Use of draft tube in Francis turbine to increase output

In summary, the draft tube in a Francis turbine increases the power output by increasing the available head due to a decrease in static pressure at the runner outlet. This is because the power output is dependent on the blade RPM, which is constant, and the inlet velocity, which is directly proportional to the tangential velocity component. Without a draft tube, cavitation can occur within the turbine, resulting in lower power generation.
  • #1
the_kool_guy
37
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Work output(deal) for a hydraulic machine per kg of incoming fluid is V(TANGENTIAL)*SPEED OF BLADE (assuming zero out swirl velocity.

Draft tube causes static pressure ont runner outlet to fall below zero. Thus available head is increased by an amount equal to this drop. But power output from francis turbine is dependent on blade rpm( which is constant as per electricity frequency) and inlet velocity.
How does draft tube increases power output of a francis turbine if flow of water is constant . ( this flow rate is directly proportional to tangential velocity component )...

My alternate question is on the values of power generated in the absence of draft tube ...
 
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  • #2
See the website below. It states that the Draft Tube is used to reduce or eliminate cavitation within the turbine.

http://www.learnengineering.org/2014/01/how-does-francis-turbine-work.html
 

1. How does a draft tube increase the output of a Francis turbine?

A draft tube is a conical or gradually expanding tube that is attached to the outlet of a Francis turbine. As the water exits the turbine through the draft tube, it expands and slows down, converting the kinetic energy of the water into potential energy. This increase in potential energy leads to a higher output of the turbine.

2. What is the purpose of a draft tube in a Francis turbine?

The main purpose of a draft tube is to increase the efficiency and output of a Francis turbine. It also helps to reduce the pressure at the outlet of the turbine, which can help to prevent cavitation and damage to the turbine blades.

3. How does the design of a draft tube impact its effectiveness?

The design of a draft tube, including its length, angle, and shape, can greatly impact its effectiveness. A longer and more gradually expanding draft tube can create a smoother flow of water and increase the potential energy gained. The angle of the draft tube should also be carefully designed to prevent backflow and maintain a consistent flow of water.

4. Are there any disadvantages to using a draft tube in a Francis turbine?

Although a draft tube can greatly improve the efficiency of a Francis turbine, there are some potential disadvantages to consider. The installation and maintenance of a draft tube can be costly and require additional space. In some cases, the draft tube may also cause unwanted vibrations or noise.

5. Can a draft tube be used in other types of turbines?

While draft tubes are commonly used in Francis turbines, they can also be utilized in other types of turbines such as Kaplan and Pelton turbines. However, the design and placement of the draft tube may vary depending on the specific type of turbine and its operating conditions.

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