Heat transfer augmentation in two-phase flow heat exchanger using porous microstructures and a hydrophobic coating

In this study, we improved the thermal performance of a slightly inclined tube for a two-phase flow heat exchanger by means of surface modification techniques. The exchanger condenses pure steam inside the tube while boiling takes place outside the tube in a pool of saturated water. First, appropriate surface modification techniques for each boiling and condensation surface were separately investigated. An electroplating technique with hydrogen bubbles was utilized to create porous microstructures as cavities on a boiling surface, which remarkably promoted heterogeneous bubble nucleation and resulted in significant enhancement on average 107% in the boiling heat transfer coefficient. Hydrophobic thin films of Teflon were coated on a condensation surface, which considerably enhanced heat transfer on average 100% by promoting dropwise condensation. Secondly, the selected surface modification techniques were applied to outer boiling and inner condensation surfaces of a single-tube two-phase flow heat exchanger and remarkable improvement in heat transfer performance (>60%) due to the surface treatments was experimentally demonstrated.