Flow boiling of HFE-7100 over graphene coated sintered porous copper surfaces in a minichannel

• The effects of sintered layer thickness and graphene coating on flow boiling of HFE-7100 were investigated. • An optimal sintered layer thickness for the best performance was obtained. • 0.5 mm sintered layer thickness could enhance heat transfer by up to 145 %. • Graphene coating could enhance heat transfer even further by up to 34 %. • Graphene coating could prevent dry spots in minichannels. High power dense electronic devices demand efficient heat removal and thermal management. Phase change heat transfer with the application of graphene coating offers superior heat dissipation. In this study, the effects of sintered copper powders and monolayer graphene coating on flow boiling of HFE-7100 were investigated for a minichannel. Bare copper surface and surfaces with additional sintered layers of thicknesses of 0.5 mm, 1.0 mm, and 2.0 mm were compared in terms of flow boiling heat transfer. Additionally, graphene coatings were applied to each surface, and the effects of graphene coating on flow boiling heat transfer were assessed at atmospheric pressure. The experiments were conducted at different heating fluxes and two different mass fluxes (120 kg/m²s and 180 kg/m²s) for each surface. Novec HFE-7100, a dielectric fluid having a high potential for the use in electronics cooling applications, was used as the working fluid in flow boiling experiments. The results indicated that the sintered layer improved the flow boiling heat transfer performance. The sintered layer thickness of 0.5 mm offered the best heat transfer performance with an enhancement up to 145 % relative to the bare surface at high heat fluxes. It was also observed that graphene coatings further enhanced the heat transfer performance of the sintered surfaces up to 34 %. When 0.5 mm sinter thickness and graphene coating were combined, the maximum heat transfer enhancement was recorded as 227 % compared to the bare surface. In the light of high-speed camera images, flow boiling characteristics and effects of graphene coating on flow patterns were displayed. Accordingly, the graphene coating increased the nucleation site density, improved the stability of bubble formation and led to HTC enhancement for the sintered surfaces.