A Note on Enhancing Aeration via a Vortex-Based Cavitation Device

Abstract

There is growing interest in generating micro- or nanobubbles for enhancing aeration. Small bubbles not only enhance the interfacial area for gas–liquid mass transfer but also may enhance the equilibrium solubility if the size of the bubbles is small enough. In this note, we demonstrate the use of a vortex-based hydrodynamic cavitation device (VD) for generating small bubbles and enhancing aeration. Experimental results for conventional aeration and aeration with VD operated under three different conditions are presented. A reference case of potential degassing because of the low pressure generated in the cavitation device was also investigated. Experiments were carried out in a bubble column using DI water as the liquid phase. The dissolved oxygen (DO) concentration was measured using a precalibrated dissolved oxygen probe. Measurements of transient profiles of dissolved gas concentrations were carried out under different operating conditions. A generalized framework to analyze mass transfer in the presence of degassing, absorption, and desorption (via top surface or large bubbles) is developed and used for interpreting the experimental data. The per-pass degassing factor of VD was found to increase with the power dissipation [∝ (P–P_c)^0.4, where P is power dissipation and P_c is the critical power beyond which degassing starts]. The aeration generated by VD was found to realize 30% higher DO concentration beyond the equilibrium solubility at atmospheric conditions. The bubble sizes estimated from the steady-state DO concentration were in the range from 80 to 200 μm for the operating parameters considered in this work. The presented results demonstrate the effectiveness of VD for enhancing aeration and will be useful for intensifying gas–liquid processes.