Cavitational bubbles

Fig. 3. Liquid jet produced during collapse of a cavitation bubble near a solid surface. The width of the bubble is about 1 mm.

Spectroscopic Probes of Cavitation Conditions. Determination of the temperatures reached ia 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 sUicone oU under Ar have been reported and analy2ed (7). The observed emission comes from excited state has been modeled with synthetic spectra as a... 

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the coUapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of apphcations of ultrasound to enhancing chemical reactivity has been explored, with important apphcations in mixed-phase synthesis, materials chemistry, and biomedical uses. 

Fig. 17. Cavitation phenomenon in pumps showing cavitation bubble distribution and rate of weight loss as a function of cavitation coefficient at constant...

Temperature, air content, pressure, and chemical composition of the fluid can affect the tendency of the fluid to cavitate. For example, the presence of minute air bubbles in the fluid can act as nucleation sites for cavitation bubbles, thereby increasing the tendency of the fluid to cavitate. Increasing pressure decreases susceptibility to cavitation decreasing pressure increases susceptibility to cavitation. 

The cavitation damage in this spacer was due to vibrations from operation of the engine. The localized nature of the damage in this case is an illustration of a common feature of cavitation. Pits formed by initial cavitation damage become preferred sites for the development of subsequent cavitation bubble formation due to the jagged, irregular contours of the pit. This tends to localize and intensify the cavitation process, especially in later stages of pit development. 

This case vividly illustrates the potential effect of surface roughness on the propagation of cavitation bubbles. Figure 12.18 represents a surface... 

Note that localized corrosion having the appearance illustrated in Figs. 12.18 through 12.20 could be associated with brief exposure to a strong acid. In this case, however, all available information indicated that the tubes had never been exposed to an acid of any type. Cavitation was caused by high-frequency vibration of the tubes. The vibration apparently induced a threshold cavitation intensity such that rough or irregular surfaces produced cavitation bubbles, and smooth internal surfaces did not. 

When vapor bubbles eollapse inside the pump the liquid strikes the metal parts at the speed of sound. This is the elicking and popping noise we hear from outside the pump when we say that eavitation sounds like pumping marbles and roeks. Sound travels at 4,800 ft per second in water. The velocity head formula gives a elose approximation of the energy contained in an imploding cavitation bubble. Remember that implosion is an explosion in the opposite direction. 

In pump terminology, the approximate energy in an imploding cavitation bubble is 358,209 ft. To convert this energy into pressure ... 

The process by which a material becomes eroded when exposed to the forces resulting from the collapse of cavitation bubbles proceeds in a number of stages according to Thiruvengadam and Preiser . In the first of these, the... 

Morch, K. A., Dynamics of Cavitation Bubbles and Cavitating Liquids, Treatise on Materials Science and Technology, Vol 16, Erosion, Academic Press, London, p. 309-353 (1979)... 

An 87 13 mixt with benz gave a Trauzl test of 134% of TNT (Ref 26). Mixts of from 65 to 90% by wt in benz had a card gap test of over 300 cards, indicating them to be very sens (Ref 36). The sensy of mixts with benz was found to be due to the formation and collapse of cavitation bubbles in the liq (Ref 43) Ethylene Glycol. A 50% soln by wt had a card gap test of 270 cards (Ref 36)... 

An interesting way to retard catalyst deactivation is to expose the reaction mixture to ultrasound. Ultrasound treatment of the mixture creates local hot spots, which lead to the formation of cavitation bubbles. These cavitation bubbles bombard the solid, dirty surface leading to the removal of carbonaceous deposits [38]. The ultrasound source can be inside the reactor vessel (ultrasound stick) or ultrasound generators can be placed in contact with the wall of the reactor. Both designs work in practice, and the catalyst lifetime can be essentially prolonged, leading to process intensification. The effects of ultrasound are discussed in detail in a review article [39]. 

