Effect of porous graphene networks and micropillar arrays on boiling heat transfer performance

We constructed composites featuring micropillar arrays and a porous graphene networks to enhance nucleate boiling performance. The surface was prepared by dry-etching of silicon wafers with a photoresist (PR) pattern and nucleate boiling using reduced graphene oxide (rGO) colloids as the working fluids. On boiling, a porous graphene network initially formed on the heads of micropillars, and then merged to fill the area between the pillars, forming a composite of a micropillar array with a porous graphene network. As graphene coating proceeded, the enhancement ratios of the critical heat flux (CHF) and the boiling heat transfer coefficient (HTC) of the composite decreased, probably because the porous graphene network occupied the vacant space between the pillars. This space served as a liquid inflow channel in terms of CHF, and a liquid circulatory path in terms of HTC. When the porous graphene coated only the heads of the pillars, HTC was enhanced by early maturation of nucleate boiling and increased nucleation in the low heat flux region close to the onset of nucleate boiling (ONB). Accordingly, we conclude that the composite may be optimized by local coating of the heads of micropillars with porous graphene to ensure early ONB and enhance the HTC and CHF.