Prevention of Cavitation in Propellers

Mojtaba Taseidifar,John Joseph Antony,Richard Pashley

doi:10.36253/substantia-821

Abstract

It is well known that cavitation phenomena affect the efficiency of propellers. It is a major world wide economic problem for the transport industry. The speed of fast, ocean going vessels is liimted by cavitation effects on hydrofoils and propulsion systems. The main approaches of industry to mitigate the detrimental effects of cavitation on propellers is restricted to varying operating conditions, geometric design and choice of wear resistant materials. We here develop a simple solution to the problem. It has been known for over a century that dissolved gases reduce the tensile strength of liquids by orders of magnitude. De-gassing of a iquid dramatically reduces the ability of a fluid to cavitate. Propeller cavitation in ships and submarines is typically controlled by reducing rotation rate and/or blade pitch. We here demonstrate the astonishing fact that cavitation can be completely prevented by releasing de-gassed water adjacent to the low pressure side of a rotating propeller, without varying blade speed or pitch. Practical implementation is simple and cheap.

Highlights

At the end of the 19th century it was realised that ships were not attaining their projected design speed
It is generally assumed that local suction pressures just below the vapour pressure of a fluid, at a given temperature, will nucleate bubbles which implode once local hydrostatic pressure returns
Nano-sized cavities are usually the smallest structures which can be considered as a separate phase, and their growth or collapse controls the extent of cavitation.[4]

Summary

Introduction

At the end of the 19th century it was realised that ships were not attaining their projected design speed This was eventually found to be due to cavitation.[1] The effects of the collapse of a spherical cavity within a fluid were first considered by Besant in 1859.2 Cavitation in fluids has been studied for over a century since this pioneering work and that of Reynolds in 1886 and Lord Rayleigh in 1917.3 There are two main types: inertial cavitation, created by differences between boundary and bulk fluid flow in pumps, valves and propellers; and non-inertial cavitation, created by oscillatory processes such as simple shaking and sonication.[1] Cavitation occurs in fine cavities between solid surfaces.[4] In many diverse processes, cavitation reduces the efficiency of fluid systems but the collapse of the created bubbles near surfaces creates shock waves. The cavitation index C (σ or sometimes k) is used as a measure of cavitation potential and is defined as:[5]

Methods

Results

Conclusion