FTFLD OF INVENTION
The present invention relates to an improved runner blade for Francis type hydroturbine employed in hydroelectric Power Plants. More particularly, the invention relates to an improved shape of the Francis turbine runner wherein the runner is capable of allowing greater discharge to flow through it for a given runner size, hence resulting in greater power output for a smaller runner diameter.
BACKGROUND OF INVENTION
The power output achieved from a runner of a given size depends upon the discharging capacity of the runner i.e. the quantity of water that the runner allows to flow through itself. Hence, a runner of a larger size will deliver a higher power output than that which has a lower size for a given amount of discharge available. But, the cost of the power house and associated equipment increase with an increase in the size of the runner.
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. The energy transmitted to the runner depends upon

the flow rate of water flowing through it. Larger the flow rate of water, higher is the power output. But in order to increase the flow rate of water flowing through the runner, the size of the runner needs to the increased. And as the size of the runner increases, the cost of the power house and associated equipment also increases.
OBJECTS OF THE INVENTION
Hence, it is an object of this invention to provide an improved shape of Francis turbine runner blade which allows a higher discharge of water to flow through itself at high efficiency and hence, delivers higher power output for a given diameter as compared to prior art. Effectively, this would mean that the improved shape of Francis turbine runner will deliver a given output with smaller runner diameter. This would result in smaller overall size of the electro-mechanical equipment (turbine) and less cost. Hence, an improved Francis turbine runner will allow greater discharge to flow through for a given diameter, thereby increasing the power output without increasing the size and cost of the Power house and associated equipment.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig.1a - Shows a meridional view of a Francis turbine Runner blade.

Fig. lb - Shows definition of turbine blade angle
Fig.2 - Shows the relation between rate of change of and as per the
present invention and as per the prior art.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
A Francis turbine runner comprises of a crown, a plurality of blades and a band. The shape of runner blades can be constructed as a function of blade angles and a parameter Where, is defined as (Fig la)

And is Turbine blade angle as depicted in (Fig lb)
As per the present invention, Francis turbine runner blades are designed and positioned between the crown and the band such that the rate of change of blade angle follows an accelerating trend towards the band, as
against decelerating trend in case of prior art. This results in a smaller diameter runner (diameter reduced by at least 10%) for a given discharge resulting in

reduction of cost and overall size. Further, the efficiency of turbine is strongly dependent on magnitude of acceleration and this acceleration varies with specific speed of turbine.
An advantage of the present invention is that the size of the turbine is reduced for a given discharge and power output.
In order to achieve this following two conditions are to be essentially met: Turbine blade angle should be defined such that the rate of change of towards the band should have an accelerating trend i.e. the rate of change of should increase with towards the band, where is defined as

Where, M= Meridional length
R = Distance from the axis of rotation
The definition of all above parameters has been explained in Figure la In order to achieve a runner with higher discharging capacity as compared to present art, the rate of change of blade angles should increase (accelerating trend) with respect to especially towards the band side of the blade (Fig 2).

Expressing mathematically,

Further, the magnitude of acceleration varies with specific speed of turbine and efficiency is dependent on this magnitude. A careful consideration of maximum allowable acceleration for a given specific speed turbine is required by carrying out detailed CFD analysis.
Quantification of for the improved Francis turbine:-
a) The value of especially towards the outlet side of the band beyond a location corresponding to a value of equal to 7.5
b) The value of should not be increased beyond 0.5 in order to prevent the chances of cavitation
Hence quantifying,

One imminent possibility of having an accelerating is that such runners can be prone to cavitation. To avoid chances of cavitation another necessary condition is that the value of at Band should be greater than 10.

Expressing mathematically,

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 higher discharging without sacrificing anything with respect to performance parameters like efficiency or cavitation. 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 have a higher discharging capacity as compared to prior art. Consequently to pass a given discharge the runner diameter is reduced by around 10%. Quantification for for the improved Francis turbine
The value of V should be in the range of at band side of the blade.
are geometrical parameters used for defining the geometry of a
Francis turbine blade.
is defined as the blade angle i.e. the angle between the tangent to the blade surface at a particular point and the circumferential direction.
is a non-dimensional parameter used to define the meridional distance of a particular point on the blade surface from the runner axis.

are geometrical parameters which can be used to represent any runner blade of the Francis turbine blade family. This means using any Francis blade can be represented.
The uniqueness in the present invention is not in the value of but the
novel conditional relationship of both these parameters and the correlation between these two. All Francis turbine blades including the prior art and the present invention can be defined using the parameters but the stated
conditional relationship in form of a mathematical relation and a trend curve between these parameters is the basic idea in the present invention. The values of are not significant here, but the trend of variation of with respect
to is unique.
A meridional plane is a plane containing the axis of rotation of the runner. Meridional view is a half cross-sectional view along a meridional plane i.e. a plane containing the center line of the Francis turbine runner. R is defined as the distance from the axis of rotation M is the Meridional length
Ri is defined as the distance of a typical point from the axis of rotation Ri+1 is defined as the distance of the next point from the axis of
rotation
Mi is the distance between two consecutive points i.e. point in the
meridional view

In order to achieve a runner with higher discharging capacity as compared to present art, the rate of change of blade angles should increase (accelerating trend) with respect to especially towards the band side of the blade. Figure 2 shows that in case of the prior art, the rate of change of decreases as we move towards the band. However, as per the present invention, the rate of change of should increase towards the band.

If we divide the blade into a number of sections in the meridional view and each section into a number of points, then a is evaluated by the summation of

The value of 10 has been arrived upon based upon our experience and experimentation with a number of different designs. Based upon our experience, we have found that if the value of is increased beyond 10, then the velocity of water towards the band increases too much and leads to increased chances of cavitation. Hence, in order to achieve high discharging and still avoid cavitation problems, the values of should not increase beyond 10.

WE CLAIM
1. An improved runner blade for Francis type hydroturbine employed in
hydroelectric Power Plants, comprising and characterized in that,
a runner blade with changed blade angles with an accelerating trend of increase of the said angle with respect to especially toward the band side of the blade wherein the value of at band is made greater than 10 for avoiding the chances of cavitation where the said runner blades positioned between the crown and the band, has an accelerating trend of the rate of change of blade angle with wherein the said improved runner allows a higher discharge of water to flow through for delivering higher power output.
2. An improved runner blade as claimed in claim 1, wherein the value of
especially towards the outlet side of the band beyond a location
corresponding to a value of equal to 7.5 where the value of does not increase beyond 0.5 for preventing the chance of cavitation wherein for 0 <
has been established for an improved Francis turbine.

3. An improved runner blade as claimed in claim 1, wherein value of is in the range of at band side of the blade.