Cavitation

Cavitation occurs when a pressure drop occurs within a region of a fluid to a point below the vapor pressure of the fluid at the current temperature. At this point, the state change from liquid to gas occurs, creating a bubble. The pressure drop can be achieved in numerous ways including the acceleration of the liquid, as in a waterfall or propeller, or from subjecting the liquid to an energy input, such as laser pulses, electrical discharges or acoustic fields.

After the cavities are created, the low-pressure bubbles cannot sustain their form because they are surrounded by a higher pressure fluid, and they subsequently collapse. The collapse of the bubble causes the gas inside to pressurize and heat up, and a shockwave forms with high pressure and temperature. Noise and light may also form. As mechanical parts may cause cavitation, these shockwaves can occur close to the part’s surface, causing damage and pitting. The pitted surface causes more fluid turbulence and in turn causes more cavitation. Thus, the cavitation continuously damages the surface and facilitates the corrosion of the metal.

Given the dangers of cavitation, engineers have created methods to reduce its occurrence and extend the lifetime of mechanical parts. One option is to redesign the highest risk components based on huge pressure drops and high temperatures. In some cases the difference between the static pressure and the vaporization pressure can be increased, which decreases the chance for the liquid to reach the vaporization pressure and form low-pressure bubbles. Finally, placing mechanical components in colder areas of the system or lowering the temperature of the system lowers the likelihood of cavitation because the vapor pressure is temperature dependent.