Patent Application
20110232291
The present invention generally involves an exhaust diffuser for a gas turbine. More specifically, the present invention describes a system and method that adjusts the camber of a strut in the exhaust diffuser to improve the efficiency of the gas turbine.
Gas turbines are widely used in industrial and commercial operations. A typical gas turbine includes an axial compressor at the front, one or more combustors around the middle, and a turbine at the rear. The compressor includes multiple stages of rotating blades and stationary vanes. Ambient air enters the compressor, and the rotating blades and stationary vanes progressively impart kinetic energy to the working fluid (air) to bring it to a highly energized state. The working fluid exits the compressor and flows to the combustors where it mixes with fuel and ignites to generate combustion gases having a high temperature and pressure. The combustion gases exit the combustors and flow to the turbine where they expand to produce work. An exhaust diffuser downstream of the turbine converts the kinetic energy of the flow exiting the last stage of the turbine into potential energy in the form of increased static pressure. This is accomplished by conducting the flow through a duct of increasing area, during which the generation of total pressure loss is to be minimized. The exhaust diffuser typically includes one or more aerodynamic airfoils which surround structural struts that support the bearing.
Combustion gases enter the exhaust diffuser with a wide range of inlet swirl conditions across the load range of the gas turbine. The varying swirl conditions cause the combustion gases to intercept and flow over the struts at varying incidence angles, resulting in significant aerodynamic losses. In addition, high swirl at the inlet of the diffuser has the potential for causing mechanical excitation within the diffuser due to vortex shedding from the strut. Therefore, it is desirable to be able to adjust the diffuser struts according to existing swirl conditions of the combustion gases to enhance the aerodynamic performance of the gas turbine.
Aspects and advantages of the invention are set forth below in the following description, or may be obvious from the description, or may be learned through practice of the invention.
One embodiment of the present invention is an exhaust diffuser that includes a shroud and a wall radially separated from the shroud to define a fluid passage between the shroud and the wall. A strut extends between the shroud and the wall, and the strut includes a first surface having an adjustable camber.
Another embodiment of the present invention is an exhaust diffuser that includes a shroud and a wall radially separated from the shroud to define a fluid passage between the shroud and the wall. A strut extends between the shroud and the wall, and the strut includes a first side camber, a second side camber, and means for adjusting at least one of the first side camber or the second side camber.
The present invention also includes a method for adjusting air flow across a strut having a first side camber and a second side camber. The method includes determining an incidence angle between the air flow and the strut and adjusting the first side camber of the strut.
Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the specification.
A full and enabling disclosure of the present invention, including the best mode thereof to one skilled in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which:
Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention.
Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.
Various embodiments of the present invention provide means for reducing aerodynamic losses across diffuser struts at high incidence angles. Embodiments of the present invention effectively add a small amount of camber near the leading edge of the strut to better align the leading edge of the strut with the incident flow of combustion gases. In the context of the present invention, the term “camber” refers to the amount of curvature in a surface. An inflatable bladder, hydraulic or pneumatic piston, threaded rod, or equivalent mechanical device may be used to create a bulge in the pressure side and reduce a bulge in the suction side of the strut to effectively bend the leading edge of the strut into the incident flow of combustion gases, reducing the pressure drop across the strut and axially aligning the flow of combustion gases. Benefits obtained by embodiments of the present invention may include improved diffuser aerodynamic performance in the presence of high swirl conditions and reduced vortex shedding due to high incidence angles which can lead to mechanical excitation problems.
The struts 16 extend between the shroud 12 and the wall 14 to orient the wall 14 with respect to the shroud 12. In the context of the present invention, the term “strut” includes any structure or supporting member that extends between the shroud 12 and the wall 14. The struts 16 generally include a first surface 24 and a second surface 26 that combine to form an aerodynamic surface.
Pneumatic or hydraulic pressure may be directed to or from each bladder 38, 40 through tubing 42, piping, or similar structures to increase or decrease the pressure and thus the associated volume of each bladder 38, 40. For example, a three-way valve 44 may be used to increase the pressure in one bladder while simultaneously decreasing the pressure in the other bladder to change the camber of each surface 24, 26 of the strut. In alternate embodiments, a separate valve, port, or other flow control device may be used for each bladder to independently change the pressure in each bladder.
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Numerous other means are known to one of ordinary skill in the art for moving the first and/or second surfaces to adjust the camber of the first and/or second surfaces of the strut. For example, various assemblies of hydraulic or pneumatic components, electrical motors, gears, or other mechanical devices may be used to change the shape of the first and/or second surfaces to produce the desired change in camber.
In operation, the exhaust diffuser having struts according to various embodiments of the present invention provides a method for adjusting air flow. The incidence angle between the flow of combustion gases and the strut can be determined through empirical observations or based on operating experience. For example, various instruments known in the art, including, but not limited to, pilot tubes and/or differential pressure detectors, may be used to determine the direction and velocity of the combustion gases. Alternately, prior operating experiences may be available to correlate current operating power levels with the direction and velocity of the combustion gases. Once the incidence angle is known, the camber for one or both surfaces of the strut may be adjusted to reduce the angle of incidence between the flow of the combustion gases and the leading edge of the struts. By reducing the angle of incidence between the flow of combustion gases and the leading edge of the struts, embodiments of the present invention reduce the flow resistance and resulting pressure drop across the struts. As a result, the present invention allows for larger variations and exit swirl of combustion gases while minimizing the negative effect on the thermodynamic efficiency of the gas turbine.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
