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Acoustic Cavitation as Process Intensifier: A Phenomenological Study

Excerpt

When an acoustic field is coupled to a liquid and transient cavitation is induced, it can significantly intensify ongoing processes, making them run faster, cheaper, better. Surface cleaning is a classic example that has been investigated in various studies over the years. Sonochemical reactors are increasingly being deployed in industry. Cavitational enhancement of mass transfer phenomena such as diffusion, dissolution and leaching has been quantified in literature, and its effect on heat transfer from fluids to adjoining solids has been reported as well. Nano-particle synthesis by “sono-fragmentation” is widely practised as a top-down alternative to bottom-up methods of nano-particle production with tons-per-day throughput. Cavitation-induced destratification has been investigated earlier in this laboratory as a method to keep cryogenic fuels well-mixed during storage on the ground. More recently, cavitation has been evaluated as a mechanism to remove ash, sulfur and alkalis from coal prior to combustion, thereby greatly mitigating the downstream propensity for slagging, fouling, corrosion and erosion. Practical applications abound, as the additional energy expenditure associated with the acoustic field is easily compensated by savings in process time and productivity enhancement.

Unification of these cavitation-induced effects in multiple domains is a worthwhile exercise in order that the effect may be fully understood and optimized. In particular, ultrasonic frequency is a tunable parameter that can have dramatic effects on the outcome. The transition from the cavitation regime to acoustic streaming (as frequency is increased from the ultrasonic to the megasonic regime) is one that needs to be characterized and managed with care. In many applications, acoustic streaming can provide a synergistic effect when deployed in conjunction with cavitation, as in, for example, a dual-frequency system that can operate in the high (> 100 kHz) and low (<100 kHz) rage. Alternatively, an intermediate frequency such 300 kHz which combines both cavitational and streaming features may be employed as well.

The definition of a “process intensification factor” and assessment of its dependence on ultrasonic frequency for various thermodynamic and transport phenomena constitutes a valuable contribution for practitioners, and this has been attempted in this paper. Based on the frequency dependence established, processes may be categorized with respect to their sensitivity to acoustic cavitation. This categorization enables optimum application of cavitational fields to achieve desired results with minimum expenditure of energy, and mitigation of undesirable side effects.

Introduction
Cavitation as a Process-Intensifier: Applications
Cavitation as a Process-Intensifier: Mechanistic Understanding
References
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