BHE: average heat vs T pipe out at top

I'm trying with some simulations with a 4th kind heat transp. b.c. (BHE 2U-pipe) and I always find a slight difference between the average heat plot and the pipe out temperature profile of the BHE.
As I read in your White paper n°5, at page, 54, the "average heat" plot shows the time history of temperature at the top of the outlet pipe, so the aforementioned plots (avg heat and BHE pipe out T) should agree, as shown in your examples (i.e. figure 1.10); but, in my calculations, a slight difference between them occurs. Why?

I am using the analytical method of Eskilson and Claesson; I have a free and movable top slice (should I turn it into a phreatic one?).

Thank you,
Alessandro

Dear Alessandro, you are correct, the average temperature is the outlet temperature. Do you use the most current version of FEFLOW? There was a bug related to this issue in a previous version.

It's the 5.4.5412-x86 version, I think it's the most recent one.

Dear Alessandro, the patch 12 of 5.4 gives correct results, in general. If you say "slight differences" - how large are they in Degree Celsius? Kind regards Wolfram

Dear Wolfram,
thank you for your help!

Here you can see an example:
[img width=200 height=160]http://areeweb.polito.it/ricerca/groundwater/troubles/slight_differences.GIF[/img]

[url=http://areeweb.polito.it/ricerca/groundwater/troubles/slight_differences.GIF] http://areeweb.polito.it/ricerca/groundwater/troubles/slight_differences.GIF [/url]

I am simulating a 100m deep 2U borehole heat exchanger.
The reference temperature is 10°C, and the prescribed inlet temperature is of about 14.7 °C.
For every other parameter, I kept the default value.
No hydraulic gradient is present, as I put a uniform fixed hydraulic head value on the boundary.
The aquifer is unconfined with "phreatic" option.

A slight difference of about 0.3 °C occurs.

This bug occurs only with Feflow 5.4; in the 6.0 version (with the "old fashion" Motif GUI), everything is ok with outlet temperatures...

Kind regards,
Alessandro