Discussion Forum

Hi,

I am working on modeling of a borehole geothermal heat exchanger. My model is in 3D and consists of 2 cylinder solid domains (soil and grout) and a line element (bezier curve) for modeling the pipe and carrier fluid using the pipe flow module. The pipe size is 1 inch; inlet and outlet of the pipe is at the top and it has a U-bend at the bottom of the borehole.

I have two physics: Heat transfer in solids (T2), assigned to the domains and non-isothermal pipe flow (T) assigned to the pipe. These two are coupled by setting the external temperature of the pipe wall to the temperature of heat transfer in solids physics. (T_ext = T2)

After running a time dependent analysis for 32[h], I plotted the temperature distribution of the borehole on a x-z plane, where y=0 and averaged them with respect to z direction. Thus, I obtained the average temperature distribution of the borehole on x-x cross-section.

Then, using the pipe flow physics, I acquired the temperatures at the outer pipe wall, at the inner pipe wall and the fluid temperature, respectively. I did this for both down and up legs and these values were also averaged over the depth. Finally, I added these temperature data with respect to their position, on the same plot.

I was expecting to have the same temperature (T=T2) at the pipe outer walls. Instead, there is a jump at that location and what is more interesting to me is that, the temperature of the grout domain at the pipe axis is equal to the temperature of the outer pipe wall obtained from the pipe flow physics. Which makes sense in a way, because that was the coupling condition, but to me, the location is not right.

In reality, I should not be able to get the temperature of the grout domain at locations where there is a pipe, because grout is not physically there. However, I understand that due to simplification and using a line element to model the pipe flow, I am able to get those.

I am still confused about the temperatures that pipe flow physics is giving me. Are they reliable?

One last thing to add, I approximated this 3D model and created a 2D model (X-Y cross-section) for validation. I obtained very close heat fluxes in pipe and borehole boundaries and very close temperature distribution in the borehole, except the pipes and water.

The geometry and the temperature plots are attached. Any comments will be mostly appreciated. Thanks.

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Tolga