enhance mixing and thus improve Specific Fuel Consumption (SFC). Although the design is intended for performance improvement, it also considerably reduces low frequency noise because of the lower relative mixed jet velocity compared to a confluent nozzle. In reality, the presence of the mixer adds complexity to the jet flow fields with additional high frequency source noise resulting to the small scale flow structures generated by the mixing induced within the nozzle by the forced mixer.
The influence of mixer parameters such as lobe penetration, lobe number, scalloping etc. on
noise has not been fully understood, and continues to be an active area of research for scientists and engineers. The academia primarily focuses their research efforts towards the development and the utilization of Computational-Aeroacoustics methods or some other hybrid methods using Computational Fluid Dynamics (CFD), while the engineering community focuses on establishing the delicate balance between SFC and noise reduction.
At P&WC, the GARDN noise research program was tailored to address both aspects of the
exhaust noise reduction process, as shown in Figure 2. The initial work focussed on improving the understanding of the mixer parameters, while the later phases focussed on mixer design, scaled model testing in an aeroacoustic wind tunnel, full-scale engine testing, and maturation of the aero-acoustic prediction and design tools, thereby advancing the Technology Readiness Levels (TRL) from TRL3 to TRL6, taking the process to the desired state as shown in Figure 1.

The complete noise characterization work is discussed in this paper, including some results from scale-model mixers tested in a large scale aeroacoustic wind tunnel, and final validation results carried with a full engine test. Finally, the impact at the aircraft level in term of noise benefits is also presented. This project has been a great success with key design features and the associated noise reduction technologies already seeing application in a new turbofan product. The product will get to TRL8-9 with the customer aircraft noise certification complete.