In Fenix+ 3, you can create a fire source in several ways.
Fire Source Element
The first method to create a fire source is to place an object with a horizontal top surface (Wall, Solid, Platform, Floor slab) made of combustible Material in the scenario,
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and then place a Fire Source element on top of this object.
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This object made of combustible material can have any shape, but the Fire Source element placed on top of it is always rectangular. This is due to the VENT parameter group in the FDS input file used by Fenix+ 3 to simulate the fire source, which has a rectangular shape. The size of the rectangular fire source can be arbitrary but must not exceed the maximum rectangle that can be inscribed within the contours of the object on top of which the source is placed.
The fire source in Fenix+ 3 is characterized by the following thermal parameters, calculated based on the material’s properties and the area of the source:
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Specific power (kW/m²);
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Maximum power (kW), – the power equal to the product of the specific power and the area of the fire source’s horizontal part. If the option “Flame Spread to Side Surfaces” is enabled, the side surfaces are not included in the calculation of the maximum power.
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Specific power is calculated as the product of the lower heating value, specific mass burning rate, and combustion efficiency, which characterize the material of the object on which the “Fire Source” element is placed. These material properties are set in the Flammable Properties tab in the Substances and Materials Editor.
In the properties of the Fire Source element, it is also possible to set the start and end times of its functioning (burning).
If the option “Specify Maximum Burning Area” is not enabled, the fire spreads from the central point of the fire source. The propagation rate is equal to the linear flame spread rate, specified in the Substances and Materials Editor for the material of the object where the fire source element is placed.
If the linear flame spread rate is set to zero, the fire source starts burning over its entire area at once, and reaches maximum power within a few seconds (see below). A zero flame spread rate is set by default in the Substances and Materials Editor for liquid combustible materials.
For a fire source that ignites with a finite rate, the FDS input file includes code similar to the following:
&SURF ID='SURF_1' HRRPUA=180.792 COLOR='RED' RAMP_Q='RAMP_1'/
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=0.5 F=1/
&VENT XB= -1.5, 0, 1.5, 3.25, 0.5, 0.5 SURF_ID='SURF_1' SPREAD_RATE=0.0154
XYZ= 0.625, 2.375, 0.5 CTRL_ID='CTRL_2'/
&CTRL ID='CTRL_2' FUNCTION_TYPE='CUSTOM' INPUT_ID(1:1)='CLOCK' RAMP_ID='RAMP_2' LATCH=.False./
&RAMP ID='RAMP_2' T=29 F=-1/
&RAMP ID='RAMP_2' T=31 F=1/
&RAMP ID='RAMP_2' T=599 F=1/
&RAMP ID='RAMP_2' T=601 F=-1/
Here, the HRRPUA parameter of the SURF group corresponds to the specific power of the fire source, the SPREAD_RATE parameter of the VENT group to the linear flame spread rate, and XYZ is the initial burning point, located on the plane of the VENT element, at the center of the facet of the nearest computational mesh cell to the geometric center of VENT.
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The CTRL and RAMP group parameters control the start and end times of burning, which in this case corresponds to the 30th and 600th seconds.
If the linear flame spread rate is zero, then the SPREAD_RATE and XYZ parameters are absent in the VENT group.
The RAMP_Q parameter of the SURF group allows the power of the fire source to vary during the simulation. This parameter is used both to achieve the desired law of power increase (see the section Specify maximum burning area) and to reduce the burning rate after the fire alarm activation (see the section Reduce burn ate by half).
For all fire sources, the RAMP_Q parameter is linked to the RAMP group. In the case of the above, it includes two entries that define the gradual activation of the source over 0.5 seconds.
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=0.5 F=1/
These entries are necessary because the RAMP group associated with the RAMP_Q parameter cannot be empty. You may need to reduce the power of the fire source by half after the Automatic fire suppression systems (AFSS) initiates (see below).
If the linear flame spread rate is zero, then the following entries are placed in the RAMP group associated with RAMP_Q:
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=5 F=1/
These entries ensure a gradual ignition of the fire source over five seconds from zero power to maximum power. Despite the zero linear flame spread rate corresponding to a fire source that ignites all at once at full power, simulating perfect instantaneous ignition is impossible. In particular, the faster we try to ignite the source in FDS, the more instability there is at the beginning of the simulation, leading to an excessively large burst of burning power in the first few seconds. To increase the stability of the simulation and the correctness of the results obtained, the ignition of such fire sources is delayed by 5 seconds.
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The picture shows a graph of the power increase with the ignition delay of 5 seconds. It can be seen that the power increases gradually over five seconds, and there is a spike in power in the first few seconds. However, the height of the spike is significantly less than the final power. The increase in power without a strong spike is concidered satisfactory.
For comparison, the graph below shows the power that is obtained without the ignition delay. It can be seen that the power spike at the beginning of the burning significantly exceeds the final burning power, which can lead to an overestimation of the dangerous fire factors. For example, such a power spike can lead to a spike in the magnitude of the heat flow.
