Abstract
Heat pipes are metal-wick equipped systems that circulate a phase-changing liquid and transfer heat more effectively than solid-metal compact heat spreaders. Metal wicks are effective fluid transporters, but they can face capillary limits due to significant pressure drops and pore clogging. Wettability-patterned (WP) surfaces, on the other hand, are not limited by these constraints and, as demonstrated in this work, may offer a viable supplement for metal wicks in heat pipes, which come in many shapes and sizes. The use of WP surfaces facilitates the collection and efficient transport of condensate, thus improving heat transfer from the hot (evaporator) to the cold (condenser) side of the device. Most commonly, flat heat pipes consisting of a fully wick-lined plate opposed to a wickless surface, are evaluated considering a uniform adiabatic condition on the wickless plate, which does not promote condensation. However, in a practical scenario, the heat pipe portions in contact with an external heat sink could induce condensation on the wickless side, thus affecting the heat pipe's operation and thermal resistance. To explore this scenario, a flat heat pipe of 10 cm operating length with axially graded copper-wick evaporator and non-adiabatic wickless WP surface is designed, fabricated and tested. A wettability pattern is imparted on the wickless surface to control the maximum droplet size of the condensate and condensation mode via spatially designed superhydrophilic/hydrophobic juxtaposed areas to regulate both Dropwise Condensation (DwC) and Filmwise Condensation (FwC) towards enhanced heat transfer with reduced overall system thermal resistance. Low capillary pressure domains are allocated in this design to collect condensate on the wickless plate and return it to the evaporator's wick. On the evaporator side of the system, variable-porosity copper wicks are being used to improve liquid conveyance from the cold to the hot domains. Four heat-transfer fluid charging ratios are examined with the best thermal resistance 0.27 K/W achieved at 105 W. The effect of wettability patterning on the wickless surface and the wick characteristics of the wick-lined evaporator are investigated.
NSTL
Elsevier