Clarification about ratiative heat flux

[D D]

Hello all,

when calculating heat flux due to radiation to compare against empirical limits (say for example 2.5 kW/m2), what is the appropriate measure to make and check, the result of

eq. 21.32 (FDS manual)

or just the first part of it?

I would appreciate a guidance on this as it is confusing what is my skin tolerable against; what I get (e*q-inc) or what I guess minus what I give (qr)?

Thank you.

[D D]

My last statement above may be confusing. What I mean really is, what is to be followed for tenability investigations, the results of the upper equation or the lower?

Thank you.

[Kevin McGrattan]

The output "GAUGE HEAT FLUX" is typically used to compare with heat flux measurements. If you just include q_inc, then in a completely ambient environment you will have a non-zero heat flux because q_inc will by sigma*T_amb^4 which is not zero. So you want the first expression with T_s set to ambient temperature.

[D D]

Thanks a lot for the clarification. Makes total sense. I was doing FED-heat calculations for the radiative portion only and had problems having my measurement getting steadily increased just by standing in a room far away from the fire.

[D D]

As stated, I 'm a doing a FED analysis for the radiative portion of a moving occupant using eq. 63.43 of SFPE HB, 5th edition.

    and    

I am using a particle which starts away from the fire then at some point around 65 sec enters the room of the fire walks a bit and then goes outside. This is just for test purposes.

The following shows how I coded the a case.

&SURF ID='OCCUPANT', LENGTH=1., THICKNESS(1:1)= 0.01, WIDTH=1.0  /
&INIT ID='Occupant 1', N_PARTICLES=1, PART_ID='Occupant 1', PATH_RAMP= 'Occupant - X', 'Occupant - Y', 'Occupant - Z', XYZ=35.6,33.6,1.8  /
&PART ID='Occupant 1', SAMPLING_FACTOR=1.0, SURF_ID='OCCUPANT'  /

&DEVC ID='TEMP', QUANTITY='TEMPERATURE', INIT_ID='Occupant 1' /
&DEVC ID='RHF', QUANTITY='RADIATIVE HEAT FLUX GAS', INIT_ID='Occupant 1' /

The graph of the temperature seems to be giving legit results, but the radiative heat flux device does not. It remains stuck at 0.0 no matter how near the fire the particle comes.

                

Do I miss something? Appreciate any help on this.

Thank you.

[dr_jfloyd]

The particle has an ORIENTATION and RADIATIVE HEAT FLUX GAS is the net gas radiation in the 2 pi angles centered on the ORIENTATION. The default ORIENTATION is down (down is the default since typically a DEVC with an ORIENTATION is something like a ceiling mounted sprinkler or smoke/heat detector).  Try changing so the particle is pointed towards the fire. If that doesn't work, then the next step would be to open an issue with your test case.

[D D]

I tried playing with the orientation but did not help. There were results affected which was expected but the RHFG device remained fixed at zero. I opened the issue here

https://github.com/firemodels/fds/issues/11843

hoping that it is not due to erroneously written fds file.

thank you.

[D D]

Is it possible to define the ORIENTATION vectors of the particle as variables? The intent is to have the orientation pointing to the same direction as the particle moves in space, kinda like simulating the face of an evacuee. 

&PART ID='Occupant 1', SAMPLING_FACTOR=1.0, SURF_ID='OCCUPANT', ORIENTATION= x,y,z  /

x,y,z defined by a ramp

If not, would you consider implementing something like this in the future?

[dr_jfloyd]

The face may not represent the maxium exposure seen by the evacuee.  It could be the side of head, the back of the head, etc. One option is you could have mulitple particles all with different ORIENTATION that follow the same path. Compute the FED for each and then have a MAX CTRL to pick the maximum FED. You could use 5 particles with N,E,S,W, and up or 9 with N,NE,E,SE,S,SW,W,NW, and up.

[D D]

Thanks, sounds like a good workaround.

[fred ebersohn]

Good afternoon. 

Looking for a little help plse

Anyone has a presentable matrix I could share with my FDS model showing calculations on smoke height vs time and density vs temp plse.

Thanks for your help.

Sent from Yahoo Mail on Android

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