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Note that if the simulation time is less than 10 seconds, then the ignition time will be reduced to half of the simulation time. However, with such a short simulation time, the result obtained may be unreliable due to the instability of the calculations related to the too rapid ignition of the fire source.
Additional Fire Source Settings
In the Fire Simulation Parameters window, you can specify additional settings for the fire source:
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Reduce the burn rate by half;
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Spread of flame to the side surfaces.
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“Reduce the Burn Rate by Half” Option
The ability to reduce the burn rate by half will take effect if the automatic fire suppression systems (AFSS) are implemented according to standards.
For a project of the Civil Facility type, information about the AFFS is specified in the Building properties.
For a project of the Indurtrial Facility type, information about AFFS is specified for the Room in the Room explication window.
Moreover, the AFSS must be specified according to the standards, if available.
Reducing the burn rate by half during fire risk calculation occurs only after the fire-fighting system has been activated. The way the change in burn rate reflects in the description of the fire source in the FDS file is described in the section Reduce the burn rate by half.
“Spread of Flame to the Side Surfaces” Option
Note that the shape and size of elements might slightly change when forming the input file for FDS, as any object in FDS must have dimensions that are multiples of the mesh cell size. The fire source in the FDS file is represented using VENT. A VENT object has a rectangular shape and occupies an integer number of computational mesh facets. The element on which the fire source is placed is represented in the FDS file by OBST objects, which occupy an integer number of cells.
Dimensions of objects in FDS depend on the original dimensions of the scenario elements, as well as on the cell size of the computational mesh. Consequently, after converting scenario elements into FDS objects, the fire source and the object on which it is placed may or may not have common boundaries.
Side surfaces of the fire source, just like its horizontal surface, are represented by VENT and only appear on those side facets of OBST where the horizontal VENT boundary coincides with the boundary of the OBST on top of which it is located.
Below is an example of a Fire Source placed on top of a Solid of non-rectangular shape.
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Below is an example of how such a scenario is represented in FDS. Side facets of the fire source appear only where the horizontal VENT boundary coincides with the OBST boundary.
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A side surface of the fire source also will not appear where another object, for example, a wall, adjoins the object on top of which the fire source is located.
Side surfaces do not ignite simultaneously with the upper surface of the fire source. The picture below shows a development view of the fire source when the flame can spread to the side surfaces.
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In this example we denote the linear flame spread rate as V.
Initially, the flame spreads from the center of the upper surface (point C) in all directions. After a time of a/2/V, the flame will reach points B1 and B2. At this moment, according to the model implemented in Fenix+ 3, combustion of two corresponding side facets starting from points B1 and B2 will begin. Similarly, after a time of b/2/V, the flame will reach points A1 and A2. At this moment, combustion of two corresponding side facets starting from points A1 and A2 will begin.
Just as with the upper surface of the fire source, the side facets in the FDS file correspond to a group of parameters called VENT. The linear speed (SPREAD_RATE) on the side facets is the same as on the upper facet, and the start points (XYZ) correspond to the coordinates of points A1, A2, B1, and B2. The start times of combustion at these points are delayed in relation to the beginning of combustion at point C by the amounts explained above.
In the FDS input file, a fire source with side facets is represented in the following way:
General Settings
“Building 1” - “Solid object 1”
&OBST XB= -1.75, -0.5, 2.25, 3.5, 0, 0.5 RGB=191,191,191/
Fire Source 1
&SURF ID='1' FYI='Fire source 1' HRRPUA=180.792 COLOR='RED'/
Upper surface
&VENT XB=-1.75,-0.5,2.25,3.5,0.5,0.5 SURF_ID='1' SPREAD_RATE=0.0154 XYZ= 1.125,2.875,0.5 CTRL_ID='2'/
&CTRL ID='2' FUNCTION_TYPE='CUSTOM' INPUT_ID(1:1)='CLOCK' RAMP_ID='3' LATCH=.False./
&RAMP ID='3' FYI='Fire source 1' T=-1 F=-1/
&RAMP ID='3' FYI='Fire source 1' T=1 F=1/
One pair of side surfaces
(Note that if another object adjoins the solid object on top of which the fire source is located on one side, there can only be one side surface. Also, there may be no side surfaces at all if external objects adjoin the fire source from both sides)
&VENT XB=-1.75,-1.75,2.25,3.5,0,0.5 SURF_ID='1' SPREAD_RATE=0.0154 XYZ=-1.75,2.875,0.25 CTRL_ID='4'/
&VENT XB=-0.5,-0.5,2.25,3.5,0,0.5 SURF_ID='1' SPREAD_RATE=0.0154 XYZ=-0.5,2.875,0.25 CTRL_ID='4'/
&CTRL ID='4' FUNCTION_TYPE='CUSTOM' INPUT_ID(1:1)='CLOCK' RAMP_ID='5' LATCH=.False./
&RAMP ID='5' FYI='Fire source 1 side X ramp' T=35.4 F=-1/
&RAMP ID='5' FYI='Fire source 1 side X ramp' T=37.4 F=1/
The side VENTs appear at the moment of 36.4 seconds
Second pair of side surfaces
&VENT XB=-1.75,-0.5,2.25,2.25,0,0.5 SURF_ID='1' SPREAD_RATE=0.0154 XYZ=-1.125,2.25,0.25 CTRL_ID='6'/
&VENT XB=-1.75,-0.5,3.5,3.5,0,0.5 SURF_ID='1' SPREAD_RATE=0.0154 XYZ=-1.125,3.5,0.25 CTRL_ID='6'/
&CTRL ID='6' FUNCTION_TYPE='CUSTOM' INPUT_ID(1:1)='CLOCK' RAMP_ID='7' LATCH=.False./
&RAMP ID='7' FYI='Fire source 1 side Y ramp' T=41 F=-1/
&RAMP ID='7' FYI='Fire source 1 side Y ramp' T=43 F=1/
“Specify Maximum Burning Area” Option
If the “Specify Maximum Burning Area” option is enabled for a fire source, you can set a maximum burning area in the “Maximum Burning Area” field, which may differ from the area of the defined fire source.
