Flashover is a complex and potentially very dangerous phenomenon. The NSW Fire Brigade currently conducts training courses in flashover and backdraft using a test cell made from a shipping container where chipboard is set alight in the test cell and the fire is allowed to develop into flashover. As part of a collaborative project with the NSWFB, computational models are being developed to aid in the training procedure. CFD models of the test cell in advancing flashover scenarios, using the code FDS are compared with qualitative experimental data, with good agreement shown for the fire behaviour. Models with different configurations of the test cell were also compared, with particular consideration on the effect on time to flashover and temperature trends

Barber, T.

Leonardi, E.

Mackay, D.

University of Queensland eSpace

16th Australasian Fluid Mechanics Conference 
Crown Plaza, Gold Coast, Australia 
2-7 December 2007 
 
CFD Model of a Specific Fire Scenario 
 
D. Mackay, T. Barber and E. Leonardi 
School of Mechanical and Manufacturing Engineering 
                University of New South Wales, New South Wales, 2052 AUSTRALIA 
             
Abstract 
Flashover is a complex and potentially very dangerous 
phenomenon. The NSW Fire Brigade currently conducts training 
courses in flashover and backdraft using a test cell made from a 
shipping container where chipboard is set alight in the test cell 
and the fire is allowed to develop into flashover. As part of a 
collaborative project with the NSWFB, computational models are 
being developed to aid in the training procedure. CFD models of 
the test cell in advancing flashover scenarios, using the code FDS 
are compared with qualitative experimental data, with good 
agreement shown for the fire behaviour. Models with different 
configurations of the test cell were also compared, with particular 
consideration on the effect on time to flashover and temperature 
trends. 
 
Introduction  
Flashover is a transition from a slow growth period to a fully 
developed fire in a compartment. Figure 1 shows the expected 
fire development for a compartment fire as presented by Graham 
et al [2]. In the initial growth period the fire develops as the fuel 
burns provided enough oxygen is present, allowing hot gases 
with unburnt fuel to build-up on the ceiling of the compartment. 
Fire progression occurs as the compartment transitions into high 
temperatures, ignition of the unburnt fuel particles and flames 
spread throughout the compartment with all combustible material 
involved. Although flashover is rapid, it cannot be easily defined 
as a discrete event and hence quantifying the ‘flashover event’ is 
difficult. Waterman [9] conducted a series of experiments to 
determine the criteria for the onset of flashover and Liang [4] 
notes that these experiments lead to the two common definitions 
used in literature which are when the heat flux at the floor 
reaches 20kW/m2 or when the temperature of the hot layer at the 
ceiling reaches 600oC.  
 
 
Figure 1. Stages in a compartment fire temperature history. Graham et al 
[2].   
 
Flashover and other rapid fire phenomena have potentially 
disastrous consequences for the fire brigade and those they try to 
protect. Grimwood [3] notes that “during a tragic five week 
period between June and July 2007, twenty-two US Firefighters 
have been killed or badly burned through events associated with 
rapid fire phenomena”. The NSW Fire Brigade has commenced 
compartment fire behaviour training (CFBT), to train fire fighters 
in rapid fire phenomena, the practical component of which 
involves using a large demonstration cell which is internally set 
alight and allowed under controlled conditions to reach flashover.  
 
This study examines the flashover event in the CFBT 
demonstration cell using the computational fluid dynamics (CFD) 
program Fire Dynamics Simulator (FDS) [6, 7], and the 
visualisation program Smokeview [1]. Results of the field model 
FDS will be compared to qualitative thermocouple data with 
comparisons made for temperature and time to flashover. 
Variations of the CFBT demonstration cell set-up, including 
modifying ventilation conditions will be compared and the 
influence on temperature and time to flashover examined. 
 
Demonstration Cell 
An outline of the CFBT demonstration cell modelled in FDS is 
shown in figure 2.  
 
Figure 2. CFBT demonstration cell.  
 
The cell is adapted from a shipping container with dimensions 
2.4m by 12m by 2.6m. A doorway exists at 2.5m from the entry 
and the floor toward the back of the cell is made from brick and 
backing board lines the back corner including the roof as shown 
in figure 2. A small crib made from chipboard is used as the 
ignition source. A 0.6m baffle exists 6.5m from the cell entry and 
a vent 1.76m by 0.59m exists on one wall of the cell opposite to 
the crib but is not shown in figure 2. The vent is controlled by the 
instructor who may open it during a burn if desired. The baffle is 
used to contain smoke in the front portion of the room until 
sufficient build-up has occurred, however it is not always 
utilised. 
 
 
Four thermocouples are attached to the side and ceiling of the 
cell, allowing CFBT inspectors to examine the temperature 
profiles at these locations after a training session has been 
completed. Since these measurements are taken during a training 
session, which includes having the inspector and fire fighters in 
the cell (near door), fire suppression takes place as the fire 
develops. It includes short pulses which are used to cool the 
smoke on the inside of the door, however no water is applied 
directly to the flames. The aim of this study is to model the 
demonstration cell without fire suppression taking place as the 
main focus is flashover development. Only qualitative 
comparisons will therefore be made with the CFBT thermocouple 
data. Figure 3 shows typical temperature plots for the CFBT cell 
during a training session, thermocouple 1 is located on the roof of 
the cell approximately one metre from the crib, thermocouples 2, 
3 and 4 are attached to a wall on the same side of the cell as the 
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crib 300mm closer to entry than the baffle at 150mm from 
ceiling, kneeling height and 150mm from floor respectively.     
 
