Geothermal Reservoir CFD Simulation, ANSYS Fluent Tutorial

Description

 Description

This project aims to simulate a geothermal reservoir using a single U-tube, a Downhole heat exchanger (DHE), which consists of a system of tubes or a U-tube located in a single wellbore, through which the working fluid is circulated to extract heat.

The simulation model is based on a small-scale field case and consists of three parts: the U-tube, borehole, and ambient geothermal reservoir. In an actual situation, the process occurs 200 meters under the ground. We scaled the model in which the depth of the ground zone and u-tube is 6 and 3.2 meters, respectively.

We designed the U-tube with an inlet and outlet diameter of 0.0875 meters. It is surrounded by a borehole of 0.35 diameter and 6 m height inside the ground zone, a cylinder with a diameter of 3 m.

In the solid zone, the temperature varies linearly from top to bottom, which makes the hole temperature change the same way. This causes free convection heat transfer, and the heat is extracted from the ground and transferred to the tube so that the outlet temperature of tube water increases.

The temperature of the solid zone is defined as a linear function of temperature in Named Expression.

We modeled the geometry of the project using Design Modeler software. Then we meshed the model with ANSYS Meshing software. The mesh model is polyhedra, and the number of cells equals 1,749,097.

Geothermal Reservoir CFD Simulation Methodology

In this project, we consider gravity since the water inside the hole moves due to natural convection. It takes the heat from the bottom of the hole and gives it to the pipe and the water flowing inside.

In order to simulate the turbulence of the fluid, we used the Realizable k-epsilon model with a standard wall function.

The polynomial method defines water heat conductivity and heat capacity as temperature-dependent. The flow behavior is steady.

Conclusion

After the solution, we obtained the contour of temperature, pressure, and velocity vectors for tube and hole zones.

The results show that the convective heat transfer between the hole and the tube increases the tube outlet temperature to 305.47 k.

As the velocity vectors show, the generated vortex at the bottom of the hole consequently increases flow turbulence and heat transfer. Since the temperature of water flow near the wall increases, its density decreases, so it goes up and transfers the heat to the upper layers of water. Its temperature decrease and comes back down; natural convection happens, and heat is transferred to the tube.