• BHE Virtual Radius and Ideal Element Size


    I am building a 3D model of a Borehole Heat Exchanger that crosses 3 geological units (out of which two are aquifers). The BHE should run with a time-varying heating load with annual cicles. Thus it has been modelled with imposed Power and flow rate (in terms of time series, linearly varying).

    As the simulation starts, it works well until the first few days. Then the time step collapses to extremely slow values and the Inlet and Outlet temperatures suddenly decrease of several °C.

    I noticed that the virtual radius of the BHE is very large (30 m more or less) while the real radius is 0.08 m. Anyway the mesh discretization is in good agreement with the ideal element size (Delta was computed with the formula suggested in White papers Vol5.). I can also see that the mesh nodes around the BHE are coincident with the Ideal Element Size circle that can be drawn as an attribute of BHE. Furthermore, moving the nodes around the BHE, does not seem to result in variation of the virtual radius.

    Do you think that the problem is the big difference in the radii? Any suggestion about ways to solve this issue ?

    Thanks you in advance for any help!