Here the variation of a liquid-film thickness at small Reynolds numbers is discussed. The film thickness measurement by laser interferometry corresponds to the liquid flowing on the inner surface of a small-bore glass tube. An adequate theoretical background for the techniques used in this experiment is discussed to demonstrate the capability of the experimental technique. An advantage of this method is that it shows the shape of the thin film on the inner surface of the vertical tube at a point in a horizontal cross section. The results obtained from this experiment show that the flow of liquid films on vertical surfaces is inherently unstable and three dimensional even at a Reynolds number smaller than 1.

Measurement of liquid-film thickness by laser interferometry.

Self-drainage of viscous liquids in vertical and inclined pipes

Falling film reactors are a very effective geometry in which to carry out fast exothermic gas-liquid reactions, because they allow easy removal of the heat produced. To optimise and improve falling film reactors, soundly based physical models are required. Existing models assume a flat liquid film combined with empirically-determined mass transfer coefficients, but there is growing evidence that this is unsatisfactory, since the interface is dominated by ripples and larger disturbance waves which have a significant effect on the mass and heat transfer rates through and into the film. 

The main objective of this study was the better understanding through experimental study of the interfacial characteristics of liquid falling films in the presence of a co-current down flow of air, with emphasis on the physical and hydrodynamic conditions encountered in current falling film reactor technology, so that the information obtained can be implemented in the enhancement of falling film reactor modelling. The liquids chosen were water, for validation of the measurement techniques through comparison with the literature, and two common falling film sulphonation reactor feedstocks; dodecylbenzene and ethoxylated alcohol.

The experimental study consisted of film thickness and disturbance wave celerity measurements in a purpose built flow facility with a flat plate flow surface. A novel spatial film thickness measurement technique, the Light Absorption Imaging Technique (LAIT), was developed to give unparalleled spatial information on the interfacial characteristics of liquid films. A computer algorithm-based technique has been developed for categorisation of the spatial film data obtained by LAIT into substrate and disturbance wave regions, from which globally averaged characteristics of the interfacial structure have been obtained. Disturbance wave celerity measurements were obtained utilising the cross-correlated output signal from two light sources and a manual time-of-flight measurement technique using the images from a high-speed video camera.

The study has provided a comprehensive set of data on the hydrodynamic structure of the interface, with and without the influence of a co-current down flow of air, for both water and actual sulphonation feed stocks as the liquid phase. The dramatic influence of both flow conditions and physical properties on interfacial characteristics has been demonstrated In particular, a transition has been observed of the disturbance wave structures in falling films due to the influence of a co- current airflow. Differences in the disturbance wave structure have been identified between the organic and water films after this transition.

The interfacial characteristics of falling film reactors

An optical method for measuring the thickness of a bottle in real time has been developed by a combination of image and reflection methods. The system consists of two subsystems: one is a sensor head consisting of a laser light source, two CCD arrays, a focusing lens and a half mirror, and the other is a data-processing subsystem consisting of a personal computer employing an A/D converter and amplifier, and a monitor. The resolution of this method, i.e., the minimum measurable change in thickness, for a CCD array with a pixel resolution of 10 µm was between 10 and 50 µm depending on the optical arrangement as determined by a simulation experiment with the computer software 'Zemax®'. This resolution is sufficient for practical application.

Real time measurement of a bottle thickness by means of both image and reflection methods

Real Time Measurement of a Bottle Thickness by Means of Both Image and Reflection Methods

A method is described whereby a laser beam irradiates the wall of a hollow glass cylinder, and reflections from the inner and outer curved surfaces of the wall interfere to produce a fringe pattern which is analysed to give the wall thickness. The method is dependent on the fact that a curved surface presents an incident laser beam with a continuously varying angle of incidence. A comparison is made with experiment. The method is also applied to a film of water draining down the interior of a vertical cylindrical tube.

https://iopscience.iop.org/article/10.1088/0957-0233/6/9/017/pdf

F E Irons

Papers

An interferometric measurement of the wall thickness of a cylindrical glass tube with application to a draining liquid film