Experimental study on boiling heat transfer of a self-rewetting fluid on copper foams with pore-density gradient structures

Light-weight and high-surface-area metal foams used in phase change heat transfer may suffer flow resistance from the porous matrix and cause boiling deterioration. To alleviate the flow resistance, metal foams with pore-density gradient was proposed and significant enhancement of pool boiling heat transfer was achieved for fluids such as water and refrigerants. In this work, a self-rewetting fluid (aqueous n-butanol solution) was used for boiling on copper foams with pore-density gradient structures formed by using several layers of foam covers. The experimental results show that, comparing with the one-layer foam, the bubble departure phenomenon was substantially attenuated due to the largely increase of pore density and hence the bubble moving resistance when using a two- or three-layer foam structure. However, the increase of pore density can enhance the pool boiling of water when the foam thicknesses are the same due to more active cavity sites being formed in a denser metal foam. While the enhancement for the solution is not obvious especially for that in the foam structure with higher pore density and heat transfer deterioration may emerge at high heat fluxes, the boiling heat transfer of the solution can generally be enhanced by using the 110 ppi foam and its gradient structures as compared to the polished surface. This provides new insight into enhancing the boiling heat transfer utilizing both the surface properties formed in the pore-density gradient structure and the unique interfacial properties of the self-rewetting fluids.