Date of Award

May 2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Ryoichi S Amano

Committee Members

John R Reisel, Yongjin Sung, Ilya V Avdeev, Istvan G Lauko

Keywords

Cavitation, Cavitation Treatment, Hydro Power, Hydrofoils, Pressure Control, Temperature Control

Abstract

Treating the cavitation by optimizing two different methods, and scops to increase the efficiency and the life span of the hydrofoils and hydro turbines. Cavitation can cause a lot of damage to the turbo machinery when the vapor pressure fall below the saturation vapor pressure, phase change occurs and bubbles are formed, bubbles will then hit the surface causing vibration, noise, and corrosion that reduces the life span and efficiency of any turbomachinery. Two methods are introduced in this study to optimize the treatment of cavitation and increase the life span.It is necessary to create and generate cavitation at first and study the cavitation phenomena by utilizing Computational Fluid Design (CFD) analysis at different Angles of Attack (AoA) at different water inlet velocities and test the effect of changing the AoA with the formation of the vapor that causes the cavitation. Then test the same hydrofoil with the two methods of pressure and temperature control to treat the cavitation. The Validation took multiple stages; first is the visual validation, then the CFD simulation results, and lastly the image processing. CFD simulations were conducted using StarCCM+ software and the cavitation can be identified by a unique number called (Averaged Vapor Volume Fraction) (VVF), which indicates the amount of vapor formed compared to the whole volume of water and vapor. The CFD results are compared to previously processed images done by research at the same conditions to validate the CFD tool used. The multiphase physics model was used in the numerical solving technique. The volume of Fluid (VOF) is used for the multiphase model to express the large volumes of water, water vapor, and later, the air injected and to track the interface between them, as this case involves phase change. The unsteady and turbulent phenomenon was simulated using Large Eddy Simulation (LES). As an implicit unsteady simulation for all cases, LES with Wall-Adapting Local Eddy (WALE) is chosen with a time step of 5E-5 seconds and a total solution time of 0.3 seconds. Courant–Friedrichs–Lewy number (CFL) and y+ were compared with the computational time to decide on the best time step and solution time. The first method tested was Pressure Control by injecting pressurized air at different pressure levels to study and test the effect of pressurized air on the cavitation phenomenon. The hydrofoil is tested in a square water tunnel with water entering the tunnel at different velocities for each AoA. While the cavitation can be identified by a unique number (Averaged Vapor Volume Fraction) in the CFD tool, the work done experimentally studied the amount of air being injected and compared to the amount of air in the CFD tool for validation purposes using the High-Speed Camera (HSC) with the same previous stages. The air was introduced at 0psig (1.01 kPa) at (0, 6, 9, and 12 AoA), and at higher pressures of 3, 6, and 9 psig for the 6AoA and the 12AoA case. The second method tested numerically and experimentally was Temperature Control, by cooling down the hydrofoil surface and maintaining the surface temperature lower than the flowing water temperature. Multiple Temperatures were tested (25, 20, 15, and 10°C) at different AoA (degrees) for the 16 m/s and 22 m/s numerically, while experimentally it was tested on 16 m/s and then validated with the CFD image contours using High-Speed Camera (HSC) at multiple AoA. The data acquired from the simulation and the experiment was compared after image processing to find a small percentage of error between the numerical solution and the experiment, in this case validating the CFD tool. Overall, most of the study investigated the cavitation behavior over a hydrofoil involved developing a numerical tool to validate the experimental, for the same model to be used later to investigate other designs and other methods of cavitation treatment to eliminate the need to perform expensive experiments. Both methods showed a great reduction in the vapor formed around the hydrofoil, the temperature control method showed an average reduction in all cases of AoA, and all cases of temperatures at a velocity of 16 m/s of 80% while injecting air at 0Psig showed an average reduction of the vapor formed of about 98% in all AoA and all velocity cases. 3, 6, and 9Psig showed the same trend as the 0Psig trend and the same reduction of an average of 99% for the two cases of 6 & 12 AoA.

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