Heat Transfer Characteristics of Falling Film Process on Coated Division Tubes: Effect of the Surface Configurations

In general, mixing of solution could refresh the interface of bulk liquid, and hence lead to the more efficient heat transfer within the liquid film. By using the high speed video camera and thermal imaging technique, the flow patterns and heat transfer characteristics of liquid film on coated division tubes, on which the hydrophobically coated regions and the bare regions presented alternately, were investigated experimentally. Flow visualization revealed that the minimum wetting rate increases with the increase in the effective contact angle of this surface. The cumulative liquid ring was observed at the boundary of bare region and coated region. The fluctuations of liquid film temperature were measured at different liquid film mass flow rates and the initial fluid temperatures. The temperature fluctuation periods on the coated division tube increased with the decrease of flow rate and initial liquid temperature. The liquid film surface temperature on the coated division tubes was higher than that on the uncoated bare tube at the same falling film position; that is, the heat transfer in the liquid film on the coated division tube was more efficient than that on the uncoated bare tube. The disturbance of falling liquid film was strengthened from the existence of the coated region, resulting in stronger mixing that drove the liquid particles from the internal of the liquid film to the interface, which greatly contributed to the enhancement of heat and mass transfer.