Boiling heat transfer enhancement on heterogeneous copper foams in the presence of transverse flow

Open cell copper foams are widely employed to enhance boiling heat transfer, but homogeneous foams face a trade-off between promotion in wetted area and reduction in resistance to bubble departure. In this work, heterogeneous copper foams with horizontal gradient are proposed to assure both enlarged wetted area and facilitated bubble departure. Boiling of HFE-7100 on the heterogeneous foams consisting of 30 pore per inch (PPI) square core and 60 PPI frame with different projection area fractions and thicknesses is conducted under the conditions of forced transverse flow. A peak heat transfer coefficient (HTC) of 5.4 W cm⁻² K⁻¹ and a critical heat flux (CHF) of 91.4 W cm⁻² are obtained on the heterogeneous foam having a core thickness of 5 mm and a frame thickness of 2 mm. The difference in thickness delays the formation of vapor blanket on the foam to promote CHF and the difference in pore density favors balanced surface wetting and vapor venting to enhance HTC. Vapor bubbles on the copper foams are visually observed and a semi-empirical model correlating CHF with heterogenous geometric parameters of the foam is proposed. This work offers an optional strategy to further enhance boiling heat transfer on porous mediums.