'RADIATIVE HEAT FLUX GAS' device

[Stefano Capra]

Hi everyone, 

I am trying to monitor the radiative heat flux produced by a 500kW fire and compare it with an analytical relation like the one described here at page 58:

Principles of Fire Behavior

To do that I have placed a series of devices in my simulation following the suggestions on the User Manual (Sec. 16.10.5):

&DEVC ID='flux_0.8', QUANTITY='RADIATIVE HEAT FLUX GAS', XYZ=18.6,12.6,0.8,

ORIENTATION=-1,0,0 /

which should be a shortcut for the radiative heat flux on a spherical particle of radius 0.001m

Unfortunately the values out of the FDS simulation are way lower than what approximated by the equaltion:

(HRR*radiatian_fraction) / (4*pi*distance^2)

Even if I am using the same radiant fraction of I have got out of FDS

On top of that the device values change if I change its orientation, which is strange for a sphere

I am missing something?

Thanks

Ste

[Kevin]

Add

&BNDF QUANTITY='RADIATIVE  HEAT FLUX' /

to your file. You will probably see that as you get far from the fire, the flux field develops a star-like pattern that is related to the number of angles you have selected for the radiative transport equation. The default is approximately 100. You may need to increase the number of angles to get a more uniform field away from the fire.

[dr_jfloyd]

This device only integrates radiation over the half sphere centered on the ORIENTATION, so changing the ORIENTATION will change the flux. Make sure you have pointed the device at the fire.

[Stefano Capra]

Hi dr_jfloyd and  Kevin, 

many thanks for the useful tips.

I figured out the orientation issue last night and I am now running a couple of tests with the device oriented towards the fire.

I will post the comparison with the analytical relation has soon as possible.

I will also try to investigate the effect of increasing NUMBER_RADIATION_ANGLES and RADI NMIEANG

I have also tried to use "INTEGRATED INTENSITY" but I find it a bit confusing.

It should integrate the incoming radiation over the whole 4pi sphere. 

Can I use that as well subtracting the 1.67kW/m2 background value?

Is the integration the only difference between the two devices?

Thanks

Ste

[Kevin]

NMIEANG is number of Mie angles for water droplets. This is not relevant here.

INTEGRATED INTENSITY is also not useful here. It is the average heat flux over all angles.

[Stefano Capra]

Hi everyone,

I have attached the results I obtained increasing the number of rays

The difference is noticable but not significant 

Orienting the device properly was the key factor to get reasonable radiative fluxes out of FDS

thanks to both dr_jfloyd and Kevin