Ultra-efficient heat pipes enabled by nickel-graphene nanocomposite coatings: Concept and fundamentals

Heat pipes are a type of passive and efficient two-phase devices. Despite of its wide applications in solar collectors, electronic cooling and energy recovery systems, the heat pipe technology has been stagnant in last two decades. Though, numerous designs to enhance the evaporation and mass transportation have been explored since its discovery, the intrinsic limitation induced by filmwise condensation (FWC) in the condenser section has not been effectively addressed. In this study, sustainable dropwise condensation (DWC), which can be 10 folds more efficient than the traditional FWC, was successfully induced on the condenser of heat pipes. The effective thermal conductivity (k_eff) of wickless heat pipes can be enhanced more than 7 times higher than the one without Ni-Gr nanocomposite coatings. Besides the drastically reduced thermal resistance resulted from DWC by fundamentally altering continuous film liquid flow into discrete droplet sliding, the liquid return from the condenser to the evaporator by gravity has been significantly accelerated. The new liquid return mode can also effectively manage the instability of geyser boiling with enhanced evaporation rate in the evaporator section and further, promote the heat pipe k_eff. This study shows a new feasibility to elevate heat pipe technologies to the next level.