Heat Transfer and Evaporation of Falling Liquid Films on Surfaces With Advanced Three-Dimensional Periodic Structures: Experiments and Numerical Simulations

Thin liquid films are widely used in many technological applications. Heat and mass transfer in falling liquid films can be controlled and enhanced by using walls with advanced three-dimensional topographies that influence the film hydrodynamics, stability and wavy pattern or promote evaporation in a very thin film region. Furthermore, capillary suction on structured surfaces leads to a significant increase of the critical heat flux. In this work, heat transfer in laminar falling water films on heated plates with herringbone structure and with meandering grooves has been studied experimentally for different heat fluxes (up to 24 kW/m²), inclination angles and flow rates under reduced pressure, so that evaporation has a significant impact on heat transfer. The flow patterns and temperature gradients on the liquid-gas interface are visualized by high-speed infrared thermography. The wall temperature distribution is measured with thermocouples. The experimental data are compared with the results of numerical simulations. The predicted effect of micro region evaporation on heat transfer has been confirmed experimentally for the first time for partially wetting films on a plate with meandering grooves. This effect manifests itself in a significant decrease of the local wall temperature after the film rupture and consequent transition from a continuous film flow to rivulet flow regime.