In a turbine, relative velocity refers to the velocity of the working fluid (e.g., steam, gas, or water) as observed from the perspective of the rotating turbine rotor blades.
Understanding Relative Velocity
The relative velocity is crucial for understanding how energy is transferred from the fluid to the turbine rotor, causing it to spin. It differs from the absolute velocity, which is the velocity of the fluid as seen by a stationary observer.
- Relative Velocity (w): The velocity of the fluid relative to the moving rotor blades. Its direction aligns with the tangent of the blade curvature at the point of observation.
- Absolute Velocity (v): The velocity of the fluid as measured by a stationary observer.
- Rotor Circumferential Velocity (u): The speed of the rotor blades at a specific radius. This is consistent at both the inlet and outlet of the rotor.
These three velocities are related by vector addition:
v = w + u
Where:
- v is the absolute velocity
- w is the relative velocity
- u is the rotor circumferential velocity
Importance of Relative Velocity
The design of turbine blades depends heavily on the relative velocity. Turbine blades are shaped to optimize the angle at which the fluid strikes the blades (angle β1 at the inlet, angle β2 at the outlet). Correctly managing the relative velocity ensures efficient energy transfer, minimizing losses and maximizing turbine performance. The blade angles are designed according to the relative inlet and outlet velocities.
Example
Imagine standing on a moving merry-go-round (the rotor blade) and watching someone throw a ball (the working fluid) at you.
- The speed and direction the ball appears to have to you is the relative velocity (w).
- The actual speed and direction of the ball as seen by someone standing still on the ground is the absolute velocity (v).
- Your speed on the merry-go-round is the rotor circumferential velocity (u).
The angle at which the ball hits you is critical for whether you can catch it effectively. Similarly, the turbine blades are angled to best "catch" the fluid based on its relative velocity.
In Summary
Relative velocity in a turbine is the velocity of the working fluid as experienced by the rotating blades. Understanding and managing this velocity is essential for efficient turbine design and operation.
