Short Answer:

The efficiency of a turbine is said to be maximum when it converts the maximum possible energy of the fluid into mechanical work. For achieving maximum efficiency, the velocity of the turbine runner must be properly matched with the velocity of the jet or fluid striking the blades. In general, the blade or runner speed should be about half of the jet speed for impulse turbines. The angles of the blades and the smooth flow of fluid also play an important role in maintaining high efficiency.

When these ideal conditions are met, energy losses due to friction, turbulence, and velocity mismatch are minimized. Hence, the turbine runs smoothly and delivers the highest possible output for the given head and flow rate. These conditions depend on the type of turbine, such as Pelton, Francis, or Kaplan turbine, and are determined through design and experimental optimization.

Detailed Explanation:

Conditions for Maximum Efficiency in Turbines

The main purpose of a turbine is to convert the energy of a fluid, such as water or steam, into mechanical work that can be used to generate electricity. The efficiency of a turbine depends on how effectively it converts the available energy into useful work. To achieve maximum efficiency, several conditions related to velocity, blade design, and fluid flow must be satisfied. These conditions vary slightly with different types of turbines, but the basic principle remains the same — maximum efficiency occurs when energy conversion losses are minimum.

1. Velocity Relationship Between Jet and Runner

For maximum efficiency, the velocity of the runner or blade (V) should be a certain fraction of the velocity of the jet (V₁) striking it.

• In an impulse turbine (like the Pelton wheel), maximum efficiency occurs when the runner speed (V) is half of the jet speed (V₁/2).
• In this condition, the relative velocity of the fluid after striking the blade becomes nearly zero, meaning that most of the fluid’s kinetic energy is transferred to the runner.
• Mathematically, this can be expressed as:

This condition ensures that there is no wasted kinetic energy in the outgoing jet.

2. Perfect Jet Striking and Smooth Flow

The jet of water or fluid should strike the bucket or blade exactly at the designed angle.

• The entry angle of the jet should be such that it strikes the blade tangentially and smoothly, without splashing or turbulence.
• Any misalignment of the jet or rough surface of the blade can cause energy loss due to friction or turbulence, which reduces efficiency.
• The flow of the fluid should remain smooth and uniform throughout the passage over the blades.

3. Correct Blade Angles

The blade angles or deflection angles of the turbine play an important role in energy transfer.

• For maximum efficiency, the blade must be designed to deflect the jet by nearly 180° (for impulse turbines like Pelton wheel).
• In reaction turbines such as Francis or Kaplan turbines, the inlet and outlet angles of the blades must be adjusted so that the fluid leaves the blades without shock or eddy formation.
• When the fluid leaves the blades with minimum velocity and without turbulence, energy losses are minimized, ensuring higher efficiency.

4. Reduction of Mechanical and Hydraulic Losses

The efficiency of a turbine is also affected by mechanical and hydraulic losses.

• Mechanical losses occur due to bearing friction and windage losses in rotating parts.
• Hydraulic losses occur due to leakage, turbulence, and shock at the blade entry and exit.
• To achieve maximum efficiency, both types of losses should be reduced by proper lubrication, accurate alignment, and smooth surface finishing of components.

5. Proper Head and Flow Rate

The head (height of water level) and discharge rate (flow of fluid) must be suitable for the turbine design.

• For a given turbine, there exists an optimum combination of head and flow rate at which efficiency is maximum.
• Operating the turbine outside this range (either too low or too high flow) results in energy losses and reduced performance.

6. Matching of Speed with Load

The turbine should run at a speed corresponding to the designed load conditions.

• If the turbine runs too fast or too slow compared to its design speed, it can lead to unbalanced forces and energy losses.
• Governors and control systems are used to maintain constant speed under varying load conditions, which helps maintain maximum efficiency.

7. Clean and Well-Maintained Working Conditions

For consistent maximum efficiency, the turbine must be kept clean and well-maintained.

• Sediments, dirt, or corrosion on the blades can increase surface roughness and reduce the effective energy transfer.
• Regular inspection and cleaning help maintain the smoothness of the blades and the flow passage, ensuring higher efficiency over time.

Conclusion:

Maximum efficiency in turbines is achieved when all energy losses are minimized, and the energy of the fluid is converted into mechanical work as effectively as possible. The ideal condition is when the velocity of the runner is a proper fraction of the jet velocity, the blade angles are correct, and the flow remains smooth and steady. Proper design, alignment, maintenance, and operation under rated conditions help maintain this efficiency. Therefore, understanding and maintaining these conditions is essential for achieving the best turbine performance and long-term energy savings.