The methods of film thickness measurement in annular flow can be classified into two groups: methods involving contact with the fluid and methods without contact.
The methods with contact are: quick-closing valve method; methods based on measurement of conductance or conductance and capacitance of the film; electrical contact method.
The methods without contact are: methods based on measurement of the absorption of light, β, or γ beams; optical methods; fluorescence method.
The liquid volume V is measured in a channel with length L by quickly closing valves at the start and end of the length. The mean film thickness δ = Vπ/DL is determined. The complexity of method consists of the realization of instantaneous and simultaneous closure of the valves. It is usually assumed that the part of the liquid volume associated with liquid drops is negligible.
In the conductance method, the conductance of the liquid film is measured between electrodes flush with the channel wall. An electrolyte is often added to liquid to make it more conductive. The measurement section must be electrically insolated. A wide variety of electrode designs have been employed [see Hewitt, (1982)]. The closer the electrodes, the more localized the measurement but the more restricted the linear range of the relationship between film thickness and conductance. Less commonly, for nonaqueous systems, the capacitance of the film is measured.
Here, a needle is moved in a direction normal to the liquid film and the distance from the wall is determined when electrical contact between the needle and the film appears. If there are the waves on the film surface, short-time contacts with the wave crests arise. An uninterrupted contact exists if the needle touches the wave trough. The film can stick to the needle and the needle can influence the flow. The electric contact method is not usually suitable for small film thickness (δ ≤ 0.1 mm).
In this method, a light beam is passed through the film. The light absorption depends on film thickness. The intensity of transmitted radiation (determined with aid of a photodiode or photomultiplier) decreases not only due to absorption. If the film surface is wavy, the light can scatter, refract or reflect. These effects must be considered and may decrease the usefulness of the method.
In these methods, β or γ beams are passed through the film. The transmitted intensity is given by:
where I0 = intensity of incident radiation, ρ = liquid density, k = absorption coefficient, y = layer thickness. The method allows measurement of the mean film thickness. There are obvious problems in many systems of gaining interrupted access of the beam to the film.
When investigating the liquid film flow on outer cylinder surface, the thickness can often be measured using shadowgraphy or photography.
This method is based on using a light beam with a given wavelength that passes through the liquid film. A fluorescent substance is added to the circulated liquid and the incident light stimulates fluorescence with a different wavelength. The quantity of emitted fluorescent light increases with increasing of film thickness. This value is measured with the aid of a photomultiplier and spectrometer. The method allows highly localized measurements.
Hewitt, G. F, Hall-Taylor, N. S. (1970) Annular Two-Phase Flow, Pergamon Press, Oxford.
Hewitt, G. F. (1978) Measurement of Two-Phase Flaw Parameters, Academic Press, London.
Hewitt, G. F. (1982) Measurement Techniques. Chapter 10 of Handbook of Multiphase Systems, G. Hetsroni. Ed., McGraw-Hill Book Company, New York.