Numerical Investigation of the Effect of Nanoparticle Diameter and Sphericity on the Thermal Performance of Geothermal Heat Exchanger Using Nanofluid as Heat Transfer Fluid

The geothermal heat exchanger system is one of the most energy-efficient and environmentally friendly building service systems. In the present study, CuO/water nanofluid was used as the heat transfer fluid to enhance the energy efficiency of the geothermal heat exchangers. A three-dimensional numerical model was employed to investigate the effect of nanoparticle diameter and sphericity on the thermal performance of the geothermal heat exchanger, and it was well validated against the experimental results of nanofluids in the geothermal heat exchangers. The numerical results showed that nanoparticles with a diameter of 5 nm and 50 nm were not recommended for the nanofluids used in the geothermal heat exchangers due to the performance efficiency coefficient lower than 1, and the optimum diameter was 40 nm, which had the highest performance efficiency coefficient (1.004875). Moreover, the spherical particle-based nanofluid was characterized by the 8.55% higher energy efficiency, in comparison to rod-shaped particle-based nanofluid. Therefore, the application of nanofluid in the geothermal heat exchanger can enhance heat transfer, and the proposed optimum particle diameter and sphericity could contribute to higher energy efficiency.