Cavitation

Bubble Trackers

Yves Lecoffre

• Publisher: Routledge
• Year: 2021
• eBook ISBN: 9781351462167
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

This text deals with the concept that cavitation is the main limitation to the performance of hydraulic components. Topics covered include the vaporization of liquids due to high velocities or pressure fluctuations, and the effects of cavitation on the performances of rotary machinery. One chapter is devoted to cavitation noise which concerns many users, including surface ships and submarines, and the author finishes with some examples of the use of cavitation and subject-specific measuring techniques.

1

Phenomenon of Cavitation

1.1 INTRODUCTION

Cavitation is the phenomenon of vaporisation of fluids under the effect of depressurisation, i.e., low pressure caused by high flow velocity. It occurs in many hydraulic machines and components whose effective range of operation is limited by the appearance or development of the phenomenon.

Generally, the following undesirable effects are produced by cavitation:

1. — generation of intense noise. When cavitation occurs the noise level increases by up to 20 to 40 dB;
2. — damage to surfaces. Erosion necessitates frequent repair of machines even when they are correctly designed from a strict point of view of hydraulic performance;
3. — loss in level of performance. When cavitation is well developed the machines undergo large losses in performance and may even become totally unusable.

One of the objectives of this book is to provide designers and users of hydraulic machines and components sufficient information that will give them a better understanding of the phenomenon of cavitation in practical situations.

After explaining the universal aspects of the phenomenon of cavitation, we shall describe the classical methods of characterisation of machines and components as well as the general aspects of similitude. Thereafter, we shall discuss the current state of theoretical and experimental knowledge of various aspects of cavitating flows in accordance with such characterisation. Next, we shall discuss the developments in the field of erosion and similitude. A chapter is devoted to cavitation noise and another to the thermodynamic effects. Finally, we shall present the methods of testing and the general manner of their implementation.

1.2 VAPORISATION

Vaporisation of a liquid is the change of phase that converts it into the vapour state. This transformation is mostly brought about by adding heat, as in the case of boiling in heating systems or more commonly in a casserole. It can also be achieved by depressurisation, for example in evaporators in desalination factories. In some cases, vaporisation is accomplished by a combination of heat addition and reduction in pressure.

The two types of vaporisation are illustrated in the thermodynamic p − v diagram of Fig. 1.1, showing pressure as a function of specific volume.

Fig. 1.1. Modes of vaporisation of a liquid.

The transformation AC represents vaporisation at constant pressure attained by heat addition Q · AB represents vaporisation due to reduction in pressure. Both transformations need heat addition to provide for the latent heat of change of phase.

In the first case (AC) the heat is provided entirely by the surroundings and the mass of vapour produced is given by

$\text{m}=\frac{\text{Q}}{\text{L}}$

where L is the latent heat of vaporisation of the liquid.

In the second case (AB) the transformation is globally adiabatic, i.e. there is no exchange of heat with the surroundings. Therefore, in order to produce vapour, the unevaporated liquid must get cooled; this is schematically shown in the figure by a shift from one isotherm (through A) to another (through B).

The pressure at which vaporisation occurs depends on the temperature. Fig. 1.2 shows the dependence of the vapour pressure of water on temperature.

Table 1.1 gives various physical properties of water of interest in the study of cavitation: pressure, temperature, latent heat, enthalpy of liquid and vapour, density of liquid and of vapour.

Table 1.1 Some properties of water

T °K	Pv bar	ρv kg/m^3	ρL kg/m^3	HLkJ/kg	HG kJ/kg
273.15	0.00611	1.047 E-10	1000	0	2502
290	0.01917	0.0143	999	70.7	2532
320	0.1053	0.0715	989	196	2586
373.15	1.0133	0.596	958	419.1	2676
420	4.370	2.35	919	618.6	2742
470	14.55	7.35	868	838.2	2789
570	82.16	43.86	718	1382	2757

Fig. 1.2. Vapour pressure of water.

It may indeed be noted that the vapour pressure for water is very low at ordinary temperatures; hence, the density of vapour is extremely low. Therefore, production of a significant volume of vapour at low temperature needs a very small amount of heat addition and very often, in the case of cavitation in cold water, such heat addition is negligible.

All liquids are susceptible to cavitation as soon as the pressure attains a value equal to the vapour pressure. Water occupies a very important position, both because of its occurrence in numerous technical applications and because it is water that is used to simulate the behaviour of other fluids in most experiments.

The thermodynamic characteristics of several substances currently used in technical applications are given at the end of the book.

1.3 CAVITATION: SOME EXAMPLES...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Foreword to the English Edition
• Foreword to the French Edition
• Acknowledgements
• List of Symbols
• Table of Contents
• Introduction
• 1. Phenomenon of Cavitation
• 2. Parameter σ of Cavitation
• 3. Types of Cavitation
• 4. Single Bubble Life
• 5. Cavitation of a Hydrofoil
• 6. Travelling Bubble Cavitation
• 7. Fixed or Attached Cavitation
• 8. Other Types of Cavitation
• 9. Cavitation Noise
• 10. Thermodynamic Attenuation of Cavitation
• 11. Cavitation Erosion
• 12 Cavitation in Rotary Machines
• 13. Test Facilities
• 14. Instrumentation
• 15 Applications of Cavitation
• Some Physical Properties of Fluids
• References
• Index