Mapping the thickness of falling liquid films under ionic wind by a light absorption technique

The paper shows the elaboration of a photometric technique with the purpose of obtaining thickness maps of liquid films, subject to corona wind. Light absorption occurring through a liquid layer with the addition of dyestuff is used to evaluate its thickness. The layer is illuminated from the back of the wetted wall and the transiting light intensity is measured by means of a digital camera. Non-uniformities in the original intensity field suggest building a calibration curve for each measurement point. The filtering and interpolation procedure necessary to transform the measured light intensity into the thickness field is described in detail. The technique is economical and non-intrusive. It can determine the local film thickness with an accuracy below 10 % of the measuring range. Application to obtaining a liquid thickness map for falling films under ionic wind in different regimes and at different waviness is illustrated. It is particularly interesting to study this phenomenon, since an ionic wind, with electrical currents lower than 1 mA, can create additional waves and ripples on the liquid/gas interface, thus, in practical applications, it increases mass and heat transfer between the two phases with a negligible energy expense. The thickness of the falling film is measured with and without the generation of corona wind by a thin wire. The pattern of surface ripples can be observed over the entire illuminated area and the beneficial effect on surface waviness can be evaluated at both polarities of the applied high voltage.