Gas turbine inlet silencer

Abstract

A system for attenuating sound emissions from an inlet to a flow path for an air inducting machine including an inlet duct structure having inlet and outlet passages. A planar vortex generator is located adjacent to the outlet passage and creates vortices that interact with a specific tonal acoustic frequency emitted from the inlet of the machine to effect formation of a standing wave at the vortex generator. The standing wave has an upstream propagating component that reflects off an acoustic reflector wall to form a reflected component that interferes with the upstream propagating component to attenuate the specific tonal acoustic frequency from the inlet of the machine.

What is claimed is: 
     
         1 . A system for attenuating sound emissions from an inlet to a flow path for an air inducting machine, the system comprising:
 an inlet duct structure having an inlet passage and an outlet passage downstream from the inlet passage, the outlet passage defining an outlet plane extending span-wise generally perpendicular to flow through the outlet passage;   a vortex generator located adjacent to or upstream of the outlet passage, the vortex generator extending across the outlet passage and defining a plane, the vortex generator creating vortices that interact with a specific tonal acoustic frequency emitted from the inlet of the machine to effect formation of a standing wave;   an acoustic reflector wall located on the inlet duct between the inlet and outlet passages, upstream of the vortex generator and oriented generally parallel to the plane of the vortex generator; and   wherein the standing wave has an upstream propagating component that reflects off the acoustic reflector wall to form a reflected component that interferes with the upstream propagating component to attenuate the specific tonal acoustic frequency from the inlet of the machine.   
     
     
         2 . The system of  claim 1 , wherein the vortex generator includes a plurality of vortex producing rods extending in a row parallel and in spaced relation to each other in the span-wise direction across the outlet passage, and the rods form wake shed vortices on downstream sides thereof in a plane generally parallel to the outlet plane. 
     
     
         3 . The system of  claim 2 , wherein the rods have a circular cross-section defining a diameter, and at least one of the rods has a different diameter than at least another of the rods. 
     
     
         4 . The system of  claim 3 , wherein the diameters of particular rods are selected with reference to an average velocity of air flow at the location of each of the particular rods. 
     
     
         5 . The system of  claim 4 , wherein a distance from the rods to the acoustic reflector wall varies, depending on the diameter of the rod. 
     
     
         6 . The system of  claim 2 , including two or more rows of rods spaced from each other in the direction of flow through the outlet passage wherein rods in the first row of rods are aligned with rods in the second row of rods in a direction extending perpendicular to the outlet plane, and form either an in-line or staggered array. 
     
     
         7 . The system of  claim 1 , wherein the acoustic reflector wall is located at a junction where the flow direction changes between the inlet and outlet passages, parallel to the plane of the vortex generator. 
     
     
         8 . The system of  claim 1 , wherein the air inducting machine is a gas turbine engine having a compressor including rotating blade rows, and the specific tonal acoustic frequency is a blade passing frequency created by at least one of the rotating blade rows. 
     
     
         9 . The system of  claim 8 , wherein the standing wave has a frequency that corresponds to the blade passing frequency, and the vortex generator is positioned relative to the acoustic reflector wall such that a distance, d, between the standing wave and the acoustic reflector wall is equal to n(λ/4), where n is an odd integer and λ is the wavelength of the blade passing frequency. 
     
     
         10 . The system of  claim 9 , wherein at least one of the vortex generator and the acoustic reflector wall is movable relative to the other of the vortex generator and the acoustic reflector wall to adjust the distance, d. 
     
     
         11 . The system of  claim 1 , wherein interior surfaces of the inlet duct structure, except for the acoustic reflector wall, are lined with an acoustic absorptive structure. 
     
     
         12 . A method of attenuating sound emissions from an inlet to a flow path for an air inducting machine, the method comprising:
 providing a flow of air through an inlet duct structure having an inlet passage and an outlet passage, the outlet passage defining an outlet plane extending span-wise generally perpendicular to flow through the outlet passage;   directing the flow of air over a vortex generator located adjacent to or upstream of the outlet passage to create vortices that interact with a specific tonal acoustic frequency emitted from the inlet of the machine to effect formation of a standing wave;   providing an acoustic reflector wall located on the inlet duct between the inlet and outlet passages upstream of the vortex generator and oriented generally parallel to the plane of the vortex generator; and   wherein the standing wave has an upstream propagating component that reflects off the acoustic reflector wall to form a reflected component that interferes with the upstream propagating component to attenuate the specific tonal acoustic frequency from the inlet of the machine.   
     
     
         13 . The method of  claim 12 , wherein directing the flow of air over a vortex generator comprises providing a plurality of rods extending in a row parallel and in spaced relation to each other, and directing the flow of air through spaces between the rods. 
     
     
         14 . The method of  claim 13 , wherein the standing wave is formed in a plane spaced downstream from the row of rods. 
     
     
         15 . The method of  claim 14 , wherein a plurality of the rods are located at different distances from the plane of the standing wave. 
     
     
         16 . The method of  claim 12 , wherein the standing wave has a frequency that destructively interferes with the specific tonal acoustic frequency after the upstream propagating component reflects off the acoustic reflector wall. 
     
     
         17 . The method of  claim 16 , including moving at least one of the vortex generator and the acoustic reflector wall relative to the other of the vortex generator and the acoustic reflector wall to adjust the distance between the vortex generator and the acoustic reflector wall to tune the reflected component so as to destructively interfere with the specific tonal acoustic frequency. 
     
     
         18 . The method of  claim 12 , wherein the air inducting machine is a gas turbine engine having a compressor including rotating blade rows, and the specific tonal acoustic frequency is a blade passing frequency created by at least one of the rotating blade rows. 
     
     
         19 . The method of  claim 18 , wherein the standing wave has a frequency that corresponds to the blade passing frequency, and the vortex generator is positioned relative to the acoustic reflector wall such that a distance, d, between the standing wave and the acoustic reflector wall is equal to n(λ/4), where n is an odd integer and λ is the wavelength of the blade passing frequency. 
     
     
         20 . The method of  claim 19 , including moving at least one of the vortex generator and the acoustic reflector wall relative to the other of the vortex generator and the acoustic reflector wall to adjust the distance, d.