Short Answer:

Guide vanes are fixed or adjustable blades placed around the runner of a reaction turbine. Their main function is to control and direct the flow of water onto the turbine runner blades at the correct angle. They also regulate the quantity of water entering the runner, depending on the load conditions of the turbine.

In simple words, guide vanes act like flow controllers that ensure the water strikes the turbine blades efficiently. By controlling both the direction and amount of water, guide vanes help in achieving smooth operation and better efficiency of the hydraulic turbine.

Detailed Explanation :

Function of guide vanes

Guide vanes play a very important role in the efficient working of reaction turbines such as the Francis or Kaplan turbine. They are arranged uniformly around the runner and are usually adjustable, allowing the flow rate and direction of water to be controlled according to the power requirement. The proper functioning of guide vanes ensures that water enters the runner blades at the correct angle and velocity, which is essential for smooth turbine operation and high efficiency.

The main function of guide vanes is to regulate and direct the water flow entering the runner. In a reaction turbine, the energy of water consists of both kinetic and pressure energy. The guide vanes help in converting part of the pressure energy into kinetic energy by controlling the velocity and direction of flow before it strikes the runner blades. This ensures that the turbine operates under the desired flow condition for maximum power output.

Guide vanes are mounted inside a component called the guide mechanism or spiral casing, which distributes water evenly around the turbine. Each guide vane is attached to a pivot so that its angle can be changed by means of a control ring or regulating mechanism. This system allows the operator or automatic governor to adjust the vane opening depending on the load on the turbine. When the power demand increases, the vanes open wider to allow more water to flow through; when the demand decreases, they close partially to reduce flow.

The functions of guide vanes can be explained in the following points:

1. Direction Control:
   Guide vanes direct the water flow so that it enters the runner blades at the most efficient angle. Incorrect direction can cause losses due to shock and turbulence, reducing turbine efficiency.
2. Flow Regulation:
   The guide vanes act as regulating devices that control the amount of water entering the runner. By adjusting the opening of the vanes, the discharge through the turbine can be increased or decreased depending on the load requirement.
3. Energy Conversion:
   As the water passes through the guide vanes, its pressure energy is partially converted into kinetic energy. This prepares the water for efficient impact on the runner blades, improving energy transfer.
4. Protection Against Overload:
   The guide vanes prevent excessive water from entering the turbine during low load conditions, thereby protecting the runner and other components from mechanical stress and damage.
5. Maintaining Efficiency:
   By ensuring that water always enters the runner at the correct velocity and angle, the guide vanes maintain the overall hydraulic efficiency of the turbine across different operating conditions.

The working of guide vanes can be described as follows: when water enters the spiral casing, it is distributed around the runner. The guide vanes are positioned between the casing and the runner. As the water passes through the adjustable guide vanes, its flow direction and speed are modified. This controlled jet of water then strikes the runner blades, producing torque and causing rotation of the turbine shaft. The governor mechanism automatically adjusts the vane opening to maintain a constant turbine speed even when the load changes.

The design of guide vanes is based on hydraulic and mechanical considerations. The vanes must be strong enough to withstand the high pressure of flowing water, and their shape should minimize flow separation and energy loss. Generally, the vanes are made of stainless steel or bronze for strength, durability, and resistance to corrosion. The surfaces are made smooth and streamlined to ensure laminar flow and reduce friction losses.

Guide vanes are particularly important in Francis turbines, where both pressure and velocity energy of water are utilized. The turbine operates efficiently only if the water enters the runner without shock, which depends on the precise setting of guide vane angles. In Kaplan turbines, the guide vanes work together with adjustable runner blades to achieve efficient performance under varying load conditions.

The performance of a turbine can be severely affected if the guide vanes are not functioning properly. Improper vane alignment can cause cavitation, vibration, and loss of efficiency. Therefore, proper maintenance and regular inspection of the guide vane mechanism are essential for reliable operation.

The advantages of using guide vanes include:

• Better control over turbine speed and output.
• Improved hydraulic efficiency.
• Reduction of energy losses due to turbulence.
• Flexibility to operate efficiently under variable load conditions.
• Extended turbine life due to controlled water impact.

Thus, guide vanes serve as both regulators and directors of water flow in reaction turbines. They form an essential part of the turbine’s control system and ensure that the turbine works efficiently, safely, and smoothly under all operating conditions.

Conclusion:

Guide vanes are an essential part of reaction turbines, responsible for directing and regulating the flow of water to the runner blades. They convert pressure energy into kinetic energy, control flow rate, and ensure smooth water entry at the correct angle. Proper design and adjustment of guide vanes help in achieving maximum efficiency, stable operation, and protection of turbine components.