FIELD OF THE INVENTION

The present invention relates to Francis turbines employed in hydroelectric
power plants with specific speed greater than 225 m-kW. More particularly, the
invention relates to a Francis turbine runner blade modified to mitigate pressure
pulsations and noise associated with hydraulic instabilities inside the turbine.
BACKGROUND OF INVENTION
Francis turbines are employed in hydroelectric power plants for extracting the
potential energy of water and converting it into the rotational energy of the
turbine shaft. Water flows past the turbine runner blades such that it transmits
its' energy to the runner. This process is associated with a lot of hydraulic
instabilities leading to pressure pulsation and noise. This problem is further
aggravated in the case of low head turbines due to the formation of the draft
tube vortex rope which causes a lot of pressure pulsations
Figure 1 shows a meridional view of a typical Francis turbine consisting of a
spiral casing, plurality of stay vanes, guide vanes, plurality of at least one guide

vane, runner leading edge, multiple runner blades, at least one runner trailing
edge, a hub, a shroud and a draft tube. A Francis turbine runner consists of a
crown, a band and a plurality of blades located in between the crown and band
such that water coming from the inlet guide vanes enters the runner from
leading edge and leaves the runner from the trailing edge into the Draft tube. In
a Francis turbine, high pressure water enters the Spiral casing and is guided by
the stay vanes and guide vanes before entering the runner. The water transmits
its' energy to the runner and leaves the runner into the draft tube.
The entire flow from the spiral casing inlet to the draft tube outlet is highly
turbulent in nature, which causes instabilities and disturbances in the flow
leading to pressure pulsations, noise etc. The pressure pulsations in low head
Francis turbines are the result of:
(a) Draft tube vortex rope formed at the exit of the runner.
This is the major cause of pressure pulsations in Francis turbines having specific
speed greater than 225 m-kW. Especially at off-design operating points, a strong
vortex rope is formed at the exit of the runner which travels down into the draft
tube. It is an objective of this patent to provide for an improved shape of the
runner which can reduce the severity of these pulsations.
(b) Interaction of the guide vane trailing edge vortex with the runner blade
leading edge (Rotor-stator interaction)

As the water exits from the guide vanes, each runner blade passing behind the
guide vanes leads a surge in the pressure. Further, water exiting from the
guide vanes experience the Classical Karman vortices with a unique vortex
shedding frequency, which upon interaction with the runner inlet edge causes
unsteady pressure pulsations. The cumulative effect of the above phenomenon
gives rise to undesirable pressure pulsations.
(c) Pulsations formed due to the Vortices exiting the trailing edge of the runner
Vortices are created when water exits from the entire span of runner blades.
Further the vortices are unsteady in nature. The severity of the pressure
fluctuations created by these vortices depends, to some extent, on the time
interval between the shedding of these vortices from different sections along the
span-wise length of the blades. If most of the vortices exit from the blade at the
same instant of time, the severity of pressure fluctuation is larger. It is an
objective of the present invention to curb the pressure fluctuations and
vibrations caused due to this phenomenon.
OBJECTS OF INVENTION:
It is therefore an object of this invention to propose a Francis turbine runner
blade modified to mitigate pressure pulsations and noise associated with
hydraulic instabilities inside the turbine.

Another object of the invention is to propose a Francis turbine runner blade
modified to mitigate pressure pulsations and noise associated with hydraulic
instabilities inside the runner which reduces the evolved pressure pulsations due
to interaction of the guide vane trailing edge vortex with the runner blade
leading edge.
A still another object of the invention is to propose a Francis turbine runner blade
modified to mitigate pressure pulsations and noise associated with hydraulic
instabilities inside the runner which reduces pressure pulsations generated due
to draft tube vortex rope formed at the exit of the runner as well as due to the
runner trailing edge vortices.
SUMMARY OF THE INVENTION
As per the present invention, the runner blade is configured such that:
a) the sections of the runner blade are stacked in a particular fashion at the
leading edge such that the intensity of interaction between the guide vane
trailing edge vertex and the leading edge is reduced
b) the meridional shape and the outlet blade angles of the blade are designed
such that the absolute outlet circumferential velocity follows a particular
relationship from the hub to shroud, hence mitigating the severity of the draft
tube rope & the pressure fluctuations associated with it.

c) the meridional shape and blade outlet angles have been designed such that
the fluid column travel length and velocity along the streamwise direction at
different blade sections is varied to ensure the vortex shedding at the runner
trailing edge occurs at different instants of time from different sections in the
spanwise direction.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Rgure 1 shows a typical Francis turbine cross-section.
Figure 2 shows a leading edge of a turbine blade as per Prior Art (Plan View of
blade).
Rgure 3 shows a leading edge of the turbine blade as per Present Invention
(Plan view of blade)
Rgure 4 shows an illustration of parameter 'φ'.
Rgure 5 shows a comparison of Prior art & Present Invention in respect of
spanwise location of the leading edge.
Rgure 6 shows an embodiment of the Present Invention (modified turbine
runner blade).
Figure 7 shows a comparison of Prior art & Present Invention in respect of
spanwise location of the trailing edge.

