CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Application No. 61/771,116, filed March 1, 2013.

BACKGROUND

[0002] This application relates to a method of reducing the noise associated with discharging air from an anti-surge valve associated with a gas turbine engine.

[0003] Gas turbine engines are known and, typically, include a compressor compressing air and delivering the air into a combustion section. The air is mixed with fuel and ignited, and products of this combustion pass downstream over turbine rotors driving them to rotate. The turbine rotors, in turn, drive a compressor rotor.

[0004] One type of gas turbine engine is an auxiliary power unit ("APU"). The APU is typically utilized on an aircraft to provide power before the main engines are started, and further to assist in starting the main engines. An APU is typically provided with a gear box also driven by the turbine rotors, and which drive an electrical generator.

[0005] Under certain conditions, the pressure downstream of the compressor rotor may become unduly high. As an example, when the APU is first starting a main gas turbine engine on an aircraft, the loads on the APU are very high. The compressor may be delivering very high pressures and this can lead to a condition called "surge." This is undesirable and can lead to damage.

[0006] Thus, an anti-surge valve is typically included downstream of the compressor and upstream of the combustor. The anti-surge valve is selectively opened when a control determines that surge is a possibility. The anti-surge valve in the prior art taps air downstream of the compressor into an exhaust for the APU.

[0007] A problem with simply delivering the anti-surge air into the exhaust is that the air generates noise over a very broad band, and at frequencies within the range of human hearing. As an example, broadband noise between 500 and 8,000 hertz is not atypical for APUs. [0008] Since the APUs are particularly utilized when associated aircraft is on the ground, this is undesirable.

SUMMARY

[0009] In a featured embodiment, an anti-surge mechanism has a duct for tapping compressed air and delivering that air into an exhaust flow. A selectively open valve allows the tapped air to flow into the exhaust flow through a plurality of holes. The holes are sized to tune a frequency of a sound created by the tapped air to a frequency range outside of normal human hearing.

[0010] In another embodiment according to the previous embodiment, the hole size is equal to or less than about .1 inch (.254 centimeters) in hydraulic diameter.

[0011] In another embodiment according to any of the previous embodiments, the holes are in a piccolo tube including a plurality of axially spaced ones of the holes.

[0012] In another embodiment according to any of the previous embodiments, the piccolo tube extends into the exhaust flow.

[0013] In another embodiment according to any of the previous embodiments, the piccolo tube extends into a silencer.

[0014] In another embodiment according to any of the previous embodiments, the piccolo tube extends into an acoustic chamber positioned radially outwardly of a main exhaust flow chamber in the silencer downstream of an exhaust duct.

[0015] In another embodiment according to any of the previous embodiments, the holes are formed within a turbine catcher.

[0016] In another embodiment according to any of the previous embodiments, the turbine catcher includes at least three extending arms. The holes are formed in each of the at least three arms.

[0017] In another featured embodiment, a gas turbine engine has a compressor including an anti-surge system. The anti-surge system includes a duct for tapping compressed air downstream of a compressor rotor and delivering that air into an exhaust flow. The anti-surge system further includes a selectively open valve for allowing the tapped air to flow from the location downstream of the compressor rotor into the exhaust flow. The tapped air moves into the exhaust flow through a plurality of holes. The holes are sized to tune a frequency of sound created by the tapped air outside of normal human hearing.

[0018] In another embodiment according to the previous embodiment, the hole size is equal to or less than about .1 inch (.254 centimeters) in hydraulic diameter.

[0019] In another embodiment according to any of the previous embodiments, the gas turbine engine is an auxiliary power unit.

[0020] In another embodiment according to any of the previous embodiments, the holes are in a piccolo tube that include a plurality of axially spaced holes.

[0021] In another embodiment according to any of the previous embodiments, the piccolo tube extends into the exhaust flow.

[0022] In another embodiment according to any of the previous embodiments, the piccolo tube extends into a silencer.

[0023] In another embodiment according to any of the previous embodiments, the piccolo tube extends into an acoustic chamber positioned radially outwardly of a main exhaust flow chamber in the silencer downstream of an exhaust duct.

