Enhanced boiling heat transfer by gradient porous metals in saturated pure water and surfactant solutions

•Pool boiling heat transfer of gradient metal foams was investigated experimentally.•The main influencing factors are layer number and foam material gradient.•Departure bubble becomes smaller when it is colliding with the nickel foam skeleton.•Surfactant effect is dependent on concentrations and nanoparticle deposition condition.

Pool boiling heat transfer of gradient metal foams has been investigated in saturated pure deionized water and surfactant solutions at atmospheric pressure. Pore density gradients are from 5 PPI to 100 PPI, while the porosity remains at the fixed value of 0.98. The parametric study is performed by varying foam layer number and material. Alumina nanoparticle and surfactant (SDS and Triton X-100) effects on pool boiling heat transfer of gradient metal foams are also investigated. Images of nanoparticle-deposited foam fiber are captured by SEM. Bubble growth inside the gradient metal foams is captured by a high-speed camera. For no-nanoparticle-deposited gradient metal foams, pool boiling heat transfer in deionized water is heavily dependent on foam layer number and material gradient. For nanoparticle-deposited gradient metal foams, pool boiling heat transfer in surfactant solutions is dependent on surfactant concentrations and nanoparticle deposition condition on the metal skeletons. The visualization results show that departure bubble size first increases and then decreases in the gradient copper–nickel foam.