Automated optical liquid film thickness measurement method

Abstract

The need to measure the thickness of thin liquid films is evident from the number of methods that have been developed to do so. Many of these methods have significant drawbacks, such as intrusive probes or the dependence on a conductive liquid. A non-intrusive, automated, optical film thickness measurement technique has been developed to be used with a wide range of fluids and with virtually any flow configuration. In this method, light is reflected from the surface of a liquid film flowing over a transparent wall. This reflected light generates an image on the outside of the wall which is captured and digitized using a CCD camera and framegrabber card in a desktop computer. The image is processed to determine the positions of the reflected light rays, with which the film thickness and film slope are calculated. The entire process is automated and can be performed in less than 9 seconds on a 486 PC, allowing many data points to be collected efficiently. Film thicknesses as small as 0.03 mm can be determined using inexpensive components, with the possibility of greater precision using more advanced imaging equipment. An automated calibration procedure allows for the determination of the necessary physical parameters automatically, so the index of refraction of the test fluid or the test section wall need not be known a-priori. A prototype of the automated system generates static liquid measurements that agree to within 3% of measurements made using the needle-contact method. Film thickness data are also presented for an air-water system in cylindrical, annular, two-phase flow and compared with data from the literature.111

Adiabatic and diabatic measurements of the liquid film thickness during spray cooling with FC-72

“…The diameter of the light ring is proportional to the total distance that the light travels on its round trip through the wall and liquid. Thus, if the wall thickness is known, the thickness of the film can be determined once the radius of the light ring has been found [6,7]. It should be noted that if the liquid thickness is approximately 1.5 times the thickness of the transparent wall, the light will strike the solid/liquid interface at an angle greater than the critical angle for that interface and a constant light ring corresponding to the thickness of the wall will appear [6].…”

Characterisation of liquid film in a microstructured falling film reactor using laser scanning confocal microscopy

“…Examples include conductivity probes [15][16][17], ultrasound detectors [18], fibre-optic sensors [19], microwave techniques [20,21], laser interferometry [22,23], fluorescence [24], laser-induced fluorescence [25,26], optical measurement methods [27][28][29], rainbow refractometry [30] and infrared thermography [31]. The use of conductivity probes was not considered for this investigation because they require physical contact with the liquid film which would greatly affect flow characteristics due to the small flow channels and liquid film thicknesses encountered.…”

A Critical Review of Advanced Experimental Techniques to Measure Two- Phase Gas/Liquid Flow

“…Another study [47] used U-tube water manometers to measure pressure drops. In addition, an optical measurement was used to measure local liquid film thickness [48,49]. The flow characteristics were: annular, horizontal two-phase flow, air and water as working fluid, round (12.7 mm and 25.4 mm ID), square (15.2 22.7 mm) tube was used, both 6.5 m long.…”