FIELD OF THE INVENTION:
The present invention relates to the field of exhaust diffusers. Particularly  the present invention relates to exhaust diffusers provided between last stage of low pressure module blades and condenser in steam turbines.

BACKGROUND OF THE INVENTION:
The fundamental definition of the diffuser is a device that controls the characteristics of the fluid at the entrance to a thermodynamic open system. In the context of the exhaust diffuser located between the last stage of low pressure module blades and the condenser  the thermodynamic open system is condenser. The condenser will be at a pressure of 0.1 ata and the steam flowing from last stage low pressure module blades through diffuser should achieve the pressure of 0.1 ata at the inlet of the condenser. The purpose of the diffuser is to facilitate in increase of pressure of the steam flowing from the last stage blades to achieve the constraint pressure of 0.1 ata at the inlet of the condenser.

Figure 1 illustrates a front sectional view of low pressure module blades along with conventional diffuser. The inlet area of the diffuser is the exit area of the last stage low pressure module blade. The exit area of the last stage low pressure module blade is 2.0 m2 (i.e  p[(radius at the tip of blade=921.38mm)2 – (radius at the hub of the blade=470mm)2]). Another constraint in the optimization of the diffuser is to select diffuser contour which prevents flow separation. The other tradeoff parameters in optimization are 1) Optimized axial length of the diffuser 2) Optimized pressure recovery.

Optimized axial length of the diffuser implies simple diffuser and the diffuser that can manufactured economically. Optimized pressure recovery in the diffuser implies improved energy recovery from the last stage low pressure module blades and thereby resulting in efficient turbine.

Several techniques have been disclosed in the prior art illustrating the optimization of exhaust diffusers in steam turbines to prevent flow separation and achieve improved pressure recovery.

US Patent No: 6261055 filed on Aug 3  1999 titled “Exhaust flow diffuser for a steam turbine” discloses an annular diffuser having its inlet located at the exit of a last row of blades of a steam turbine having initially very slowly increasing cross-sectional area with distance to accommodate the diffusion produced by the decaying wakes in the diffuser so as to prevent flow separation from diffuser walls and as a result to foster the diffusion process and to increase the efficiency of the steam turbine. The rate of increase of cross-sectional area  which is much smaller than that appropriate in diffusers having uniform incompressible flow at their inlets  allows wakes which form near the trailing edges of the last turbine blades to dissipate while avoiding flow separation. In the diffuser of this invention  whether it is one of fixed shape or one whose cross-sectional area can be changed by making use of an adjustable guide vane which surrounds at least a portion of the bearing cone  at a distance from inlet of one half of diffuser height at inlet  the cross-sectional area increase is smaller than 5.0% of the inlet cross-sectional area. This is equivalent to the corresponding two-dimensional straight-wall diffuser angle of  approximately  2.9 degrees. For the diffuser whose cross sectional area can be changed as required depending on its inlet flow conditions  the above limit applies for preferably most of the travel path of the adjustable guide vane but at least for the adjustable guide vane position closest to the turbine last blades. The length of the diffuser of this invention  in its preferred embodiment  measured along its mean line  is larger than or at least equal to 90% of the length of last turbine blades. The outer flow guide which defines the outer wall of the diffuser should have radius of curvature at its beginning larger than one half of the length of turbine last blades and should have a horizontal tangent there.

US Patent Publication No: 20020127100 published on Sep 12  2002 titled “Low Pressure Steam Turbine with Multi Channel Diffuser” discloses an axial/radial three-channel diffuser provided with two guide plates for dividing the diffuser into three partial diffusers that are distributed so that the distribution of the surface area over the three partial diffusers in the inlet surface area is uneven. The guide plates are oriented in accordance with the total pressure field after the last rotating blade row and are arranged at a minimum distance from the trailing edge of the last rotating blade row. Because of its
long extension in relation to the channel heights of the partial diffusers  the three-channel diffuser brings about a gentle deflection of the diffuser flow. The diffuser according to the invention results in an improved pressure recovery and increased turbine performance.

All the prior art mentioned above includes diffusers which are complex and not economical.

Therefore  there is a felt need for development of a simple and economical diffuser which ensures in the smooth flow without formation of vortices and eddies and produces an optimized pressure recovery.

OBJECTS OF THE INVENTION:
An object of the present invention is to provide an efficient exhaust diffuser for steam turbines.

Another object of the present invention is to provide an exhaust diffuser that produces an optimized pressure recovery.

One more object of the present invention is to provide a simple diffuser with optimized axial length.

