Short Answer:

Draft tube efficiency is the ratio of the actual increase in kinetic energy head recovered by the draft tube to the total kinetic energy head available at the inlet of the draft tube. It shows how effectively the draft tube converts the kinetic energy of water leaving the turbine into useful pressure energy. A well-designed draft tube reduces velocity losses and helps improve the overall efficiency of the reaction turbine system.

In simple words, draft tube efficiency tells us how much of the water’s velocity energy is recovered and utilized instead of being wasted. A high draft tube efficiency means that less energy is lost at the turbine exit, leading to better performance of the turbine.

Detailed Explanation:

Draft Tube Efficiency

The draft tube is an essential part of reaction turbines such as Francis and Kaplan turbines. It is a gradually expanding pipe fitted at the outlet of the turbine runner. Its main purpose is to convert the kinetic energy of the water leaving the runner into pressure energy, thereby allowing the turbine to be set above the tailrace level without loss of head. The performance of the draft tube is measured in terms of draft tube efficiency, which indicates how effectively it performs this energy conversion.

1. Meaning of Draft Tube Efficiency

Draft tube efficiency is defined as the ratio of the energy actually recovered by the draft tube to the total kinetic energy available at the turbine outlet.
It can be expressed mathematically as:

Where,

•  = Draft tube efficiency
•  = Head recovered or increase in pressure head due to the draft tube
•  = Kinetic energy head at the inlet of the draft tube

This efficiency measures how much of the fluid’s velocity head (kinetic energy) is successfully converted back into pressure head. The higher the value of draft tube efficiency, the better the draft tube design and performance.

2. Function of Draft Tube

The main function of a draft tube is to recover part of the kinetic energy of the water leaving the runner and to discharge it at a lower velocity into the tailrace.

• When water leaves the runner, it still possesses considerable velocity. If discharged directly into the tailrace, this velocity energy would be lost.
• By using a gradually diverging draft tube, the water slows down, and its velocity head is converted into pressure head.
• This helps in maintaining a low-pressure region at the runner exit, improving turbine efficiency.

Thus, the draft tube not only helps recover energy but also allows the turbine to be placed above the tailrace level, which simplifies construction and maintenance.

3. Working Principle of Draft Tube

The draft tube works on the principle of conversion of kinetic energy into pressure energy as water flows through an expanding passage.

• At the runner exit, water has high velocity and low pressure.
• As it passes through the expanding draft tube, the velocity decreases due to the increase in the cross-sectional area.
• According to Bernoulli’s theorem, when velocity decreases, pressure increases.
• This pressure rise enables recovery of part of the velocity head, which would otherwise be wasted.

The energy recovered in this way improves the total head available for power generation.

4. Factors Affecting Draft Tube Efficiency

Several factors influence the efficiency of a draft tube:

1. Shape and Design:
   • A properly shaped and smoothly diverging draft tube gives high efficiency.
   • Common types are conical (straight divergent), elbow, and Moody spreading tubes.
2. Angle of Divergence:
   • The divergence angle should not be too large. Ideally, it lies between 6° and 10°.
   • A large angle may cause flow separation and energy losses due to eddies.
3. Surface Roughness:
   • A smooth inner surface ensures less friction loss and better flow recovery.
4. Discharge Velocity:
   • Lower discharge velocity at the outlet of the draft tube results in higher pressure recovery and thus higher efficiency.
5. Operating Conditions:
   • Proper alignment, water level in the tailrace, and correct turbine speed also affect the draft tube performance.

5. Typical Efficiency Values

The draft tube efficiency usually ranges between 70% and 90%, depending on the type and design of the tube:

• Conical draft tubes: around 80–90% efficiency
• Elbow draft tubes: around 70–85% efficiency
• Moody spreading tubes: around 85–90% efficiency

Higher efficiencies indicate better energy recovery and improved overall turbine performance.

6. Importance of Draft Tube Efficiency

Draft tube efficiency is very important for the performance of reaction turbines because:

• It reduces the loss of kinetic energy of water exiting the runner.
• It increases the effective head on the turbine, thus enhancing the overall efficiency of the system.
• It allows the turbine to be installed above the tailrace level, making maintenance and inspection easier.
• It improves the power output of the turbine for the same flow conditions.

Hence, maintaining high draft tube efficiency is crucial for achieving good hydraulic performance in turbines.

Conclusion:

Draft tube efficiency represents how efficiently the draft tube converts the velocity energy of water leaving the turbine into useful pressure energy. It is defined as the ratio of head recovered by the draft tube to the total kinetic energy head at the inlet. A well-designed and properly maintained draft tube ensures maximum energy recovery, reduces losses, and increases the overall efficiency of the turbine system. In modern reaction turbines, achieving a draft tube efficiency above 85% is considered very good for practical operation.