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How It Works
The cavitation number measures how close a flow is to forming vapor bubbles. It compares the difference between the local pressure and the fluid's vapor pressure to the dynamic pressure of the flow. A lower cavitation number means the fluid is closer to boiling locally, which creates destructive vapor cavities.
Engineers use it to design pumps, propellers, and valves that avoid cavitation damage. Keeping the cavitation number above a critical threshold for the specific geometry ensures reliable, erosion-free operation.
Example Problem
A pump operates with an inlet pressure of 200 kPa. The water temperature gives a vapor pressure of 2.34 kPa. The fluid density is 998 kg/m³ and the velocity at the impeller is 12 m/s. What is the cavitation number?
- σ = 2(p − pᵥ) / (ρV²) = 2(200,000 − 2,340) / (998 × 144)
- σ = 395,320 / 143,712 = 2.75
A value well above 1 suggests cavitation is unlikely at these operating conditions.
Frequently Asked Questions
What does a low cavitation number mean?
A low cavitation number means the local pressure is close to the vapor pressure relative to the flow's kinetic energy. When σ drops below a critical value (typically between 0.5 and 3 depending on geometry), vapor bubbles form and collapse violently, causing noise, vibration, and surface erosion.
How do you prevent cavitation in pumps?
Increase the inlet pressure (NPSH available), reduce fluid temperature to lower vapor pressure, or decrease flow velocity. All three actions raise the cavitation number. Most pump manufacturers specify a minimum required NPSH that corresponds to an acceptable cavitation number.
What is the difference between cavitation number and NPSH?
NPSH (Net Positive Suction Head) expresses the margin above vapor pressure in meters of head, while the cavitation number normalizes that margin by the dynamic pressure. They convey the same physical idea in different units. NPSH is more common in pump catalogs; the cavitation number is preferred in general fluid-dynamics research.
Can cavitation occur in any fluid?
Yes. Any liquid can cavitate if its local pressure drops below the vapor pressure. Water cavitates around 2.3 kPa at 20 °C, while hydraulic oil may cavitate at even lower pressures. Even liquid metals in nuclear reactors can experience cavitation under extreme flow conditions.
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