Lobe mixer for gas turbine engine
BACKGROUND OF THE INVENTION 
     This invention relates to turbine types of power plants and particularly to lobed mixers. 
     The mixing of two streams in a jet engine, particularly of the turbofan variety has been evolving over the past several years. For example, much work has been carried on to suppress sound of a turbofan engine by mixing the fan discharge flow with the core engine flow and lobe mixers have been heretofore utilized with much success. However, their design always included a compromise between mixing performance and pressure losses where a high degree of mixing is traded for pressure losses with consequential good sound suppression and poor engine performance and a low degree of mixing resulted in just the converse. 
     This invention contemplates improving lobed mixers by scalloping the side walls of the mixer, thereby increasing the mixing length and introducing trailing vortex sheets. This has proven to not only increase sound suppression but also improve propulsion performance or at least did not deteriorate it. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide for a turbine type power plant an improved lobed mixer. Such mixer design includes having the side walls recessed to define a generally scalloped effect in the sinusoidal curvature of the mixer. 
    
    
     Other features and advantages will be apparent from the specification and claims and from the accompanying drawings which illustrate an embodiment of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial view of a turbofan engine showing the improved lobe mixer. 
     FIG. 2 is a perspective view of the &#34;scalloped&#34; lobe mixer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is shown in FIGS. 1 and 2 as being utilized in a fan-jet engine generally illustrated by reference numeral 10 which engine can be model JT-9D manufactured by the Pratt &amp; Whitney Aircraft Division of United Technologies Corporation (the assignee) as being exemplary. As will be appreciated by one skilled in the art, this invention would have application for any application where two flow streams are mixed. The fan flow stream is in the outer annulus 14 adjacent the engine casing 12 and the core stream is in the inner annulus 16 adjacent the plug 18. The lobe mixer 20 separates the two annular streams at this juncture of the engine and serves to provide a means for efficiently mixing the two. As is well known the lobe mixer comprises an annular portion 22 on the upstream end and a lobed or sinusoidal wave forms 24 on the downstream end. As noted, the lobe is basically a plurality of convoluted valleys 25 on the outer surface and convoluted peaks 26 on the inner surface each defining open ended channels for alternately receiving the fan stream and the core stream. Heretofore, these streams would be guided to the downstream end of the mixer where mixing would ensue upon the stream discharging from its channel. 
     In accordance with this invention the side wall 28 adjacent each valley 25 and peak 26 are recessed a portion downstream of the annular portion 22, and the depth of the recess 30 is determined by the following design procedure. The design flow path contours of the convolute produce flow areas in the fan and core streams that give static pressure balance at the lobes trailing edge and at the leading edge of the scalloped region where the flows of the two streams meet, such that minimal flow adjustment occurs. The sides of the convolute are scalloped such that the overhang of peak 26 is of sufficient length and curvature to allow turning of the core stream gases from a near radial direction to axially aft, which then develops trailing vortex sheets for increased mixing of the gases. 
     It should be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the spirit or scope of this novel concept as defined by the following claims.