In Fig. 1.1, the parameter space for transient and stable cavitation bubbles is shown in R0 (ambient bubble radius) - pa (acoustic amplitude) plane [15]. The ambient bubble radius is defined as the bubble radius when an acoustic wave (ultrasound) is absent. The acoustic amplitude is defined as the pressure amplitude of an acoustic wave (ultrasound). Here, transient and stable cavitation bubbles are defined by their shape stability. This is the result of numerical simulations of bubble pulsations. Above the thickest line, bubbles are those of transient cavitation. Below the thickest line, bubbles are those of stable cavitation. Near the left upper side, there is a region for bubbles of high-energy stable cavitation designated by Stable (strong nf0) . In the brackets, the type of acoustic cavitation noise is indicated. The acoustic cavitation noise is defined as acoustic emissions from... 

Fig. 1.1 The regions for transient cavitation bubbles and stable cavitation bubbles when they are defined by the shape stability of bubbles in the parameter space of ambient bubble radius (R0) and the acoustic amplitude (p ). The ultrasonic frequency is 515 kHz. The thickest line is the border between the region for stable cavitation bubbles and that for transient ones. The type of bubble pulsation has been indicated by the frequency spectrum of acoustic cavitation noise such as nf0 (periodic pulsation with the acoustic period), nfo/2 (doubled acoustic period), nf0/4 (quadrupled acoustic period), and chaotic (non-periodic pulsation). Any transient cavitation bubbles result in the broad-band noise due to the temporal fluctuation in the number of bubbles. Reprinted from Ultrasonics Sonochemistry, vol. 17, K.Yasui, T.Tuziuti, J. Lee, T.Kozuka, A.Towata, and Y. Iida, Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles, pp. 460-472, Copyright (2010), with permission from Elsevier...

From Fig. 1.1, it is seen that stable cavitation bubbles are tiny bubbles of a few pm in ambient radius or relatively large bubbles of about 10 pm or more in radius at 515 kHz. The range of ambient radius for transient cavitation bubbles becomes... 

The third mechanism for nucleation is the fragmentation of active cavitation bubbles [16]. A shape unstable bubble is fragmented into several daughter bubbles which are new nuclei for cavitation bubbles. Shape instability of a bubble is mostly induced by an asymmetric acoustic environment such as the presence of a neighboring bubble, solid object, liquid surface, or a traveling ultrasound, or an asymmetric liquid container etc. [25-27] Under some condition, a bubble jets many tiny bubbles which are new nuclei [6, 28]. This mechanism is important after acoustic cavitation is fully started. 

Fig. 1.7 Three sites for chemical reactions for a cavitation bubble...

Cavitation bubbles work as nucleation sites of particles. For example, in a supercooled sucrose solution, nucleation of ice crystals induced by cavitation bubbles has been experimentally observed [72], This phenomenon has been called sonocrys-tallization [73]. Although there are some papers on the mechanism of sonocrystal-lization, it has not yet been fully understood [74, 75]. It has been reported that the distribution of crystal size in sonocrystallization is narrower than that without ultrasound [73]. It may be related to the narrower size distribution of sonochemi-cally synthesized particles compared to that without ultrasound [76, 77]. Further studies are required for the mechanism of particle nucleation by ultrasound. 

In a bath-type sonochemical reactor, a damped standing wave is formed as shown in Fig. 1.13 [1]. Without absorption of ultrasound, a pure standing wave is formed because the intensity of the reflected wave from the liquid surface is equivalent to that of the incident wave at any distance from the transducer. Thus the minimum acoustic-pressure amplitude is completely zero at each pressure node where the incident and reflected waves are exactly cancelled each other. In actual experiments, however, there is absorption of ultrasound especially due to cavitation bubbles. As a result, there appears a traveling wave component because the intensity of the incident wave is higher than that of the reflected wave. Thus, the local minimum value of acoustic pressure amplitude is non-zero as seen in Fig. 1.13. It should be noted that the acoustic-pressure amplitude at the liquid surface (gas-liquid interface) is always zero. In Fig. 1.13, there is the liquid surface... 

Ashokkumar M, Lee J, Iida Y, Yasui K, Kozuka T, Tuziuti T, Towata A (2009) The detection and control of stable and transient acoustic cavitation bubbles. Phys Chem Chem Phys 11 10118-10121... 

Bremond N, Arora M, Dammer SM, Lohse D (2006) Interaction of cavitation bubbles on a wall. Phys Fluids 18 121505 (10 pages)... 

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