A TECHNIQUE FOR QUANTITATIVE MEASUREMENTS OF SHEAR RATE AND FLOW CHARACTERISTICS IN COATING SYSTEMS

The flush-mounted hot-film anemometer may be used to investigate the f low charateristics in coating systems. This device has a hot-film sens or heated by an electric current and cooled by the incidental flow, wh ich acts by virtue of its mass flux and its temperature With this syst em, we measure the current (or resistance) which correlates with the w all shear rate. The resulting heat transfer between the hot-film senso r and the fluid is detected electrically as a function of flow paramet ers. We have used this setup to obtain quantitative measurements of th e critical machine speed, or the critical Reynolds number, for the tra nsition from steady-state to time-periodic and unsteady flow in a cavi ty simulating the pond of a short-dwell coater. The results confirm th e previous flow visualization experiments and provide additional infor mation on the dynamics of the system and the shear rate in the cavity. The technique developed in this study can be used to measure the wall shear rate in any coating system. This technique is nonintrusive, rel atively easy to apply and can be used with opaque fluids such as coati ng colors.