Cavitation sounds

Many heterogeneous reactions are accelerated by the enhanced micromixing properties of cavitating sound fields. Oscillating and transient bubbles create intense microstreaming in the vicinity of suspended solids. Macromixing is induced by acoustic streaming and the oscillation of bubbles in the sound field. In most cases, a locally different mass-transport coefficient is observed. A tenfold increase in mass-transfer coefficients compared with silent reactions was measured [18]. 

The last piece for the model is the bubble-size distribution function and the limits for the rest radii of bubbles in the sound field. The cavitation thresholds as a function of applied sound pressure indicate the upper and lower size limits for bubbles in a cavitating sound field. A simplifying point of view would differentiate between a) transient bubbles, b) stable bubbles and c) dissolving bubbles. 

Only bubbles with the attributes stable or transient are Ukely to exist in a steady-state cavitating sound field. The lower size of a bubble at a given sound pressure is defined by the lower limit for stable cavitation. Any bubble smaller than this size would dissolve rather quickly under the action of its own surface tension. 

The first recorded observation of cavitation was made in 1754 by Euler. However, serious research into the character and mechanism of cavitation did not begin until 1919. Research since that time has established a reasonably sound basis for describing and explaining the phenomenon. 

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. 

Along with the sounds, evidence and signs of cavitation, there is a broad range of other information and signals available to the maintenance mechanic. Almost all mechanics have seen the gouge and scratch marks, and signs of heat on the pump when disassembled in the shop. Sadly, most mechanics are never trained to interpret these marks. 

When a liquid is exposed to ultrasonic irradiation, a variety of phenomena can be observed originating from streaming and cavitation. High intensity sounds in liquids are accompanied by wave radiation pressure which results in steady-state... 

The role of cavitation in ultrasound degradation has been confirmed repeatably in most experiments where cavitation was prevented, either by applying an external hydrostatic pressure, by degassing the solution, by reducing the sound intensity or the temperature, polymer chain scission was also largely suppressed [117]. 

For general aspects on sonochemistry the reader is referred to references [174,180], and for cavitation to references [175,186]. Cordemans [187] has briefly reviewed the use of (ultra)sound in the chemical industry. Typical applications include thermally induced polymer cross-linking, dispersion of Ti02 pigments in paints, and stabilisation of emulsions. High power ultrasonic waves allow rapid in situ copolymerisation and compatibilisation of immiscible polymer melt blends. Roberts [170] has reviewed high-intensity ultrasonics, cavitation and relevant parameters (frequency, intensity,... 

Abstract Acoustic cavitation is the formation and collapse of bubbles in liquid irradiated by intense ultrasound. The speed of the bubble collapse sometimes reaches the sound velocity in the liquid. Accordingly, the bubble collapse becomes a quasi-adiabatic process. The temperature and pressure inside a bubble increase to thousands of Kelvin and thousands of bars, respectively. As a result, water vapor and oxygen, if present, are dissociated inside a bubble and oxidants such as OH, O, and H2O2 are produced, which is called sonochemical reactions. The pulsation of active bubbles is intrinsically nonlinear. In the present review, fundamentals of acoustic cavitation, sonochemistry, and acoustic fields in sonochemical reactors have been discussed. 

Dahnke S, Keil F (1998) Modeling of sound fields in liquids with a nonhomogeneous distribution of cavitation bubbles as a basis for the design of sonochemical reactors. Chem Eng Technol 21 873-877... 

Acoustic cavitation In this case, the pressure variations in the liquid are effected using the sound waves usually ultrasound (16 kHz to 100 MHz). The chemical changes taking place due to the cavitation induced by the passage of sound waves are commonly known as sonochemistry. 

For cavitation to occur in a liquid, it has to overcome the natural cohesive forces present in the liquid. Any increase in these forces will tend to increase the threshold pressure and hence the energy required to generate cavitation. In highly viscous liquids, severe attenuation of the sound intensity occurs and the active cavitating zone gets reduced substantially. Moholkar et al. [56] have confirmed this fact with experiments with different liquids and reported that for highly viscous liquids, cavitational effects are not observed. 

Acoustic cavitation based reactors create much higher sound energy/pressure... 

The interpretation of trends in MBSL and sonochemical yield with electrolyte concentration needs to be revised in light of the aforementioned finding as changes in bubble size distribution and number population not only determine the number of cavitation events occurring but will have a marked effect on sound wave transmission and the local environment surrounding bubbles, influencing collapse symmetry. 

However, cavitation may result in an even more serious condition than vapor lock. When the pressure at any point within the pump drops below the vapor pressure of the liquid, vapor bubbles will form at that point (this generally occurs on or near the impeller). These bubbles will then be transported to another region in the fluid where the pressure is greater than the vapor pressure, at which point they will collapse. This formation and collapse of bubbles occurs very rapidly and can create local shock waves, which can cause erosion and serious damage to the impeller or pump. (It is often obvious when a pump is cavitating, because it may sound as though there are rocks in the pump )... 

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