Effects of Surface Wettability on Cavitation Inception and Cloud Cavitation

Abstract

Cavitation, the phenomenon involving the formation and collapse of bubbles due to pressure variations in liquid flows. This causes noise, vibrations, surface damage, and reduced efficiency in hydraulic systems. Cavitation is strongly dependent on the surface of the systems involved. However, the effect of surface wettability on controlled flow conditions is largely unexplored. The experiment in this study investigates the impact of cavitation formation and development over Venturi geometry. The Venturi models were engineered using three techniques: spray-coating, chemical etching, and initiated chemical vapor deposition, to induce different surface wetting characteristics. The models were characterized through contact angle and roughness measurements. High-speed imaging, with synchronized pressure data, was obtained to study cavitation: inception bubble statistics and cloud cavitation dynamics. Space-time diagrams, power spectral density, spatial occupancy maps, and RAFT-based optical flow velocimetry were performed to quantify the vapor and liquid-phase dynamics. The results revealed that the surface with increased hydrophobicity was incepted earlier and demonstrated localized near-wall inception at the Venturi throat. For example, the carbon black spray-coated surface, with a contact angle of 140 deg, promoted inception at 45 L/min, whereas the clear surface (Contact angle of 84 deg) incepted at 50 L/min. In addition, the hydrophobic surface led to reduced modal bubble size, nearly half the size as seen over the clear surface. The hydrophobicity also influenced the peak inception event density, with inception events 10 times higher than in the clear case. In the cloud cavitation analysis, hydrophobic and superhydrophobic surfaces consistently produced thinner and shorter cavities with higher shedding frequencies. At the same Reynolds numbers, the carbon black spray-coated surface reduced the cavity length by about 4.9 mm in comparison to the cavities over the clear surface. The etched Aluminum, with increased surface roughness and hydrophobicity, showcased suppression of cavitation cycles. In contrast, the iCVD superhydrophilic surface produced larger, longer cavities, reaching 16.78 mm at Re = 238697, compared to cloud lengths of 15.61 mm and 12.9 mm for the clear and iCVD hydrophobic surfaces, respectively. The RAFT-based velocimetry illustrated more attached shear layers, with reduced upstream circulation for hydrophobic surfaces. On the other hand, the surfaces at the other end of the wettability spectrum, clear and superhydrophilic surfaces, led to broader and detached vapor structures. As a whole, the findings indicate that the surface wettability acts as an important parameter for altering the cavitation flow dynamics, both at initial and advanced stages of cavitation. The experimental results guide engineering strategies focused on delaying, suppressing, or controlling cavitation in related engineering systems.