Radiative Heat flux Calculations
254 views
Skip to first unread message
Robert Peart
Feb 28, 2012, 2:51:15 AM2/28/12
to FDS and Smokeview Discussions
I am attempting some thermal analysis of relatively small sectioned
structural elements (100 mm x 100 mm angle) with large external fires
(170 MW) where the the heating of the structural elements is by
radiation. As the elements heat up convective heat transfer has some
effect. Because of the course grids (0.2 m and 0.4 m) the net heat
flux results are based on model structural element section areas that
are double to 4x the actual section areas. I have elected to distill
the 'RADIATIVE HEAT FLUX GAS' data from the FDS model and perform a
time step spreadsheet calculation to estimate the structural member
time temperature profile. To this end, part of that calculation
involves calculating the radiation heat transfer from the fire to the
structural elements. I need to calculate the configuration factor (F).
Does the FDS radiation model simplify the fire emitter temperature as
is implied by the RADI default parameter value for RADTMP (900C)?
If the fire radiation temperature is assumed to be 900C (Default
'RADTMP') then the configuration factor for the 'RADIATIVE HEAT FLUX
GAS' detector should be:
F= 'RADIATIVE HEAT FLUX GAS' / (Stefan-Boltzmann Constant x
(900+273.15)^4)
Please advise if my assumption that the default radiator temperature
(900C) for the FDS gray gas radiation model is valid.
structural elements (100 mm x 100 mm angle) with large external fires
(170 MW) where the the heating of the structural elements is by
radiation. As the elements heat up convective heat transfer has some
effect. Because of the course grids (0.2 m and 0.4 m) the net heat
flux results are based on model structural element section areas that
are double to 4x the actual section areas. I have elected to distill
the 'RADIATIVE HEAT FLUX GAS' data from the FDS model and perform a
time step spreadsheet calculation to estimate the structural member
time temperature profile. To this end, part of that calculation
involves calculating the radiation heat transfer from the fire to the
structural elements. I need to calculate the configuration factor (F).
Does the FDS radiation model simplify the fire emitter temperature as
is implied by the RADI default parameter value for RADTMP (900C)?
If the fire radiation temperature is assumed to be 900C (Default
'RADTMP') then the configuration factor for the 'RADIATIVE HEAT FLUX
GAS' detector should be:
F= 'RADIATIVE HEAT FLUX GAS' / (Stefan-Boltzmann Constant x
(900+273.15)^4)
Please advise if my assumption that the default radiator temperature
(900C) for the FDS gray gas radiation model is valid.
Robert Peart
Feb 28, 2012, 5:11:43 PM2/28/12
to FDS and Smokeview Discussions
Belay that question. By varying the assumed fire emitter temperature
between 700C and 900C the spreadsheet timestep model only shows a
member temperature variation of 10C after 3 hours. Essentially the
spreadsheet model is not sensitive to the emitter temperature.
shostikk
Feb 29, 2012, 1:58:11 AM2/29/12
to FDS and Smokeview Discussions
RADTMP is only used for the calculation of some material properties
related to the gases ans particles.
The emission source is based on the real temperature of the CFD
solution OR the prescribed radiative fraction of combustion. See the
guides for more.
related to the gases ans particles.
The emission source is based on the real temperature of the CFD
solution OR the prescribed radiative fraction of combustion. See the
guides for more.
Robert Peart
Feb 29, 2012, 4:29:35 PM2/29/12
to FDS and Smokeview Discussions
Thanks Simo
I realised I had asked a dumb question after looking at the wall to
wall vector calculus in Ch 7 of the Technical Reference Guide.
In the end with the spreadsheet model properly set up, 'RADIATIVE HEAT
FLUX GAS' was the only input required from the FDS model.
Regards
Robert
I realised I had asked a dumb question after looking at the wall to
wall vector calculus in Ch 7 of the Technical Reference Guide.
In the end with the spreadsheet model properly set up, 'RADIATIVE HEAT
FLUX GAS' was the only input required from the FDS model.
Regards
Robert
Reply all
Reply to author
Forward
0 new messages