Reynolds and Throat Cavitation Numbers

CaviMax is a constant, high-flow-rate, single-orifice instrument with a maximum pressure of 8 MPa and orifices that can be interchanged in the range of 1.25 to 2.90 mm. The bubble dynamics can be further controlled through the use of a flow-throttling valve located just after the cavitation recovery zone. Other high-volume processors are available but were not used in this study. 

Cavitation occurs when bubbles containing either vapor or gas are formed by reduction in the local pressure at constant temperature (Brennan, 1995 Knapp et al., 1979 Young, 1989) such as the rapid movement of the fluid past an impeller blade, through a pump, or in this case through a restriction (orifice) at near supersonic velocities. Quantifying the cavitation number is debatable, but it can be derived from the Bernoulli equation. Lush (Young, 1989) uses a throat cavitation number (aT) defined as 

Although the onset of continuous cavitation occurs at cavitation numbers aT near 0.6, bursts of cavitation occur at 0.3, which begin to cause damage to surfaces. As the number increases, cavitation still may exist, but the bubble dynamics—i.e., size, internal pressure, and periodicity of formation-collapse-reformation—will be different. 

Experiments in the CaviMax to characterize the fluid flow varied the orifice size in the single-orifice processor from 1.75 to 6 mm (orifice removed). As mentioned before, the instrument operates at constant flow thus, the pressure of the measurement decreases as the applied orifice size increases. The flow measurements using this processor resulted in the ability to vary Reynolds numbers between 60,000 and up to 175,000, whereas the range of the available throat cavitation number was 0.85 to 1.7. 

In subsequent metal oxide synthesis experiments, the relationship between cavitation numbers and Reynolds numbers to synthesis results, such as crystallographic strain, primary grain size, agglomerate size, or phase purity, was examined to develop an understanding of the effects of different bubble dynamics on crystal properties. 

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CaviMax and CaviPro processors, 23-26 cavitational devices, 23-24 characterization of Reynolds and throat cavitation numbers, 25-27 classical approach, 18 copper-modified zinc oxide with crystallographic strain, 38-39 crystallographic strain by hydrodynamic cavitation, 34-39 engineered synthesis, 18-19 equipment for hydrodynamic cavitation, 21-22... 

Fig. 11. Three-dimensional surface plot showing the correlation of applied pressure, crystallographic strain, and Reynolds number-throat cavitation number for the synthesis of titania using the CaviMax processor.

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