What is a Reaction turbine?
A reaction turbine generates power from the combined forces of pressure and moving water. A runner is placed directly in the water stream, allowing water to flow over the blades rather than striking each individually.
Reaction turbines are generally used for sites with lower head and higher flows and are the most common type currently used in the United States.
A reaction turbine is a type of turbine that develops torque by reacting to the pressure or weight of a fluid. The operation of reaction turbines is described by Newton’s third law of motion (action and reaction are equal and opposite).
In a reaction turbine, unlike in an impulse turbine, the nozzles that discharge the working fluid are attached to the rotor. The acceleration of the fluid leaving the nozzles produces a reaction force on the pipes, causing the rotor to move in the opposite direction to that of the fluid.
The pressure of the fluid changes as it passes through the rotor blades. In most cases, a pressure casement is needed to contain the working fluid as it acts on the turbine; in the case of water turbines, the casing also maintains the suction imparted by the draft tube.
Alternatively, where a casing is absent, the turbine must be fully immersed in the fluid flow as in the case of wind turbines. Francis turbines and most steam turbines use the reaction turbine concept.
How Does a Reaction Turbine Work?
In a reaction turbine, the blades sit in a much larger fluid volume and turn around as the fluid flows past them. A reaction turbine doesn’t change the direction of the fluid flow as drastically as an impulse turbine: it simply spins as the fluid pushes through and past its blades.
Wind turbines are perhaps the most familiar examples of reaction turbines.
If an impulse turbine is like kicking soccer balls, a reaction turbine is more like swimming—in reverse. Let me explain! Think of how you do freestyle (front crawl) by hauling your arms through the water, starting with each hand as far in front as you can reach and ending with a “follow-through” that throws your arm well behind you.
What you’re trying to achieve is to keep your hand and forearm pushing against the water for as long as possible, so you transfer as much energy as you can in each stroke.
A reaction turbine is using the same idea in reverse: imagine fast-flowing water moving past you so it makes your arms and legs move and supplies energy to your body! With a reaction turbine, you want the water to touch the blades smoothly, for as long as it can, so it gives up as much energy as possible.
The water isn’t hitting the blades and bouncing off, as it does in an impulse turbine: instead, the blades are moving more smoothly, “going with the flow.”
Turbines capture energy only at the point where a fluid touches them, so a reaction turbine (with multiple blades all touching the fluid at the same time) potentially extracts more power than an impulse turbine the same size (because usually only one or two of its blades are in the path of the fluid at a time).
types of reaction turbines
The two most common types of reaction turbines are Propeller (including Kaplan) and Francis. Kinetic turbines are also a type of reaction turbine.
Propeller Turbine
A propeller turbine generally has a runner with three to six blades. Water contacts all of the blades constantly. Picture a boat propeller running in a pipe. Through the pipe, the pressure is constant; if it wasn’t, the runner would be out of balance.
The pitch of the blades may be fixed or adjustable. The major components besides the runner are a scroll case, wicket gates, and a draft tube.
There are several different types of propeller turbines:
- Bulb turbine: The turbine and generator are a sealed unit placed directly in the water stream.
- Straflo: The generator is attached directly to the perimeter of the turbine.
- Tube turbine: The penstock bends just before or after the runner, allowing a straight-line connection to the generator.
- Kaplan Turbine: Both the blades and the wicket gates are adjustable, allowing for a wider range of operation. This turbine was developed by Austrian inventor Viktor Kaplan in 1919.
Francis Turbine
The Francis turbine was the first modern hydropower turbine and was invented by British-American engineer James Francis in 1849. A Francis turbine has a runner with fixed blades, usually nine or more.
Water is introduced just above the runner and all around it which then falls through, causing the blades to spin. Besides the runner, the other major components include a scroll case, wicket gates, and a draft tube.
Francis turbines are commonly used for medium- to high-head (130- to 2,000-foot) situations though they have been used for lower heads as well. Francis turbines work well in both horizontal and vertical orientations.
Kinetic Turbine
Kinetic energy turbines, also called free-flow turbines, generate electricity from the kinetic energy present in flowing water rather than the potential energy from the head. The systems can operate in rivers, man-made channels, tidal waters, or ocean currents.
Because kinetic systems utilize a water stream’s natural pathway, they do not require the diversion of water through man-made channels, riverbeds, or pipes, although they might have applications in such conduits.
Kinetic systems do not require large civil works because they can use existing structures, such as bridges, tailraces, and channels.
Advantages of Reaction Turbine
- It has high hydraulic efficiency.
- This requires less space.
- The reaction turbines use oil-free exhaust systems.
- It has a small size.
- It has a high capacity to use high temperature and high pressure.
- This type of turbine has a high working speed.
- It is easy to construct.
- The blade has a higher efficiency than an impulse turbine.
Disadvantages of Reaction Turbine
- This type of turbine generates thrust force.
- It confronts the problem of the cavity.
- It does not have symmetrical blades.
- These turbines require higher maintenance than impulse turbines.
- Higher maintenance costs are required.
Applications of Reaction Turbine
- Apart from cross-flow turbines, this is the only turbine to achieve optimum power output at low peak water head and high velocity, which is inefficient.
- Reaction turbines are used in wind power mills to generate electricity.
- It is the most widely used turbine for generating electricity in hydroelectric plants.
FAQs.
What is the difference between a reaction turbine and an impulse turbine?
The main difference between the impulse turbine and the reaction turbine is that, in the former, there is a pressure drop across the fixed blades only, whereas in the latter, there is a pressure drop across both the fixed and the moving blades.
Why is the Kaplan turbine called a reaction turbine?
The Kaplan turbine is an inward flow reaction turbine, which means that the working fluid changes pressure as it moves through the turbine and gives up its energy.
Is a steam turbine a reaction turbine?
Nozzles move due to both the impact of steam on them and the reaction due to the high-velocity steam at the exit. A turbine composed of moving nozzles alternating with fixed nozzles is called a reaction turbine or Parsons turbine.
Is Pelton a reaction turbine?
Unlike other types of turbines which are reaction turbines, the Pelton turbine is known as an impulse turbine. This simply means that instead of moving as a result of a reaction force, water creates some impulse on the turbine to get it to move.
What are the three types of reaction turbines?
The two most common types of reaction turbines are Propeller (including Kaplan) and Francis. Kinetic turbines are also a type of reaction turbine.