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Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
The Potential of CFD Simulations for the
Determination of Smoke Motion in case of Fire
Prof. Bart Merci
Ghent University, Belgium
Faculty of Engineering and Architecture
Department of Flow, Heat and Combustion Mechanics
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Overview
– Setting
– ACR - RCR
– Fire safety systems design
– Example: Car park
– Fire Safety Science Research
– Fire forecasting
– Concluding remarks
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Setting
– This talk is NOT intended to be a review of the
SOTA.
– Detailed assessment of quality of simulations
itself is NOT the central issue of this talk.
– Important challenges, such as pyrolysis modeling
and interaction of water with smoke/fire, are NOT
addressed.
– So, what IS addressed?
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Setting
– Use of computer (gas phase) simulations in the
context of smoke motion:
• Fire safety systems design
• Fire safety science development
• Fire forecasting
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
ACR - RCR
– ACR: Available (or ‘Applied’) Computing Resources
– RCR: Required Computing Resources
– What determines RCR?
– Is ACR > RCR?
– If not, what are the consequences?
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
ACR - RCR
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
ACR - RCR
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Safety Systems Design
– RCR: determine what is necessary to tackle the
problem at hand.
– Subsequently, set ACR equal to, or higher than,
RCR.
– Example: user-defined fire with soot yield and
radiative loss fraction CFD calculations for
smoke dynamics are feasible (as long as species
are not considered), unless the geometry is very
complex CFD, with SOTA models, has the
potential of providing accurate results for smoke
dynamics CFD can ‘help’ developing or
assessing a fire safety system design.
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Safety Systems Design
– State-of-the-art:
• CFD (note: zone modeling not to be ruled out!)
• User-defined fire (in design calculations)
• Providing source terms (valid assumption?)
and BCs: smoke concentrations can be
calculated in reliable manner.
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Example: Car park fire safety
– Fire Safety Journal, Vol. 57 (2013).
– Full-scale experiments (> 100!), model-scale
experiments and CFD (>500!) simulations.
Fire source
Jet fans
Support beams (and columns)
Thermocouple lines
Extraction fans
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Example: Car park fire safety
– Flow patterns at z = 1.2m (low) and z = 2.65m
(near ceiling), 500kW fire.
OOOOO XXOXX OXXXO XXXXO
1.2m
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Example: Car park fire safety
– Flow patterns at z = 2.65m (near ceiling), 500kW
and 4MW fire.
OOOOO XXOXX OXXXO XXXXO
4MW
500kW
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Safety Systems Design
– But: the investment in the computational mesh
and/or subgrid scale modeling is essential.
– With classical LES: one or two orders of
magnitude higher than what we use, even in
academic research.
– Why? No technical reason… Reason: time and/or
money pressure.
– In other words: ACR < RCR is possible.
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Safety Systems Design
– Important questions:
• How reliable are the results if ACR < RCR?
• Do we still capture trends correctly?
• What safety margins do we need?
– Note: the proof of the pudding is often not in the
eating.
– Research needed.
– Bench-marking studies can provide important
insight.
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Safety Science Research
– Idea: bench-marking studies on cases relevant for
fire.
– Idea is not new. Much effort made and ongoing:
• FDS Verification and Validation (with
repository):
– Open (44 sets of experiments, 5000+ data points)
– Key target: make FDS code more ‘fail-safe’ clear
benefit
– Yet, response is reported1 to be ‘disappointing’.
1https://groups.google.com/forum/#!topic/fds-smv/jmIyOy2wzFA
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Safety Science Development
– Idea is not new. Much effort made and ongoing:
• FM Global Open Source CFD Fire Modeling
Workshop :
– Open to all of us
– ‘Come together and share data’ – ‘collaboration’
– Key target: ‘to develop the capability of predicting
fire growth behavior and fire protection requirements
within the framework of physics-based models’2
2https://sites.google.com/site/firemodelingworkshop/
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Safety Science Development
• Initiative: ‘MaCFP’ (International Workshop on
Measurement and Computation of Fire
Phenomena) series.
