Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – With indicator or control of power plant
Reexamination Certificate
1999-12-13
2002-03-05
Nguyen, Tan (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
With indicator or control of power plant
C060S039370, C060S039520, C060S238000, C415S076000, C340S521000, C356S370000, C324S207160
Reexamination Certificate
active
06353789
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to predicting the amount of noise in a gas turbine engine, and more particularly to a computer-implemented method that predicts the amount of broadband noise from the stator vane cascade in a gas turbine engine.
BACKGROUND ART
An aircraft gas turbine engine includes a fan section, a compression section, a combustion section, and a turbine section. An annular flow path for the working medium gases extends axially through the engine. The fan section has a rotor assembly that includes an array of rotor blades that are angled with respect to the approaching gas flow. The fan section also has a stator assembly that includes an array or cascade of stator vanes. Each vane is typically arranged in a radial direction outward from the center axis of the engine. The stator vane cascade is disposed axially downstream of the rotor blade array in the gas flow path. The vanes receive and guide the direction of the flow of gases exiting from the upstream rotor blade array. The stator vanes connect between an outer duct wall and an inner duct wall within the engine. The duct walls extend circumferentially with respect to the flow path to form the boundaries for the working medium gases in the fan section.
As the rotor assembly rotates, the blades do work on the gases to increase the pressure of the gases. The rotor blades also increase the velocity of the gases and direct the flow of gases from the engine axial direction to the blade rotation direction. The gases are then flowed past the rotor blade array to the stator vane cascade, which redirects the flow of gases to the axial direction. By reorienting the flow in this manner, the stator vane cascade increases the recovery of the flow energy of the gases into thrust.
As the working medium gases travel along the engine flow path, the gases are pressurized in the rotating fan and compression sections, which causes the gas temperature to rise. The hot, pressurized gases are burned with fuel in the combustion section to add energy to the gases. The gases are then expanded through the rotating turbine section to produce useful work for pressurizing the gases in the fan and compression sections. The expanding gases also produce thrust for propelling the aircraft forward.
The gas flow through the engine generates acoustic energy or noise. Aircraft engine manufacturers are concerned with the adverse effect of excessive noise levels on passengers, aircraft personnel, and residents in close proximity to airports. Due to increasingly stringent noise restrictions placed upon aircraft that operate in certain geographic areas and at certain times, there is a persistent need for quieter aircraft engines.
The principal sources of noise in an aircraft gas turbine engine are jet or exhaust noise, core noise, and fan noise. Jet noise results from mixing of the high-velocity engine exhaust gas stream with the ambient air. A considerable amount of turbulence is generated when these two streams, which are traveling at different velocities, mix together. This turbulence generates jet noise. In a turbofan engine, there are two exhaust streams; therefore, there are two sources of jet noise. One noise source is the turbulent mixing of the fan exhaust stream with the ambient air. The other noise source is the turbulent mixing of the engine core exhaust stream with the fan exhaust stream and the ambient air.
Core noise consists of compressor noise, combustion noise and turbine noise. Compressor and turbine noise are caused by the unsteady blade forces and fluid stresses when fluids are compressed for driving the turbines. Combustion noise results from the turbulence generated by the burning of fuel in the combustion chamber.
Fan noise is often the predominant noise source in a high-bypass ratio turbofan engine. Fan noise is caused by non-uniform gas flow exiting the rotor blades and impinging upon the stator vanes in the fan section. As each rotor blade passes through the gases, the blade leaves a wake or track of turbulent gases behind it at the trailing edge of the blade. This wake is commonly referred to as the rotor wake turbulent flow. Also, tracks of non-turbulent flow exist between the rotor blade array and stator vane cascade in the areas not directly behind the blades. The turbulent flow impinges on the stator vanes and generates much of the noise radiated by the engine. This fan noise is also commonly referred to as rotor/stator interaction noise.
In addition, secondary flow patterns exist adjacent the tips of the rotor blades due to the interaction of the blade tip with the boundary layer of the engine outer duct wall. This interaction introduces further turbulence (i.e., “tip vortex”) into the wake turbulent flow at the blade tip region. Also, turbulent flow emanates from the hub or root portion of the rotor blade (i.e., “hub vortex”). The wake turbulent flow (i.e., “downwash”) from rotor blades sweeping past stator vanes produces pressure fluctuations on the vane surfaces. These fluctuating aerodynamic pressures on the vane surfaces produce forces that generate noise.
The rotor blade wake turbulent flow has a steady component and a random component. The steady component is also commonly referred to as the harmonic or periodic wake component.
The random component is also commonly referred to as the broadband wake component. The random component is represented by the turbulent kinetic energy, which varies dramatically across the span of the stator vane inlet. The turbulent kinetic energy is often greater at the root or hub and tip regions of the rotor blade as opposed to the mid-span region of the rotor blade. The turbulent kinetic energy present in the blade root region is typically attenuated as it is absorbed downstream in the low-pressure compressor section of the engine.
The harmonic and broadband components of the wake turbulent flow are related to the noise spectrum components. The harmonic wake component causes harmonic or tonal noise and the broadband wake component causes broadband noise. Tonal noise is noise at specific frequencies, which are multiples of the rotor blade passage frequency. This tonal noise has a distinct sound that can be heard above the background noises. The amount of tonal noise generally depends upon the number of stator vanes and rotor blades, the geometry of the duct walls bounding the flow path for the working medium gases, the velocity of the gases, and the rotor speed. On the other hand, broadband noise is distributed over a wide range of frequencies, rather than being at specific, discrete frequencies. The noise signatures of modem turbofan engines tend to be dominated by broadband noise.
It is difficult to suppress or attenuate fan noise because of the interdependence of the mechanisms that contribute to this noise and the basic aerodynamic operation of the fan section of the engine. The prior art contains noise suppression structures adapted specifically for retrofit or original fit on an aircraft gas turbine engine. Typically, the noise suppression structure consists of sound attenuating liners applied to the nose cowl, the nose dome and the fan flow path components of the engine. In typical constructions, the sound absorption material lines the inlet duct and nozzle of a turbojet or turbofan engine to suppress the noise generated within the flow path. However, significant aerodynamic losses (e.g., thrust) result from the addition of noise suppression structures that provide for acceptable levels of tonal and broadband noise.
Another known method of attempting to reduce the fan noise involves selecting blade/vane ratios to satisfy the cut off criterion for propagation of noise at the fundamental rotor frequency. It is also known to increase the axial spacing between the rotor assembly and the stator assembly to reduce fan noise.
The aerospace community is aware of the potential for reducing the tonal noise component of the turbulent rotor wake by adjusting the angular physical positioning of the stator vanes in one or both of two different angular directions. This angu
Bachman & LaPointe P.C.
Nguyen Tan
To Tuan C
United Technologies Corporation
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