C.S. Lewis

C.S. Lewis

Thursday, July 3, 2014

Cavitation: How Bubbles Cause Massive Destruction



            Cavitation.  Not a cavity in your teeth, but an extremely powerful, destructive, explosive bubble of high-pressure gas that can destroy even the strongest of metals.  My guess is that you haven’t heard of it..  Often invisible, this occurrence can tear apart rockets and other high-powered liquid fuel projectiles.  


A vacuum pressure is any pressure that is below the atmospheric pressure (approximately 14.7 psi).  Cavitation is a vacuum force that can be up to approximately -15954 psi (Pounds per square inch) of pressure given a water flow at about 100⁰ C.  For a comparison point, a generic Air-Gun (pellet gun) operates at about 800 psi, a paintball gun operates at about 4,496 psi, and the average human blood pressure is between about 1.5 and 2.3 psi.  Obviously, this is a powerful force with immense destructive capabilities.

A vacuum can be formed from water flowing at a high pressure through a pipe, an orifice of a smaller size and/or shape than the original pipe, and back into the larger pipe diameter.  When the water is forced at a high speed through said orifice, it can cause a turbulent flow on the far side of the orifice.  As the flow lines go into the larger diameter again, a vacuum is formed in the corners of the pipe as a result of the water being forced to burst at a high pressure through the small orifice.  Temperature is a form of energy output.  When the pressure changes in the water flow occur the water absorbs and releases the energy resultant from the pressure changes.  This results in rapidly changing temperatures in the water as it flows through an orifice.  Essentially, the change in the pipe shape causes a rapid and drastic pressure and temperature change in the water flow.


It’s pretty common knowledge that a high temperature can cause a liquid to boil.  However, it is less well known that a high pressure can also cause a liquid to boil.  The increased pressure breaks the polar covalent bonds holding water molecules together and causes the liquid to vaporize. 

Cavitation is a phenomenon that happens when a pressure is lower than the liquid’s vapor pressure.  This can be seen when a high pressure rapidly releases into a low pressure region.  The rapid change in pressure can cause vaporization of the water to occur, causing bubbles to form.  These bubbles grow in size and, at a certain point, shrink and implode, releasing a high amount of energy into the surrounding area. The shrinkage is a result of the low pressure bubble being surrounded and pressed upon by a high-pressure surrounding region.  This phenomenon is called “Bubble Fusion,” or “Sonofusion.”  It is a hypothesis that a nuclear fusion reaction happens inside of extraordinarily large vapor bubbles that collapse after cavitation.  



These bubbles can flow out with the water and cause minor to major instability in the water, called a turbulent flow.  Turbulent flow can cause the water to release in spurts or randomized flow lines.  If the purpose of the liquid flow is to service an engine of some kind, the unstable flow can result in inconsistent combustion of the fuel.  In a rocket, this can cause the flight path to be altered because the combustion occurs in spurts and/or lessen the altitude that the rocket might reach.




The second way cavitation can cause problems happens if the bubbles get trapped in a pocket around which the flow lines go.  When the temperature decreases or liquid pressure surrounding the bubbles increases, the bubbles collapse in on themselves.  The energy release from the implosion is a shock wave that, when occurring repeatedly, causes a cyclic stress on any nearby surface.  Over time, this constant strain on a material can do damage to the point that it can burst under high pressure.  If this were to occur in an engine, it runs a major risk of explosion.  On a more positive note, cavitation can be used as a cleaning agent.  The shock waves loosen and remove any contaminants or particles on a surface and allow for a very intensive cleaning.



               Long story short, it really is the little things in life!  You can look at the “Big-Picture” all day long, but your amazing, futuristic, over-arching plans can be easily demolished by something on the atomic-level.