Damage effect cavitation

The pumps used in handling these high-pressure liquids can suffer considerable damage from cavitation. Incompressible liquids will not compress, nor will they withstand tension thus if the suction inlet to a pump is restricted the fluid will release any contained air to form cavities. This condition seriously affects the performance of the pump, can cause damage to its rotor and generates a great deal of noise. Gas or air entrained in a hydraulic fluid is detrimental to its effectiveness as a power transmission medium. 

The damage effect due to cavitation in the elastomeric sample iPP5 stretched at 900 % deformation (Fig. 11.9e and profile f of Fig. 11.10b) is irreversible. In fact, in mechanical cycles of stretching and relaxing the tension no significant changes in... 

In the case of the elastomeric sample iPP5 the values of the scattering Io(q) and order parameter of P2(cos x) of the stress-relaxed specimens are also reported in Fig. 11.10a, b, a", b". They are similar to those of the fully stretched fibre at 900 % deformation, indicating that the damage effect caused by cavitation is irreversible. 

Erosion and cavitation both can degrade materials simply by mechanical means or by combining the effects of mechanical deterioration and corrosion to produce a synergistic result. However, the mechanisms by which erosion and cavitation operate, and the resulting damage, are quite distinct. 

Weld overlays of stainless steel or cobalt-based wear-resistant and hard-facing alloys such as Stellite may salvage damaged equipment. In addition, weld overlays incorporated into susceptible zones of new equipment may provide cost-effective resistance to cavitation damage. 

Cavitation Formation of transient voids or vacuum bubbles in a liquid stream passing over a surface is called cavitation. This is often encountered around propellers, rudders, and struts and in pumps. When these bubbles collapse on a metal surface, there is a severe impact or explosive effect that can cause considerable mechanical damage, and corrosion can be greatly accelerated because of the destruction of protective films. Redesign or a more resistant metal is generally required to avoid this problem. 

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

As we have mentioned before, acoustic streaming, cavitation and other effects derived from them, microjetting and shock waves take also relevance when the ultrasound field interacts with solid walls. On the other hand, an electrochemical process is a heterogeneous electron transfer which takes place in the interphase electrode-solution, it means, in a very located zone of the electrochemical system. Therefore, a carefully and comprehensive read reveals that all these phenomena can provide opposite effects in an electrochemical process. For example, shock waves can avoid the passivation of the electrode or damage the electrode surface depending on the electrode process and/or strength of the electrode materials [29]. 

Surface Damage and Reaction Rates. Erosion of surfaces resulting in higher surface area and removal of inhibiting impurities are two effects of cavitation on solids in liquid media, both of which lead to increased reaction rates. The high temperatures and pressures are sufficient to deform and pit metal surfaces (even cause local melting of some metals) and to fracture many nonmetal lie solids, in particular, brittle materials. 

Low power ultrasound offers the possibility of enhancing the effects of chlorine. The results of a study of the combined effect of low power ultrasound and chlorination on the bacterial population of raw stream water are shown (Tab. 4.2). Neither chlorination alone nor sonication alone was able to completely destroy the bacteria present. When sonication is combined with chlorination however the biocidal action is significantly improved [10]. The effect can be ascribed partly to the break-up and dispersion of bacterial clumps and floes which render the individual bacteria more susceptible to chemical attack. In addition cavitation induced damage to bacterial cell walls will allow easier penetration of the biocide. 

Figure 13.9 Sequence of events in a croid formation, (a) Initial state at the crack tip. (b) Cavitation ofthe rubber particles dueto loading head of the crack tip. (c) Cavitation of rubber particles near the already cavitated particles due to stress-concentration effect. The croid is forming, (d) Croids are propagating ahead ofthe crack and inside the craze-like damaged zone many shear bands develop between cavitated rubber particles. (Sue, 1992 with kind permission from Kluwer Academic Publisher.)...

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