Cavitation

Hey Everyone!

So today I will be writing about a very interesting phenomenon called Cavitation. Cavitation is a very destructive effect that occurs in fluids and can cause a catastrophic failure of any solid surface in direct contact with the fluid over a short period of time.

Cavitation_Propeller_Damage
Pump Impeller damaged by Cavitation

 

To understand Cavitation, we must first understand what exactly is boiling. Boiling is the process of transition from a liquid state to a vapor state. Cavitation is directly related to the formation of bubbles during boiling.

The bubble formation during boiling a liquid, such as water, causes a decrease in the solubility of the air within it. This causes the air contained within it to be released in the form of bubbles. Due to the lighter weight of the bubble than the water surrounding it, the bubble rises rapidly to the water surface. In this instance, boiling is caused due to the introduction of heat to the water.

But, there exists another way to boil a liquid: by, lowering its pressure. It may sound counter-intuitive, but it is true. Look at the Pressure vs Temperature plot for water below:

VPGRAPH.jpg

As you can see, the black parabolic curve represents the boundary between the fluid existing as a liquid or as a vapour. The liquid can be made to convert to its vapour form by heating (red line), reduction in pressure (blue line) or by a combination of both (yellow line). Also, the pressure at any point on the black line is the vapour pressure for that temperature.

A major difference between cavitation and boiling is that the bubbles formed in boiling are due to release of air and in cavitation, the bubbles are steam bubbles.

Cavitation is the abrupt formation of vapour bubbles within any fluid due to reduction of its pressure at that point below the corresponding vapour pressure for that fluid temperature. Now, cavitation can occur in any fluid application such as pumps, turbines, propellers, etc.

In such applications, there is a rotating body such as the impeller in the pump or the propeller or runner in a turbine which rotates within the fluid. The rotation of this body causes a difference in pressure above and below the surface in contact with the liquid . If this difference in pressure causes a pressure lower than the vapour pressure of that fluid for that temperature, the fluid boils almost instantaneously and forms steam bubbles in the liquid.

So, this seems like a harmless phenomenon,right? NO. The formation of the bubbles is followed by their subsequent implosion on impacting with the rotating surface. As soon as there is an increase in the pressure of the surrounding fluid, the bubbles explode! This causes ultra-high localized pressures on the surface, leading to erosion of the surface. The pressures can be as high as 3000-4000 times the standard atmospheric pressure!

Parts damaged by cavitation generally have an eroded surface as shown in the following photos:

Cavitation damage on Turbine Blade
Cavitation damage on Pelton Wheel Bucket

Another effect of cavitation is the noise and the trail that the bubbles formed cause when a submarine travels underwater at a high speed. To counter this and to remain hidden submarines use a propeller with more blades than other ships to prevent cavitation.

Cavitation is also observed (under slow motion video) when a Mantis Shrimp attacks its prey. Check out the link at the bottom of this page to learn more about that.

To summarise, in practical applications of bodies moving through fluids, cavitation is a very prominent occurrence and special care must be taken to avoid this from happening.

Well, I hope this was interesting and helpful.

Until next time!

-Chaitanya 🙂

References and Further Links:

  1. http://web.mit.edu/hml/ncfmf/16CAV.pdf
  2. http://www.engineeringtoolbox.com/cavitation-d_407.html
  3. Cavitation Causes and Effects: https://youtu.be/oRYYP4F8LTU
  4. https://en.wikipedia.org/wiki/Cavitation
  5. http://www.mobilehydraulictips.com/7-ways-avoid-cavitation/
  6. https://pateklab.biology.duke.edu/high-speed-video-cavitation-during-mantis-shrimp-strike

    MANTIS MURDER SHRIMP (Slow Motion) – Smarter Every Day 121

    https://youtu.be/LXrxCT0NpHo

 

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