vapor pressure such that it becomes equal to or greater than the operating pressure at the liquid surface. For example, if water at room temperature (about 77?F) is kept in a closed container and the system pressure is reduced to its vapor pressure (about 0.52 psia), the water quickly changes to a vapor. Also, if the operating pressure is to remain constant at about 0.52 psia and the temperature is allowed to rise above 77?F, then the water quickly changes to a vapor.
Just like in a closed container, vaporization of the liquid can occur in centrifugal pumps when the local static pressure reduces below that of the vapor pressure of the liquid at the pumping temperature.
Step Two, Growth of bubbles Unless there is no change in the operating conditions, new bubbles continue to form and old bubbles grow in size. The bubbles then get carried in the liquid as it flows from the impeller eye to the impeller exit tip along the vane trailing edge. Due to impeller rotating action, the bubbles attain very high velocity and eventually reach the regions of high pressure within the impeller where they start collapsing. The life cycle of a bubble has been estimated to
Collapse of a Vapor Bubble
be in the order of 0.003 seconds。
Step Three, Collapse of bubbles，As the vapor bubbles move along the impeller vanes, the pressure around the bubbles begins to increase until a point is reached where the pressure on the outside of the bubble is greater than the pressure inside the bubble. The bubble collapses. The process is not an explosion but rather an implosion (inward bursting). Hundreds of bubbles collapse at approximately the same point on each impeller vane. Bubbles collapse non-symmetrically such that the surrounding liquid rushes to fill the void forming a liquid microjet. The micro jet subsequently ruptures the bubble with such force that a hammering action occurs. Bubble collapse pressures greater than 1 GPa (145?106 psi) have been reported. The highly localized hammering effect can pit the pump impeller. The pitting effect is illustrated schematically in this the figure.
After the bubble collapses, a shock wave emanates outward from the point of collapse. This shock wave is what we actually hear and what we call \ The implosion of bubbles and emanation of shock waves (red color) . In nutshell, the mechanism of cavitation is all about formation, growth and collapse of bubbles inside the liquid being pumped. But how can the knowledge of mechanism of cavitation can really help in troubleshooting a cavitation problem. The concept of mechanism can help in identifying the type of bubbles and the cause of their formation and collapse.
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Selected from :
[1] J.M.Coucson, J.F.Richardson :Chemical Engineering Butterworth-Heinemann Ltd.,1995. [2] O.C.Patella, R.F.Reboud :Experimental and Numerical Studies in a Centrifugal Pump with Two-Dimensional Curved Blades in Cavitating Condition.Journal of Fluids Engineering,vol.125,P970―978,(2003).
[3] Zhang,J.F., Yuan,S.Q., Fu,Y.D. :Numerical Forecast of the Influence of Splitter Blades on the Flow Field and Characteristics of a Centrifugal pump,Chinese Journal of Chemical Engineering,vol.45,P131-137,(2009).
[4] P.D. Lyapkov, Trudy VNII :No.5, Gostoptekhizdat, Moscow (1959).
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