The experimental determination of wall-fluid mass transfer coefficients using plasticized polymer surface coatings

Abstract

A novel profilometric technique for determining local mass transfer coefficients at solid surfaces is described, and the conditions for its successful and advantageous practical application are deduced from basic principles. Instead of the volatile or soluble surface coatings customarily employed in profilometric work, the coatings used in the method described here are composed of involatile polymer capable of absorbing volatile or soluble swelling agents. Rates of transfer of swelling agent to or from the surface by the experimental fluid stream can then be deduced from measurements of the swelling or shrinking rate of the coating. This system has the advantage that the coating itself does not require frequent renewal; for each fresh experiment it can be re-activated by discharge of accumulated swelling agent or by re-immersion in the swelling bath. A further advantage is the ease with which the Schmidt number can be varied; this can be accomplished by a change of swelling agent alone, without change of experimental fluid or polymer coating. Moreover, this technique offers the possibility of investigating wall-wall mass transfer situations, which cannot readily be studied by conventional means. In a general analysis of the case in which the coating is initially charged with swelling agent, it is shown how suitable polymer-swelling agent systems and operating conditions may in practice be chosen so that local rates of coating shrinkage are an accurate measure of local fluid-side mass transfer coefficients. Essential data are given for a practical coating system, which is shown to behave as the analysis predicts.