Recent advances in heat transfer enhancement with gradient porous materials

Gradient porous materials (GPMs) by meticulous design of pore-scale structures and sizes indicate high specific surface area and thermal conductivity, and show great potential in mitigating the drawbacks commonly observed in uniform porous materials (UPMs), including significant rise in pressure drop and flow resistance, and the need for high pumping power to provide effective heat transfer enhancement. Therefore, GPMs have been proposed to improve heat transfer performance and are now widely applied in heat dissipation devices, geothermal heat extraction systems and concentrated solar systems. The pore-scale structures and their corresponding heat transfer properties of GPMs have attracted broad interest from multiple disciplines in recent years. This paper presents a comprehensive review of recent progress in heat transfer enhancement with GPMs in order to provide insight on the heat transfer mechanisms and thereby promote industrial applications of GPMs. The structural characteristics, thermal properties, heat transfer mechanisms, and applications of single-layer, double-layer and multi-layer GPMs are summarized and analyzed in detail. And the conjugated heat transfer techniques by combination of GPMs with nanoparticle addition, metal fin insertion, applied magnetic fields, and microchannels are also examined and discussed. Finally, the potential future research topics of GPMs in structural design, material synthesis, numerical simulation, and composite heat transfer enhancement are highlighted.