Spatio-temporal analysis of cavitation dynamics and decolorization of methylene blue in an energy-efficient orifice-type hydrodynamic cavitation reactor

Hydrodynamic cavitation, though a promising advanced oxidation technology for wastewater treatment, is often limited by high energy demands in its industrial implementation. In this study, an optimised orifice-type HC reactor geometry was arrived at using numerical simulations and optimization techniques to simultaneously maximise cavity collapse intensity and minimise energy requirement. The resulting geometry was subjected to experimental characterisation, focusing on the spatiotemporal evolution of cavitation dynamics. These analyses reveal that the cloud cavitation regime, characterised by periodic shedding and collapse-induced shock waves, offers the greatest treatment potential. Intense cavity collapse, quantified primarily by pressure fluctuations, results in the chemical degradation of methylene blue (MB). Pressure fluctuations and MB removal efficiency are correlated, thereby providing a rapid, cost-effective diagnostic framework for predicting reactor performance. A comparison of the present results with those available in the literature indicates that the optimised geometry is significantly more energy-efficient (10.2 kJ/L), achieving comparable degradation (11% MB decolourisation at pH 7.8 after 90 passes). Hence, this approach establishes it as a robust candidate for scaling up for wastewater treatment applications.