Short Answer:

Draft tube efficiency is the ratio of the actual conversion of kinetic energy into pressure energy within the draft tube to the total available kinetic energy at the exit of the runner. It shows how effectively the draft tube recovers the velocity head of water leaving the turbine runner. In simple terms, higher draft tube efficiency means more energy is recovered from the water before it leaves the turbine, leading to better overall performance of the turbine system.

The draft tube helps reduce energy loss by converting the high-velocity water leaving the runner into low-velocity water at the outlet. This process increases the effective head acting on the turbine. If the draft tube efficiency is low, a portion of the kinetic energy is wasted, reducing the turbine’s total efficiency. Hence, it is an important factor in the design and performance evaluation of reaction turbines such as the Francis and Kaplan turbines.

Detailed Explanation :

Draft Tube Efficiency

Meaning of Draft Tube Efficiency
Draft tube efficiency represents how efficiently the draft tube converts the kinetic energy of water exiting from the turbine runner into pressure energy. When water leaves the runner, it still possesses considerable velocity. If this kinetic energy is not properly recovered, it will be lost. The draft tube is designed to slow down the water and recover part of this energy by converting velocity head into pressure head. The efficiency of this process is called draft tube efficiency.

It is expressed mathematically as:

In other words, it shows the fraction of kinetic energy successfully recovered by the draft tube before the water is discharged to the tailrace.

Purpose of Draft Tube

The draft tube is an essential part of reaction turbines like Francis and Kaplan turbines. It serves three main purposes:

1. It allows the turbine to be placed above the tailrace level without losing head.
2. It converts the velocity energy of the water leaving the runner into pressure energy, thus improving efficiency.
3. It helps in discharging water smoothly into the tailrace with minimum energy loss.

If the draft tube does not perform effectively, much of the kinetic energy will be wasted as turbulence and frictional losses. Hence, the efficiency of the draft tube plays a vital role in the total efficiency of the turbine.

Working Principle Related to Efficiency

When water leaves the runner, it possesses a high velocity and low pressure. The draft tube has a gradually increasing cross-sectional area. As the water flows through it, the velocity decreases, and due to the continuity principle, pressure increases. This process helps recover part of the kinetic energy.

However, during this conversion, some energy is lost due to friction and turbulence. The ratio of recovered energy to total available kinetic energy at the runner exit gives the draft tube efficiency. Ideally, the efficiency should be as high as possible (usually around 90–95%) for effective operation.

Factors Affecting Draft Tube Efficiency

1. Shape of the Draft Tube:
   The efficiency depends on the shape and design of the draft tube. Common types include straight divergent tubes, elbow-shaped tubes, and Moody spreading tubes. A well-designed shape with a small divergence angle ensures smooth flow and reduces energy loss.
2. Divergence Angle:
   If the divergence angle of the draft tube is too large, flow separation occurs, creating vortices and turbulence, which reduce efficiency. The ideal angle is generally kept between 5° to 10° to maintain smooth flow.
3. Surface Roughness:
   Smooth internal surfaces help reduce frictional losses. Rough surfaces increase energy loss and reduce the draft tube efficiency.
4. Flow Conditions:
   Flow should remain steady and laminar within the draft tube. Sudden changes in velocity or direction cause turbulence and pressure loss, lowering efficiency.
5. Height of Installation:
   The height difference between the turbine and the tailrace also affects efficiency. If the turbine is placed too high, it may create cavitation and reduce the pressure recovery.

Significance of Draft Tube Efficiency

1. Improves Turbine Efficiency:
   By recovering a major portion of the kinetic energy, the draft tube increases the net head acting on the turbine, thus improving overall turbine efficiency.
2. Reduces Energy Loss:
   Efficient draft tube design ensures that less kinetic energy is lost as the water exits the turbine.
3. Allows Higher Turbine Placement:
   It permits the turbine to be installed above the tailrace, reducing construction difficulties and making maintenance easier, without losing head energy.
4. Ensures Stable Operation:
   High draft tube efficiency ensures smooth discharge and reduces vibration and noise caused by turbulence or cavitation.

Typical Values

The draft tube efficiency generally ranges from 85% to 95%, depending on the design and working conditions. Modern Kaplan and Francis turbines are designed with optimized draft tubes to achieve higher efficiency values.

Conclusion

Draft tube efficiency is a key performance parameter in reaction turbines. It defines how effectively the draft tube converts the remaining kinetic energy into useful pressure energy. High draft tube efficiency ensures better energy recovery, improved turbine performance, and reduced energy loss. Proper design, smooth surface, correct divergence angle, and controlled flow help in achieving high efficiency. Therefore, maintaining good draft tube efficiency is essential for maximizing the overall output of a hydroelectric power plant.