Effective cavitation temperature

Spectroscopic Probes of Cavitation Conditions. Determination of the temperatures reached in a cavitating bubble has remained a difficult experimental problem. As a spectroscopic probe of the cavitation event, MBSL provides a solution. High resolution MBSL spectra from silicone oil under Ar have been reported and analyzed (7). The observed emission comes from excited state C2 and has been modeled with synthetic spectra as a function of rotational and vibrational temperatures, as shown in Figure 7. From comparison of synthetic to observed spectra, the effective cavitation temperature is 5050 =L 150 K. The excellence of the match between the observed MBSL and the synthetic spectra provides definitive proof that the sonoluminescence event is a thermal, chemiluminescence process. The agreement between this spectroscopic determination of the cavitation temperature and that made by comparative rate thermometry of sonochemical reactions is surprisingly dose (6). 

In general an increase in intensity (I) will provide for an increase in the sonochemical effects. Cavitation bubbles, initially difficult to create at the higher frequencies (due to the shorter time periods involved in the rarefaction cycles) will now be possible, and since both the collapse time (Eq. 2.27), the temperature (Eq. 2.35) and the pressure (Eq. 2.36) on collapse are dependent on P i(=Ph + PA)> bubble collapse will be more violent. However it must be realised that intensity cannot be increased indefinitely, since (the maximum bubble size) is also dependent upon the pressure amplitude (Eq. 2.38). With increase in the pressure amplitude (P ) the bubble may grow so large on rarefaction (R g, ) that the time available for collapse is insufficient. 

Temperature dependency of the rate of destruction is determined by the effect of temperature upon the cavitation process there is an optimum temperature at which the intensity of cavitation is maximum similarly, the rate and depth of destruction depend upon the frequency of acoustic vibrations. 

The concentration of volatile compounds in the cavitation bubbles increases with temperature thus, faster degradation rates are observed at higher temperatures for those compounds [23]. Conversely, in the case of nonvolatile substrates (that react through radicals reactions in solution), the effect of temperature is somehow opposed to the chemical common sense. In these cases, an increase in the ambient reaction temperature results in an overall decrease in the sonochemical reaction rates [24]. The major effect of temperature on the cavitation phenomenon is achieved through the vapor pressure of the solvent. The presence of water vapor inside the cavity, although essential to the sonochemical phenomenon, reduces the amount of energy... 

A second spectroscopic thermometer comes from the relative intensities of atomic emission lines in the sonoluminescence spectra of excited-state metal atoms produced by sonolysis of volatile Fe, Cr, and Mo carbonyls. Sufficient spectral information about emissivities of many metal atom excited states are available to readily calculate emission spectra as a function of temperature. Because of this, the emission spectra of metal atoms are extensively used by astronomers to monitor the surface temperature of stars. From comparison of calculated spectra and the observed MBSL spectra from metal carbonyls, another measurement of the cavitation temperature was obtained.6 The effective emission temperature from metal atom emission during cavitation under argon at 20 kHz is 4,900 250 K. 

The values given in Table 1 refer to the cavitation characteristics of the pure solvents. When reagents are present, other factors may have some importance, such as the structure of the solution or diffusion effects. The temperature at which all the factors have a balanced influence corresponds to the optimum, which does not necessarily coincide with Tmax- The indications given above... 

The effects of microchannel size, mass flow rate, and heat flux on boiling incipience or bubble cavitation in a microchannel were investigated by Li and Cheng [56], The effects were also estimated of contact angle, dissolved gas, and the existence of microcavities and corners in the microchannel on bubble nucleation and cavitation temperature. 

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). 

Cavitation. The subject of cavitation in pumps is of great importance. When the Hquid static pressure is reduced below its vapor pressure, vaporization takes place. This may happen because (/) the main stream fluid velocity is too high, so that static pressure becomes lower than vapor pressure (2) localized velocity increases and static pressure drops on account of vane curvature effect, especially near the inlets (J) pressure drops across the valve or is reduced by friction in front of the pump or (4) temperature increases, giving a corresponding vapor pressure increase. 

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