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Reviews in Chemical Engineering, Vol.17, No.1, 1-85, 2001
Hydrodynamic cavitation reactors: A state of the art review
Hydrodynamic cavitation is produced by pressure variation in a flowing liquid caused by the velocity variation in the system. The present review aims at understanding the various stages in hydrodynamic cavitation starting with discussion about the methods of generation and advantages of the hydrodynamic cavitation over acoustic cavitation in general, followed by the most important part of bubble dynamics as a function of operating and geometric parameters of the hydrodynamically cavitating system and the scope of optimizing the same. Various hydrodynamic cavitation reactors have been described and comparison with the acoustic counterparts indicate higher energy efficiency for the hydrodynamic cavitation reactors, for certain applications. The effects of various operating and geometric parameters viz. inlet pressure/operating speed, physico-chemical properties of liquid, geometry of the constriction, initial radius of the cavitation bubble nuclei on the overall cavitational efficiency have been discussed and recommendations regarding the selection of these parameters for efficient design of the reactors have been made. Further, the effects of hydrodynamic cavitation are compared with its counterpart i.e. acoustic cavitation where the generation of cavities is due to the ultrasonic irradiation of the cavitating medium. Similarities between the two types of cavitation have also been documented for better understanding and various advantages of hydrodynamic cavitation reactors have been stressed with some experimental confirmations. Various applications of hydrodynamic cavitation, though on a pilot plant/laboratory scale, have been highlighted. Some recommendations for future work have been made which can transfer the various laboratory scale applications into commercial scale operations. Based on the detailed analysis done in the present work, it has been conclusively proven that conditions similar to acoustic cavitation can be generated in hydrodynamic cavitation reactors in a much more energy efficient way and the reactors based on the hydrodynamic cavitation phenomena can be scaled up to industrial scale, bypassing the problems associated with acoustic cavitation reactors.