Traveling cavitation

Traveling cavitation is a type of cavitation in which individual transient cavities or bubbles form in the liquid and move with it as they expand or shrink during their life cycles. To the naked eye traveling cavitation may appear as sheet cavitation. 

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. 

Some solutions may not be practical, or economical, or timely and consistent with production. You could be forced to live with cavitation until the next plant shutdown to make the neces.sary corrections. In the meantime, the cavitation shock waves and vibrations will travel through the impeller, down the shaft to the mechanical seal faces, and onto the shaft bearings. We offer some. specific recommendations for surviving cavitation shock waves and vibrations in Chapters 13 and 14 on Mechanical Seals. 

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. 

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... 

The lifetime of the bubble, which is reflected in the distance travelled and hence the extension of the zone of cavitational influence from point of its inception... 

Initiation by Precursor is a phenomenon encountered in low velocity detonations, LVD, in liquid explosives. It depends primarily on cavitation of the liquid by the shock traveling in the container ahead of the shock in the liquid. For a description of this effect, see Low Velocity Detonation in this Vol... 

Cavitation begins at much smaller intensities when low sound frequencies are applied. Fig. 5 describes how the threshold intensity increases with increasing frequency. Drawing a vertical line at approximately 20 kHz, as one moves up this vertical line, wave intensity increases [W/cm2]. The first thing one encounters as the intensity is increased is the curve for aerated water, or water saturated with air. The intensity at this point is sufficient to produce cavitation as desorbed air contributes to bubble nucleation. As one continues to increase intensity, one will encounter the curve for degassed cavitation. This intensity is the absolute maximum intensity allowed (at standard conditions) for sound traveling in water at this frequency. Most of sonochemistry are performed at intensity levels between these two values. 

Scenario Hydrofoils traveling at high rates of speed are damaged by the cavitational action of water on the hydrofoil. To prevent the destructive cavitational forces acting on the surface of the foil, a thin layer of ice is introduced by refrigerating the surface of the foil, thus forming a renewable... 

The formation of free radial OH and H in a naturally air-saturated aqueous solution exposed to traveling ultrasonic wave of 820 kHz was investigated using a spin-trapping agent, 5,5-dimethyl-l-pyrroline-l-oxide (DMPO) and ESR techniques [75]. It was shown that the cavitation threshold occurred at 0.537-0.632 W cm-2, and no further increase was observed above 3 W cm-2. At a fixed sound intensity the yield of OH increased linearly with the sonication time. 

Only the first two types of cavitation are of suitable intensity for chemical or physical processing. In the case of cavitation reactors, two aspects of cavity dynamics are ofmain importance, the maximum size reached by the cavity before its violent collapse and the life of the cavity. The maximum size reached by the cavity determines the magnitude of the pressure pulse produced on the collapse and hence the cavitation intensity that can be obtained in the system. The life of the cavity determines the distance traveled by the cavity from the point where it is generated before the collapse and hence it is a measure of the active volume of the reactor in which the actual cavitational effects are observed. 

Figure 8.3 Showing transients for valve travel, x, valve opening, /, and choking and cavitation coefficients Km and Kc.

---