Patent Application
20120163969
The present invention relates to steam turbines. More specifically, the present invention relates to embodiments to provide improved exhaust hood performance in a steam turbine.
Steam turbines are widely used for power generation and primarily include a casing, a rotor extending in a longitudinal axis of a steam turbine, and a plurality of rows of buckets transversely disposed on the rotor. During operation, exhaust gases leave a last row of buckets and flow through an annular passage. Typically, the annular passage is defined by a steam guide which extends from the casing and a bearing cone that surrounds a portion of the rotor. Depending on various configurations of the steam turbine, the annular passage acts as an exhaust hood to diffuse the exhaust gases and direct the exhaust gases towards a condenser.
Particularly in a downward exhaust hood configuration, the exhaust gases are required to turn 180 degrees after leaving the annular passage and prior to reaching the condenser. Due to the simultaneous diffusion and turning in the annular passage, the high velocity exhaust gases experience turbulence and/or flow separation at the walls of the annular passage. This leads to the formation of vortices, decreases pressure recovery in the annular passage, and affects the overall performance of the steam turbine.
Various prior art solutions have been proposed to improve the pressure recovery in a downward exhaust hood. For example, a set of adjustable guide vanes may be provided on the bearing cone to guide the exhaust gases. The adjustable guide vanes change the cross-sectional area of the annular passage and improve pressure recovery in the annular passage. Further, the geometry of the annular passage may be modified to increase pressure recovery. However, geometry modifications lead to a complex and uneven construction of the exhaust hoods.
In light of the foregoing, there exists a need for an improved exhaust hood.
In accordance with one embodiment of the present invention, a turbine includes an outer casing surrounding an inner casing, a rotor enclosed by the inner casing that extends along a longitudinal axis of the turbine, and multiple rows of buckets transversely disposed on the rotor. The turbine further includes a bearing cone that surrounds a portion of the rotor and a flow guide that extends from the inner casing such that an annular passage for flow of exhaust gases is defined by the flow guide and the bearing cone. The bearing cone and the flow guide form an inner wall and an outer wall of the annular passage respectively. A guide cap having a streamlined surface is provided in a downstream direction of the flow guide.
In accordance with another embodiment of the present invention, the turbine includes one or more tip leakage flow injection channels which inject the exhaust gases at the inner surface of the flow guide. The injection channels are provided to energize flow of the exhaust gases in the annular passage and prevent the boundary layer separation at the inner surface of the flow guide.
In accordance with another embodiment of the present invention, the casing of the turbine surrounding the last stage buckets is inclined relative to the longitudinal axis of the turbine. The angle of inclination may range from about 5 degrees to 15 degrees relative to the longitudinal axis.
In yet another embodiment of the present invention, a first portion of the annular passage includes a substantially constant surface area and a second portion of the annular passage includes a progressively increasing surface area.
Illustrative embodiments of the invention will now be described more fully hereinafter with reference to the accompanying drawings. The invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.
Embodiments presented herein enable the guiding of exhaust gases in a downward exhaust hood configuration of a turbine.
Turning now to the drawings, and referring first to
Generally, the turbine 100 includes an outer casing 104 and an inner casing 106 such that the outer casing 104 encloses the inner casing 106. A rotor 108 is enclosed by the inner casing 106 and extends along a longitudinal axis AA′ of the turbine 100. The inner casing 106 and the rotor 108 form a flow path 110 for a working fluid such that the working fluid flows from a high pressure region to a low pressure region within the turbine 100. Further, multiple buckets 112 are transversely disposed with respect to the longitudinal axis AA′, and mechanically coupled to the rotor 108. More specifically, the buckets 112 are arranged in multiple rows which are arranged circumferentially around the rotor 108. Moreover, multiple nozzles 114 may extend from the inner casing 106 to circumferentially surround the rotor 108 and are axially positioned between the adjacent rows of the buckets 112. The buckets 112 and nozzles 114 work together and form multiple turbine stages, thus defining a portion of the flow path 110 leading to the exhaust hood 102.
The exhaust hood 102 may include an annular passage 116, such that the exhaust gases leaving the last row of buckets 112, are diffused in the annular passage 116. In an embodiment of the present invention, the annular passage 116 is defined by a flow guide 118 and a bearing cone 120 wherein the flow guide 118 and the bearing cone 120 may form the outer and inner walls, respectively, of the annular passage 116. As illustrated in
In an embodiment of the present invention, a guide cap 124 is provided in a downstream direction of the flow guide 118 (that is, in a location such that the exhaust gases pass over the guide cap 124 after passing through the annular passage 116 on the exhaust gas path towards an outlet 22 of turbine 100). In one embodiment of the present invention, the guide cap 124 may be integrally formed with the flow guide 118. In another embodiment of the present invention, the guide cap 124 may be a separate component which is attached to the flow guide 118 by welding, riveting, or fastening, for example. In one embodiment of the present invention, the guide cap 124 may have an airfoil shape.
During operation of the turbine 100, the exhaust gases turn by more than 180 degrees after leaving the annular passage 116 and are directed towards a condenser (not shown) through an outlet 122. Particularly at a top portion of the turbine 100, the exhaust gases exit the last stage turbine buckets 112 in an axial direction substantially parallel to the longitudinal axis AA′ of the turbine 100. Subsequently, the exhaust gases turn by almost 90 degrees to a radial direction in the annular passage 116, as illustrated by an arrow 126. In the annular passage 116, the exhaust gases may diffuse and cause pressure recovery. Subsequently, the exhaust gases turn by another 90 degrees over the guide cap 124, as illustrated by an arrow 128. Finally, the guide cap 124 turns the exhaust gases towards the condenser in a downwards direction. The guide cap 124 may have a substantially streamlined surface 130. As used herein, a “streamlined surface” is a surface which is contoured in a manner to enable a smooth flow of the exhaust gases after exiting the flow guide 118. Thus, formation of vortices and re-circulation regions in the exhaust hood 102, while taking the turn illustrated by the arrow 128, is substantially reduced.
The boundary layer separation of exhaust gases may be substantially prevented by injecting high momentum exhaust gases near the inner surface 302. This energizes the boundary layer flow and prevents the flow separation. As illustrated in
As described above, the turbine 100 includes buckets 112 which may be mechanically coupled to the rotor 108, and arranged circumferentially around a longitudinal axis AA′ of the turbine 100 to form multiple turbine stages along with a set of fixed nozzles 114. Multiple turbine stages are axially arranged on the rotor 108 with a predetermined axial clearance between any two turbine stages. Further, as illustrated in
The present invention has been described in terms of several embodiments solely for the purpose of illustration. Persons skilled in the art will recognize from this description that the invention is not limited to the embodiments described, but may be practiced with modifications and alterations limited only by the spirit and scope of the appended claims.
