Incident radiative heat flux

[Priscilla Mariani]

Hi all,

I've got a question on measuring incident radiative heat flux.

I have created a "dummy" wall along the location where I need to check the radiative heat flux tenability at, relying on radiative heat flux boundary output (BNDF) on it.

My question is how should I set the properties of this dummy wall in order to get the pure incident radiative heat flux from the fire, excluding any reflected/outgoing radiative heat flux?

Do I set it as adiabatic? 

Do I set it as a general surface with emissivity of 1? How about the boundary condition model of this surface then? (fixed temperature at TMPA or absolute zero?)

Or is there a better way to get this result?

Thank you in advance! :)

Regards,

Priscilla

[TimoK]

FDS User Guide:

20.10.7 Heat Flux
There are various output quantities related to heat flux.

’INCIDENT HEAT FLUX’
This is the term qdot''_inc,rad in Eq. (20.11).

Table 20.3: Summary of frequently used output quantities.

INCIDENT HEAT FLUX Section 20.10.7 kW/m2 B,D

So, INCIDENT HEAT FLUX is the incident radiative heat flux. It does not depend on the emissivity of the surface.

I.e., qdot''_net = emissivity * (qdot''_inc,rad - sigma*T_s^4) + convective part

TimoK

[Ben Ralph]

I wonder why you have created your "dummy" wall? Simply to capture your desired output quantity? Or does it represent something "in real life"?

If it's just to capture output, there is no need. As Timo says, you can just use devices to output incident heat flux.

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[Priscilla Mariani]

Yes we were hoping to present the result in snapshots, but we can certainly use the devices. :)
There was also the option of incident heat flux in the boundary quantity.

However, doesn't the incident heat flux include the convective part? How do we separate it out?

Thanks!

Pris

[fde]

Must not a DEVC of  INCIDENT HEAT FLUX be placed on a surface?

[Ben Ralph]

@Pris: You want to output incident radiant heat flux? Why do you want this quantity specifically? If you are carrying out a human life safety assessment then a human body would experience heat transfer via both radiation and convection, if you are estimating whether a material may ignite then similarly the material would experience both convective and radiative heat transfer. If you wanted just the incident radiative heat flux, you could output INCIDENT HEAT FLUX and CONVECTIVE HEAT FLUX and then post-process the data... 

@fde: one can use a &DEVC line as a shortcut to the creation of a lagrangian particle. Check out Heat Fluxes and Thermal Radiation section of user guide.

[fde]

Ah, yes, I remember the particle example. Earlier, I understood that you suggest there is no need for a surface. 

[TimoK]

No, the "INCIDENT HEAT FLUX" does not include the convective part. See the equations in the users guide (and my previous post).

The incident heat flux output in FDS is really the incoming radiation flux. Some of this incoming flux is absorbed by the surface and some are reflected out. (Or at least this is what is written in the users guide).

TimoK

[Tom]

Hi All,

@PriscillaI, I have some of the same questions you are asking too. If you are wanting to measure the incident heat flux then I don't believe that your surface emissivity is of any importance. I'm thinking of specifying my surface as inert.

@Ben, the incident heat flux is of specific importance for assessing occupant tenability. Both incident heat flux and time are used in assessing pain and second-degree burns (SFPE 5th ed, pg 2721). Only the radiative heat flux can be determined by device alone (without a surface, as per the fds user guide). Please can you explain further as to how to only use a device to measure incident heat flux.