Towards a durable polymeric internal coating for diabatic sections in wickless heat pipes

Heat pipe characteristics are linked to the surface properties of the diabatic surfaces, and, in the evaporator, surface properties influence both the onset boiling temperature (T_ONB) and the critical heat flux (CHF). In this work, the effect of surface wettability in pool boiling heat transfer is studied in order to understand if there could be a path to increment heat pipe thermal performance. This work analyzes the effects of surface wettability on boiling (tested fluid is pure water) and proposes a new super-hydrophobic polymeric coating (De Coninck et al., 2017, “Omniphobic Surface Coatings,” Patent No. WO/2017/220591), which can have a very important effect in improving the heat pipe start-up power load and increasing the thermal performance of heat pipes when the flux is lower than the critical heat flux. The polymeric coating is able to reduce the T_ONB (-11% from 117 ^∘C to about 104 ^∘C) compared with the uncoated surfaces, as it inhibits the formation of a vapor film on the solid-liquid interface, avoiding CHF conditions up to maximum wall temperature (125 ^∘C). This is realized by the creation of a heterogeneous surface with superhydrophobic surface (SHS) zones dispersed on top of a hydrophilic surface (stainless steel surface). The proposed coating has an outstanding thermal resistance: No degradation of SH properties of the coating has been observed after more than 500 thermal cycles.