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.
