Experimental study of convective heat transfer between nanofluids containing different shaped nanomaterials in contact with high temperature geothermal granite

In the process of geothermal energy exploitation, an important factor restricting the development of geothermal energy is the low efficiency of convective heat transfer between circulating mediums, such as water and glycol with high-temperature rocks, which results in low energy utilization of geothermal resources. Nanofluids have a broad application prospect in the field of geothermal reservoir heat transfer due to their excellent heat transfer performance and high specific surface area. Currently, there is a lack of in-depth research on the application of nanofluids in the field of geothermal energy, especially in dry-heat rock geothermal reservoirs. The research on contact heat transfer of nanofluids with different shape of nanoparticles needs to be strengthened. In this paper, Al 2 O 3 nanoparticles are selected and classified as sheet-like, rod-like and near-spherical according to the size of the specific surface area of the nanoparticles. The optimal mass ratio of dispersant to nanoparticles was optimized through systematic testing experiments, among which the optimal dispersant mass ratios corresponding to rod-like, sheet-like and near-spherical nanoparticles were 5:2, 5:4 and 10:1, respectively. The thermal conductivity and specific heat capacity of the nanofluids corresponding to the three nanoparticle shapes at different mass fractions were measured, and the optimal mass fractions of nanoparticles were preferred to be 2 wt%. Subsequently, the contact convective heat transfer characteristics between the nanofluids and high-temperature granite were tested and analyzed. The results show that the shape of nanoparticles affects the degree of enhanced heat transfer of nanofluids, in which the larger the specific surface area of nanoparticles, the higher the degree of enhanced heat transfer of nanofluids. When the Reynolds number is 2000, the heat transfer coefficients of the nanofluids containing sheet-like nanoparticles are increased by 7.4 % and 18.5 %, and the heat transfers by 5.3 % and 13.3 %, respectively, compared to the nanofluids containing rod-like and near-spherical nanoparticles. When the Reynolds number was increased to 6000, the heat transfer coefficients increased by 9.7 % and 22.5 % and heat transfer increased by 5.5 % and 10.6 %, respectively. In addition, the effect of nanofluids on the local heat transfer intensity in the flow channel is also investigated in this paper. At the highest point of heat transfer intensity in the flow channel, the nanofluid containing sheet-like nanoparticles increased the heat transfer intensity by 3.6 % and 4.7 % compared to rod-like and near-spherical nanoparticles, respectively. Compared with deionised water, the maximum temperature difference in the flow channel of the nanofluid containing sheet-like nanoparticles was found to be reduced by 61.5 % when circulating up to 240 s. It indicates that the nanofluid formulation effectively improves the problem of uneven local heat transfer intensity between fluid and high-temperature granite. Finally, this study also calculated the net energy gain of the nanofluids by integrating the heat transfer rate and pumping power. Specifically, for the nanofluid containing sheet-like nanoparticles, the increased heat transfer rate is 16 times the increased pumping power under laminar flow conditions, and nearly 8 times that under turbulent flow conditions. This provides significant reference value for promoting the application of nanofluids in practical geothermal energy extraction.