3D-Modeling of a Coaxial Borehole Heat Exchanger in Sahand Field, Northwest Iran Considering the Porous Medium and Presence of Nanofluids

The purpose of this study is the 3D CFD numerical modeling of a coaxial borehole heat exchanger. The operating fluid inlet velocity, the groundwater seepage velocity, the soil porosity, and the use of nanofluids instead of pure water are investigated. Ansys Fluent software is used for numerical simulation and the k-ε turbulence model is employed for turbulent flow modeling. The results show that they significantly increase the operating fluid temperature. The presence of groundwater seepage decreases the temperature of the working fluid which is related to the groundwater flow velocity. High soil and backfill porosity affect the thermal performance of CBHE, and increase thermal resistance, and decrease thermal conductivity. The nanofluids utilization with a higher thermal conductivity than pure water increases the temperature growth rate along the outer pipe. Kriging optimization method suggested that the best operating conditions for the system are inlet water velocity 0.03 m/s, groundwater velocity 5 m/d, soil porosity 0.28, backfill thermal conductivity 3.3 (W/m.K) and CuO/water nanofluid. By considering the mentioned operating conditions, the working fluid temperature increases by about 6% at the depth of 60 m.