Understanding Cavitation in Turbines

Cavitation is a phenomenon that occurs in fluid flow systems, including hydraulic turbines. It involves the formation of vapor bubbles within the liquid (usually water) when the local pressure drops below the vapor pressure of the liquid at that temperature. These bubbles are carried along with the flow to regions of higher pressure, where they rapidly collapse (implode). This collapse generates intense shock waves and microjets, which can cause significant erosion and damage to the turbine surfaces, leading to reduced efficiency and structural failure over time.

Where Cavitation Occurs on Turbine Blades

In a turbine, water flows over the blades, and the shape of the blades influences the velocity and pressure distribution. According to Bernoulli's principle, as the velocity of the fluid increases, its pressure decreases. On the surface of a turbine blade, the flow accelerates over certain areas, causing the pressure to drop.

• The blade has two sides: the concave side (pressure side) and the convex side (suction side).
• The flow velocity is typically higher on the convex side than on the concave side.
• The blade also has an inlet (leading edge) and an outlet (trailing edge).

The highest flow velocities and thus the lowest pressures often occur on the convex side of the blade, particularly towards the outlet (trailing edge). If this pressure drops below the vapor pressure of water, cavitation bubbles will form in this region. As these bubbles are swept towards higher pressure areas, they collapse, causing damage.

Therefore, cavitation in a turbine is most likely to occur in the region of lowest pressure, which is typically found near the outlet on the convex side of the blades.

Why this Location is Prone to Cavitation

The combined effect of high velocity and the specific curvature of the blade near the outlet on the convex side creates a low-pressure zone. This low pressure makes the liquid susceptible to vaporizing and forming cavitation bubbles. The subsequent collapse of these bubbles against the blade surface causes pitting and erosion.

Based on the principles of fluid dynamics and typical pressure distributions on turbine blades, the region most susceptible to cavitation is the outlet on the convex side of the blades.