Heat transfer of nanostructure coating on commercially micro-enhanced refrigerant tubes under pool boiling and falling film boiling conditions

Abstract

Refrigeration tube manufacturers commercially produce micro-enhanced tubes for the refrigeration industry with the aim to enhance outside heat transfer coefficients. A new engagement on commercially micro-enhanced tubes is to nanocoat these tubes in an inexpensive attempt to further enhance the heat transfer coefficients. Decreased nucleation site dryout because of increased wettability imposed by a hydrophilic copper oxide nanocoating is hypothesised to enhance heat transfer. In this study, commercially micro-enhanced tubes were nanocoated with copper oxide and tested in R134a refrigerant in pool boiling, and falling film boiling conditions; and additionally in condensation and dryout performance tests. Relevant literature indicated high surface wettability from hydrophilic nanocoatings increasing heat transfer coefficients under pool boiling conditions. This was said to be due to less surface dryout because of the surface’s liquid affinity, however, numerous studies resulted in heat transfer coefficient degradation because of nucleation site flooding and high surface energy requirement. This study proved through scanning electron microscopy that copper oxide nanocoatings successfully coated all microstructured tubes evenly by using a coating procedure and a dedicated tube coating machine without impeding the surface features. For the uncoated tubes in pool boiling at 5°C, the EHPII and GEWA-B5 tubes were the most independent from heat flux with flat heat transfer coefficient curves. They performed the best with heat transfer coefficients of 299% and 318% higher than a plain roughened tube, whereas the low-finned GEWA-KS performed moderately well with heat transfer coefficients 57% higher than the heat transfer coefficients of the plain roughened tube. An increase in saturation temperature to 25°C decreased the EHPII’s heat transfer coefficients by 10%, whereas the GEWA-KS’s heat transfer coefficients increased by 25%. The copper oxide nanocoated tubes in pool boiling at 5°C performed similar to the uncoated tubes in pool boiling. However, the copper oxide nanocoating generally decreased the heat transfer coefficients from the uncoated case, where the EHPII, GEWA-KS and plain roughened tube had heat transfer coefficients approximately 89%, 91% and 85% of the uncoated heat transfer coefficients respectively. The GEWA-B5 was affected the most with heat transfer coefficients approximately 60% of the uncoated heat transfer coefficients. For the uncoated tubes in falling film boiling at 5°C, the EHPII and GEWA-B5 tubes were the most independent from heat flux with flat heat transfer coefficient curves. They had a similar high heat transfer performance, whereas the GEWA-KS performed moderately well. The plain roughened tube’s heat transfer coefficients had an average heat transfer coefficient of 8.6 kW/m2 · K. The increase in saturation temperature to 25°C decreased the EHPII’s heat transfer coefficients with 7%, whereas the GEWA-KS’s heat transfer coefficients increased with 7% on average. The copper oxide nanocoated tubes in falling film boiling at 5°C performed with marginal improvement compared to the uncoated tubes in falling film boiling. The copper oxide nanocoating generally had a degrading effect on the GEWA-B5 and plain roughened tube achieving about 66% of the uncoated heat transfer coefficients. Moderate enhancement for the EHPII tube with a peak enhancement of 110% at 100 kW/m2was seen, however remained steady at achieving 99% of the uncoated heat transfer coefficients on average. The greatest enhancement was achieved by the GEWA-KS with an average of 119% of the uncoated heat transfer coefficients, and a peak enhancement of about 160% at 20 kW/m2, similarly seen at 25°C saturation temperature. The dryout performance tests showed no improvement for all tubes through the addition of the nanocoating and further experimental research is required to deduce an optimal multiscale enhancement to increase dryout performance. The addition of the copper oxide nanocoating is therefore not a reliable option to enhance heat transfer coefficients except for the GEWA-KS tube under falling film conditions. Degradation of the heat transfer coefficients are thought to be due to the flooding of the nucleation sites and the degradation in the hydraulic bubble pumping action of the microstructure capillary channels facilitating the sensible and latent heat transfer. The condensation tests showed a consistent degradation in heat transfer coefficients and is likely due to the inefficient dry surface exposure because of inadequate liquid expulsion from microstructure cavities and general surface liquid retention by the hydrophilic copper oxide nanocoating.