Francis Turbine

By Mohit Uniyal|Updated : August 26th, 2022

Francis turbines are the most prevalent water turbines in use today, with an efficiency of about 95%. Before we go into the details of the Francis turbine, let's first define a turbine. A turbine is a rotational mechanical device that transfers energy from a fluid flow into practical work. When paired with a generator, the work performed by a turbine can be used to generate electrical power. A turbine is a turbomachine with at least one moving component, the rotor assembly, which consists of a shaft or drum with blades attached. The blades move and transfer rotational energy to the rotor due to the moving fluid. Windmills and waterwheels are examples of early turbines.

Water turbines, such as the Francis turbine, are a form of water turbine. It's a reaction turbine with an inward flow that blends radial and axial features. From 1848 through around 1920, the modern Francis runner was developed. Around 1920, it was dubbed the Francis turbine after British-American engineer James B. Francis, who invented a revolutionary turbine design. Francis turbine is most commonly used to generate power. Let's look at the fundamentals of the Francis turbine and its benefits, drawbacks, and applications.

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What is the Francis Turbine?

The Francis Turbine is a hybrid of an impulse and reaction turbine. The blades revolve using the reaction and impulse force of water flowing past them to produce additional power. In hydro power plants, the Francis turbine is utilized to generate energy. The two main flow patterns on which turbines operate are radial and axial flow principles. James B. Francis, an American civil engineer from Lowell, Massachusetts, comes up with the idea of merging impulse and reaction turbines, with water entering the turbine and exiting axially.

The Francis turbine is a reaction turbine, a type of turbine in which the working fluid enters the turbine under tremendous pressure, and the energy is taken from the working fluid by the turbine blades. The fluid gives a portion of the energy due to pressure fluctuations on the turbine blades, as measured by the expression of the degree of reaction. In contrast, the turbine's volute casing extracts the remainder. Water acts on the spinning cup-shaped runner features at the exit, resulting in a low velocity and low swirl with very little kinetic or potential energy remaining.

Francis Turbine Efficiency

Francis Turbine is a highly efficient turbine that can achieve an efficiency ranging from 90% to 95%. The design of Francis turbine blades, which rotate using both reaction and impulse force of water flowing through them, is the primary reason for its superior efficiency.

The blade efficiency formula for Francis Turbine is:

ηb = e/(e + V2f2/2)

Components of Francis Turbine

Due to the utilization of these turbines, the main issue of water head availability is eliminated, as the turbine generates power using kinetic and potential energy. The Francis turbine is also known as a Mixed Flow Turbine.

Francis Turbine Diagram

Francis Turbine Diagram

Figure: Francis Turbine Diagram

The Francis turbine's main components are as follows:

  • Spiral Casing
  • Guide and Stay Vane
  • Runner Blades
  • Draft Tube

Spiral Casing

The volute casing, also known as the scroll case, is a spiral casing that wraps around the turbine's runner. It contains several openings at regular intervals along its length to allow the working fluid to impact the runner blades. Before the fluid impinges on the blades, these apertures turn the fluid's pressure energy into kinetic energy. Despite the multiple openings for the fluid to enter the blades, the velocity remains constant because the cross-sectional area of the casing diminishes uniformly around the circumference.

Guide and Stay Vane

The guide and stay vane's principal role is to transform the fluid's pressure energy into kinetic energy. It also directs the flow to the runner blades at specified angles.

Runner Blades

Runner blades are the heart of any turbine. The fluid contacts these points, and the tangential force of the impact provides torque, causing the turbine shaft to rotate. Blade angles at the inlet and outflow must be carefully designed because they are important determinants of power output.

Draft Tube

The draft tube connects the runner exit to the tail race, where the water from the turbine is released. Its primary purpose is to lower the velocity of released water to reduce kinetic energy losses at the output. This allows the turbine to be positioned above the tail water without significantly reducing the available head.

Applications, Advantages, and Disadvantages of Francis Turbine

The design of the runner blades determines the turbine's performance and efficiency. The runner blades of a Francis turbine are separated into two segments. The lower part is shaped like a little bucket and rotates the turbine using the water's impulse motion. The reaction force of water flowing through the upper half of the blades is used. The runner rotates due to the interaction of these two forces. The Francis turbine's benefits, drawbacks, and applications are described below.

Applications of Francis Turbine

  • It is one of the most energy-efficient hydro-turbines.
  • It is effective throughout a wide range of head and flow rates.
  • It is one of the most extensively utilized turbines for energy generation.

Advantages of Francis Turbine

  • Even with variable heads, it's simple to control.
  • There isn't much of a difference in efficiency over time.
  • It comes with a small runner.

Disadvantages of Francis Turbine

  • It is relatively tough to inspect.
  • The issue of cavitation.
  • Francis turbines can be severely harmed by water contaminated with dirt.
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FAQs on Francis Turbine

  • The Francis turbine is the most common turbine used to generate electricity in hydropower plants. Irrigation water pumping units also use mixed flow turbines to pump water from the earth for irrigation. It works well at a variety of water heads and flow rates. It's the most efficient hydro-turbine we've seen so far.

  • The Kaplan turbine has better efficiency than the Francis turbine. In comparison to the Francis turbine, the Kaplan turbine has a smaller cross-section and a lower rotating speed. Water flows axially in and axially out in a Kaplan turbine, but radially in and axially out in a Francis turbine.

  • The Pelton wheel is a type of impulse turbine that converts the static pressure head of water into mechanical energy. Francis and Kaplan turbines, on the other hand, are reaction turbines that turn the dynamic pressure head of water into mechanical energy.

  • The Francis turbine is a hybrid of an impulse and reaction turbine, in which the blades revolve using both the reaction and impulse force of water flowing past them to produce additional power.

  • The draft tube's main function is to transform kinetic energy from water into pressure energy. The pipe is used to gradually increase the cross-sectional area of the water to reduce the velocity and pressure of the water before it enters the tailrace.

  • The specific speed range of the Francis turbine is 60-300 rpm.

  • The Francis Turbine is a type of reaction turbine

  • The number of blades in the Francis turbine runner is between 16 - 24.

  • The type of water flow experienced by the Francis turbine is a radial flow.

  • Francis turbine is usually used for the medium head.

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