tenability and choice of the device

[GGfire]

Dear Experts,

I want to determine the radiative heat flux due to a fire source impinging an hypotetical occupant of the building. A possibility can be doing this using RADIATIVE HEAT FLUX GAS device at 1.8 m from the ground. However the difficulty of this approach is to choose the orientation of the device ( a large number of devices is used, and every device should have a different orientation). An other option I have thinked is to set in every point the 6 orientations  (up, down, east west, nort south), but it could not be exhaustive, it still could not been the best radiation. 

Which would be the best practice to do that? Are there other devices that can help me in this task?

Thank you in advance

[o...@aquacoustics.biz]

From above (hence 1.8 m).  If the radiant heat exposure is horizontal then occupants will not approach, they'll retreat.  If the whole space is burning (post flashover) then they will already be fatalities.  The primary concern here is radiation from the hot upper layer.

There is at least one exception where a single egress is route is compromised or close to a potential fire. 

But you should also note that:

     radiation exposure (other than the absolute limits) is not assessed at a point and an instant.  It is more appropriately assessed as a Fractional Effective Dose (FED) over time and path.

     in assessing tenability radiation exposure is usually the last metric to fail.  The usual sequence is visibility, followed by FED CO, followed by radiation.

[GGfire]

I agree. Unfortunately, showing the results of this parameter is mandatory for our regulation and I would like to show something reasonable.

Do you think that the orientation should be UP far away the fire source and maybe horizontal near the fire source ? 

I was thinking also to use the integrated intensity device? I don't know if it is proper for this calculation.

[dr_jfloyd]

Integrated intensity would be conservative. If that value passes, then the flux in any one direction would pass.

[Tim O'Brien]

We have a similar requirement embodied in C/VM2 here is New Zealand.  The purpose of this metric is for occupant egress.  The direction is UP.  The locations are on egress routes, often just one  at the point of closest proximity to the design fire in the compartment of fire origin.  Where there is more than one escape route the metric is redundant as occupants will not move into smoke and heat (Purser et al. on human behaviour in fires).

There are also times when one might want to assess emergency responder FED thermal hazards.  The threshold increases due to PPE.  The location should be rationalized based on anticipate response (research FBIM – Fire Brigade Intervention Model).

 

t.

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[mst-parkhachev]

Is this true? Isn't the flow averaged in all directions when calculating the integated intensity?

пятница, 15 марта 2024 г. в 21:51:39 UTC+3, dr_jfloyd:

[Tim O'Brien]

I suspect that you need to understand the concept of Fractional Effective Dose (FED).  The effects of heat on people are cumulative, but simultaneously decay over time.  In occupant egress people are moving along a path a defined rate.  So the FED is calculated over the travel path where the radiation from the hot layer above varies in time and space.

 

Dr Floyd’s comment is also correct.  If an occupant were to stand still at the point of maximum received radiation flux from above on the egress path for the total evacuation time and not be incapacitated, then thermal radiation should not prevent evacuation under the fire scenario.

 

Note that many codes do not require FED thermal, but have a go/no go maximum heat flux (often expressed as upper layer temperature).    

 

From: fds...@googlegroups.com [mailto:fds...@googlegroups.com] On Behalf Of mst-parkhachev
Sent: Sunday, 17 March 2024 9:01 PM
To: FDS and Smokeview Discussions
Subject: [fds-smv] Re: tenability and choice of the device

 

Is this true? Isn't the flow averaged in all directions when calculating the integated intensity?

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[dr_jfloyd]

@mst-parkhachev

Here is a simple case with two hot surfaces (-x and -y) on two sides of a box and seven devices - one INTEGRATED INTENSITY and 6 radiative heat flux pointing at each face. 

II: 93 kW/m2

-x: 63 kW/m2

+x: 3.52 kW/m2

-y: 23 kW/m2

+y: 14 kW/m2

-z: 18 kW/m2

+z: 15 kW/m2

The integrated intensity is larger than any of the six faces and also larger than the norm of -x and -y which is 67 kW/m2.

&HEAD CHID='test'/

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

&TIME T_END=10/

&VENT PBZ=2,SURF_ID='OPEN'/

&VENT PBX=0,SURF_ID='HOT1'/
&VENT PBY=0,SURF_ID='HOT2'/

&SURF ID='HOT1',TMP_FRONT=1000/
&SURF ID='HOT2',TMP_FRONT=500/

&DEVC ID='UII',XYZ=1,1,1,QUANTITY='INTEGRATED INTENSITY'/

&DEVC ID='R100',XYZ=1,1,1,ORIENTATION=1,0,0,QUANTITY='RADIATIVE HEAT FLUX GAS'/
&DEVC ID='-R100',XYZ=1,1,1,ORIENTATION=-1,0,0,QUANTITY='RADIATIVE HEAT FLUX GAS'/
&DEVC ID='R010',XYZ=1,1,1,ORIENTATION=0,1,0,QUANTITY='RADIATIVE HEAT FLUX GAS'/
&DEVC ID='R0-10',XYZ=1,1,1,ORIENTATION=0,-1,0,QUANTITY='RADIATIVE HEAT FLUX GAS'/
&DEVC ID='R001',XYZ=1,1,1,ORIENTATION=0,0,1,QUANTITY='RADIATIVE HEAT FLUX GAS'/
&DEVC ID='R00-1',XYZ=1,1,1,ORIENTATION=0,0,-1,QUANTITY='RADIATIVE HEAT FLUX GAS'/

[Kevin McGrattan]

Divide by 4 pi

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[GGfire]

Hi,

If  INTEGRATED INTENSITY is conservative, it could be perfect for me. By reading the user guide I thought it is an averaged value, so as highlighted by mst-parkhachev I thought it wasn't conservative. Dr. McGrattan should I divide the INTEGRATED INTENSITY by 4pi? Could you specify what should I divide by 4pi? Thanks 

[Kevin McGrattan]

The word INTEGRATED in INTEGRATED_INTENSITY means that the radiation INTENSITY has been INTEGRATED over a sphere of 4 pi solid angles. The radiation INTENSITY can be thought of as sigma*T^4/pi. When you multiply by 4 pi, you get 4 sigma T^4. So I misspoke above, divide the INTEGRATED_INTENSITY by 4 to get a value comparable to an average heat flux at a point in space.

More can be found in the FDS Tech Guide.

[Tim O'Brien]

See Dr Floyd’s early comment.  FED thermal at the point of maximum received radiation flux (in the compartment of fire origin) should be higher than the FED thermal calculated along an egress path over travel time.  So for a fire safety design the point measure will be conservative as it should fail tenability criteria earlier.

You should justify your preferred method, and you might want to talk with your regulator or examine the Code that you are interested in as this measurement is often prescribed.

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