Thermal contact theory for estimating the thermal conductivity of nanofluids and composite materials

Recently, extensive researches are going on the heat transfer enhancements of low thermal conductivity materials by adding a few percentage of high thermal conductivity materials, especially in the topic of the nanofluids and polymers. Also, metal meshes have been used in the thermal storage systems to enhancing the thermal conductivity of the systems. A few theories have been suggested on the heat transfer enhancement in nanofluids or composite materials. However, the discrepancies in the published experimental data are wide and results are not conclusive. In this work, the most popular models are discussed and critically analyzed. Also, a model is developed based on thermal resistance analysis and compared with those models. Furthermore, numerical analyses are performed to closely understand the effect of adding a few percentage of high thermal conductive material to a low thermal conductive material. The findings are very interesting, where more than 30% enhancements can be achieved if a small percentage of high thermal conductivity of material embedded in a low thermal conductivity materials, provided that the particles are long enough and align with the temperature gradient. However, if the particles align perpendicular to the temperature gradient, the heat transfer enhancement is insignificant. Hence, aspect ratio and orientation of the particles are the defining factors in heat transfer enhancement.