[berkmm]

Hello everyone, I'm working on a Geothermal Borehole Heat Exchanger simulation and it involves modelling a soil(earth ground). I needed to implement the temperature gradient of soil during modeling of soil because its temperature is changing according to the depth.

My model is a transient axisymmetric model and can be seen in the Figure 1 and Figure 2. It has a 200m wide and 2140 m depth ( with a 3 porous zone section at 2000 m 2090 m and 2140 m) and the temperature is changing by depth. Temperature formation values in well can be seen in the Figure 3 and Figure 4. According to the literature(temperature gradient is modeled with an UDF) , simulation's initial temprature gradient were given in Figure 5.

In order to do modelling these temperature gradient in my soil model i've used named expression. I used a piecewise linear expression for each of the soil segments. (According to the tutorial https://www.youtube.com/watch?v=GVSLyb7buEU&t=627s ) Temperature values of given figure can be seen in the below.
Depth(m) Temperature (K) Expression (Unitless)
0<x<150 283.15<Temp<433.15 T1=10+273.15+0.9333[/m]*(x)
150<x<288 433.15<Temp<463.15 T2= 150+273.15+0.2899[/m]*(x)
288<x<1437 463.15<Temp<473.15 T3=190+273.15+0.0087[/m]*(x)
1437<x<1500 473.15<Temp<493.15 T4=273.15+200+0.3175[/m]*(x)
1500<x<1588 493.15<Temp<588.15 T5=273.15+220+1.0795[/m]*(x)
1588<x<1650 588.15<Temp<608.15 T6=273.15+315+0.3226[/m]*(x)
1650<x<2000 608.15<Temp<618.15 T7=273.15+335+0.0286[/m]*(x)
2000<x<2090 618.15<Temp<783.15 T8=273.15+345+1.8333[/m]*(x)
2090<x<2140 783.15<Temp<923.15K T9=273.15+510+2.8[/m]*(x)

Note that= Depth given in meter, temperature Kelvin and expression is unitless because I've given the temperature values in cell zone condition( Fixed values - temperature) and it only support the unitless parameters.

Since my model has 3 section (Porous zone 1 2 and 3), I need to adapt these temperature variations depth my depth. Therefore, I used a IF conditional expression according to the depth down until 2000m. Because of I have 3 sectional figure.
Temperature Gradient (0-2000m) for Porous Zone 1: IF(x>=1650[m],p7,IF(x>=1588[m],p6,IF(x>=1500[m],p5,IF(x>=1437[m],p4,IF(x>=288[m],p3,IF(x>=150[m],p2,p1))))))
Temperature Gradient 2 ( 2000-2090m) for Porous Zone 2:
273.15+345+1.8333[/m]*(x)
Temperature Gradient 3 for Porous Zone 3:
273.15+510+2.8[/m]*(x)

Note: p1 p2 p3 p4 p5 p6 p7 values are T1 T2 T3 T4 T5 T6 T7. Moreover, I've defined the wall boundary conditions for soil wall and I made it by changing unitless temperature values to [K] and changing named expression while assigning it to the walls except Soil top surface defined as 283.15 K and bottom surface is defined as 923.15 K constant temperature.

However, After I run a transient simulation ( Simulation is performing very quickly because there is no flow) , I'm not getting an expected results. Temperature is increasing very rapidly and slip occurs in values (Temperature is going up to 5000K ). Also contours and XY Plot values are not matching. Results can be seen in the Result 1 figure (Total Temperature contour result of internal zone) and Result 2 figure (XY Plot of Total Temperatue of internal zone) .

Therefore, I need help to model the temperature gradient of ground. How can I model and get the correct results according to the Figures for this model?