Still another object of the present invention is to provide an exhaust diffuser that can be manufactured economically.

Further another object of the present invention is to provide an exhaust diffuser profile which ensures in smooth stream line flow path and prevents recirculation in the diffuser.

BRIEF DESCRIPTION OF THE DRAWINGS:
The invention will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates a front sectional view of low pressure module blades along with conventional diffuser;

Figure 2 illustrates a plot showing Diffuser area ratio on x-axis vs Axial length of low pressure module along with diffuser on y-axis in accordance with the present invention;

Figure 3 illustrates a plot showing Diffuser area ratio on x-axis vs ratio of length/height of the diffuser on y-axis in accordance with the present invention;

Figure 4 illustrates a plot of smooth stream line flow of the steam in the diffuser at the optimized diffuser exit annulus area in accordance with the present invention;

Figure 5 illustrates a plot of annulus area of the diffuser along the axial length of the diffuser in accordance with the present invention;

Figure 6 illustrates a plot of optimized pressure recovery at the optimized diffuser area ratio in accordance with the present invention; and

Figure 7 illustrates the geometry of an optimized diffuser in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
Figure 2 illustrates a plot showing Diffuser area ratio on x-axis vs Axial length of Low pressure module along with diffuser on y-axis. The plot shows a directly proportional relationship between diffuser area ratio and axial length of the low pressure module along with the diffuser.

Figure 3 illustrates a plot showing Diffuser area ratio on x-axis vs ratio of length/height of the diffuser on y-axis. The plot shows a directly proportional relationship between diffuser area ratio and length/height ratio of the diffuser.

All the iterations of the diffuser area ratio from 1 to 3 are considered and simulations are run to achieve an optimized diffuser area ratio. The optimized diffuser area ratio at which the design requirements of pressure recovery of at least 10%  optimized axial length  optimized manufacturing cost and prevention of occurrence of recirculation zones are met is 1.58.

The axial length of the low pressure module along with diffuser at optimized diffuser area ratio of 1.58 is 1430 mm as shown in figure 2. The ratio of length/ height of the diffuser at optimized diffuser area ratio of 1.58 is 1.52 as shown in figure 3.

Figure 4 illustrates a plot of smooth stream line flow of the steam in the diffuser at the optimized diffuser exit annulus area. It indicates that there is no occurrence of recirculation zone in the diffuser.

Figure 5 illustrates a plot of annulus area of the diffuser along the axial length of the diffuser. It is evident that the exit annulus area of the diffuser is 3.15 m2.

Figure 6 illustrates a plot of optimized pressure recovery at the optimized diffuser area ratio. It is evident from the blue colour plot ‘B’ that pressure recovery at the optimized diffuser area ratio is 10%.

Figure 7 illustrates the front cut sectional geometry of an optimized diffuser. In accordance with the present invention  there is provided a diffuser which resembles the hollow trumpet shape. The distance of outer edge of the inlet of the diffuser from rotor axis is 926.43 mm and is generally indicated with a reference numeral Y1. The distance of inner edge of the inlet of the diffuser from rotor axis is 470 mm and is generally indicated with a reference numeral Y2. The distance outer edge and inner edge of the exhaust of the diffuser from rotor axis is 1200 mm and is generally indicated with a reference numeral Y3. The exhaust passage width of the diffuser is 420.53 mm and is generally indicated with a reference numeral X1. The thickness of the diffuser is of 11.72 mm and is indicated by a reference numeral X2.

Although the invention has been described herein above with reference to the embodiments of the invention  the invention is not limited to the embodiments described herein above. It is to be understood that modifications and variations of the embodiments can be made without departing from the spirit and scope of the invention.

We claim:
1) An exhaust diffuser for steam turbines comprising a hollow trumpet like shape element located at the end of last stage of low pressure module blades having an inlet area of 2.0 m2 and exit area of 3.15 m2 wherein the geometry defining parameters are tabulated in Table 1 below:

Table 1
Representation Value (mm)
Y1 926.43
Y2 470
Y3 1200
X1 420.53
X2 11.72

2) An exhaust diffuser for steam turbines as claimed in claim 1  wherein the inlet passage width of the diffuser is of 456.43mm.

3) An exhaust diffuser for steam turbines as claimed in claim 1  wherein the exhaust passage width of the diffuser is of 420.53mm.

4) An exhaust diffuser for steam turbines as described herein the description and accompanying drawings.

Dated this 21st day of Jan  2012 (for Triveni Turbine Ltd)


Dr.Sunil Jajit GM-IPR