Enhancement in Boiling Heat Transfer for Water Using a Polished Plate Surface

The thermal characteristic of a plate evaporator for boiling water have been experimentally investigated. Stainless-steel plates with five kinds of surface roughness (a mirror-polished surface and four kinds of polished plate surface, F2, F0, F-1, and F-2 polishing) were used for the investigation. The roughness order is F-2>F-1>F0>F2>mirror. The local boiling heat transfers were measured on the evaporator surface using ten thermocouples inside the evaporator. For a comparison of the heat transfer characteristic of the evaporator, the local boiling heat transfer coefficient was measured when the polishing direction was perpendicular or parallel to the water flow at a range of pressures (0.3–0.4 MPa) and flow rates (20–60 mL/min). The results indicate that the local heat transfer coefficient increased with increasing vapor quality at all surface conditions. For F0 polishing, the heat transfer coefficient on the surface with perpendicular polishing was much larger than that on the surfaces with parallel and mirror polishing. In particular, the heat transfer coefficient of the perpendicularly polished surface (F2 and F0 polishing) was increased six-fold in comparison with that of the mirror surface at a vapor quality of 0.35. However, for the F-1 and F-2 perpendicularly polishing plates, the heat transfer coefficients were smaller than those of the F0 and F2 perpendicularly polishing plates. For the F2 perpendicularly polished plate, heat transfer enhancement was confirmed, along with an increase in the number of bubbling points, on increasing the system pressure. For the polished surface, the heat transfer coefficient increased with increasing flow rate. This indicates that the bubble formation cycle was promoted by an increased flow rate.