Thermal entry length in a falling liquid film at high reynolds numbers

Abstract

Using the fluorescence and the infrared technique of thickness and temperature measuring, experimental study is performed on variation of the thermal entry length in a falling water film at high Reynolds numbers. The data obtained by means of a high-speed infrared camera show that, in the interrivulet area occurring between the crests of the developed synchronous 3D waves, even at low heat fluxes, at distances of 7–10 mm from the upper heater edge, zones are formed with the temperature above the initial film temperature. The thermal entry length is shown to be larger within the area of the rivulets forming over the synchronous 3D wave crests, but it decreases rapidly with the heat flux density increase. Analysis of the film thickness data obtained by the fluorescent technique shows that, within the areas between the synchronous 3D waves, the averaged film thickness is less. Respectively, the values of the local film Reynolds number decrease, as do the distances to the localities of the temperature non-uniformities. In those places, even at relatively low heat fluxes, the influence of the thermo-capillary force takes place thus causing a significant decrease in the thermal entry length.