Cavitation

Cavitation is the phenomenon where small and largely empty cavities are generated in a fluid, which expands to large size and then rapidly collapse, producing a sharp sound. In simple terms, when a liquid is subjected to a low pressure above a threshold it ruptures and forms vaporous cavities. This phenomenon is termed as Cavitation. Cavitation occurs only when a liquid is subjected to rapid changes of pressure causing the formation of cavities in the lower pressure regions of the liquid. Formation of vapour bubbles within a liquid at low-pressure regions occurs in places that have been accelerated to high velocities, as in the operation of centrifugal pumps, water turbines and marine propellers.

Problems

Cavitation is undesirable because it produces extensive erosion of the rotation blades, additional noise from the resultant knocking and vibrations and a significant reduction of efficiency because it disturbs the flow pattern. When the cavitation collapses, they focus liquid energy to very small volumes. Thereby, Spots of high temperature are created and shock waves are emitted which are the source of noise. The noise created by cavitation is a particular problem in submarines. The collapse of cavities involves very high energies and can cause major damage.

Types

Some of the types of capitation may include inertial cavitation, non-inertial cavitation, hydrodynamic cavitation, chemical engineering applications, biomedical application, suction cavitation, discharge cavitation, cavitation in engines, vascular plants, marine life and coastal erosion.

 Reasons for Cavitation

The cavities forms due to five basic reasons and it’s common to all general classification of cavitation. They are Vaporization, air ingestion, internal recirculation, flow turbulence and the wane passing syndrome. A fluid vaporizes when its pressure becomes too low or high. All centrifugal pumps have a required head at the suction side of the pump to prevent to prevent this vaporization. A centrifugal pump can handle 0.5% air by volume. At 6% air the results can be disastrous. This condition is visible on the leading edge of the impeller, close to the outside diameter, working its way back to the middle of the vane. It can also be found at the suction eye of the pump. Liquid has to flow through a piping at a constant velocity. Corrosions or obstructions can change the velocity of this liquid. Vane passing syndrome is caused when the OD of the impeller passes too close to the pump cutwater.

 Benefits

In military, super cavitating torpedoes which are used by the military can move faster in underwater, perhaps even at supersonic speed. In industries cavitation can be a boon in cleaning ultrasonic devices. Cavitating water purification devices have also been designed.

Questions:

• What is “Cavitation”?
• What are the basic reasons for cavitation?