Switchable heat transfer mechanisms of nucleation and convection by wettability match of evaporator and condenser for heat pipes: Nano-structured surface effect

A novel strategy was proposed for the improvement of vapor chamber heat pipe (HP) performance. The mastoid process array, wettability control of heat pipe and control of liquid charged into heat pipe are comprehensively used to manage the phase distribution. Thus, the two dominant heat transfer mechanisms of nucleation and convection can be switched. The objective of this strategy is to maintain nucleation mechanism but suppress convection mechanism. Holding the nucleation mechanism, our newly developed heat pipe behaves excellent thermal sensitivity with respect to heating loads. Our idea was thoroughly verified by experiments, in which three HP samples were fabricated and tested: #1 equal wettability sample, #2 moderate wettability difference sample and #3 opposite wettability sample. The wettability difference was generated with nano-structured surface. The #1 sample shows convection mechanism with heat transfer coefficients (HTCs) not changed versus heat fluxes, due to evaporator porous wick occupied by larger vapor content and liquid filmwise condensation on condenser surface. The super-hydrophilic evaporator and super-hydrophobic condenser (#3 sample) shows nucleation mechanism with HTCs significantly increased with increases in heat fluxes, due to large driving force to flood evaporator porous wick by water and expose condenser by vapor. Surprisingly, opposite wettability match HP had overall thermal resistances of only 1/3 to those of equal wettability HP. The comprehensive use of mastoid process array and controls of wettability and water amount charged into HP lowers heater temperatures by 30–40°C at high heat fluxes such as ~100W/cm2 on a large heater area of 1.4cm2, paving a new road for heat pipe performance improvement.