Cavitation And Surface Tension

CAVITATION  In flowing liquids it is possible for the pressure in localized regions to reach vapor pressure thereby causing cavitation.  An important reason for vapor pressure lies in the common observation that in flowing fluids it is possible to develop very low pressure due to the fluid motion,and if the pressure is lowered to the vapor pressure.For example,this phenomenon may occur in flow through the irregular,narrowed passages of a valve or pump.When vapor bubbles are formed in a flowing fluid they are swept along into regions of higher pressure where they suddenly collapse with sufficient intensity to actually cause structural damage.The formation and subsequent collapse of vapor bubbles in a flowinf fluid,called cavitation.  Cavitation occurs when the pressure is reduced to the vapor pressure.  In such situations boiling occurs(though the temperature need not be high),vapor bubbles form,and then they collapse as the fluid moves into a region of higher pressure(lower velocity).This process can produce dynamic effects(imploding)that cause very large pressure transients in the vicinity of the bubbles.Pressures as large as 100,000psi(690Mpa) are believed to occur.If the bubbles collapse close to a physical boundary they can,over a period of time,cause damage to the surface in the cavitation area.  Cavitation can cause damage to equipment.

Cavitation damage on the hub or base plate of a centrifugal pump impeller. The two photographs are of the same area, the one on the left showing the typical cavitation pattern during flow and the one on the right the typical cavitation damage.

SURFACE TENSION The storage of energy at the surface of liquids.Surface tension has units of erg cm-2 or dyne cm-1.It arises because atoms on the surface are missing bonds.Energy is released when bonds are formed,so the most stable low energy configuration has the fewest missing bonds.Surface tension therefore tries to minimize the surface area, resulting in liquids forming spherical droplets and allowing insects to walk on the surface without sinking.Temperature and connection area effect the surface tension. =Surface Tension =>For water