Heat Transfer at Evaporation and Boiling of Refrigerant Mixture Film Falling Down a Vertical Heater with a 3D-Printed Dual-Scale Coating

In the proposed study, experiments were conducted to investigate heat transfer enhancement during evaporation and boiling of R114-R21 refrigerant mixture film flowing down a vertical surface. To improve heat transfer, a dual-scale coating with macroscale longitudinal ribbing and a microscale porous internal structure of sintered bronze particles was printed by combined SLS/SLM (Selective Laser Sintering/Selective Laser Melting) on a flat rectangular substrate ( \(70\times80\)  mm). The film Reynolds number ranged from 400 to 1300, indicating a change in the film flow regime from the laminar wave to the undeveloped turbulent one. Heat flux density varied from zero to pre-crisis values. The results showed that the presence of the modulated capillary-porous coating can increase heat transfer at nucleate boiling of the falling film by up to four times as compared to a smooth surface. To evaluate the obtained results, the authors compared them with experimental data previously gathered for a flat 3D-printed capillary-porous coating and a microstructured surface created by deformational cutting. The microcharacteristics of the obtained coating were also compared with the active centre size ranges predicted by models of Hsu and Liu et al.