• Timing: final preparation in 2015, with first workshop
in 2017 (Lund).
• Systematic evaluation of the accuracy and reliability
of CFD simulation results with state-of-the-art models
as used now and with typical numerical settings
(including typical computational mesh cell size): high
on the priority list and crystallized into target test
cases.
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Safety Science Development
• Note: repetitive small-scale experiments and full-
scale experiments also remain important.
• Small-scale, intermediate-scale and full-scale set-ups
provide complementary information for the engineer.
• Full-scale experiments: analyze ‘crashes’ (recall: the
proof of the pudding is often not in the eating).
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Forecasting
• The need for speed:
– Forecasting simulations must be faster than real-
time a clear need for speed.
– Current levels of ACR and RCR: use of CFD for
forecasting purposes practically impossible.
– Two (or more) zone modeling approach: far more
time efficient solution.
– CFD on very coarse mesh: not useful (key
advantage of CFD (solving momentum equations)
is lost if there is no accuracy in solving the
momentum equations).
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Forecasting
• The need for speed:
– Thus, it is better to use two-zone model
calculations and remedy deviations by taking
advantage of sensor data.
– Note: uncertainties in boundary conditions
(including the fuel and fire heat release rate)
there is a need for assistance by (sensor) data ‘on
the fly’ during the calculations anyway.
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Forecasting
• Sensor-assisted simulations:
– Necessary data during the simulations, in order to
circumvent uncertainties in boundary conditions.
– User-defined fire, in combination with thermal (and
ventilation) boundary conditions, as in fire safety
design calculations.
– Key difference from design calculations: UDF is
varied on the run, taking advantage of sensor
data, monitoring the fire as it evolves.
– Most important parameter: HRR (in combination
with thermal (and ventilation) boundary
conditions).
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Rabot2012
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Forecasting
• Example: Rabot2012 full-scale tests
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Forecasting
• Example: Rabot2012 full-scale tests: flame spread
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Forecasting
• Example: Rabot2012 full-scale tests: flame height
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Forecasting
• Example: Rabot2012 full-scale tests: smoke height
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Forecasting
• Example: Rabot2012 full-scale tests: use in 2-zone
modelling
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Forecasting
• Example: Rabot2012 full-scale tests: use in CFD
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Fire Forecasting
• Example: Rabot2012 full-scale tests: fire forecasting
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Concluding remarks
– CFD simulations are a big help for:
• Fire safety system design calculations,
• Improving our understanding in fire safety
science (theory and model development and
validation),
• Fire forecasting.
– … if performed in a proper manner.
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Concluding remarks
– Focus of talk was on gas phase phenomena.
– But: much research effort is required for multi-
phase phenomena (condensed phase):
• Pyrolysis,
• Interaction of water with flames / smoke,
• Fire suppression,
• …
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Concluding remarks
– ACR (Available Computing Resources) vs. RCR
(Required Computing Resources):
• Determine the required accuracy, complexity
and modeling dimensionality;
• Carefully map it to what can be achieved with
available resources;
• In other words, the investment in complexity,
accuracy and modeling dimensionality must be
done with great care.
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Concluding remarks
– ACR (Available Computing Resources) vs. RCR
(Required Computing Resources):
• Making the ACR and the RCR more
quantitative and assessing the consequences
if ACR < RCR are considered major and
imminent tasks for the fire safety science
community.
Universiteit Gent - UGent
Vakgroep Mechanica van Stroming, Warmte en Verbranding
www.FloHeaCom.UGent.be
Concluding remarks
• Improvements in CFD fire simulations:
– By carefully importing knowledge and progress
from other research communities.
– Focus on fire specific aspects as a community.
– Bench-marking studies at all levels and scales
should play a central role in this effort, in order to
evaluate in a systematic manner the reliability of
CFD simulations of fires, with special attention to
cell sizes in the computational meshes used.
• Sensor-assisted fire forecasting simulations very
promising (with zone modeling for the time being).