This invention concerns apparatus and methods for measuring the quantity of bubbles present in a liquid containing gas bubbles. The method and apparatus are particularly useful in determining the gas bubble content of liquids turbulently flowing through pipes. The determination is made at a fixed location in the pipe so that the gas bubble content of the flowing liquid can be continuously monitored.
Techniques for determining the presence or absence of gas bubbles in liquids are known. Apparatus for determining the onset of the formation of bubbles, i.e. cavitation or boiling, but not the proportion of bubbles, has been described by a number of patentees. In U.S. Pat. No. 3,381,525 to Kartluke et al., sound waves are launched into a liquid. The liquid is monitored for sound waves at subharmonic frequencies of the launched sound waves. When subharmonic waves are detected, cavitation is imminent or has begun. In U.S. Pat. No. 3,240,674 to Ledwidge, a similar technique is used. No sound waves are added to the liquid. Instead, the frequencies of sound waves in the liquid are monitored for a selected spectrum peak that indicates localized boiling, a prelude to boiling of the entire liquid volume. U.S. Pat. No. 3,622,958 to Tucker et al. discloses a number of methods of detecting the existence of gas bubbles in a liquid. Waves at a fundamental frequency are launched into a liquid by a first transducer and waves at harmonic frequencies are detected by a second transducer. Detection of harmonic frequency signals indicates the presence of gas bubbles. Alternatively, reflected waves at harmonic frequencies are detected by the same transducer that launches the fundamental frequency wave. In still another embodiment, multiple frequency sound waves are launched into the liquid which is monitored for waves having frequencies equal to a sum or difference of two of the frequencies of the launched waves.
A complex method of determining the fraction of steam in a steam/water mixture was disclosed by Arave in "An Ultrasonic Void Fraction Detector Using Compressional Stress Waves in A Wire Helix" published October 1970 by the Idaho Nuclear Corporation for the U.S. Atomic Energy Commission. In this method, a stress wave is propagated on the surface of a wire helix that is immersed in a liquid-gas mixture. The attenuation of the stress wave from one end of the helix to the other is measured to determine the "void fraction", i.e. bubble content, of the liquid in the local volume adjacent the surface of the helix.
The known technology does not provide a simple, reliable method of quantitatively measuring the bubble content of a liquid-gas mixture. The invention provides a simple method and apparatus for measuring the bubble content in a liquid and especially in a turbulently flowing liquid.