In hydrodynamics, liquid flows are encountered which are referred to as being "cavitating" because a local pressure drop may give rise to bubbles of vapor which may suddenly disappear by implosion due to a subsequent rise in pressure or which may attach themselves to a solid wall and grow in volume, thus forming a cavity. Cavitation, i.e. the appearance of such bubles, is an important phenomenon: for example it is cavitation which oftens limits the maximum propulsive power of a ship's propeller.
The behavior of cavitating flows is highly influenced by the concentration of nuclei in the liquids which constitute such flows. These nuclei are weak points in the structure of said liquids. They may be of known or unknown nature, but in practice they may always be measured in terms of their critical pressures. That is to say by that pressure which is less than the vapor pressure and is often negative, to which the liquid surrounding a nucleus must be subjected in order to give rise to a bubble of vapor.
In several practical situations, for example when testing a propeller in a laboratory, it is necessary to be able to measure the concentration of such nuclei at various critical pressures in the liquids being used. This concentration of cavitation nuclei is generally shown in the form of a histogram showing a cumulative concentration as a function of decreasing critical pressure.
The present invention enables such measurements to be performed.
The concentration of nuclei in a liquid is currently measured using two types of method: there are so-called "non-cavitating" methods which are used when the nature of the nuclei is known. The nuclei are generally micro-bubbles in this case. A magnitude is measured representative of their size. A transposition formula is used to deduce the critical pressure therefrom. Such methods include: holography, Coulter's electrical method, and the light diffusion method.
In so-called "cavitating" methods, the liquid is forced to pass at high speed through a venturi (i.e. a Venturi tube). A conventional venturi comprises a converging portion leading to a throat and followed by a diverging portion, the flow cross-section is circular throughout and the minimum diameter is at the throat.
A region of minimum adjustable pressure Pt is thus created at the throat of the tube, which pressure may be less than the vapor pressure of the liquid. When a nucleus passes through the tube it may be excited or it may not depending on whether its critical pressure Ps is greater than or less than the pressure Pt. The pressure Pt may be varied by varying the water flow rate passing through the venturi, and classes of nuclei having different critical pressures can thus be excited, thereby enabling the histogram representative of the nucleus population to be determined, by detecting and counting excited nuclei and by measuring the pressure Pt and the water flow rate. Such cavitating methods are described in particular in the following documents:
O.N.R. Symposium, Ann Arbor University (Michigan) 1981, in a communication by Y. Lecoffre and J. P. Legoff (Nuclei and cavitation); and
A.I.R.H. Bulletin, Amsterdam 1982 "Aspects pratique du controle de germes de cavitation en moyens d'essais" by Lecoffre, Marcoz, Valibouze, (Practical aspects of monitoring cavitation nuclei in test equipment).
Apparatus in accordance with the present invention uses a cavitating method.
Presently existing apparatuses using such a method suffer, at present, from three major defects:
1. There is a high degree of interaction between the bubbles developing in the venturi throat and the incident flow. Thus, at a given water flowrate, the throat pressure varies as a function of time and the accuracy of the measurements is poor, in particular when the concentration of active nuclei is high or when the pressure Pt is very low;
2. Measurable concentrations are limited to a few nuclei per cm.sup.3, which is insufficient in some applications; and
3. Venturi tubes are difficult to make and their reliability can pose serious problems. More precisely, their surface state must be excellent in order to obtain reliable measures, and this requires sophisticated and expensive machining methods to be used such as certain kinds of mechanical and electrolytic polishing.
The aim of the present invention is to provide apparatus for measuring the concentration of cavitation nuclei in a liquid in such a manner as to obtain accurate and reliable measurements at low cost even when the concentration of nuclei is high.