Experimental investigations of bubble behaviors and heat transfer performance on micro/nanostructure surfaces

Abstract

The multi-level hierarchical surfaces combining different characteristics of single modified surfaces such as expanded heat transfer area, nucleation site density and capillary wickability can further enhance the heat transfer performance. The pool boiling experiment of FC-72 with 35 K subcooling was conducted on the hybrid micro/nanostructure surface (NPF50-60) with nanoforest structure fabricated on the top and bottom of micro-pin-fins using the dry etching and plasma repolymerization techniques. As a comparison, experiments were also conducted on the smooth surface (S), the micro-pin-finned surface (PF50-60) and the nanoforest surface (NS). The novel phenomenon of bubble oscillation on heating surface was observed, which is considered as the result of interactions between evaporation and condensation effects. The predictions of bubble center position during oscillation from forces analysis agree well with the experimental results. In addition, the bubble jumping induced by coalescence was also observed. The results indicated that the three micro/nanostructure surfaces can significantly enhance the boiling heat transfer performance compared to the smooth surface. The lower wall superheat and greater heat transfer coefficient (HTC) with relatively large critical heat flux (CHF) were achieved on the hybrid micro/nanostructure surface for the larger cavity size of nanoforest structure. The wicking velocity of different surfaces obtained from the capillary wickability tests shows a good linear relationship with the CHF. It was concluded that the mechanism of CHF enhancement on micro/nanostructure surfaces is the liquid replenishment with capillary wickability to prevent the expansion of dry spots and maintain a higher critical heat flux.