Cavitation refers to the formation of cavities in fluids. Cavities start to from when the local static pressure within a fluid falls below a critical value. This critical value is generally about as high as the vapour pressure for the fluid.
Cavitation may take the form of:
- flow cavitation
- oscillation cavitation
- bubble cavitation.
Flow cavitation is of particular significance as far as fluid technology is concerned. It mainly occurs where cross sections narrow or widen or where there is a sudden change in flow direction. When cross sections narrow, the static pressure decreases as the flow velocity increases and the static pressure is therefore converted into dynamic pressure (flow energy). Vortices will mainly form after cross sections widen, with static pressure falling to the critical value at their centre due to the fluid's high circumferential velocity. A similar effect occurs after sudden changes in direction.
A distinction is made between vapour and gas cavitation, depending on whether the cavitation bubbles contain fluid vapour or a gas such as air dissolved in the fluid. For example, water only dissolves small quantities of air. This is why the cavitation to be found in water is generally vapour cavitation. By contrast, mineral oil dissolves much larger quantities of air (around 9% by volume under normal conditions in terms of the Bunsen coefficient). As far as cavitation in mineral oil is concerned, it is possible to assume therefore that a mixture of vapour and gas cavitation is involved.
Significant effects of cavitation are:
- cavitation erosion,
- undissolved gas in the hydraulic system
- loud, high-frequency noise
- high temperatures at certain places in the fluid and
- changes in the resistance characteristics of hydraulic resistances