Sound Suppression System

Abstract

System includes a shroud immediately downstream of turbine exhaust nozzle to form confined zone to receive turbine discharge. Conduit, which preferably surrounds nozzle, supplies fan air to mix with exhaust gas in zone, cool it, and increase mass flow. Shroud is corrugated to form peripherally spaced, radially extending lobes to discharge gaseous mixture. Spaces between lobes define flow paths for ambient air to flow between lobes and mix with gaseous mixture at fluted exit margin to further attenuate noise.

Description

BACKGROUND OF THE INVENTION

This invention lies in the field of sound suppression of gas turbine or jet engines, which produce reaction thrust by ejecting a high-velocity stream of gas from the exhaust nozzle or tailpipe of the gas turbine. One of the problems of airplanes equipped with jet engines on which a great deal of effort has been expended is that the exhaust stream creates a very high level of sound energy or "noise" in a wide range of frequencies, and a portion of this noise reaches the ground from low-flying airplanes, as during takeoff and climb, at an energy level which is not acceptable to the public.

The invention is directed to a system in which air is mixed with the exhaust gas to cool it and add mass and to raise the sound frequencies to levels which are more readily attenuated. It is directed particularly to a system of this type applied to a fan-jet engine in which maximum use is made of the fan air in reducing the noise level. While various schemes have been proposed for applying sound suppression systems to fan-jet engines, so far as known none of them has made use of both fan air and ambient air in a manner which will produce the most effective results.

SUMMARY OF THE INVENTION

The present invention provides a very satisfactory solution to the problem with a minimum amount of equipment and with no moving parts requiring repair and maintenance. Generally stated, the system includes a shroud which is positioned immediately downstream of the engine exhaust nozzle to form a confined discharge zone for the turbine exhaust. The fan air is fed into this zone and mixes thoroughly with the exhaust gas, cooling it, and increasing the mass flow. The turbulence of mixing also raises the frequencies. Preferably the fan air conduit completely surrounds the exhaust nozzle, providing maximum area of mixing contact.

The forward end of the shroud is preferably circular or cylindrical to form a virtual continuation of the fan air conduit and the balance is formed with gradually deepening corrugations defining a plurality of peripherally spaced radially extending lobes which constitute rearwardly directed discharge passages for the gaseous mixture of exhaust gas and fan air. The spaced lobes define between them rearwardly converging flow paths for ambient air and the streams come into contact at the fluted exit margin of the shroud. The peripheral extent of the exit margin is about twice as great as the circumference of the forward end of the shroud, thus doubling the junction line and doubling the mixing effectiveness.

Thus, there is a first thorough mixing of the exhaust gas and the fan air in a confined discharge zone followed by contact of the gaseous mixture along a practically maximum line with the ambient air to achieve the greatest possible mixing and cooling of the jet stream and maximum sound suppression.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other advantages and features of novelty will become apparent as the description proceeds in conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic side elevational view of a fan-jet engine incorporating the invention; and

FIG. 2 is a schematic rear elevational view of the engine.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The general arrangement of the invention is illustrated in FIG. 1, in which a typical jet engine 10 having an exhaust nozzle 12 is circumscribed by a double-walled fan air duct or conduit means 14 enclosed in a housing 16 which may be a nacelle or other airborne body. The duct and housing preferably are generally circular in cross section. A shroud 18 has a forward end 20 which is also generally circular or cylindrical to complement the housing and is secured thereto by any suitable means to form a virtual continuation of the duct. The shroud serves as a confined discharge zone 22 to contain and direct the flow of the gaseous mixture.

The fan air flowing from duct 14 completely surrounds the gas stream issuing from the engine nozzle to provide the maximum area of mixing contact. Thus the exhaust gas is cooled and the mass of the air is added to it before the gaseous mixture is discharged, and the sound energy level is already greatly reduced.

Starting adjacent to the forward end 20, the balance of the shroud is formed with gradually deepening corrugations defining a plurality of peripherally spaced and radially extending lobes 24. These lobes are open at their aft ends to constitute rearwardly directed discharge passages for the gaseous mixture of exhaust gas and fan air. They also define between them an equal number of flow passages 26 for ambient air, which passages are of maximum depth at the fluted exit margin 28 of the shroud.

With this construction and arrangement, the total peripheral extent of the exit margin is approximately twice as great as the circumference of the forward end 20 or a reference circle circumscribing the lobes. Thus the junction line of mixing contact between the gaseous mixture and the ambient air is twice as great as if the shroud were formed as a cylinder.

In addition, a streamlined plug 30, coaxial with the engine centerline, may be located with its maximum diameter in the plane of exit margin 28. The plug is about one-third of the diameter of the shroud and the radial extent of each lobe is about two-thirds of the radius of the shroud. The location of the plug tends to force the exhaust gas outwardly and the convergence of the ambient airflow passages 26 tends to force some of the fan air inwardly in discrete portions. These two factors ensure a thorough mixing of the exhaust gas and the fan air.

Thus it will be apparent that a system has been devised to take advantage of the availability of fan air in a fan-jet engine by first mixing it directly and thoroughly with the turbine exhaust gas in a confined zone to appreciably lower the sound level, and then mixing the resultant gaseous mixture with ambient air with the maximum practical area of contact for further noise reduction. The improvement has been accomplished with a minimum amount of material and weight and the system functions with complete reliability.

Claims

What is claimed as new and useful and desired to be secured by U.S. Letters Patent is:

1. A sound suppression system for a fan-jet engine having a turbine exhaust nozzle of circular configuration in the region of its exit plane, said nozzle having a discharge zone extending immediately downstream of its exit plane, comprising: conduit means surrounding said nozzle to supply fan air to said discharge zone of said nozzle for mixing with the turbine exhaust gas stream to cool the stream and increase the mass flow; and a shroud extension of said conduit means surrounding the discharge zone to contain and direct the flow of the gaseous mixture; said shroud being corrugated to define a plurality of peripherally spaced, radially extending lobes; each lobe constituting a rearwardly directed discharge passage for a portion of the gaseous mixture; each pair of adjacent lobes defining between them a flow passage for ambient air; the fluted exit margin of said shroud providing an elongated junction line for mixing of the ambient air with the gaseous mixture.

2. A system as claimed in claim 1; the peripheral length of said exit margin being approximately twice the circumference of a reference circle circumscribing said lobes.

3. A system as claimed in claim 1; said lobes each having a radial extent equal to approximately two-thirds of the radius of their circumscribing circle.

4. A system as claimed in claim 3; including a streamlined plug coaxial with the engine and occupying the space between the inner ends of said lobes.

5. A system as claimed in claim 1; said conduit means being generally cylindrical and surrounding the turbine exhaust nozzle to supply fan air to the entire periphery of the turbine discharge.

6. A system as claimed in claim 5; the forward end of said shroud being substantially cylindrical to form a virtual continuation of said conduit; and the flow passages for ambient air converging rearwardly to achieve full depth at the exit margin of the shroud.

7. A method of reducing the noise level of a fan-jet engine, comprising: discharging the turbine exhaust gas stream into a confined zone; discharging the fan air into the same zone and dividing the fan air into a plurality of separate streams therein to mix with the exhaust gas and cool it; and discharging the gaseous mixture into further mixing relation with a flow of ambient air effectively enhanced by said division of the fan air to further cool the mixture and raise the frequency level.