Surface wettability modifications and applications in wickless heat pipes

Abstract

Proper methods of surface fabrication and characterization are essential for various applications. For example, wickless heat pipes, such as flat plate pulsating heat pipes (FPPHP) and a compact flat thermosyphon (CFT), have massive potential for commercialization owing to simple construction, flexibility in using different working fluids, and better thermal contact with flat heat sources. In this work, various surface treatment methods such as chemical etching, electrochemical deposition, and laser texturing on a copper sample are investigated to prepare superhydrophilic or superhydrophobic surfaces, and the thermal durability of the as-prepared surfaces is tested through heating and boiling experiments for 15 days. The wettability modification methods are implemented on the surfaces of the FPPHP and CFT, with the different wettability combinations as either bare evaporator and condenser, completely superhydrophilic (that is, all surfaces including that of evaporator, condenser, and adiabatic section) or completely superhydrophobic or hydrophilic evaporator and superhydrophobic condenser. The thermal performance of the FPPHP and CFT subjected to different wettability combinations on the surface is investigated. Results of FPPHP are reported for water as the working fluid at 60 % filling ratio with power inputs given from 40 to 200 W. Similarly, for CFT, the results are reported for water filled at 50 % filling ratio with heat flux input from 2 to 20 W/cm2. On the FPPHP, the best thermal performance and lowest resistance were obtained when the entire surface was superhydrophilic with a maximum reduction of 60 % at 200 W. The CFT gives the best thermal performance with a bare evaporator and superhydrophobic condenser with a maximum decrease in thermal resistance of 42 % at 20 W/cm2. The best-performing wettability combination for FPPHP and CFT is contrasting and has to be chosen appropriately depending on the heat transfer device.