Intrinsic Heat Transfer Enhancement Mechanisms in Boiling With Nanoscale Surface Features

Boiling heat transfer results from a number of multiscale phenomena that are activated by the inception of a nucleating bubble that induces motion of the liquid and vapor phases. Heat is transferred by conduction and convection to the liquid from the heated surface, and subsequently by evaporation at different liquid-vapor interfaces. Nanoscale features on the surface lead to boiling heat transfer enhancement by influencing the surface morphology, nucleation characteristics, localized heat transfer processes, motion of the three-phase contact line, growth and departure of the bubble, and liquid flow over the heated surface. This paper discusses the intrinsic mechanisms associated with nanoscale features that are responsible for enhancement in critical heat flux (CHF) and heat transfer coefficient (HTC) in pool boiling. High wettability surfaces provide CHF enhancement through improved bubble dynamics. Improved contact line motion and liquid circulation around a bubble are responsible for enhanced bubble growth rate. High wickability of nanostructures or nanoparticles on the heated surface is able to supply liquid to the evaporating thin film underneath a bubble. Other techniques lead to enhancing heat transfer to liquid through roughness and lateral conduction in high thermal conductivity particles such as graphene and carbon nanotubes. Enhancement in contact line region heat transfer has been effectively utilized at both nano-and microscales. However, for refrigerants, the enhancement with nanoscale features is only modest at best in improving the heat transfer. These mechanisms are discussed in detail and areas for future research are identified.