Which of the following statements best describes the impact of a draft tube's design on the efficiency of a reaction turbine?

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Question

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BHEL Supervisor Trainee Mechanical 21 Jan 2024 Official Paper

The design of the draft tube has no significant impact on the efficiency of a reaction turbine.
A draft tube with a constant cross-section throughout its length optimizes efficiency by maintaining steady flow velocity.
A draft tube with a rapidly expanding cross-section enhances efficiency by significantly reducing the velocity of the fluid.
A draft tube with a gradually expanding cross-section increases efficiency by converting kinetic energy into pressure energy.

Answer 1

Explanation:

Draft Tube:

The design of the draft tube plays a crucial role in the efficiency of a reaction turbine.
A draft tube with a gradually expanding cross-section increases efficiency by converting kinetic energy into pressure energy.
A draft tube is a conduit that connects the outlet of the turbine to the tailrace (the waterway that carries water away from the turbine).
Its primary function is to recover the kinetic energy of the water exiting the turbine and convert it into pressure energy, which can be utilized to improve the overall efficiency of the turbine.

Principle of Operation:

In a reaction turbine, water flows through the runner blades and imparts energy to them. As the water exits the turbine, it still possesses a significant amount of kinetic energy, which, if not recovered, would result in energy losses. The draft tube, with its gradually expanding cross-section, serves to decelerate the water flow gradually, allowing the kinetic energy to be converted into pressure energy. This pressure recovery process helps in enhancing the overall efficiency of the turbine by reducing the energy lost in the form of high-velocity water jets.

Key Points:

Gradual Expansion: The gradual expansion of the draft tube ensures a smooth transition of water flow, minimizing turbulence and energy losses. This design allows for a more efficient conversion of kinetic energy into pressure energy.
Pressure Recovery: By converting the kinetic energy into pressure energy, the draft tube helps maintain a higher pressure at the exit of the turbine, which can be beneficial for the subsequent stages of energy conversion or for reducing the energy required to pump the water back to a higher elevation.
Reduction in Velocity: The gradually expanding cross-section of the draft tube reduces the velocity of the water as it exits the turbine. This reduction in velocity helps in minimizing the energy losses associated with high-velocity water jets, thereby improving the overall efficiency of the turbine.