Measurements Of The Thickness Of Liquid Films Utilising Total Reflection Of A Laser Beam At The Phase Boundary

Abstract

In this work, a measurement technique for measuring liquid film thickness based on the total reflection of a laser is investigated. The thickness of liquid films is a key parameter in modeling certain two-phase flow regimes like annular or slug flow. Its measurement usually requires extensive optical or mechanical access to the flow. However, in situations where optical access is limited and radical modifications on the measurement object are impossible, e.g. in an operating fuel cell, these conventional techniques are not easily applicable. A key feature of the technique presented in this work is that it requires no modifications other than an optical access at the surface on which the liquid film of interest is deposited on. A laser beam is introduced through a prism to the optical access and consequently the liquid film in an angle that causes the beam to be totally internally reflected at the free surface of the film. An imprint of the reflected beam, which leaves the optical access again through the prism, is captured on a scope. If the thickness of the film changes, the laser beam’s optical path changes with it, causing a displacement of its imprint on the scope. This displacement is a direct measure for the film thickness. In this work, a three dimensional linear algebra model describing the experimental setup is presented and used to derive the basic working principle. It is successfully shown in a proof of concept study that the measurement technique correctly estimates the thickness of a static liquid film deposited on a prism in combination with a cross-correlation based image processing routine. To understand the restrictions of the measurement technique, the most important error sources with regards to the measurement of dynamic film flows are discussed using a mixed analytical and numerical approach based on the previously derived working model. The technique is likely applicable to dynamic film flows. However, concerns regarding high measurement uncertainty caused by curved free surfaces demand further investigation.