The temperature in the back portion of the room which contains 
thermocouple 1 is allowed to reach much higher temperatures as 
fire fighters will not enter this area. As the backing boards begin 
to burn and flames begin to spread in this region a rapid increase 
in temperature is found, limited initially to approximately 400oC 
by fire suppression. The fire is allowed to burn steadily for 
approximately 10 minutes while fire fighters observe the fire 
behaviour. Appropriate cooling techniques are used in the front 
portion of the room to ensure conditions at kneeling height (T3) 
do not exceed 150oC.  
 
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Figure 3. NSWFB thermocouple graphs for the demonstration cell. 
 
The Model 
Recently flashover has been modelled using CFD with some 
success [5, 8, 10]. Zou and Chow [10] successfully used FDS to 
model a compartment fire with gasoline pool fires as an ignition 
source and found good agreement between full scale experiments 
and FDS output. FDS is a field model which numerically solves 
the Navier-Stokes equations. It assumes low-speed, thermally 
driven flow and utilizes the Smagorinsky form of Large Eddy 
Simulation (LES) to solve the flow. Direct Numerical Simulation 
(DNS) is also available as an alternate solution option. The 
Smagorinsky model models the turbulent viscosity μLES as : 
 
( ) SCsLES 2Δ= ρμ    
where ( )2
3
22 uSSS ijij ⋅∇−⋅=  
And is typically given by ( )Δ 31__ elementvolumecontrol .  
 
The default value for the Smagorinsky constant Cs of 0.2 was 
used in this study.  
 
FDS includes a combustion model where gas species are 
described by its mixture fraction. At each point in time in each 
volume the mixture fraction is given for the gas species. The 
mixture fraction is the ratio of mass for the species compared to 
the total mass.   
 
Finite volume methods are applied to thermal radiation transport 
and Lagrangian particles are used to simulate the smoke 
movement. A rectilinear grid is applied which all geometry must 
conform to [7]. 
 
In this study the FDS model was matched as closely as possible 
to the demonstration cell. The crib was modeled as a solid block 
with boundary condition set as a heat release rate per unit area of 
1200kW/m2, which ramps to full value in 700 seconds. As shown 
in figure 2 the back portion of the floor was modeled as brick 
which is the same as the demonstration cell. The backing board 
was modeled as a similar material to the chipboard used during 
burns. 
 
A grid sensitivity study was undertaken on all models tested. 
Grid sizes of 0.15, 0.1 and 0.075 were compared. The FDS 
Technical Reference Guide [7] states “The grid size is the most 
important numerical parameter in the model, as it dictates the 
spatial and temporal accuracy of the discretized partial 
differential equations”. Information within a cell is constant and 
it is expected that a finer grid will produce more accurate results 
as it should increase the resolution, aiding in the accurate 
representation particularly of flame spread and fire growth.  
 
The effect of time step was also investigated and initial time steps 
were varied for the cell from 0.05 to 0.1 to determine the effect 
on the solution. The default time step for FDS is set by dividing 
the size of a grid cell by the characteristic velocity of the flow 
[6].  
 
The placement of computational boundaries or domain size can 
have a significant effect on the flow. Originally the boundaries 
were set to the confines of the demonstration cell. The 
computational domain was then extended 1m in each direction.  
 
The model was solved both with and without the vent which can 
be opened and closed by an instructor during a demonstration cell 
burn. The baffle as shown in figure 2 was removed to determine 
what effect it had on temperature and time to flashover. 
 
 
Results and Discussion 
Model Optimisation 
A grid sensitivity study was performed with default time steps 
and the computational domain was bound by the shipping 
container. The baffle was not included in these cases. Figure 4 
shows a comparison of the temperature plots for the three grids 
for thermocouple 2. Significant differences can be seen in the 
0.15m and 0.1m grids however the 0.1m and 0.075 grid are 
within 5% of each other which is considered to be in good 
agreement. Figure 5 shows the percentage difference between the 
0.1m and 0.075m grid for thermocouple 1 which had the worst 
agreement of all the thermocouples but is still considered to be 
adequate for determining fire behaviour. 
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Figure 4. Temperature profiles at thermocouple 2 for varying grid sizes 
 
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Figure 5. Temperature profiles at thermocouple 2 for varying grid sizes 
 
The placement of the computational boundaries was examined at 
three locations. In figure 6 Domain1, Domain2 and Domain3 
refer to an extension of 2.5m, 0.8m and 0m respectively of all 
domain boundaries. As can be clearly seen in figure 6 reducing 
the domain to only include the cell itself reduced the temperature 
by up to 12%. In this case vents are placed at any exit to the cell 
but any flow dynamics occurring at or near the boundary may not 
be fully captured. Minimal difference was found between 
Domain 1 and Domain 2 and hence the domain was extended to 
0.8m for all subsequent cases. 
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Figure 6. Temperature profiles at thermocouple 2 for varying domain 
sizes.  
 
