Experimental study for multi-lobed mixer high bypass exhaust systems for subsonic jet noise reduction. II - Acoustic results

Several multi-lobed m ixers were tested in high bypass exhaust systems to develop a design methodology and noise reduction concepts. These m ixers along with appropriate confluent m ixers were tested with several fan nozzles of different lengths with and without acoustic treatment in GEAE’s freejet anechoic facility. Extensive amount of farfield acoustic data and flow-field laser Doppler anemometer data were acquired. Significant noise reduction is achieved by using a lower core velocity engine cycle for multi-lobed and confluent m ixer exhaust systems. Low frequency noise is significantly reduced due to multi-lobed m ixers. The objective of 3 EPNdB reduction relative to 1992 technology level is demonstrated that a 12-lobed m ixer configuration (F9B) resulted in a 3.5 EPNdB reduction compared to the confluent m ixer at Vti=lOOO ft/sec 1.0 Introduction The objective was to develop a design methodology and noise reduction concepts for high bypass exhaust systems which could be applied to both existing production and new advanced engine designs. Special emphasis was given to engine cycles with bypass ratios in the range of 4:l to 7:1, where jet m ixing noise was a primary noise source at full power takeoff conditions. The goal of this effort was to develop the design methodology for m ixed-flow exhaust systems and other novel noise reduction concepts that would yield 3 EPNdB noise reduction relative to 1992 baseline technology. The experimental study was carried out in two phases utilizing m & -lobed m ixers. In the first phase, three different 1Zlobed m ixers and a confluent m ixer (see Figure 1) were designed, fabricated, and tested in GEAE’s freejet anechoic facility. The second phase was aimed at improving the noise suppression capabilities of the 12-lobed m ixers. In this phase, one 22lobed axisymmetric, a 21-lobed with a unique lobe, and a confluent m ixers(see Figure 2) were designed, fabricated, and tested with several fan nozzles of different lengths with and without Figure 1. Confluent and 12-lobed E’ m ixers tested in GEAE’s Anechoic Freejet Facility. w Copyright 6iJ I999 by GE Aircraft Engmes. ~acoustic treatment in GEAE’s freejet anechoic Published by the American Institute of facility. Extensive amount of fat-field acoustic Aeronautics and Astrqnautics, Inc., with data and flowfield laser Doppler anemometer perm ission. data were acquired in both the phases and are