Experimental investigation of cavitation dynamics at mesoscale and microscale in swirled and non-swirled venturi tubes

Hydrodynamic cavitation has been widely used in various applications, namely, peening, surface cleaning, and wastewater treatment. Recent studies have demonstrated that introducing swirl in hydrodynamic cavitation can substantially enhance process efficiency. However, a knowledge gap remains regarding the comparative characteristics of hydrodynamic cavitation in swirled and non-swirled flows at the mesoscale and microscale. In this study, we utilize shadowgraph and high-speed phase-contrast x-ray imaging techniques alongside spectral proper orthogonal decomposition to resolve such characteristics in venturi tubes. The analyses show that imposing swirl reduces the hydraulic power delivered by the pump to initiate cavitation by 71.9%. It also makes the cavitating length of the flow less dependent on the operating condition. Investigations indicate that the non-swirled venturi tube is dominated by sheet cavitation followed by cloud cavitation. Introducing swirl shifts the cavitation toward the cloud regime established at the center of the tube while changing the coherent motions of the cavitating flow. In non-swirled flow, coherent motions are dominated by disk-like structures, while in the swirled flow, they include helical and double-helical coherent structures. Both mesoscale and microscale analyses reveal that swirl shifts the cavitation dynamics toward low-frequency coherent motions. Microscale results suggest that microbubbles from cavitation cloud collapse could trigger cavitation when they are involved in high-velocity motions.