Evolution from Francis turbine to Kaplan turbine

Last updated November 05, 2019

Francis turbine converts energy at high pressure heads which are not easily available and hence a turbine was required to convert the energy at low pressure heads, given that the quantity of water was large enough. It was easy to convert high pressure heads to power easily but difficult to do so for low pressure heads. Therefore, an evolution took place that converted the Francis turbine to Kaplan turbine, which generated power at even low pressure heads efficiently.

The Francis turbine is a type of water turbine that was developed by James B. Francis in Lowell, Massachusetts. It is an inward-flow reaction turbine that combines radial and axial flow concepts.

A turbine is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating electrical power when combined with a generator. A turbine is a turbomachine with at least one moving part called a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades so that they move and impart rotational energy to the rotor. Early turbine examples are windmills and waterwheels.

Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure.

Changes

Turbines are sometimes differentiated on the basis of the type of inlet flow, whether the inlet velocity is in axial direction, radial direction or a combination of both. Francis turbine is a mixed hydraulic turbine (the inlet velocity has Radial and Tangential Components) while the Kaplan turbine is an Axial hydraulic turbine(the inlet velocity has only Axial Velocity Component). The evolution consisted of the change in the inlet flow mainly.

The velocity of an object is the rate of change of its position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed and direction of motion. Velocity is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of bodies.

The image describes the changes in velocity triangles on decreasing the Specific speed or decreasing the Pressure Head and finally shows the evolution from Francis Hydraulic turbine to Kaplan Hydraulic turbine. Different Velocity Triangles on increasing the Specific speed.jpg — The image describes the changes in velocity triangles on decreasing the Specific speed or decreasing the Pressure Head and finally shows the evolution from Francis Hydraulic turbine to Kaplan Hydraulic turbine.

Nomenclature of a Velocity Triangle:

A general velocity triangle consists of the following vectors:^[1]^[2]

In turbomachinery, a velocity triangle or a velocity diagram is a triangle representing the various components of velocities of the working fluid in a turbomachine. Velocity triangles may be drawn for both the inlet and outlet sections of any turbomachine. The vector nature of velocity is utilized in the triangles, and the most basic form of a velocity triangle consists of the tangential velocity, the absolute velocity and the relative velocity of the fluid making up three sides of the triangle.

V : Absolute velocity of the fluid.
U : Tangential velocity of the fluid.
V_r: Relative velocity of the fluid after contact with rotor.
V_w: Tangential component of V (absolute velocity), called Whirl velocity.
V_f: Flow velocity (axial component in case of axial machines, radial component in case of radial machines).
α: Angle made by V with the plane of the machine (usually the nozzle angle or the guide blade angle).
β: Angle of the rotor blade or angle made by relative velocity with the tangential direction.

Generally, the Kaplan turbine works on low pressure heads (H) and high flow rates (Q). This implies that the Specific speed (N_s) on which Kaplan turbine functions is high as Specific speed (N_sp) is directly proportional to Flow (Q) and inversely proportional to Head (H). On the other hand, Francis turbine works on low Specific speeds i.e., high pressure heads.

Specific speedN_s, is used to characterize turbomachinery speed. Common commercial and industrial practices use dimensioned versions which are of equal utility. Specific speed is most commonly used in pump applications to define the suction specific speed —a quasi non-dimensional number that categorizes pump impellers as to their type and proportions. In Imperial units it is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute against one foot of hydraulic head. In metric units flow may be in l/s or m³/s and head in m, and care must be taken to state the units used.

The Kaplan turbine is a propeller-type water turbine which has adjustable blades. It was developed in 1913 by Austrian professor Viktor Kaplan, who combined automatically adjusted propeller blades with automatically adjusted wicket gates to achieve efficiency over a wide range of flow and water level.

In the figure, it can be seen that the increase in Specific speed (or decrease in Head) have following consequences:

A reduction in inlet velocity V₁ .
The flow velocity V_f1 at inlet increases, and hence allows a large amount of fluid to enter the turbine.
V_w component decreases as moving to Kaplan turbine, and here in the figure, V_f represents axial (V_a) component.
The flow at inlet, in the figure, to all the runners, except the Kaplan impeller, is in radial (V_f)and tangential (Vw) directions.
β₁ decreases as the evolution proceeds.
However, the exit velocity is axial in Kaplan runner, while it is the radial one in all other runners.

Hence, these are the parameter changes that has to be incorporated in converting a Francis turbine to Kaplan turbine.

General differences between Francis and Kaplan turbines

Efficiency of Kaplan turbine is higher than Francis turbine.
Kaplan turbine is more compact in cross-section and has lower rotational speed to that of Francis turbine.
In Kaplan turbine, the water flows axially in and axially out while in Francis turbine it is radially in and axially out.
The runner blades in the Kaplan turbine are less in number as the blades are twisted and covers a larger circumference.
Friction losses in Kaplan turbine are less.
Position of Shaft Francis Turbine generally vertical but Some time horizontal also, and Kaplan Turbines are only vertical Position.
The Francis Turbines specific speed is high (75 - 1000rpm) and Kaplan Turbines are medium (54.5 - 450rpm).

Notes

↑ Venkanna, B.K. (2011). Fundamentals of Turbomachinery. Prentice Hall India. ISBN 978-81-203-3775-6.
↑ Govinde Gowda, M.S. (2011). A Text book of Turbomachines. Davangere: MM Publishers.

