Cavitation number, boiling

This is similar to a traditional cavitation number except that the pressure within the vapor cavity is strongly affected by both local fluid mechanics and thermal factors. It was shown by Smith and Katsanevakis (1993) that the RPD in a boiling agitated system can be described adequately by relationships of the form... 

A number of terms in this area will be unfamiliar to most chemists. Cavitation is the formation of gas bubbles in a liquid and occurs when the pressure within the liquid drops significantly below the vapor pressure of the liquid. Cavitation can occur from a variety of causes turbulent flow, laser heating, electrical discharge, boiling, radiolysis, or acoustic irradiation. We will be concerned... 

Any increase in temperature will raise the vapor pressure of a medium and so lead to easier cavitation but less violent collapse (see above). This effect will be accompanied by a decrease in viscosity and surface tension. However, at temperatures approaching the solvent boiling point, a large number of cavitation bubbles are generated concurrently. These will act as a barrier to sound transmission and dampen the effective ultrasonic energy from the source which enters the liquid medium. The combination of all these effects shows a shape of maximum and the optimum temperature depends on the experimental conditions used and reaction studied. 

The vapor pressure of liquids can cushion the bubble collapse like a high gas content. Vapor in a transient bubble can be condensed in the compression cycle and lead to higher cavitation intensities than gas-filled bubbles. Experiments with different solvents show that small vapor pressures are necessary for a sufficiently high cavitation intensity. Higher vapor pressures, especially near the boiling point of the liquid, can dampen the cavitation efficiency to nearly zero. If a substrate is subject to treatment within the collapsing bubbles, then a certain number of its molecules must be present in the bubbles and exert an at least measurable vapor pressure. The existence of molecules inside the bubble can easily be proved by means of molecules that exist as ionic or molecular species at different pH values. Ionic species do not enter the bubbles, and high-temperature pyrolysis products can therefore not be created. 

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