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Engineering

Research project: Cavitation erosion-corrosion of ship propeller materials

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Cavitation erosion is the phenomenon where the body of liquid undergoes reduction in pressure at a constant temperature by static or dynamic means due to friction in liquid causing the a state of pressure, called the vapour pressure, to be reached where the liquid starts boiling and gas/vapour bubbles form and grow, which on collision with a solid surface implode and cause wear. Marine propulsion systems (especially propellers and rudders) have been known to suffer the adverse effects of the cavitation erosion-corrosion phenomenon.

Project Overview

Cavitation is a general fluid mechanics phenomenon, which can occur whenever a liquid used in a machine inducing pressure and velocity fluctuations in the fluid e.g. Pumps, turbines, propellers, bearings, during a traumatic injury the brain agar phantom may cause pressure change, causing cavitation, and even pressure changes in the fluid in the heart and knee joints. Cavitation can cause erosion of materials it is in contact with such as ship propellers and rudders during their operation at sea. And if the eroded surface in subjected to a corrosive environment, such as in the case of ship propellers functioning under water, corrosion is inevitable. Such a process is destructive in nature and causes severe loss in performance of the ship leading to eventual costly maintenance in the form of frequent dry dockings, inspections and preventive maintenance or replacement of the damaged part itself.

The first objective of this research is to review physical mechanisms for cavitation erosion loads that have been suggested in the literature. These mechanisms are evaluated with observations on the detailed dynamics of the flow over a cavitating foil, and with observations that are available from ships where cavitation has lead to erosion damage on the rudder or the propeller.
The second objective is to review the use of Computational Fluid Dynamics (CFD) or experimental results as input for the prediction of the risk of cavitation erosion. In addition to characterization of the materials based on steady mass loss over long periods of cavitation erosion and in-situ electrochemical measurements of corrosion, this study is aimed to understand the cavitation phenomena simulated by the vibratory probe device, tunnel or cavitation jet through CFD studies. Several tests will be conducted, with and without cavitation protection, using both direct and indirect methods of cavitation erosion tests.

 

Figure 1

An experimental rig, replicating the one used by Lloyd’s Register (LR) in 2011 under ASTM standards, is set up comprising of ultrasonic sonotrode, a seawater tank and a bath, a circulatory pump and 25x25mm ship propeller material samples. The test rig set up can be seen in figure 1. The seawater is made to circulate throughout the system via the bucket and into the glass bath using pipes and cables (not shown in the picture). The sonotrode consisting of a titanium vibrating horn is meant to vibrate at a frequncy of 19.5KHz and an amplitude of 114μm, submerged in a bath of seawater along with the specimen. The samples are eroded under probe tip in the circulating seawater bath due to cavitation induced by the horn. Various cavitated specimen samples of Nickle Aluminium bronze, low alloy steel, martensitic stainless steel, low carbon steel, composites and duplex stainless steel, obtained from the previous experiments conducted by LR, were analysed for surface texture topography and scar profiling using Alicona.

Figure 1: Experimental test rig for cavitation erosion simulated by vibratory horn.

Associated research themes

Cavitation wear and corrosion

Publications

Book Chapter

Staff

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