Hello!

I need to make a 3D model of a diaphragm wall having pipes tied to the reinforcing cast inside the wall. Pipes are connected with a heat pump and they purpose is changing heat with the ground and excavation air surrounding the wall. The problem I encountered is about modeling the pipes within the wall. The inlet fluid inside them has a certain velocity and temperature according to the period of the year. (summer, winter)

As I read, I need discrete features for defining them, right? But I don't know how to use them, any help?

I thought about a different solution: Modeling a horizontal slice of the wall, 2D, containing the pipes to determine the most efficient layout afterwards extrapolating the results into 3D model, where instead of pipes, I would set a surface behaving like a heat source or sink, with a certain heat flux, obtained in 2D which represents the fluid velocity and temperature. I need the heat exchanged between the inlet and outlet and that's why I don't know if this would work. Do you think this solution works?

I'm not sure whether this has been studied in detail yet. I know of a FEFLOW user having simulated a similar situation for sheet-pile walls connected to pipes for heat exchange, but I cannot remember the details of his work. However, I think in any case the procedure will highly depend on the scale of the system and the level of detail required. For a long stretch of diaphragm wall with multiple (parallel?) connections to a heat pump a heat source/sink might be a sufficient solution, with reasonable assumptions about the heat transfer, depending on (seasonal) ground and fluid temperatures. The smaller the scale gets, the more you'll need a spatially correct solution. There the simulation using discrete features may be a good solution. On the other hand, the horizontal slice could be a valid approach, too. If heat storage in the diaphragm wall is negligible (because of the time scale), then you may even be able to derive a bulk thermal resistance between the fluid in the pipe and the interface between the diaphragm wall and the ground - and this could then be applied for a heat-transfer BC.

Many options - and I think you'll have to choose depending on the questions asked to the model and the temporal and spatial scales required.
Thank you! I will do some extra research and come with more definite questions. Where can I find the theoretical background for FELFOW in order to better understand the boundary conditions, mainly Cauchy for heat transfer? I need to find a correspondence between heat transfer coefficient and the temperature applied within this boundary condition for modelling the excavation air temperature and external air temperature.
The heat-transfer BC in FEFLOW (used without boundary fluid flow) is is simply heat conduction with a heat transfer coefficient. The theoretical background of FEFLOW is avalailable in the FEFLOW book (https://www.springer.com/de/book/9783642387388) or - with less up-to-date and less revised content - in the older Reference manual (e. .g., https://www.yumpu.com/en/document/read/20781782/reference-manual-feflow).
Hello, It's me again.

I don't understand why there is difference between a certain volume and the nodes which are forming volume in terms of Storage Capture/Release or BCs. I understand that an element has several nodes. In my model I tried with a small volume having 32 elements. When I convert the selection to 3D nodes, I obtain 50 nodes. On the Period-Budget the quantities measured in Joules mentioned above are different and I don't know why and how to find a connection between them.

What exactly Storage Capture/Release means? It means that my domain of interest is receiving heat for the surroundings/giving heat to surroundings? Based on DOI initial temperature?
Which is the difference between Storage Capture/Release and Internal Transfer? I don't understand why I can't obtain Internal Transfer at the nodes, but only at surfaces/volumes that containes those nodes!?

Thank you!
The difference is the following:
Internal transfer is the heat flow calculated across the boundaries of the DOI.
Change in storage is the change in the energy content of the volmume within the DOI or related the the selected nodes. So when you have heat leaving the DOI (assuming no sources or sinks in the DOI), there is an INPUT to the model from storage (as the volume cools down it kind of transfers its stored heat to the groundwater)
Hello,

Not linked to feflow as such, but in an EU research project some of my colleauges have been working on modelling this type of system, perhaps the papers they published may be of interest and help getting you to run your model:

Shafagh I, Rees SJ. 2019. Analytical Investigations into Thermal Resistance of Diaphragm Wall Heat Exchangers. European Geothermal Congress 2019

Shafagh I, Rees S. 2018. A Foundation Wall Heat Exchanger Model and Validation Study. IGSHPA Research Track

Best wishes,

Henk Witte

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