Heat transfer of uncoated and nanostructure coated commercially micro-enhanced refrigeration tubes under pool boiling conditions

Abstract

The heat transfer performance of commercially produced micro-enhanced tubes with and without a nanocoating was investigated under pool boiling of saturated refrigerant. These multiscale enhancements were on the outside of 19 mm horizontal copper tubes heated by water to determine the effectiveness of this multiscale enhancement technique on industrially relevant tubes and geometry. The tubes tested were a plain tube roughened by sandpaper, a low finned GEWA-KS tube and two micro-enhanced re-entrant cavity tubes, the GEWA-B5 and EHPII. The tubes were tested in R134a at saturation temperatures of 5 °C and 25 °C across a range of heat fluxes from 20 kW/m2 to 100 kW/m2 under pool boiling conditions. The nanocoating applied to the tubes produced a forest of copper oxide nanostructures on the surface, increasing wickability of the surface. A Scanning Electron Microscopy showed that copper oxide nanocoatings coated all micro-enhanced tubes evenly without impeding the surface features or significantly blocking the re-entrant cavities. For the uncoated tubes in pool boiling, the heat transfer coefficients of the EHPII was up to 519 % greater than those of the plain roughened tube, the GEWA-B5 was up to 539 % higher than the roughened tube and the GEWA-KS was at best 64 % higher than those of the plain roughened tube. Increases in the saturation temperature to 25 °C produced minor improvements in heat transfer coefficients. The application of the copper oxide nanocoating resulted in generally decreased heat transfer performance by approximately 40 % on average compared to the uncoated tubes, with the GEWA-B5 tube the worst affected. Degradation of the heat transfer is thought on plain surfaces to be due to the flooding of nucleation sites, while re-entrant cavity style enhanced surfaces were thought to experience degraded sensible and latent heat transfer due to impeded flow in the microstructure capillary channel, as bubbles were noted to be trapped in the channels. It is recommended that the heat transfer at much higher heat flux ranges be explored for possible HTC enhancement.