Figure 8 shows the illustration location of the trailing edge of Blade Outlet
Angle 'β'.
Figure 9 shows a comparison of Prior art & Present Invention in respect of
DETAILED DESCRIPTION OF THE INVENTION
An advantage of the present invention is that the pressure pulsations created
due to interference between the vortex eddies created at the exit of the guide
vane and leading edge of the runner are reduced. This is achieved by designing
the blade with an improved configuration of the leading edge as shown in figure
3.
The leading edge is defined by a parameter 'φ', which is the relative stacking
angle between the hub section and successive sections progressing from the hub
to shroud (referred to as the span-wise direction) in the plan view of the blade
as shown in figure 4. As per the prior art, the parameter 'φ' remains constant
from the hub to shroud, or varies linearly from the hub to shroud as shown in
figure 5. As per the present invention, the parameter 'φ' follows a particular
pattern from hub to shroud as shown in figure 5.

Another embodiment of the present invention provides the variation of 'φ' from
hub to shroud as shown in figure 6. The maximum value of this parameter 'φmax'
should not be greater than 3.5°.
Another embodiment of the present invention provides a range for the maximum
value of the parameter 'φs' at shroud as sown in figure 6. The maximum value of
'φs' should not be greater than 2°.
Another embodiment of the present invention provides for the location of the
maximum value of parameter 'φ' in the spanwise direction. The maximum value
of 'φ' should be located between 60 to 75% of the span-wise distance from the
hub as shown in figure 6.
Another advantage of the present invention is that the pressure pulsations
created due to the draft tube vortex rope are reduced. This is achieved by
designing the meridional geometry of the blade, especially the trailing of the
blade, the blade outlet angle distribution and the variation of the blade outlet
angle such that the absolute circumferential velocity at the runner outlet follows
a particular pattern from the hub to shroud as shown in figure 7.
As per an embodiment of the present invention, the blade outlet angle is reduced
by an angle of 2 to 3.5° as shown in figure 9 in order to achieve the desired
outlet absolute circumferential velocity distribution as shown in figure 7.
Another advantage of the present invention is that the pressure fluctuations
created due to the vortices formed at the runner trailing edge are mitigated. This
is achieved by designing the meridional geometry of the blade & the blade outlet
angle distribution in such a manner that the vortices from different spanwise
locations of the trailing edge are shed at different instants of time so that the

pressure waves do not add on to each other. The travel length of water streams
along different spanwise locations of the blade designed as per the present
invention are such that the water streams exit from different sections of the
blade at different intervals of time.
Based on detailed CFD analysis and detailed experimental model tests that have
been carried out, it is evident that Francis turbine runners which are designed
based upon the above embodiment exhibit improved behavior with respect to
pressure pulsations. It has been established using full 3-D numerical simulation
using Computational Fluid Dynamics analysis and detailed experimental model
tests that Francis turbine runners which are designed according to the above
embodiment substantially reduce the maximum amplitude of pressure pulsations.

WE CLAIM:
1. A Francis turbine runner blade modified to mitigate pressure pulsations
and noise associated with hydraulic instabilities inside the runner, the
turbine comprising a spiral casing, a plurality of stay vanes, a plurality of
guide vanes, multiple runner blades each having at least one leading edge
of the runner and at least one trailing edge, a hub, a shroud, and a draft
tube,
the runner is characterized by a modified leading edge defined by a
parameter φ representing a relative stacking angle between the hub
section and successive sections progressing from the hub to the shroud in
a span-wise direction, wherein 'φ', varies along the span in a
predetermined relationship , wherein the maximum value of 'φ' is less
than 3.5° for specific speed 225 m-kW and higher and wherein the
maximum value of 'φ's at shroud is less greater than 2°, wherein 'φ' varies
along the span in such a way that the maximum value of 'φ' is located
between 60to 75% of the span-wise distance from the hub.
2. The runner blade as claimed in claim 1, the maximum value of 'φ's at
shroud is less than 2o, wherein 'φ' varies along the span in such a way that
the maximum value of 'φ' is located between 60 to 75% of the span-wise
distance from the hub.

3. The runner blade as claimed in claim 1, wherein the outlet angle of the
blade is reduced by an angle of 2 to 3.5° in order to achieve the desired
variation of the absolute circumferential velocity at the outlet
4. The runner blade as claimed any of the preceding claims, wherein the
configuration of the blade is characterized in that the travel time of water
streams from the hub to the shroud vary so that the vortices at the blade
trailing edge do not have the same frequency.