Geothermal Reservoir

Geothermal Reservoir with U-Tube Downhole Heat Exchanger — ANSYS Fluent CFD Simulation

Description

This project simulates heat extraction from a geothermal reservoir using a single U-tube Downhole Heat Exchanger (DHE) — a U-shaped pipe set in a wellbore through which a working fluid circulates to draw heat out of the ground. It's a strong study in natural-convection-driven conjugate heat transfer, where the heat path runs from the surrounding ground, through the borehole fluid, and into the circulating tube water. Within the Renewable Energy: Beginner CFD Training Package, this project introduces a completely different energy source — geothermal heat drawn from the earth — bringing subsurface flow and coupled solid–fluid heat transfer into the package.

Methodology

The model is built from three coupled parts — the U-tube, the borehole, and the ambient geothermal reservoir — and is a scaled version of a real field case (which sits about 200 m underground). Here the ground zone and U-tube are scaled to 6 m and 3.2 m depth, with a 0.0875 m tube diameter inside a 0.35 m borehole, all within a 3 m ground cylinder. The geometry is built in Design Modeler and meshed in ANSYS Meshing as a polyhedral mesh of roughly 1.75 million cells. The physics centers on free (natural) convection: the solid ground temperature is set as a linear function of depth using a Named Expression, so the borehole heats from the bottom up. Gravity is enabled, and water's thermal conductivity and heat capacity are defined as temperature-dependent via the polynomial method — the buoyancy that drives the whole problem depends on getting these property variations right. Turbulence uses the Realizable k-ε model with standard wall functions, and the flow is solved steady.

Analysis

The results provide temperature and pressure contours plus velocity vectors for both the tube and borehole zones. Convective heat transfer raises the tube outlet temperature to 305.47 K. The velocity vectors reveal the mechanism clearly — a vortex forms at the bottom of the hole, intensifying turbulence and heat transfer; water near the hot wall warms, loses density, and rises, then cools and sinks, completing the natural-convection loop that feeds heat into the tube. By the end of this project, you'll be able to set up buoyancy-driven natural convection, define depth-dependent solid temperatures with Named Expressions, model temperature-dependent fluid properties, and run a coupled solid–fluid heat-exchange case.