Full Papers Optimization of Hydrodynamic Cavitation Using a Model Reaction By Nilesh P. Vichare, Parag R. Gogate, and Aniruddha B. Pandit* The decomposition of potassium iodide to liberate iodine, the model reaction to study cavitational effects, has been carried out under different cavitational conditions. The effect of various parameters (inlet pressure, flow geometry of orifice plates) on the iodine liberation rate has been studied. It is found that the flow geometry of the orifice plates considerably affects the rate of the iodine liberation. Recommendations are given for the arrangement of the holes in order to achieve maximum benefits from the hydrodynamic cavitation. The experimental results obtained in the present work are very much consistent with the results based on the theoretical model developed for the hydrodynamic cavitation. Due to this fact, it can be said that the model can be extended to any geometry of construction in the hydrodynamic cavitation setup and will be helpful in designing cavitational reactors. 1 Introduction In hydrodynamic cavitation, the overall cavitational effect depends on the intensity of turbulence and the number of cavities generated [1]. In the case of the orifice plates, the permanent pressure drop across the orifice plate is always high and the intensity of turbulence increases with a decrease in the orifice opening to pipe diameter ratio (). The presence of turbulence makes cavitation transient which otherwise would have been stable and the increase in turbulence makes the collapse of cavities more violent, generating large magnitude pressure pulses [1]. Yu et al. [2] made the detailed numerical study of the collapse of cavities in the shear layer formed behind the bluff bodies. A comparison of the cavity collapse in the shear layer and in the quiescent liquid shows that an increase in turbulence in the shear layer increases the rate of cavity collapse significantly. For the orifice plates having almost the same free area or flow area, the pressure drop across the orifice remains the same, resulting in the same power dissipation per unit mass of liquid (P M ). However, the same flow area can be accommo- dated by using multiple-hole orifice plates with different combinations of the number of holes and the size of the holes. Thus, the scale of turbulence, intensity and frequency of turbulence can be altered [3]. As the power input remains the same using these plates, the cavitational yield can be improved by varying the flow geometry of the orifice plates. In the earlier work [3], the effect of various parameters, such as the inlet pressure, downstream pressure, perimeter of the holes, on the cavitational yield has been studied developing a sound theoretical model for the hydrodynamic cavitation. The present work aims at confirming the results obtained earlier with the help of a model chemical reaction and also at developing some guidelines for the arrangement of the orifice plates in the design of the hydrodynamic cavitational reactor in order to achieve maximum benefits. To study the global effect of the operating parameters and the fraction of the overall flow area occupied by the shear layer on the chemical reactions in the presence of cavitating conditions, one of the reactions, i.e. the decomposition of potassium iodide, which requires a cavitational effect has been considered. The chemistry part of the model reaction has been studied in detail by Suslick et al. [4]. Senthilkumar [5], using two multiple-hole orifice plates, has studied the cavitational effect of hydrodynamic cavitation on the potassium iodide decom- position. Turbulence in the shear layer and the area occupied by the shear layer were found to be important factors affecting the cavitational yield. The iodine liberation with the plate having smaller diameter holes was found to be higher than that with the plate having large diameter holes for the same flow area. In the present study, six orifice plates with different geometries have been used to study the cavitational effects in hydrodynamic cavitation and the potassium iodide decom- position to liberate iodine has been studied using these plates. 2 Salient Features of the Model In the earlier work [3], a simplified and unified model has been proposed to study the cavitation phenomena in the hydraulic devices. The dynamics of a cavity is strongly dependent on the surrounding pressure field and its time variation around the cavity. Hence, a fluid turbulence model analogous to the acoustic cavitation was used accounting for the turbulence effects due to the presence of constrictions in the flow field. The turbulent pressure obtained from the above model was then used to solve the cavity dynamics equation, i.e. the Rayleigh-Plesset equation for growth and collapse phases Chem. Eng. Technol. 23 (2000) 8, Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0930-7516/00/0808-0683 $ 17.50+.50/0 683 ± [*] N. P. Vichare, P. R. Gogate, A. B. Pandit (author to whom correspondence should be addressed), Chemical Engineering Section, University Depart- ment of Chemical Technology, Matunga, Mumbai- 400 019, India; e-mail: abp@udct.ernet.in 0930-7516/00/0808-0683 $ 17.50+.50/0 Full Paper