[0024] In another embodiment according to any of the previous embodiments, a turbine catcher is positioned in an exhaust duct. The holes are formed within the turbine catcher.

[0025] In another embodiment according to any of the previous embodiments, the turbine catcher includes at least three extending arms. The holes are formed in each of the at least three arms.

[0026] These and other features may be best understood from the following drawings and specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 schematically shows a first embodiment of this invention.

[0028] Figure 2 shows a second embodiment.

[0029] Figure 3A shows a first view of a third embodiment.

[0030] Figure 3B shows a second view of a third embodiment. DETAILED DESCRIPTION

[0031] Figure 1 shows an APU 20 having a compressor rotor 22 delivering compressed air into a combustion section 24. The air is mixed with fuel and ignited, and products of this combustion pass downstream over a turbine rotor 26 driving the turbine rotor 26 to rotate. The turbine rotor 26, in turn, drives the compressor rotor 22 and an electrical generator 34 through a gearbox 32. The features mentioned to this point are all schematically shown and all well known in this art.

[0032] Downstream of the turbine rotor 26, the products of combustion pass into an exhaust duct 28, and then through a silencer 30.

[0033] As mentioned above, an anti-surge valve 38 may be associated with a tube 36 to tap air downstream of a compressor rotor 22 and upstream of the combustion section 24. A control 39 receives operational information, and when it determines that a surge condition is possible, it will open the anti-surge valve 38 and bypass air downstream of the compressor rotor 22 to a duct portion 40. One example may be when an aircraft associate with APU 20 is at altitude, for example above 10,000 feet (3,048 meters). Under these conditions, anti-surge valve 38 may be opened. Of course, a worker of ordinary skill would appreciate other appropriate times for such control.

[0034] In an embodiment, a piccolo tube 42 receives the anti-surge air and passes the air into the outlet flow 45 through a plurality of spaced holes 44. The spaced holes 44 are all relatively small. As an example, the holes may be equal to or less than about .1 inch (.254 centimeters) and in one embodiment may be on the order of .063 inch (0.160 centimeter). The holes need not be cylindrical, and these sizes may also be true of a hydraulic diameter of other shaped holes. By utilizing such small holes, the frequency of the noise created by the discharge moves outside of the range of human hearing. As an example, with the .063 inch (0.160 centimeter) holes, the noise may be on the order of 150 kilohertz.

[0035] Thus, by utilizing a piccolo tube and, in particular, by discharging the air through a plurality of small holes, the noise associated with the operation of the anti-surge valve moves outside of the range of human hearing.

[0036] Figure 2 shows a second embodiment wherein the tube 140 delivers the anti-surge air through a plurality of holes 148 in a piccolo tube, as described in the first embodiment. However, the holes 148 and the piccolo tube 140 are placed in an outer acoustic chamber 144 of the silencer 240. The main exhaust flow flows through a main flow chamber 142 downstream of the exhaust duct 28. From the main flow chamber 142, the air may flow through perforations 146 into the outer acoustic chamber 144. The air can move between the outer acoustic chamber 144 and main flow chamber 142 and eventually exits the silencer through the main flow chamber 142. The size of the holes 148 is selected to move the noise outside of the normal range of human hearing.

[0037] Figure 3 A shows another embodiment, wherein the tube 160 is connected into the exhaust duct 28. As shown in Figure 3B, the tube 160 communicates with a plurality of supply ducts 162, each of which are connected into a leg 164, 165 and 163 of a so-called "turbine catcher." A turbine catcher 161 is a device placed at a downstream end of an exhaust duct 28. The turbine catcher 161 is typically placed in the duct 28 to catch the turbine in the unlikely event that it become separated from the engine rotor shaft.

[0038] The turbine catcher 161 includes relatively small holes 168 in tubes to discharge the anti-surge air. The size of the holes is selected to move the noise created outside the range of human hearing.

[0039] In general, all of the embodiments include holes 44/148/168, which are sized to tune a frequency of a sound created by the trapped air to a frequency range outside of normal human hearing.

[0040] Although embodiments of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.