Variation of the time steps had no measurable impact on results. 
The default setting was therefore used for all FDS cases.  
 
Comparison with Expected Results 
It is stressed that only general comparisons with the experimental 
data, shown in figure 3, can be made as fire suppression is taking 
place, this differs for each burn run as shown by the differences 
in the temperature graphs in figure 3. All CFD cases were solved 
with a 0.075m grid and extended computational domain. If we 
take the ceiling temperature value of 600oC to be the flashover 
then the experimental results do not reach flashover, this is due to 
fire suppression taking place at critical times to ensure safety for 
fire fighters. However the sharp temperature rise which occurs 
between 100 and 200 seconds could be seen as a prelude to 
flashover which may have occurred at approximately 200 
seconds. The FDS results shown in figure 7 increase steadily over 
the first 200 seconds and flashover, as taken to be when the 
temperature reaches 600oC, occurs at 210 seconds. Peak 
temperatures of close to 650oC persist as the backing boards 
burn. The time to flashover as well as the general temperature 
profile is found to be consistent with the experimental results and 
expected temperature curve for compartment fires. 
   
The experimental results for thermocouples 2, 3 and 4 are 
deliberately kept low for the safety of fire fighters. Fire 
suppression occurs in front of the baffle which is why the 
temperature for thermocouple 2 is significantly different between 
the experimental values and those in figure 7, however the 
general shape of the temperature curve is consistent with 
expected results. Since it is shielded by the baffle the temperature 
in thermocouple 2 is expected to be considerably smaller than 
thermocouple 1. Thermocouples3 and 4 do not reach temperature 
greater than 100oC indicating that flaming combustion did not 
reach all the way to the floor. 
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Figure 7. FDS temperature profiles at four thermocouple locations. 
 
Effect of Ventilation 
The model was solved under identical conditions except a vent 
was added and the baffle was removed. The vent is part of the 
demonstration cell and gives instructors control over ventilation 
in the back portion of the room. The vent is 1.76m by 0.59m and 
is located 1.7m above the floor. Figure 8 shows the temperature 
profiles for the same thermocouple locations.  
 
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Figure 8. FDS temperature profiles of the demonstration cell with vent 
open at four thermocouple locations. 
 
Figure 8 shows that with the vent in place the required flashover 
temperature is not achieved. Maximum temperatures were 
maintained close to 500oC Similar profiles are seen for 
thermocouples 1, 2 and 4 compared with those in figure 7, 
however thermocouple 3 does not increase far beyond standard 
conditions showing that the increase in temperature did not reach 
to that portion of the room. This can also be seen in thermocouple 
2, since no baffle exists to shield the thermocouple it would be 
expected that the temperature would increase beyond 300oC 
however a maximum temperature of 290oC is found after 550 
seconds.   
 
With the open vent close to the fire source smoke and heat are 
able to leave the compartment before reaching the front portion 
of the room where the three measurements are being taken. The 
heat leaving the compartment reduces the temperature around the 
fire source and backing boards such that flashover is no longer 
achieved.  Figure 9 shows fire and smoke leaving the 
compartment via the vent displayed in Smokeview. 
 
Figure 9. A Smokeview representation of the cell with vent open.  
 
Effect of Baffle 
The baffle contains the smoke in the front portion of the cell until 
sufficient build up has occurred, as with the vent the baffle can be 
removed which was modelled in FDS. With the baffle removed a 
higher temperature were recorded by thermocouple 2 which 
reached 350oC after 850 seconds, this is expected as it is no 
longer shielded. Comparison of thermocouple 2 in figures 8 and 
10 highlight the reduction in temperature brought about by the 
introduction of the vent into the demonstration cell. 
  
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Figure 10. FDS temperature profiles of the demonstration cell without the 
baffle at four thermocouple locations. 
 
Conclusions 
Grid optimisation of the FDS simulation for the NSWFB 
demonstration cell showed that a 0.075m grid with domain 
boundaries extended beyond the sides of the cell produced the 
most accurate results.  
 
Good agreement was shown for fire behaviour in comparison 
with qualitative experimental data. The FDS model produced 
higher temperatures of up to 100oC however since fire 
suppression is included in the experimental results this was 
expected. Temperature profiles produced were in good agreement 
with experimental results and expected compartment fire profiles 
for fire development. 
 
The introduction of a vent was found to remove a significant 
amount of heat from the front part of the room. Removal of the 
baffle showed increased temperatures in the front region which 
would have been otherwise shielded by the baffle. 
 
 
Acknowledgments 
The authors thank John McDonough and the NSW Fire Brigade 
for providing field data included in this paper. 
The authors acknowledge with thanks the financial support of the 
Australian Research Council.  
 
 
References 
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Version 4 – A Tool for Visualizing Fire Dynamics Simulator 
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CFD Model of a Specific Fire Scenario

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