Cavitation viscosity

The viscosity of a iiquid has been reported to affect the onset of acoustic cavitation. Viscosity is a quaiitative measure of molecular interaction in a liquid. The higher the viscosity is, the higher are the attractive forces between the molecules and hence, the higher is the threshold intensity of US where cavitation starts. Based on experimental evidence, Briggs et al. [58] developed a quantitative relation between liquid viscosity q and the experimental value of the cohesive pressure Pco. which is defined as the difference between the hydrostatic pressure Po — which coincides with ambient pressure when no... 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

Fig. 4. Typical design elements foi wet deagglomeiation in low viscosity systems (a) a high, ipm lotoi (shown below its normal position within stator) produces turbulence and cavitation as blades pass each other (b) a rotating disk creates a deep vortex to rapidly refresh the surface, and up- and downtumed teeth at the edge cause impact, turbulence, and sometimes cavitation and (c) the clearance of a high rpm rotor can be reduced as the batch...

A pump lifts water from a lake. At the pump suction entry a foot valve is fitted. Determine the maximum static delivery height the water can be raised without cavitation taking place. The saturation pressure of water is 1.23 kPa at 10 °C and the dynamic viscosity is 1.3 x 10" kg m s T The suction pipe water velocity is 2.0 m s , the internal pipe diameter is 100 mm, and the pipe roughness is 0.03 mm. The resistance of the foot valve is 4.5. 

Probably the most important single property of hydraulic oil is its viscosity. The most suitable viscosity for a hydraulic system is determined by the needs of the pump and the circuit too low a viscosity induces back-leakage and lowers the pumping efficiency while too high a viscosity can cause overheating, pump starvation and possibly cavitation. 

The important liquid phase physicochemical properties which affect the cavitation phenomena and hence the extent of cavitational effects for the given application include vapor pressure, viscosity and surface tension. 

Both chemical and physical properties of the reaction medium will dictate the required level of cavitation power. High viscosity media with low vapor pressure will require higher energy to generate cavitation. The presence of entrained or evolved gases will facilitate cavitation, as will the presence or generation of solid particles. 

Comparison with acoustic cavitation have also shown that hydrodynamically generated cavitation is far more energy efficient i.e. 76.5 J/mL as against 14,337 J/mL needed for sonically generated cavitation, for the same process, i.e., equivalent viscosity reduction. 

A liquid with a viscosity of 5cP, density of 45 lbm/ft3, and vapor pressure of 20 psia is transported from a storage tank in which the pressure is 30 psia to an open tank 500 ft downstream, at a rate of 100 gpm. The liquid level in the storage tank is 30 ft above the pump, and the pipeline is 2 in. sch 40 commercial steel. If the transfer pump has a required NPSH of 15 ft, how far downstream from the storage tank can the pump be located without danger of cavitation ... 

The dynamic process of bubble collapse has been observed by Lauter-born and others by ultrahigh speed photography (105 frames/second) of laser generated cavitation (41). As seen in Fig. 4, the comparison between theory and experiment is remarkably good. These results were obtained in silicone oil, whose high viscosity is responsible for the spherical rebound of the collapsed cavities. The agreement between theoretical predictions and the experimental observations of bubble radius as a function of time are particularly striking. 

Choice of liquid Vapor pressure Surface tension Viscosity Chemical reactivity Intensity of collapse Transient cavitation threshold Transient cavitation threshold Primary or secondary sonochemistry... 

Since it is necessary for the negative pressure in the rarefaction cycle to overcome the natural cohesive forces acting in the liquid, any increase in these forces will increase the threshold of cavitation. One method of increasing these forces is to increase the viscosity of the liquid. Tab. 2.1 shows the influence of viscosity on the pressure amplitude (Pft) at which cavitation begins in several liquids at 25 °C, at a hydrostatic pressure of 1 atm. 

The effect, though not insignificant, is hardly dramatic. Taking corn and castor oils as examples, a ten-fold increase in viscosity has only led to a 30% increase in the acoustic pressure needed to bring about cavitation. 

The final factor to be considered here, and known to affect the cavitation threshold, is the temperature. In general, the threshold limit has been found to increase with decrease in temperature. This may in part be due to increases in either the surface tension (a) or viscosity (rj) of the liquid as the temperature decreases, or it may be due to the decreases in the liquid vapour pressure (P ). To best understand how these parameters (a, q, Py) affect the cavitation threshold, let us consider an isolated bubble, of radius Rq, in water at a hydrostatic pressure (Pjj) of 1 atm. 

Let us now consider the effect of solvent viscosity on the cavitation threshold. According to Tab. 2.1, an increase in the solvent viscosity required the application of a... 

Whilst vapour pressure may be the major solvent factor involved in the degradation process, there could also be a contribution from solvent viscosity or even, yet less likely, from surface tension. It has already been argued (see Section 2.6.2) that although an increase in viscosity raises the cavitation threshold, (i. e. makes cavitation more difficult), provided cavitation occurs, the pressure effects resulting from bubble collapse... 

Such observations have been interpreted in terms of the increase in viscosity of the solution - i. e. the higher the viscosity the more difficult it becomes to cavitate the solution, at a given intensity, and the smaller is the degradation effect. 

Reduction in the apparent bulk viscosity due to a change in polymer rheology. It is well known that ultrasound can lead, via degradation, to a reduction in polymer solution viscosity. Although Fairbrother did not investigate whether degradation of the polymer, and subsequent reduction in R.M.M. and hence viscosity had occurred, it seems reasonable to assume that the polymer melt with an initial viscosity of 30000-100000 poise would certainly have resisted cavitation and thus degradation. 

Colloidal potassium has recently been proved as a more active reducer than the metal that has been conventionally powdered by shaking it in hot octane (Luche et al. 1984, Chou and You 1987, Wang et al. 1994). To prepare colloidal potassium, a piece of this metal in dry toluene or xylene under an argon atmosphere is submitted to ultrasonic irradiation at ca. 10°C. A silvery blue color rapidly develops, and in a few minutes the metal disappears. A common cleaning bath (e.g., Sono-clean, 35 kHz) filled with water and crushed ice can be used. A very fine suspension of potassium is thus obtained, which settles very slowly on standing. The same method did not work in THF (Luche et al. 1984). Ultrasonic waves interact with the metal by their cavitational effects. These effects are closely related to the physical constants of the medium, such as vapor pressure, viscosity, and surface tension (Sehgal et al. 1982). All of these factors have to be taken into account when one chooses a metal to be ultrasonically dispersed in a given solvent. 

Where the maintenance of a clear channel between sprue and the slowest freezing part of a charge is impractical, cavitation is avoided by casting charges in layers, each of which is allowed to crust over before pouring the next TNT melts at 81°. It forms eutectics with RDX, Tetryl (68°), PETN (76°), and other impurities in the mix and makes these materials more soluble at higher temps. Thus, there is a general tendency for the solid content and, hence, the apparent viscosity of most castable mixts to decrease as the temp is increased. However, a reversal of the tendency toward the reduction in viscosity has been noted in Comp B when it heated above 100°... 

By the action of hydraulic shear forces, cavitation, turbulence and impact owing to the very high flow velocity (several 100 m/s) or high differential pressure (low viscosity liquids, 300 to 400 bar, or more viscous liquids, up to 1500 bar) the liquid is turned into a very fine (homogeneous) dispersion. 

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