Hello there!
We're currently using Plaxis2D with its PaxFlow and Thermal Modules to simulate the behaviour of liquid soil as a filling material for buried power lines. In our case we got field data from a ton of sensors recording the actual soil-behaviour at a research project where liquid soil has been used as a fill material for the trench of a Gas insulated DC powerline. There is two issues that I'm currently having, that result in pretty significant discrepancies between the recorded data and the simulation:
1. My first question: Is there a way to modify the formula that is being used to calculate the soils' heat conductivity? (Page 208 in reference manual "thermal tabsheet")
Plaxis calculates the effective heat conductivity of the soil based on the assumption that it behaves like a porous solid material instead of an agglomeration of spherical soil particles. Therefore the resulting heat-conductivities easily end up in very optimistic regions of +4W/(mK) with regular parameters, plus does not take non-linear effect of saturation into account (especially coarse sand has a highly non linear increase of conductivity in the low saturated-range). This non-linearity results due to the rapid increasing of the contact surface between the spherical soil particles, as soon as the water content rises (since the adhesive waterfilms inevitably form "water-bridges" between the particles).
My solution so far was to radically change the input parameters being conductivity of the solid, water and vapour to fit the curve to the actual saturation-based conductivity. This resulted in a heat conductivity of 7 W/(mK) for the water being 10 times higher that originally supposed to be, while the solid conductivity ends up at around 0.8W/(mK). With these modifications we get realistic values of 0.5W/(mK) for dry sand and 1.5W/(mK) for Sr=0,3.
2. My second question, which results due to the focus on thermal and groundwater flow (flow-only-calculation type was chosen):
How is the temperature of infiltrating water calculated or is it just set to the temperature at the "entry-point" (for instance ground level for precipitation).
Since we recorded the precipitation in the field experiment, i implemented the precipitation as a table into the simulation. However, i could not find information on how the temperature of the infiltrating water is calculated or whether it's calculated and not just set to the ThermalBCs Temperature at the ground level (in the case of precipitation). I'm asking because our soil has a high permeability and the recorded sensordata states high heat-transport via infiltrating rainwater.
Thanks alot in advance! I hope someone can help us even though it's kind of an unusual "problem".
King regards
Louis