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Hydrodynamic Cavitation

Hydrodynamic cavitation is a phenomenon of formation, growth and collapse of vapor filled cavities (micro bubbles) within a liquid due to variation of local pressure. The cavities are generated in a low pressure region. When these cavities travel to a region of higher pressure, they implode (collapse). The cavity collapse under certain conditions results in very high pressures and temperatures near the location of collapse [1-3]. These extreme temperatures (>2000?K) and pressures (>100?MPa) result in the generation of highly reactive radical species (from water and dissolved gases). In addition, the imploding cavities also result in high velocity jets and intense shear. These extreme physio-chemical effects produced by cavitation have been of interest to engineers and scientists for over a century, largely due to its potential to damage equipment. The mechanisms of cavitation damage have been and continue to be hotly debated, and to this day the causes and effects of cavitation still present fundamental scientific challenges to engineers; for example, continuing issues in the field of water turbines described by the "three gorges puzzle" [4].

The extreme conditions generated during cavitation can also be harnessed for realizing beneficial physico-chemical transformations. In the last couple of decades, the application of cavitation has been extensively explored for a variety of physical, chemical and biological processes. Cavitation offers a novel way of intensifying these processes in an energy efficient manner [5, 6]. The in situ generation of strong oxidants like OH· radicals, local hot spots and intense shear, has a potential to become a very promising technology platform for realizing various transformations (see Figure 1.1). It can be harnessed for waste water treatment [7-12]; microbial disinfection [13-17]; desulphurisation of fuels [18-20]; biomass pre-treatment [21-23]; biodiesel synthesis [24, 25]; and in food and beverage production [26, 27]; esterification reactions [28]; and many other process intensification applications [3, 5, 6, 29, 30].

The references cited here represent only a tiny fraction of illustrative examples from published laboratory studies on applications of hydrodynamic cavitation. There are also several patents and start-up/spin-out companies commercializing cavitation based technologies and applications. See for example, reviews by Carpenter et al. [5] and Holkar et al. [6]. Despite such a large number of publications on cavitation (thousands of research papers per year - Google Scholar), the promise of hydrodynamic cavitation as a technology platform is still largely unfulfilled. One of the key reasons of holding back the realization of this promise is inadequate understanding of inception as well as resulting physico-chemical effects of cavitation. Systematic methodology for designing hydrodynamic cavitation devices and generalized framework for development and optimisation of hydrodynamic cavitation processes is still not established and available.

Figure 1.1 Cavitation for beneficial physico-chemical transformations.

Mathematical modeling of hydrodynamic cavitation and the related physico-chemical effects is rather complex even for the simplest devices and systems. Dynamics of a single cavity and subsequent collapse when exposed to adequately strong pressure fluctuations has been studied for long since Lord Rayleigh [31]. The book by Brennen [1] provides detailed models as well as results of cavity dynamics. The quest for developing better models for simulating cavity dynamics is still continuing (see for example, Pandit et al. [32] and references cited therein). Though such single cavity models are available, use of these models for simulating overall performance is still far from satisfactory. The book by Shah et al. [2] has attempted to present a methodology for systematic analysis of cavitation processes and discussed possible pathways for connecting single cavity dynamics to overall performance. Franc and Michel [33] published a book on fundamentals of cavitation. However, the focus was mainly on hydrodynamics and various applications were not discussed. About a decade ago, Ozonek [34] attempted to present a methodology for applying hydrodynamic cavitation primarily to water treatment applications. There are couple of reviews from the group of Professor Pandit [30, 35] with discussion on design aspects of orifice and venturi based hydrodynamic cavitation.

None of the available resources provide systematic basis for designing and evaluating different hydrodynamic cavitation devices or for simulating overall performance and scale-up of processes based on such devices. This book attempts to bridge this gap. This chapter provides a general introduction to hydrodynamic cavitation and devices to realize hydrodynamic cavitation. It also presents overall structure of the book to facilitate better usage of the presented contents.

Figure 1.2 (a) Boiling and cavitation illustrated in thermodynamic diagram (

Source: Reprinted from [36], with permission from Meijn/Delft University of Technology

) and (b) Graphic illustration of growth, implosion and collapse of bubbles in the cavitation process (

Source: Reprinted from [37], with permission from Elsevier

).

1.1 Hydrodynamic Cavitation

Cavitation in a simple sense is the phenomenon of formation, growth and collapse of gaseous pockets in a liquid medium due to a dynamic pressure change in the bulk of the medium. When a liquid is subjected to a pressure below its vapor pressure, there is a possibility of generating a cavity or a gaseous bubble which is called as cavitation (see Figure 1.2a). In absence of any nuclei, liquid phase is able to withstand negative pressure (with respect to its vapor pressure) without rupturing or forming vapor cavities. In such cases, cavitation may occur via homogeneous nucleation (formation of small nuclei - transient gaps between molecules caused by random motions of the molecules). However, in most of the real life cases, liquid contains tiny suspended particles or some dissolved gases. These particles or desorption of dissolved gases provide nucleation sites for cavitation. In the presence of such heterogeneous nucleation, cavitation may be assumed to occur when local pressure falls below vapor pressure of liquid.

When a cavity generated in a manner described above is subjected to varying pressure field, complex process of expansion and contraction may occur eventually leading to the collapse of the cavity. Cavity dynamics is generally quite fast and has been extensively investigated. The experimentally visualized growth and collapse of a typical cavity is shown in Figure 1.2b. The collapse of cavities give rise to high velocity jets, very high localized pressure and temperature as well as results in the generation of hydroxyl radicals [3, 32]. The intense shear and hydroxyl radicals generated by collapsing cavities and resulting physico-chemical transformations have been of interest to scientists and engineers for over a century to either develop approaches for avoiding them or harnessing them for variety of desired applications [38].

The history of this phenomenon can be traced back to it being proposed as the possible reason for reduced efficiency of a warship - HMS Darling back in 1885. The phenomenon was thought to be responsible for the formation of gaseous pockets at very high speeds in naval structures and thereby reducing its efficiency. Barnaby [39, 40] wrote papers to describe the phenomenon of creation of voids and cavities, below a certain low pressure. Lord Rayleigh formalized these discussions and theoretically proposed mechanisms for the cavitation phenomenon in 1917, which has been improved until now to develop the current understanding on the mechanism of cavitation. There is evidence that the phenomenon was anticipated as early as 1704 by Newton and further in the works of Euler and Reynolds in the nineteenth century [41]. A detailed historical account of the origin prior to Lord Rayleigh's work is succinctly provided by Young. The historical aspects such as the coinage of the word and earlier theoretical descriptions of existence of such a phenomenon exist prior to Lord Rayleigh's work [33], but for all practical purposes, theoretical investigation and the foundation of the current understanding was from the early twentieth century.

Depending on the way of generation, four types of cavitation processes are known: optical, particle, acoustic and hydrodynamic cavitation. The cavitation phenomena realized by optic and particle methods are not widely investigated, as they are not suitable for effecting change in bulk solutions and have very limited applicability to chemical processing [42, 43]. Acoustic cavitation, as evident from its name, uses ultrasound for generating cavitation. When high frequency sound waves (20?kHz to 200?MHz) of adequate intensity are passed through a liquid, local pressure may dip below the vapor pressure and cavitation may occur. Ultrasonic cavitation was first used to accelerate chemical reactions in 1927...