Francis Turbine free Study Notes for Diploma / BTech.

1. What is Francis Turbine ?

Francis turbine is a type of reaction turbine widely used for medium head and medium flow applications. It was developed by James B. Francis and is one of the most commonly used turbines in hydroelectric power plants.

It operates on the principle that both pressure energy and kinetic energy of water are converted into mechanical energy.

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2. Construction of Francis Turbine

The main parts are:

(a) Spiral Casing (Volute)

Surrounds the runner.
Its area decreases gradually to maintain uniform velocity.
Distributes water evenly around the runner.

(b) Guide Vanes (Stay Vanes & Wicket Gates)

Control the flow and direction of water.
Convert pressure energy into kinetic energy.
Adjustable to regulate load.

This image showing Francis turbine components.

(c) Runner

Heart of the turbine.
Consists of curved blades mounted on a shaft.
Converts hydraulic energy into mechanical energy.

(d) Draft Tube

Connected at the outlet of the runner.
Converts kinetic energy at outlet into pressure energy.
Helps in increasing efficiency.

(e) Shaft and Bearings

Transmit power from runner to generator.

3. Working Principle of Francis Turbine

Water enters the spiral casing under pressure. It flows through guide vanes, where:

Pressure decreases
Velocity increases
Direction is adjusted properly

Water strikes the runner blades, causing it to rotate. Both pressure and velocity energy are transferred to the runner. Water exits through the draft tube, where velocity is reduced and pressure is partially recovered.

4. Working Features

Flow is radial at inlet and axial at outlet (mixed flow turbine).
Operates fully submerged.
Suitable for medium head (30 m to 300 m).

5. Velocity Triangles

5.1. Inlet Velocity Triangle (At Runner Entry)

At the inlet of a Francis turbine, water enters the runner from the guide vanes at an angle. The velocity triangle consists of:

V₁ (Absolute Velocity): Velocity of water leaving the guide vane
Vf₁ (Flow Velocity): Component of V₁ in radial direction
Vw₁ (Whirl Velocity): Tangential component of V₁ (responsible for torque)
U₁ (Blade Velocity): Peripheral velocity of runner at inlet
Vr₁ (Relative Velocity): Velocity of water relative to moving blade

This is a schematic diagram of velocity analysis.

5.2 Outlet Velocity Triangle (At Runner Exit)

At the outlet, water leaves the runner after transferring energy:

V₂ (Absolute Velocity at outlet)
Vf₂ (Flow Velocity at outlet)
Vw₂ (Whirl Velocity at outlet)
U₂ (Blade Velocity at outlet)
Vr₂ (Relative Velocity at outlet)

6. Efficiency of Francis Turbine

(a) Hydraulic Efficiency

$\eta_h = \frac{\text{Power delivered to runner}}{\text{Water power at inlet}}$ (b) Mechanical Efficiency

$\eta_m = \frac{\text{Shaft power}}{\text{Runner power}}$ (c) Overall Efficiency

$\eta_o = \eta_h \times \eta_m$ Typical efficiency: 85% to 95%

7. Advantages and Disadvantage of Francis Turbine

Advantage	Disadvantage
High efficiency over a wide range of loads. Compact design compared to impulse turbines. Suitable for variable load conditions. Requires less space	Complex design and construction. Requires careful installation. Cavitation may occur if not properly designed.

8. Applications

Hydroelectric power plants.
Pumped storage plants.
Medium head water resources.

Examples:

Widely used in dams and hydro projects across India and worldwide.

9. Comparison with Other Turbines

Feature	Francis Turbine	Pelton Turbine	Kaplan Turbine
Type	Reaction	Impulse	Reaction
Flow	Mixed	Tangential	Axial
Head	Medium	High	Low
Efficiency	High	High	Very High