Different ways to calculate ADIABATIC SURFACE TEMPERATURE

[Hanzhou Mao]

Hello,

In a small model (linked picture), I've set 2 batches of devices : close and far from the heat source.

I'll be focusing on the batch close the fire here.

I'm trying to compare the ADIABATIC SURFACE TEMPERATURE values through 2 different ways :

- &DEVC QUANTITY='ADIABATIC SURFACE TEMPERATURE'

(directly on the ceiling surface)

- &DEVC QUANTITY='INCIDENT HEAT FLUX'

  &DEVC QUANTITY='HEAT TRANSFER COEFFICIENT'

  &DEVC QUANTITY='GAS TEMPERATURE'

(at the center of the mesh closest to the ceiling surface)

Then I used the formula linked to this message to calculate Tast (ADIABATIC SURFACE TEMPERATURE). The devices were applied on a particle placed at the center of the mesh closest to the ceiling (the mesh size has been reduced so the mesh would be as close as possible to the ceiling surface).

By doing so, the devices should show very similar results as they are very close to each other, or so I thought.

However, on the "particle x1.2" file, it appears that the temperature is underestimated by using the indirect measure : I'm forced to apply an artificial mulplier coefficient.

Do you have any ideas about the source of this issue?

Thanks in advance, 

HM

[dr_jfloyd]

Are you converting the gas temperature from C to K before doing the T_AST calculation?

Are you accounting for the emissivity of the surface?

[Hanzhou Mao]

Thanks for the reply,

Yes, I've added +273.15 to 'GAS TEMPERATURE' values before calculating T_AST, but I've assumed an emissivity ε=1.
Just in case, T_AST is converted back into into °C using the same value, and 'INCIDENT HEAT FLUX' is divided by 1000 before the calculation (kW -> W).

HM

[dr_jfloyd]

When you say that you've assumed emissivity=1, does this mean you set EMISSIVITY=1 for the SURF that applies to where you are measuring the AST?  If you didn't set a value in FDS, then the default EMISSIVITY in FDS is 0.9 and that is the value that should be used.

Below is a very simple input file. At 10 s AST=92.945 C, TG=106.585 C, h=7.142 W/m/K, and qr=0.9103 kW/m2

e ( qr - sigma AST^4) = 0.9 * (910.3 - 5.67E-8 * (273.16+92.945)^4) = -97.37 W/m2

h (TG-AST) = 7.142 * (106.585 - 92.945) = 97.43 W/m2

The sum of these is 0.06 W/m2 which is ~0. It seens like the AST is working correctly.  I'd suggest double checking your calculation to make sure you don't have an error. If you still have the case where your AST does not seem to be correct, submit a simple input file to the issue tracked. In this case, something like just the fire and enough of the geometry around it to contain the set of near devices. 

&HEAD CHID='AST'/

&MESH XB=0,2,0,2,0,2,IJK=20,20,20/

&TIME T_END=10/

&REAC FUEL='PROPANE',SOOT_YIELD=0.02/
&SURF ID='FIRE',HRRPUA=1000,COLOR='RED'/

&VENT XB=0.4,0.6,0.4,0.6,0.0,0.0,SURF_ID='FIRE'/

&DEVC XYZ= 0.2,0.2,2.0,IOR=-3,ID='AST',QUANTITY='ADIABATIC SURFACE TEMPERATURE'/
&DEVC XYZ= 0.2,0.2,2.0,IOR=-3,ID='TG',QUANTITY='GAS TEMPERATURE'/
&DEVC XYZ= 0.2,0.2,2.0,IOR=-3,ID='h',QUANTITY='HEAT TRANSFER COEFFICIENT'/
&DEVC XYZ= 0.2,0.2,2.0,IOR=-3,ID='qr',QUANTITY='INCIDENT HEAT FLUX'/

[Hanzhou Mao]

" When you say that you've assumed emissivity=1, does this mean you set EMISSIVITY=1 for the SURF that applies to where you are measuring the AST?  If you didn't set a value in FDS, then the default EMISSIVITY in FDS is 0.9 and that is the value that should be used."

I've left the default value (0.9), so I tried using the formula with  ε=0.9 instead.

Again, there's approximately a 1.2 factor between the results.

When I look at your file, I see all the devices share the same coordinates.

Do you think my issue has something to do with the devices not exactly sharing the same coordinates (AST on a mesh surface, and TG,h,qr at the center of a mesh)?

HM

[dr_jfloyd]

 The AST for a wall cell is computed using the incident flux, gas temperature, and heat transfer coefficient at that wall cell. If you are trying to use data from another location, then that is likely your issue.

[Hanzhou Mao]

Understood, thank you for your guidance!

HM