Cavitation In Depth

Part 1 - The Cavitation Process

    The Latin word "cavus" means hole or cavity, referring to one feature of the cavitation process, the existence of cavities within the liquid.  The interior conditions of these cavities are one of the important differences between cavitation "bubbles" and gas bubbles.

The multi-stage process of cavitation in a pump is described as follows:

1. Local pressure in a liquid flow falls below the vapor pressure for that liquid.

2. Molecules exposed to low pressure change phases from liquid to gas.

3. When liquid phase molecules change to gas phase molecules, the volume they occupy increases by a large amount.  For water at sea level pressure and close to the freezing point, the volume occupied by the water molecules increases by a factor of about 1,700.

4. If conditions are conducive to further or increased cavitation, gas phase molecules coalesce or accumulate into larger and larger pockets of gas.  The gas pockets eventually accumulate into large visible structures appearing as strings, sheets, and flame like shapes.  Pockets of gas may become attached to objects in the flow path.

5. Other substances in solution within the liquid may also experience phase changes or they may diffuse out of solution.  Low pressure may cause a liquid to become supersaturated with a substance, resulting in that substance coming out of solution, either as a solid or a gas.

6. The last stage of the cavitation process begins when fluid pressure increases as the fluid moves through the pump or valve, or through or by a restriction.  Increasing fluid pressure surrounding the cavities forces cavity walls inwards, compressing the gas inside the cavity, until the vapor pressure is reached, at which point the process changes dramatically from compression to phase change.  When pressure in the gas reaches the vapor pressure, near instantaneously, the gas molecules change phases from gas phase to liquid phase.  The same factor of volume expansion now becomes the factor of contraction, i.e., the volume occupied by the molecules is reduced by a factor of 1,700 for water at near atmospheric conditions.  With gas pressure no longer supporting the interior walls of the cavity, the walls of the cavity rapidly move inwards

7. The process of liquid re-occupying the cavity is properly called an implosion, because the walls of the cavity race inwards at extremely high velocities, striking each other with extreme force, releasing extreme levels of energy, on a microscopic scale, within an extremely short time frame.

8. The implosion is a chaotic process with multiple possible results depending on many variables.

9. Some of the physical phenomena that may result are: formation of a plasma which is the fourth state of matter where electrons disassociate from atoms, light emission (sonoluminescence), extreme pressures, extreme temperatures, and shock waves.

End of Cavitation - Part 1

To learn about the extreme temperatures and pressures caused by cavitation,

Follow the Link Cavitation 2  below.

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