Exhaust diffuser for turbine

Abstract

An exhaust diffuser for a turbine includes 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. A method of diffusing exhaust of a turbine includes 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.

What is claimed is: 
     
         1 . An exhaust diffuser for a turbine, comprising:
 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, wherein 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.   
     
     
         2 . An exhaust diffuser according to  claim 1 , wherein a ratio of the guide surface axial length to the active length is about 0.55. 
     
     
         3 . An exhaust diffuser according to  claim 1 , wherein a ratio of an axial length of the exhaust diffuser to the active length is between about 1.35 and 2.0. 
     
     
         4 . An exhaust diffuser according to  claim 1 , wherein the curvature of the guide surface comprises a second angle and a third angle, and the first, second and third angles correspond to a range of change in the angle over the previous 25% meridional length of the steam guide at 25%, 50% and 75% meridional distances, respectively. 
     
     
         5 . An exhaust diffuser according to  claim 4 , wherein the first angle change is in the range of 0° to 18°, the second angle change is in the range of 14° to 32°, and the third angle change is in the range of 16° to 32°. 
     
     
         6 . An exhaust diffuser according to  claim 4 , wherein the second angle is larger than the first angle and the third angle is larger than the second angle. 
     
     
         7 . An exhaust diffuser according to  claim 4 , wherein the second angle and the third angle are approximately equal. 
     
     
         8 . An exhaust diffuser according to  claim 4 , wherein the first angle corresponds to a range of change in the angle at the 25% meridional distance from a shroud tip of the last stage bucket of the turbine. 
     
     
         9 . A turbine comprising a diffuser according to  claim 1 . 
     
     
         10 . A turbine according to  claim 9 , wherein a clearance between a tip of the last stage bucket and an inner casing or shroud is between about 50 to 200 mils. 
     
     
         11 . A turbine according to  claim 10 , wherein the clearance is between about 125 to 160 mils. 
     
     
         12 . A method of diffusing exhaust of a turbine, the method comprising:
 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, wherein 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 is between about 0.45 to 0.70.   
     
     
         13 . A method according to  claim 12 , wherein a ratio of a guide surface axial length to the active length is about 0.55. 
     
     
         14 . A method according to  claim 13 , wherein a ratio of an axial length of the exhaust diffuser to the active length is between about 1.35 and 2.0. 
     
     
         15 . A method according to  claim 12 , wherein the curvature of the guide surface comprises a second angle and a third angle, and the first, second and third angles correspond to a range of change in the angle over the previous 25% meridional length of the steam guide at 25%, 50% and 75% meridional distances, respectively. 
     
     
         16 . A method according to  claim 15 , wherein the first angle change is in the range of 0° to 18°, the second angle change is in the range of 14° to 32°, and the third angle change is in the range of 16° to 32°. 
     
     
         17 . A method according to  claim 15 , wherein the second angle is larger than the first angle and the third angle is larger than the second angle. 
     
     
         18 . A method according to  claim 15 , wherein the second angle and the third angle are approximately equal. 
     
     
         19 . A method according to  claim 15 , wherein the first angle corresponds to a range of change in the angle at the 25% meridional distance from a shroud tip of the last stage bucket of the turbine. 
     
     
         20 . A method according to  claim 12 , wherein a clearance between a tip of the last stage bucket and an inner casing or shroud is between about 50 to 200 mils.