LAYER HEIGHT
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Majda
Apr 11, 2012, 2:17:53 AM4/11/12
to FDS and Smokeview Discussions
Hi!
I'm new on FDS so I'm not sure what it measures the command:
&DEVC XB=2.77,2.77, 6.65,6.65, 0.0,2.0 QUANTITY='LAYER HEIGHT',
ID='LAYER HEIGHT' /
I understood that it's the height of the level of smoke in the room!
but it's not the same height like on the simulation! so am I wrong or
why there is the difference?
thank you very much!
Majda
I'm new on FDS so I'm not sure what it measures the command:
&DEVC XB=2.77,2.77, 6.65,6.65, 0.0,2.0 QUANTITY='LAYER HEIGHT',
ID='LAYER HEIGHT' /
I understood that it's the height of the level of smoke in the room!
but it's not the same height like on the simulation! so am I wrong or
why there is the difference?
thank you very much!
Majda
BCohan
Apr 11, 2012, 8:35:06 AM4/11/12
to FDS and Smokeview Discussions
As Dr. Floyd said, have a look at section 14.3.3 in the user manual.
Layer height is an approximation that is more used in two-zone models
than CFD models. Layer height is just an approximation for where the
hot and cold gas layers interact. In real fires, this region is not a
distinct line, but estimations for a distinct elevation can be
calculated.
If you are looking at smokeview and see a difference in where the
smoke appears to be and the layer height, it's because the optical
density is based on combustion characteristics and layer height is
based on a temperature profile. The visualization of smoke will not be
exactly where the layer height is calculated. You may try a slice file
of the temperature and compare that to the layer height measured by
the device.
&SLCF PBX=2.77, QUANTITY='TEMPERATURE' /
&SLCF PBY=6.65, QUANTITY='TEMPERATURE' /
--
Brian D. Cohan
Layer height is an approximation that is more used in two-zone models
than CFD models. Layer height is just an approximation for where the
hot and cold gas layers interact. In real fires, this region is not a
distinct line, but estimations for a distinct elevation can be
calculated.
If you are looking at smokeview and see a difference in where the
smoke appears to be and the layer height, it's because the optical
density is based on combustion characteristics and layer height is
based on a temperature profile. The visualization of smoke will not be
exactly where the layer height is calculated. You may try a slice file
of the temperature and compare that to the layer height measured by
the device.
&SLCF PBX=2.77, QUANTITY='TEMPERATURE' /
&SLCF PBY=6.65, QUANTITY='TEMPERATURE' /
--
Brian D. Cohan
majda jerman
Apr 11, 2012, 9:00:55 AM4/11/12
to fds...@googlegroups.com
I have read the section 12.3.3 in the user manual, but it's not exactly explained what it is as much as I understand.
Do you maybe know what is the temperature where the hot and cold gas layers interact?
thank you very much for your time
bye bye
2012/4/11 BCohan <brian...@gmail.com>
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dr_jfloyd
Apr 11, 2012, 9:51:33 AM4/11/12
to fds...@googlegroups.com
There really is no such thing as a layer height. This is an abstraction of reality created to make the zone-model approach possible. In real life you don't have an upper layer with a 100 % uniform temperature and a lower layer with a 100 % uniform temperature. You have a hotter upper region with a temperature gradient and a colder lower region with a temperature gradient. If you have a compartment with a door and a fire that is large enough to create stratification but not so large to result in complete mixing, then there will be a region where the temperature rapidly transitions from the colder temperature to the hotter temperature. The layer equation merely attempts to define where that interface is. There is no fixed temperature where that occurs. I suggest you find and read an introductory textbook on fire dynamics.
On Wednesday, April 11, 2012 9:00:55 AM UTC-4, Majda wrote:
On Wednesday, April 11, 2012 9:00:55 AM UTC-4, Majda wrote:
I have read the section 12.3.3 in the user manual, but it's not exactly explained what it is as much as I understand.Do you maybe know what is the temperature where the hot and cold gas layers interact?thank you very much for your timebye bye
2012/4/11 BCohanAs Dr. Floyd said, have a look at section 14.3.3 in the user manual.
Layer height is an approximation that is more used in two-zone models
than CFD models. Layer height is just an approximation for where the
hot and cold gas layers interact. In real fires, this region is not a
distinct line, but estimations for a distinct elevation can be
calculated.
If you are looking at smokeview and see a difference in where the
smoke appears to be and the layer height, it's because the optical
density is based on combustion characteristics and layer height is
based on a temperature profile. The visualization of smoke will not be
exactly where the layer height is calculated. You may try a slice file
of the temperature and compare that to the layer height measured by
the device.
&SLCF PBX=2.77, QUANTITY='TEMPERATURE' /
&SLCF PBY=6.65, QUANTITY='TEMPERATURE' /
--
Brian D. Cohan
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BCohan
Apr 11, 2012, 10:08:56 AM4/11/12
to FDS and Smokeview Discussions
There is no single temperature that will determine layer height. It is
very dependent on your ambient and boundary conditions. Wall
conductivity, specific heat, density, and thickness all play into the
equation for heat removal (driving the upper gas layer temperature)
and the ambient air temp is generally used for the lower gas layer
temperature (in two zone models). Ventilation parameters will also
greatly affect the layer height. If you use a slice file along with
the layer height data, you may be able to pull a temperature that will
more or less give you a temperature to use for that specific model,
but I wouldn't put too much stock into that number.
