Internal Flow and Noise of Chevrons and Lobe Mixers in Mixed-Flow Nozzles Vinod G. Mengle * The Boeing Company, Seattle, WA, 98124-2207 Lobe mixers have been traditionally used in mixed-flow exhaust nozzles of turbofan engines to enhance mixing in order to increase thrust efficiency and reduce noise. Earlier, the author has shown how the use of internal chevrons, instead of lobe mixers, in combination with external chevrons on the mixed-nozzle lip can reduce the far field perceived noise level below that of lobe mixers for a range of typical gas conditions at take- off. In this paper, we explore experimentally the mean flow-field characteristics of internal chevrons inside the hot nozzle by developing a unique nozzle with a set of rotatable internal rakes. The inner boundary layer profile at the nozzle exit plane is also obtained and can be useful in designing immersions of external chevrons. A lower bound on the noise contribution of this internal mixing to far field noise is then found by using the technique of hard- and soft-walled nozzles. Similar internal flow and internal noise characteristics are then found for an advanced lobe mixer, as well as a simple splitter nozzle. The “gentle” mixing due to internal chevrons appears to correlate with lower contribution of internal jet noise to far field. However, the simple splitter nozzle has a fairly strong contribution from internal jet noise. Besides giving an improved understanding of these flows and noise characteristics, the experimental database generated can be used to validate predictive theories for internal flow and internal jet noise. I. Introduction N mixed-flow nozzles (or, internally-mixed nozzles) of modern turbofan engines, where the hot core flow and fan flow are mixed inside a nozzle duct, the traditional device for reducing jet noise in a thrust-efficient manner (in comparison to a simple splitter) has been the lobe mixer. However, recently the author has shown 1 for the first time that appropriately designed combinations of "internal" chevrons, in lieu of lobe mixers, and "external" chevrons added to the aft mixed nozzle lip can reduce jet noise as much as, or even better than, the quietest lobe mixers for typical high bypass ratio nozzles at take-off conditions, when compared at same thrust-level and nozzle size. Internal chevrons, by themselves, as illustrated in Fig. 1(a), also proved to be quieter than simple round splitter nozzles. I Internal chevrons reduced low frequency noise below that of simple splitter with hardly any high frequency “lift.” External chevrons further aided the internal chevrons by reducing the low frequency noise even more. Lobe mixers, on the other hand, reduce the low frequency noise tremendously to compensate for their high frequency lift, if any. Thus there appears to be a paradigm shift in the way overall perceived noise level can be reduced for internally mixed nozzles: rather than reducing the low frequency noise tremendously and accepting high frequency lift, as with lobe mixers, one can reduce the low frequency noise somewhat less but without much high frequency lift by using appropriately designed chevrons. Two key reasons were suggested for this beneficial behavior of internal chevrons in mixed-flow nozzles. To understand them note that in such internally mixed nozzles, the noise generated inside the nozzle (termed “internal” jet noise) due to the turbulent mixing between the core and the fan stream, downstream of the mixer, can also contribute to the far field noise in addition to the classic jet mixing noise generated outside the nozzle (termed “external’ jet noise). Figure 1(b) illustrates these two sources of jet noise. Then the reasons suggested for their success were: the "gentler" mixing of chevrons which supposedly reduces the contribution of internal jet mixing noise and, secondly, the higher discharge coefficient of chevrons compared to lobe mixers (the latter typically have higher blockage area ratio due to the several convoluted lobes). Although the discharge coefficients were measured and compared earlier for such internal chevrons and lobe mixers, their internal flow and internal noise characteristics were not verified. Internal flow evolution, in general, * Engineer/Scientist, Acoustics and Fluid Mechanics, P.O. Box 3707, MC: 67-ML, Senior AIAA Member American Institute of Aeronautics and Astronautics 1 44th AIAA Aerospace Sciences Meeting and Exhibit 9 - 12 January 2006, Reno, Nevada AIAA 2006-623 Copyright © 2006 by The Boeing Company. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.