Pool boiling experiments in reduced graphene oxide colloids part II – Behavior after the CHF, and boiling hysteresis

Abstract

The critical heat flux (CHF) during reduced graphene oxide (RGO) colloid pool boiling was increased by the development of coating layers from the RGO flakes. By boiling water after boiling RGO colloid, we confirmed that the CHF enhancement was due to the development of RGO coating layers such as the base graphene layer (BGL), self-assembled foam-like graphene structure (SFG), and thickly aggregated graphene layer (TGL). During RGO colloid boiling, we observed an interesting phenomenon after the CHF point: when the heat flux reached the maximum value (CHF), the wall temperature slowly began to increase while the heat flux was maintained, unlike the rapid wall temperature increase that is observed during water boiling. We examined the stability of this phenomenon and the boiling hysteresis in relation to the heat flux. We hypothesized that the BGL and SFG could induce this phenomenon by functioning as heat spreaders, owing to the greatly enhanced thermal conductivity during the formation of hot/dry spots, and the vapor escape resistance during boiling on a porous medium.