Abstract
Cavitation events create extreme conditions in a localized ‘bubble collapse’ region, leading to the formation of hydroxyl radicals, shockwaves and microscopic high-speed jets, which are useful for many chemical and physical transformation processes. Single bubble dynamics equations have been used previously to investigate the chemical and physical effects of cavitation. In the present study, the state of the art of the single bubble dynamics equations was reviewed and certain noteworthy modifications were implemented. Simulations reaffirmed previously reported collapse temperatures of the order ~5,000 K and collapse pressures well over ~1,000 bar under varying operating conditions. The chemical effects were assessed in terms of the hydroxyl radical generation rate (OHG), calculated by applying the minimization of the Gibb’s Free Energy method using simulated collapse conditions. OHG values as high as 1x1012OH molecules per collapse event were found under certain operating conditions. A new equation was proposed to assess the physical effects, in terms of the impact pressure of the water jet - termed as the jet hammer pressure (JHP), formed due to the asymmetrical collapse of a bubble near a wall. The predicted JHP were found to be within a range of ~100 to 1000 bar under varying operating conditions. Important issues such as the onset of cavitation and chaotic solutions, for a cavitating single bubble dynamics were discussed. The Blake threshold pressure was found to be a sufficient criterion to capture the onset of cavitation. The impact of key operating parameters on the chemical and physical effects of cavitation were investigated exhaustively through simulations, over the parameter ranges relevant to acoustic and hydrodynamic cavitation processes. Presented methodology and results will be useful for optimisation and further investigations of a broad range of acoustic and hydrodynamic cavitation-based applications.
Highlights
Cavitation first studied by Lord Rayleigh was identified as the cause for the damage to ship propellers rotating at very high speeds and posed a limitation for further technological improvements [1,2]
The single bubble dynamics models coupled with a detailed chemical ki netics model, enabled the first predictions of the hydroxyl radical gen eration rate due to cavitation events [7,20] and have been used recently to provide further insights into the chemical effects of cavita tion [26,27]
Considering that hydrodynamic cavi tation (HC) occurs at very low ambient pressures, pressure time trajectories obtained by tracking multiple single cavities using the discrete particle model coupled with computational fluid dynamics (CFD) sim ulations, suggest that like acoustic cavitation (AC) pressure ampli tude ratio (P’A) values up to 3 may theoretically be expected [51,52]
Summary
Cavitation first studied by Lord Rayleigh was identified as the cause for the damage to ship propellers rotating at very high speeds and posed a limitation for further technological improvements [1,2]. The single bubble dynamics models coupled with a detailed chemical ki netics model, enabled the first predictions of the hydroxyl radical gen eration rate due to cavitation events [7,20] and have been used recently to provide further insights into the chemical effects of cavita tion [26,27]. Such predictions could be made by using the Gibbs free energy minimization method [28,29,30]. The presented results and comprehensive information included in the Supporting Information will enable use of the microscale single bubble dynamics model with other macro-scale reaction engineering models
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