Experimental research of convective heat transfer between nanofluids and high-temperature dense granite in deep geothermal reservoirs

Global warming is increasing as CO2 emissions from the use of conventional fossil fuels continue to rise. The development of geothermal energy is gradually becoming the key to the sustainable development of human society in the future. Enhancing the heat transfer performance of circulating heat transfer fluids is one of the effective means to efficiently exploit deep geothermal resources. In this study, a self-developed circulating heat exchange experimental setup was established. The convection heat transfer characteristics of nanofluids in high-temperature rocks were experimentally investigated. Al2O3 nanofluid and CuO nanofluid were used as the working medium for heat transfer in high temperature granite samples. The effects of the Reynolds number (1000–6000) and parameters such as nanoparticle type (CuO & Al2O3), size (20 nm & 40 nm) and mass fraction (0 wt%-2wt%) on the heat transfer intensity were analyzed. The effect of nanoparticle accumulation on the rock surface on the local heat transfer strength was also investigated. The results indicate that the increase of Reynolds number enhances the heat transfer strength of nanofluids in rocks. Nanofluid containing 20 nm Al2O3 nanoparticles has better heat transfer performance in rock than nanofluid containing 20 nm CuO nanoparticles. Meanwhile, the heat transfer performance of nanofluid containing 20 nm Al2O3 nanoparticles in rock is better than that of nanofluid containing 40 nm Al2O3 nanoparticles. The heat transfer intensity increases and then decreases as the nanoparticle mass fraction increases. The heat transfer coefficient at Reynolds number 6000 is 52.2 % higher than that of water. For nanofluids and water, the local heat transfer intensity decreases with increasing distance from the inlet. Meanwhile, accumulation of nanoparticles on rock surfaces during convective heat transfer enhances heat transfer intensity. The results show that nanofluids have the potential to be applied to deep geothermal resource extraction. This paper provides data and technical support for the application of nanomaterials in medium and deep geothermal systems to enhance heat transfer efficiency.