Clean Water: Geothermal Reservoir

Geothermal Downhole Heat Exchanger (DHE) — Natural Convection 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 installed inside a wellbore, through which a working fluid circulates to draw heat out of the ground. The case is a strong study in natural-convection-driven conjugate heat transfer: the heat path runs from the surrounding ground, through the borehole fluid, and finally into the water circulating inside the tube.

The model consists of three coupled parts — the U-tube, the borehole, and the ambient geothermal reservoir — and represents a scaled version of a real field installation located roughly 200 m underground. In this simulation, the ground zone and U-tube are scaled to 6 m and 3.2 m depth respectively, with a 0.0875 m tube diameter inside a 0.35 m borehole, all contained within a 3 m ground cylinder.

Methodology

The geometry is created in Design Modeler, and a polyhedral mesh of approximately 1,750,000 cells is generated in ANSYS Meshing.

The physics of the case centers on free (natural) convection. The solid ground temperature is defined as a linear function of depth using a Named Expression, so the borehole is heated from the bottom upward. Gravity is enabled, and the thermal conductivity and heat capacity of water are defined as temperature-dependent using the polynomial method — an essential detail, since the buoyancy forces driving the entire problem depend on capturing these property variations correctly.

Turbulence is modeled with the Realizable k-ε model with standard wall functions, and the simulation is solved in steady-state form as a coupled solid–fluid heat exchange problem.

Analysis

At the end of the solution process, temperature and pressure contours along with velocity vectors are extracted for both the tube and borehole zones. The results show that convective heat transfer raises the tube outlet temperature to 305.47 K.

The velocity vectors reveal the heat transfer mechanism clearly: a vortex forms at the bottom of the borehole, intensifying turbulence and enhancing heat transfer. Water near the hot wall warms, loses density, and rises; as it cools near the tube, it sinks again — completing the natural-convection loop that continuously feeds heat from the ground into the circulating tube water.

By completing this project, you will learn 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 conjugate heat transfer simulation.