Thermocouple output remains at ambient temperature (20 °C) when obstruction bottoms are aligned with grid lines

[js4561207]

Dear all,
I’m new to FDS and currently learning how to use various devices, particularly thermocouples.
I have developed a simple model to test thermocouples, but the output is confusing.
when I align the bottom of the obstructions with the grid lines (as shown in Fig. 1), the output temperatures from Thermocouple03 and Thermocouple04 remain at ambient temperature (20 °C).


However, when I align the top of the obstructions with the grid lines (also shown in Fig. 2), the same thermocouples(also moved with obstructions) report significantly higher temperatures—which seems more physically reasonable.


Could someone please help me understand how thermocouples are treated in FDS and how best to position them.
I would be grateful for any insights or suggestions.
Version: FDS-6.9.1-0-g889da6a-release
Best regards,
Chu

[drjfloyd]

THERMOCOUPLE is a gas phase output. If you put a TC in a solid, there is no gas present and the TC will never change from its initial value of TMPA.

[js4561207]

I changed the position of the obstruction in Fig. 2, and now the thermocouples (highlighted in the figures and overlaid on the solid surface) are able to output temperatures—despite still being in contact with the solid, similar to the setup in Fig. 1. Here are my input files.
half_top.txt
half_bottom.txt

[drjfloyd]

Using the FDS User's Guide:

How does FDS treat an OBST that does not fill an entire grid cell?
What does FDS do with the XYZ for a point device (i.e. where does it actually place the device)?

[js4561207]

Thank you for your help. Now I understand how it works.