Heat transfer on a rotating surface with and without phase change

Abstract

This study is concerned with heat transfer to liquid films flowing across the surface of a rotating disc. Two cases of heat transfer have been considered, the first dealing with sensible heating of the liquid film, the second with heating and evaporation from the surface of the film. In both cases the heating medium was condensing steam. A model for the thermal performance of such devices, has been constructed and compared with experimental data for water and methanol. Values of the temperature of the liquid leaving the disc periphery have been measured for a wide range of liquid flow rates and disc speeds. These results compare very favourably with the temperatures predicted by the proposed model. Similar comparisons have been made for the rates of evaporation measured when methanol is heated on such discs. Again predictions compare well with measured values, except in conditions of flow where film breakdown is known to occur. If the flow on a disc surface, rotating at constant speed, is gradually reduced, a flow will occur at which the surface is no longer completely wetted. Increasing the flow rate will produce rewetting of the surface at some slightly higher value of the flow rate. This mechanism of rewetting has been studied in some detail and a theoretical model has been developed. The model has been tested under a wide range of operating conditions and comparison between measured and predicted minimum rewetting rates is quite good. Power dissipation associated with the flow of liquid film across a disc has also been considered.