If you're uncertain as to what the layer height is and what affects
it, I would suggest playing around with the Fire Dynamics Tools (FDTs)
to get a bit more familiar with it. They're just spreadsheet tools
that assume a steady state fire and use algebraic correlations to give
an estimate of properties.
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1805/
You can also play around with CFAST and it will use smokeview to
illustrate the layer height.
If you have a more specific question about the layer height or hot gas
layer, I'd be happy to try to answer it, but from your original post,
it's difficult to know exactly what you're asking. I don't know what
you're using for "height of the level of smoke in the room" or what
the difference is that you are seeing. Hopefully I've helped a bit,
but it's hard to answer a general question when there is so much that
plays into the phenomena.
very dependent on your ambient and boundary conditions. Wall
conductivity, specific heat, density, and thickness all play into the
equation for heat removal (driving the upper gas layer temperature)
and the ambient air temp is generally used for the lower gas layer
temperature (in two zone models). Ventilation parameters will also
greatly affect the layer height. If you use a slice file along with
the layer height data, you may be able to pull a temperature that will
more or less give you a temperature to use for that specific model,
but I wouldn't put too much stock into that number.
If you're uncertain as to what the layer height is and what affects
it, I would suggest playing around with the Fire Dynamics Tools (FDTs)
to get a bit more familiar with it. They're just spreadsheet tools
that assume a steady state fire and use algebraic correlations to give
an estimate of properties.
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1805/
You can also play around with CFAST and it will use smokeview to
illustrate the layer height.
If you have a more specific question about the layer height or hot gas
layer, I'd be happy to try to answer it, but from your original post,
it's difficult to know exactly what you're asking. I don't know what
you're using for "height of the level of smoke in the room" or what
the difference is that you are seeing. Hopefully I've helped a bit,
but it's hard to answer a general question when there is so much that
plays into the phenomena.
DavidShep
Apr 12, 2012, 7:38:51 AM4/12/12
to FDS and Smokeview Discussions
It was my understanding that the layer height quantity was added to
FDS for the sole purpose of providing a means to compare FDS results
with zone model results. Since FDS provides detailed information about
gas phase properties as a function of location, I cannot think of any
justification for using Layer Height in an FDS engineering analysis.
FDS for the sole purpose of providing a means to compare FDS results
with zone model results. Since FDS provides detailed information about
gas phase properties as a function of location, I cannot think of any
justification for using Layer Height in an FDS engineering analysis.
Message has been deleted
Franck Didieux @ LNE
Apr 13, 2012, 3:13:52 AM4/13/12
to FDS and Smokeview Discussions
Hi all,
In the french regulation about somke control, you have a performance
criterion about layer height, with its associated acceptance criterion
which may be simplified to something like : the layer height shall
never be less than 1,80 m (it is a bit more complicated, but let's
take this simplification).
In this case, when using a 3D model like FDS, you definitely need to
get an answer from the simulation about layer height, to be able to
compare the results with the acceptance criterion and state if a
situation is acceptable or not.
However, as previously said, if the layer height has a clear
definition in zone models, due to the underlying assumptions (i.e. an
ideal plane), there is no such situation in FDS : you have no ideal
plane, rather a continuous gradient of temperature and concentration
of smoke from the ceiling to the floor, dependent on location. In
order to be able to compare results from FDS to CFAST for example, you
at least need to define a critical concentration of smoke that is the
limit between the "smoky" and the "clear" layer, or a critical
temperature to be the limit between a "hot" and a "cold" zone, or use
an approach as in FDS. However, these approaches are somehow
arbitrary, and not really satisfying...
It would be better to focus on the safety objectives (preservation of
health and life), chose performance and acceptance criteria related
with effects of fire on people (ex:ISO13571 - FED/FEC < 1 or FED/FEC <
0.3 or whatever is convenient), and then use exposition scenarios
(i.e. fire scenarios and evacuation scenarios, linked together to
determine exposition scenarios). These exposition scenarios will
finally give sets of FED/FEC for the evacuees, and allow to state
about the level of safety. (note that you have to run many evacuation
scenarios for one fire scenario, in order to get a higher level of
confidence, as these calculations are not deterministic)
A little bit more complicated, but much more satisfying :o)
Hope this helps,
In the french regulation about somke control, you have a performance
criterion about layer height, with its associated acceptance criterion
which may be simplified to something like : the layer height shall
never be less than 1,80 m (it is a bit more complicated, but let's
take this simplification).
In this case, when using a 3D model like FDS, you definitely need to
get an answer from the simulation about layer height, to be able to
compare the results with the acceptance criterion and state if a
situation is acceptable or not.
