Étude de l'écoulement d'une émulsion deuxième partie application de l'effet fusée mécanisme de l'érosion de cavitation

A study of foam flow Second part: The "rocket effect" in gas and vapour flows in a centrifugal pumps

Maître-Assistant à la chaire d'Electrotechnique. Faculté des Sciences de Paris.

Résumé

A gas or vapor bubble moving in translation in a liquid and varying its volume propels itself by a rocket effect, which occurs in foams moving in pressure or temperature fields with a non-zero gradient. It is notably observed in boiling and centrifugal pump operation, which latter features the following two characteristic bubble types : __ 1. Gas bubbles, which are apt to emerge faster from the pump than the liquid; 2. Vapor bubbles (cavitalion). When a bubble in translatory motion implodes, energy is transferred: potential pressure energy is converted info kinetic implosion energy, which in turn becomes kinetic translation energy. The instrument for this energy transfer process is a "micro-jet" following the bubble. It is shown that a considerable increase in kinetic translation energy density occurs, which is converted into potential pressure energy on impact against a solid obstacle. The resulting pressures (10,000 kg/sq.cm) explain the mechanical aspect of cavitation erosion. Several experimental results have confirmed these theoretical considerations. The development of a rotoscope is also described, this being a device enabling a centrifugal pump impeller (for example) to be "stopped" so that only the relative motion of the flow particles remains. Unlike with a stroboscope, time exposures can he taken with this device. With more thorough knowledge of foam mechanisms, and especially of cavitalion bubble behavior, it should be possible to design cavitation erosion-proof impeller blades.