ceiling temperature vs target temperatur

[tommuel...@gmail.com]

Dear all,

I examine the natural smoke extraction of a hall (area 7500 m², height 22 m). The fire has a max. HRR of 24 MW. For the temperature of the ceiling, a value of 200 °C results after one hour. To get an idea of ​​the highest ceiling temperature in the area of ​​the plume-cl I placed a target at the ceiling level facing towards the floor. The target material corresponds to the material of the ceiling. The subsequent calculation provides a surface temperature of the target of only 110 °C. Shifting the target to other positions of the ceiling does not provide temperatures above 200 °C at any point. Why does the ceiling (on average) reach this temperature?

Thanks
Tom

[Richard Peacock]

The target and ceiling calculations have quite different view factors, but I would expect them to show similar temperatures. Targets are very position dependent, both the location and where the normal vector points can dramatically impact the temperature of a target.  I assume if you are looking to compare targets to a ceiling temperature, the normal vector should be 0,0,-1 so it's pointing down.  Highest temperatures should then be directly over the fire.

You can also see the environment near the ceiling by including a 2-D slice file on the z-axis at the ceiling.

We'd be happy to take a look at your test case if you are able to share it.

Rick Peacock

[Richard Peacock]

Here's a quick calculation for a 24 MW file in your compartment.  The target temperature (with the target placed directly over the fire) is higher than the ceiling temperature as expected. We'd need to see your input file to know what's unique about it.

[tommuel...@gmail.com]

Hi Rick,
thanks for your quick reply!
I agree: the highest ceiling temperature is to be expected at the stagnation point. And there I placed the target using a normal vector (0, 0, -1).
I add my input-file. Thank you for your help.
Best regards Tom

[Richard Peacock]

This is related to the calculation of the enhanced heat transfer due to the presence of a ceiling jet in very large rooms with a fire.  I've created a software bug report for it (see https://github.com/firemodels/cfast/issues/675).  We working on a fix.

Rick Peacock

On Friday, September 22, 2017 at 2:33:59 PM UTC-4:

[tommuel...@gmail.com]

Hello Rick,

thanks again for your support and your help!

Best regards
Tom