Experimental investigation of low-profile heat pipe with wickless wettability-patterned condenser

Heat pipe heat spreaders are metal-wick based cooling devices which circulate a phase-changing liquid to transfer heat more efficiently than solid-metal heat sinks. Although metal wicks are excellent fluid transporters, they also have capillary limitations because of their high pressure drop. On the other hand, wettability-patterned surfaces do not face these limitations and as shown in this work, could be a viable substitute for metal wicks in heat pipes. The use of wettability-patterned surfaces facilitates the efficient transport of condensate and improves heat transfer from the hot (evaporator) to the cold (condenser) side of the device. In this study, a flat heat pipe of 10 cm operating length, with copper-wick evaporator and wickless wettability-patterned condenser is assembled and tested. Although in traditional heat pipes a fully wick-lined device is of interest, in this apparatus, the condensate's size and type are being controlled via spatially designed superhydrophilic/hydrophobic juxtaposed areas to regulate both Dropwise Condensation (DwC) and Filmwise Condensation (FwC) towards enhancing the heat transfer capability while reducing the thermal resistance of the device. In this design, low-capillary pressure domains are designated to collect the condensate and return it back (via wick) to the evaporator's wick. Variable-porosity copper wicks are incorporated on the heater and the rest of the heat pipe to facilitate liquid transport from the cold to the hot side. Two charging loads are examined with the best performance achieved for a thermal resistance 0.16 K/W at 105 W. In addition, a heat pipe with a uniform hydrophilic condenser was fabricated and compared to the wettability-patterned featured heat pipes.