Modified surfaces to enhance vertical falling film heat transfer – An experimental and numerical study

Abstract

In this paper we investigate experimentally and numerically the potential improvement of heat transfer in falling film units by introducing modifications of the heat transfer surfaces. Two different modified surfaces are investigated and the obtained results are compared to the ones acquired on a smooth surface. In our experiments we work with a broad range of operating conditions with Kapitza and Reynolds numbers in the range of Ka = 167–574 and Re 18–423, respectively. We compare local heat transfer measurements conducted at multiple positions and relate them to the film thickness measurements conducted by a laser triangulation scanner. Our experiments identify both cases where the introduction of modified surfaces significantly enhances the heat transfer and conditions where such a measure does not have a significant impact. The observed differences are compared to numerical simulations providing details of the flow structures in the liquid film in different cases. For that purpose, we directly solve the full Navier-Stokes equations in two dimensions, using the volume of fluid (VOF) numerical framework. Our results reveal that the heat transfer can be enhanced by a factor of up to 2.5 due to appearance of significantly enlarged time-dependent recirculation zones occurring behind the surface modifications, as opposed to such zones present in falling film units with smooth heat-transfer surfaces.