Surface wettability effects on the behavior of air bubbles injected into water flows
Creators: name: Mullennex, Ryan
Publisher: name: University of Iowa
Year: 2023
Subjects: Frictional Drag Reduction
Laser Processing
Surface Wettability
public abstract

description: The practice of injecting air bubbles into the boundary layer of external viscous fluid flows has been studied in recent decades as a viable method of reducing frictional drag. This outcome is desired by many industries, marine industry in particular, as an approach to diminish fuel consumption and pollution of ocean-going vessels. Tremendous drag reduction has been observed by employment of air injection in model-scale experiments, yet adoption at the industrial scale has been quite limited. A large reason for this hesitation is the lack of control over the air bubbles as they travel along the solid surface. Air bubbles tend to migrate from the solid surface a short distance from where they are injected and have small effect on frictional drag reduction over large length scales. There is also a lack of control over air bubble size and shape in the streamwise direction. A solution to these setbacks must be reached before industrial-sized applications may be pursued.
description: This research investigates manipulation of the solid surface wettability by laser processing methods as a potential solution towards enhanced control of air bubble size and spatial distribution in water flows. Flat plates of size 305 mm by 152 mm were treated with a laser-based surface wettability modification technique that allows for coexistent regions of superhydrophobicity and superhydrophilicity on a single sample. Plates with various wettability patterns were created; their formation and processing mechanisms are described. All samples were subjected to water flows with air bubble injection through a water tunnel experiment. The void fraction of these flows varied between α ̅=0.428-0.566. The behavior of air bubbles as they pass over regions of superhydrophobicity and hydrophilicity were captured through video data of the flat plate surface. These results were compared with a baseline untreated flat plate. Disparate bubble behavior was observed between wettability states characterized by retainment of small circular bubbles over untreated and hydrophilic surfaces and immediate bubble coalescence over superhydrophobic surfaces. Data of air bubble spatial distribution, size, and speed (for the small circular bubbles only) were captured.
description: A multiphase computational fluid dynamics (CFD) model was formed using a commercial software to further elucidate the role of surface wettability on the behavior of injected air bubbles in water flows. A simple volume of fluid model was constructed. Verification of this model was performed by applying single-phase water flows and comparing the results with boundary layer theory. Void fraction illustrations extracted from the CFD model agree with observations of the water tunnel experiments. From these results, it is evident that surface wettability can be employed in drag reduction methods to control the size and spatial distribution of air bubbles. Such outcomes may enhance the comprehensive energy cost-benefit of the drag reduction by allowing for controlled placement of air bubbles or air layers on the hulls of ships.