Patent Publication Number: US-2013243564-A1

Title: Exhaust diffuser for turbine

Description:
The present invention relates to an exhaust diffuser for a turbine and to methods for reducing flow separation with over tip leakage flow with shrouded and unshrouded last stage buckets of the turbine. 
     BACKGROUND OF THE INVENTION 
     In the discharge of exhaust steam from an axial flow turbine, for example discharge of this exhaust steam to a condenser, it is desirable to provide as smooth a flow of steam as possible and to minimize energy losses from accumulation of vortices, turbulences and non-uniformity in such flow. Usually the exhaust from the turbine is directed into an exhaust hood and from there through a discharge opening in the hood in a direction essentially normal to the axis of the turbine into a condenser. It is desirable to achieve a smooth transition from axial flow at the exhaust of the turbine to radial flow in the exhaust hood and thence a smooth flow at the discharge opening of this hood into the condenser. 
     In the constructing of an effective exhaust hood for use with such an axial flow turbine it is desirable to avoid acceleration losses within any guide means employed therein and to achieve a relatively uniform flow distribution at the discharge opening of the exhaust hood for the most efficient conversion of energy in the turbine and effective supplying of exhaust steam to the condenser to which it is connected. 
     It is also desirable to achieve optimum efficiency at the last stage buckets of the turbine prior to exhaust from the turbine by achieving a relatively uniform circumferential and radial pressure distribution in the exit plane of the last stage buckets. Usually, attempts have been made to accomplish these results while employing a hood having as short an axial length as possible, so as to limit the axial size of the turbine train. 
     Diffusers are commonly employed in steam turbines. Effective diffusers can improve turbine efficiency and output. Unfortunately, the complicated flow patterns existing in such turbines as well as the design problems caused by space limitations make fully effective diffusers almost impossible to design. A frequent result is flow separation that fully or partially destroys the ability of the diffuser to raise the static pressure as the steam velocity is reduced by increasing the flow area. For downward exhaust hoods used with axial steam turbines, the loss from the diffuser discharge to the exhaust hood discharge varies from top to bottom. At the top, much of the flow must be turned 180° to place it over the diffuser and inner casing, then turned downward. Pressure at the top is thus higher than at the sides, which are in turn higher than at the bottom. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to an exemplary embodiment, an exhaust diffuser for a turbine comprises an inlet configured to receive exhaust gas from a last stage bucket of the turbine; and an exhaust gas guide surface configured to guide the exhaust gas. A curvature of the guide surface comprises a first angle with respect to an axis of the turbine and a ratio of a guide surface axial length to the active length of the last stage bucket which is between about 0.45 to 0.70. 
     According to another exemplary embodiment, a method of diffusing exhaust of a turbine comprises energizing a boundary layer along a curved exhaust flow guide surface of an exhaust diffuser with over tip leakage of a last stage bucket of the turbine. The curved exhaust flow guide surface is at a first angle with respect to an axis of the turbine and a ratio of a guide surface axial length to the active length of the last stage bucket which is between about 0.45 to 0.70. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts first and second exhaust flow diffusers in relation to a last stage bucket of a turbine; 
         FIG. 2  schematically depicts the steam guide length of the second exhaust flow diffuser of  FIG. 1  in relation to the last stage bucket; 
         FIG. 3  schematically depicts the angles of the steam guide surface of the second exhaust flow diffuser of  FIG. 1 ; 
         FIG. 4  schematically depicts the angles of the steam guide surface in relation to an inclined shroud; and 
         FIG. 5  schematically depicts the last stage bucket and inclined shroud. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a first exhaust flow diffuser  2  of a steam turbine low pressure section includes an inlet  18  that receives steam passing a last stage bucket  14  of the turbine. The exhaust flow diffuser  2  further includes a steam guide surface  4  that guides the steam flow passing from the last stage bucket  14 , and a diffuser end wall  6 . A ratio of the axial length L of the diffuser (as measured from a centerline  16  of the last stage bucket  14  to the diffuser end wall  6 ) to the active length AL of the last stage bucket  14  is approximately 2.0. The centerline  16  of the last stage bucket  14  is a radial line passing through the center of gravity of the bucket&#39;s root section. 
     Referring still to  FIG. 1 , a diffuser  8  of a second, different configuration from the diffuser  2  is also shown. The diffuser  8  has a ratio of axial length L (as measured from the centerline  16  to the diffuser end wall  12 ) to active length AL of the last stage bucket  14  of approximately 1.35. As shown in  FIG. 1 , the first diffuser  2  and the second diffuser  8  have the same diffuser area ratio which results in the second diffuser  8  having a sharper steam guide surface  10  curvature. Maintaining the diffuser area ratio of the second diffuser  8  the same as the first diffuser may lead to a flow separation from the steam guide surface  10  due to higher curvature. 
     The diffuser  8  uses over tip leakage from the last stage bucket  14  to energize a boundary layer along the steam guide surface  10  of the diffuser  8  to reduce, or prevent, separation of the steam from the steam guide surface  10 . Reducing, or preventing, separation of the steam from the steam guide surface  10  improves the static pressure recovery. 
     The tip clearance of the last stage bucket  14  may be reduced, or minimized, to increase, or maximize, the work of the fluid (e.g., steam or hot gases) on the rotating blades of the turbine. Some amount of clearance may be provided to reduce the possibility of rub between the blades and the inner casing. The last stage bucket  14  may have a hot radial tip clearance that varies between, for example, about 125 to 200 mils and the leakage flow rate for example, about 0.5% to 2% of the annulus flow rate. It should be appreciated that the tip clearance of the last stage bucket  14  may be otherwise, for example between about 50 to 160 mils. 
     Referring to  FIG. 2 , the diffuser  8  having a L/AL ratio of approximately 1.5 has a ratio of the steam guide axial length SGL (as measured from the centerline  16  of the last stage bucket  14  to the end of the steam guide surface  10 ) to the active length AL of the last stage bucket  14  of approximately 0.45 to 0.70, for example approximately 0.55. Referring to  FIG. 3 , the last stage bucket  14  has a tip angle A of, for example, about 0° and the curvature of the steam guide surface  10  may have a first angle B that may correspond to a range of change in the angle over the previous 25% meridional length of the steam guide from the bucket tip slope, for example, 0° to 18°, for example about 2°, 11°, or 14°. The second angle C of curvature of the steam guide surface  10  at the 50% meridional distance may be, for example, a change of 14° to 32°, for example about 20°, 22°, or 28°. The third angle D of the curvature of the steam guide surface  10  at the 75% meridional distance may be, for example, a change of about 16° to 32°, for example about 24°, 26°, or 28°. 
     Referring to  FIGS. 4 and 5 , the turbine may include an inclined shroud  20 . The last stage bucket  14  has a tip angle A of, for example, about 25° and the curvature of the steam guide surface  10  may have a first angle B that may correspond to a range of change in the angle over the previous 25% meridional length of the steam guide from the bucket tip slope, for example, 0° to 18°, for example about 2°, 11°, or 14°. The second angle C of curvature of the steam guide surface  10  at the 50% meridional distance may be, for example, a change of 14° to 32°, for example about 20°, 22°, or 28°. The third angle D of the curvature of the steam guide surface  10  at the 75% meridional distance may be, for example, a change of about 16° to 32°, for example about 24°, 26°, or 28°. 
     Although the embodiments have been described with respect to a steam turbine, it should be appreciated that the diffuser may be used with gas turbines. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.