However, as previously said, if the layer height has a clear
definition in zone models, due to the underlying assumptions (i.e. an
ideal plane), there is no such situation in FDS : you have no ideal
plane, rather a continuous gradient of temperature and concentration
of smoke from the ceiling to the floor, dependent on location. In
order to be able to compare results from FDS to CFAST for example, you
at least need to define a critical concentration of smoke that is the
limit between the "smoky" and the "clear" layer, or a critical
temperature to be the limit between a "hot" and a "cold" zone, or use
an approach as in FDS. However, these approaches are somehow
arbitrary, and not really satisfying...
It would be better to focus on the safety objectives (preservation of
health and life), chose performance and acceptance criteria related
with effects of fire on people (ex:ISO13571 - FED/FEC < 1 or FED/FEC <
0.3 or whatever is convenient), and then use exposition scenarios
(i.e. fire scenarios and evacuation scenarios, linked together to
determine exposition scenarios). These exposition scenarios will
finally give sets of FED/FEC for the evacuees, and allow to state
about the level of safety. (note that you have to run many evacuation
scenarios for one fire scenario, in order to get a higher level of
confidence, as these calculations are not deterministic)
A little bit more complicated, but much more satisfying :o)
Hope this helps,
Chris
Apr 13, 2012, 4:52:00 AM4/13/12
to FDS and Smokeview Discussions
In my opinion define critical concentrations is a good way! The point
is by using a CFD-Code you should forget the concept of Layer Height
and use to the concept of critical concentrations, critical values.
The concept of Layer Height as a strict line is not a realistic
attempt as written above in many circumstances.
Cheers
Chris
On 13 Apr., 09:13, "Franck Didieux @ LNE" <franck.didi...@lne.fr>
wrote:
is by using a CFD-Code you should forget the concept of Layer Height
and use to the concept of critical concentrations, critical values.
The concept of Layer Height as a strict line is not a realistic
attempt as written above in many circumstances.
Cheers
Chris
On 13 Apr., 09:13, "Franck Didieux @ LNE" <franck.didi...@lne.fr>
wrote:
F-Sim.de
Apr 14, 2012, 4:52:17 AM4/14/12
to fds...@googlegroups.com
"but it's not exactly explained what it is as much as I understand."
http://www.f-sim.de/?page_id=1205&did=28http://www.f-sim.de/?page_id=1205&did=29
Simon Ham, FiSEC, UK
Apr 14, 2012, 5:09:31 AM4/14/12
to FDS and Smokeview Discussions
This issue reflects a flaw in the prescriptive approach to Building
Regulations. The UK moved from prescriptive to functionally based
Regulations partly because it was recognised that you cannot prescribe
rules for all circumstances. Fire engineered solutions properly
thought through are regularly accepted as satisfying the functional
requirements of our Building Regulations. In countries where
Regulators are either not intelligent enough, or allowed to be
intelligent enough, to recognise the benefits of alternative
approaches there is little opportunity for any innovation. This in
turn may serve to stifle growth and investment
There is an approach which may prove acceptable in some circumstances
and that is to carry out a deterministic study using comparative
criteria as identified in ISO/TR 13387-1:1999. In this case it may be
possible to define a notional layer boundary based perhaps on
comparing a a generic zone model where the layer height is precisely
defined to the equivalent temperature profile generated by a generic
FDS field model. On the basis that the former is perceived to
represent an acceptable degree of safety it can be argued that the
latter is equally safe. This is poor science but may provide a
pragmatic solution.
By way of an aside I am currently engaged in an excercise comparing
the performance of generic solutions in British Standards to
variations in tne criteria in order to establish whether the
assumptions made by the committees who strung them together (of which
I was part at one time) were in fact correct and am getting some
fascinating results. The problem with Codes and Regulations is that
once established they are often difficult to change for logistic
rather than scientific reasons.
Regulations. The UK moved from prescriptive to functionally based
Regulations partly because it was recognised that you cannot prescribe
rules for all circumstances. Fire engineered solutions properly
thought through are regularly accepted as satisfying the functional
requirements of our Building Regulations. In countries where
Regulators are either not intelligent enough, or allowed to be
intelligent enough, to recognise the benefits of alternative
approaches there is little opportunity for any innovation. This in
turn may serve to stifle growth and investment
There is an approach which may prove acceptable in some circumstances
and that is to carry out a deterministic study using comparative
criteria as identified in ISO/TR 13387-1:1999. In this case it may be
possible to define a notional layer boundary based perhaps on
comparing a a generic zone model where the layer height is precisely
defined to the equivalent temperature profile generated by a generic
FDS field model. On the basis that the former is perceived to
represent an acceptable degree of safety it can be argued that the
latter is equally safe. This is poor science but may provide a
pragmatic solution.
By way of an aside I am currently engaged in an excercise comparing
the performance of generic solutions in British Standards to
variations in tne criteria in order to establish whether the
assumptions made by the committees who strung them together (of which
I was part at one time) were in fact correct and am getting some
fascinating results. The problem with Codes and Regulations is that
once established they are often difficult to change for logistic
rather than scientific reasons.
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