5.4-Cavitation, fluid mech
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Cavitation
5.4 Cavitation
Cavitation
is the phenomenon that occurs when the fluid pressure is reduced to the local vapor pressure and
boiling occurs. Under such conditions vapor bubbles form in the liquid, grow, and then collapse, producing
shock waves, noise, and dynamic effects that lead to decreased equipment performance and, frequently,
equipment failure. Engineers are often concerned about the possibility of cavitation, and they must design flow
systems to avoid potential problems.
Besides its deleterious effects on machinery, cavitation can also be beneficial. Cavitation is responsible for the
effectiveness of ultrasonic cleaning. Supercavitating torpedoes have been developed in which a large bubble
envelops the torpedo, significantly reducing the contact area with the water and leading to significantly faster
speeds. Cavitation plays a medical role in shock wave lithotripsy for the destruction of kidney stones.
Cavitation typically occurs at locations where the velocity is high. Consider the water flow through the pipe
restriction shown in Fig. 5.11. The pipe area decreases, so the velocity increases according to the continuity
equation and, in turn, the pressure decreases as dictated by the Bernoulli equation. For low flow rates, there is a
relatively small drop in pressure at the restriction, so the water remains well above the vapor pressure and
boiling does not occur. However, as the flow rate increases, the pressure at the restriction becomes progressively
lower until a flow rate is reached where the pressure is equal to the vapor pressure as shown in Fig. 5.11. At this
point, the liquid boils to form bubbles and cavitation ensues. The onset of cavitation can also be affected by the
presence of contaminant gases, turbulence and viscosity.
Figure 5.11
Flow through pipe restriction: variation of pressure for three different flow rates.
The formation of vapor bubbles at the restriction is shown in Fig. 5.12
a
. The vapor bubbles form and then
collapse as they move into a region of higher pressure and are swept downstream with the flow. When the flow
velocity is increased further, the minimum pressure is still the local vapor pressure, but the zone of bubble
formation is extended as shown in Fig. 5.12
b
. In this case, the entire vapor pocket may intermittently grow and
collapse, producing serious vibration problems. Severe damage that occurred on a centrifugal pump impeller is
shown in Fig. 5.13, and serious erosion produced by cavitation in a spillway tunnel of Hoover Dam is shown in
Fig. 5.14. Obviously, cavitation should be avoided or minimized by proper design of equipment and structures
and by proper operational procedures.
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Cavitation
Figure 5.12
Formation of vapor bubbles in the process of cavitation.
(a) Cavitation.
(b) CavitationÏhigher flow rate.
Figure 5.13
Cavitation damage to impeller of a centrifugal pump.
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Cavitation
Figure 5.14
Cavitation damage to a hydroelectric power dam spillway tunnel.
Experimental studies reveal that very high intermittent pressure, as high as 800 MPa (115,000 psi), develops in
the vicinity of the bubbles when they collapse 1. Therefore, if bubbles collapse close to boundaries such as pipe
walls, pump impellers, valve casings, and dam slipway floors, they can cause considerable damage. Usually this
damage occurs in the form of fatigue failure brought about by the action of millions of bubbles impacting (in
effect, imploding) against the material surface over a long period of time, thus producing a material pitting in the
zone of cavitation.
The world's largest and most technically advanced water tunnel for studying cavitation is located in Memphis,
TennesseeÏthe William P. Morgan Large Cavitation Tunnel. This facility is used to test large-scale models of
submarine systems and full-scale torpedoes as well as applications in the maritime shipping industry.
More detailed discussions of cavitation can be found in Brennen 2 and Young 3.
Copyright ¨ 2009 John Wiley & Sons, Inc. All rights reserved.
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