Related Research Articles

The Tesla turbine is a bladeless centripetal flow turbine patented by Nikola Tesla in 1913. It is referred to as a bladeless turbine. The Tesla turbine is also known as the boundary-layer turbine, cohesion-type turbine, and Prandtl-layer turbine because it uses the boundary-layer effect and not a fluid impinging upon the blades as in a conventional turbine. Bioengineering researchers have referred to it as a multiple-disk centrifugal pump. One of Tesla's desires for implementation of this turbine was for geothermal power, which was described in Our Future Motive Power.

A water turbine is a rotary machine that converts kinetic energy and potential energy of water into mechanical work.

Centrifugal compressors, sometimes called radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery.

An axial compressor is a gas compressor that can continuously pressurize gases. It is a rotating, airfoil-based compressor in which the gas or working fluid principally flows parallel to the axis of rotation, or axially. This differs from other rotating compressors such as centrifugal compressor, axi-centrifugal compressors and mixed-flow compressors where the fluid flow will include a "radial component" through the compressor. The energy level of the fluid increases as it flows through the compressor due to the action of the rotor blades which exert a torque on the fluid. The stationary blades slow the fluid, converting the circumferential component of flow into pressure. Compressors are typically driven by an electric motor or a steam or a gas turbine.

Turbomachinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor, a compressor transfers energy from a rotor to a fluid.

An axial-flow pump, or AFP, is a common type of pump that essentially consists of a propeller in a pipe. The propeller can be driven directly by a sealed motor in the pipe or by electric motor or petrol/diesel engines mounted to the pipe from the outside or by a right-angle drive shaft that pierces the pipe.

A centrifugal fan is a mechanical device for moving air or other gases in a direction at an angle to the incoming fluid. Centrifugal fans often contain a ducted housing to direct outgoing air in a specific direction or across a heat sink; such a fan is also called a blower fan, biscuit blower, or squirrel-cage fan. These fans increase the speed and volume of an air stream with the rotating impellers.

Radial turbine turbine in which the flow of the working fluid is radial to the shaft

Aradial turbine is a turbine in which the flow of the working fluid is radial to the shaft. The difference between axial and radial turbines consists in the way the fluid flows through the components. Whereas for an axial turbine the rotor is 'impacted' by the fluid flow, for a radial turbine, the flow is smoothly orientated perpendicular to the rotation axis, and it drives the turbine in the same way water drives a watermill. The result is less mechanical stress which enables a radial turbine to be simpler, more robust, and more efficient when compared to axial turbines. When it comes to high power ranges the radial turbine is no longer competitive and the efficiency becomes similar to that of the axial turbines.

Industrial fans and blowers are machines whose primary function is to provide and accommodate a large flow of air or gas to various parts of a building or other structures. This is achieved by rotating a number of blades, connected to a hub and shaft, and driven by a motor or turbine. The flow rates of these mechanical fans range from approximately 200 cubic feet (5.7 m³) to 2,000,000 cubic feet (57,000 m³) per minute. A blower is another name for a fan that operates where the resistance to the flow is primarily on the downstream side of the fan.

In turbomachinery, Degree of reaction or reaction ratio (R) is defined as the ratio of the static pressure drop in the rotor to the static pressure drop in the stage or as the ratio of static enthalpy drop in the rotor to the static enthalpy drop in the stage.

Compounding of steam turbines is the strategy in which energy from the steam is extracted in a number of stages rather than a single stage in a turbine. A compounded steam turbine has multiple stages i.e. it has more than one set of nozzles and rotors, in series, keyed to the shaft or fixed to the casing, so that either the steam pressure or the jet velocity is absorbed by the turbine in number of stages.

In turbomachinery, the slip factor is a measure of the fluid slip in the impeller of a compressor or a turbine, mostly a centrifugal machine. Fluid slip is the deviation in the angle at which the fluid leaves the impeller from the impeller's blade/vane angle. Being quite small in axial impellers(inlet and outlet flow in same direction), slip is a very important phenomenon in radial impellers and is useful in determining the accurate estimation of work input or the energy transfer between the impeller and the fluid, rise in pressure and the velocity triangles at the impeller exit.

An axial turbine is a turbine in which the flow of the working fluid is parallel to the shaft, as opposed to radial turbines, where the fluid runs around a shaft, as in a watermill. An axial turbine has similar construction as an axial compressor, but it operates in the reverse, converting flow of the fluid into rotating mechanical energy.

Three-dimension losses and correlation in turbomachinery refers to the measurement of flow-fields in three dimensions, where measuring the loss of smoothness of flow, and resulting inefficiencies, becomes difficult, unlike two-dimensional losses where mathematical complexity is substantially less.

Blade solidity is an important design parameter for the axial flow impeller and is defined as the ratio of blade chord length to pitch.

References

Venkanna, B.K. (2011). Fundamentals of Turbomachinery. Prentice Hall India. ISBN 978-81-203-3775-6.
Govinde Gowda, M.S. (2011). A Text book of Turbomachines. Davangere: MM Publishers.
S. K. Agrawal (1 February 2001). Fluid Mechanics & Machinery. Tata McGraw-Hill Education. ISBN 978-0-07-460005-4 . Retrieved 23 May 2013.

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[venkanna-1] Venkanna, B.K. (2011). Fundamentals of Turbomachinery. Prentice Hall India. ISBN 978-81-203-3775-6.

[govindegowda-2] Govinde Gowda, M.S. (2011). A Text book of Turbomachines. Davangere: MM Publishers.