Thanks Nick,

I'll have a look to the flux report and heater convergence. 

Yes, for wall section2 (line source) I have a shadow wall. In all cases, I always made sure both walls have the same heat rate (either in W/m2 or W/m3).

The experiment can be considered a horizontal sediment layer & a water layer (not a pipe) since the pond channel is approximately 2 m wide and the maximum heating time is only 4 hr. The cable was burried in the middle of the channel. So, the tank that I have in my 2D model represents a logitudinal section of the channel and line heat source. 

Please let me know if you need any other info. Thanks in advance.