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In this case, the fire source starts to burn completely, rather than igniting from a point. The burning power increases according to a quadratic law from zero to the maximum value which the fire source can have with the area specified in the “Maximum Burning Area” field. The law of power increase corresponds to the circular spread of the fire with the linear flame spread rate associated with the selected material. This refers to the burning power when there is a sufficient concentration of oxygen for combustion.
Knowing the linear flame spread rate (V), and the maximum burning area (S_max), it is possible to determine the time it takes for the fire to reach the specified area with circular spread.
In the FDS input file, the quadratic law of change in the power of the fire source is specified through the RAMP_Q property of the SURF parameter group:
&SURF ID='SURF_1' HRRPUA=356.748 COLOR='RED' RAMP_Q='RAMP_1'/
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=7.68 F=0.001875/
&RAMP ID='RAMP_1' T=15.36 F=0.0075/
&RAMP ID='RAMP_1' T=23.03 F=0.016875/
&RAMP ID='RAMP_1' T=30.71 F=0.03/
&RAMP ID='RAMP_1' T=38.39 F=0.046875/
&RAMP ID='RAMP_1' T=46.07 F=0.0675/
&RAMP ID='RAMP_1' T=53.74 F=0.091875/
&RAMP ID='RAMP_1' T=61.42 F=0.12/
&RAMP ID='RAMP_1' T=69.1 F=0.151875/
&RAMP ID='RAMP_1' T=76.78 F=0.1875/
&RAMP ID='RAMP_1' T=84.45 F=0.226875/
&RAMP ID='RAMP_1' T=92.13 F=0.27/
&RAMP ID='RAMP_1' T=99.81 F=0.316875/
&RAMP ID='RAMP_1' T=107.49 F=0.3675/
&RAMP ID='RAMP_1' T=115.16 F=0.421875/
&RAMP ID='RAMP_1' T=122.84 F=0.48/
&RAMP ID='RAMP_1' T=130.52 F=0.541875/
&RAMP ID='RAMP_1' T=138.2 F=0.6075/
&RAMP ID='RAMP_1' T=145.88 F=0.676875/
&RAMP ID='RAMP_1' T=153.55 F=0.75/
&RAMP ID='RAMP_1' T=161.23 F=0.826875/
&RAMP ID='RAMP_1' T=168.91 F=0.9075/
&RAMP ID='RAMP_1' T=176.59 F=0.991875/
&RAMP ID='RAMP_1' T=177.31 F=1/
Here, RAMP_Q sets the values of the coefficient (F), which are multiplied by the HRRPUA value at specified time points (T), starting from the zero moment until the moment when the power of the fire source becomes maximum. Between the time points specified in the RAMP group, the value of the coefficient is interpolated linearly. In the given example, tmax = 177.31 s.
The HRRPUA value is calculated in the following way.
Example: HRRPUA0 is the specific burning power corresponding to the material used in the fire source. It is equal to the product of the Lower Heating Value, the Specific Mass Burning Rate, and the Combustion Efficiency Coefficient. S_fact is the actual area of the fire source. S_max is the maximum burning area specified in the properties of the fire source.
Then:
HRRPUA = HRRPUA0 · S_max / S_fact
Below is a graph showing the power increase of a fire source made from the material “Administrative Space” with an actual area 𝑆_fact = 1 m², and a specified area 𝑆_max = 2 m².
HRRPUA0 = 178.374 кВт/м2
HRRPUA = 356.748 кВт/м2
V = 0.0045 м/с.
The graph shows a quadratic increase in power up until tmax = 177.31 seconds.
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When the burning power reaches its maximum value, corresponding to the “Maximum Burning Area,” any further increase in power ceases, and the fire source continues to burn at a fixed power.
If the “Spread of Flame to Side Surfaces” property is enabled in the Fire Simulation Parameters, the side surfaces burn along with the top surface of the fire source, but the described law of increase in burning power (cumulative for all surfaces) remains the same.
Best Practices of Use for the “Specify Maximum Burning Area” Property
When using the “Specify Maximum Burning Area” property, consider the following specifics. This property is often used to simulate a fire the maximum area of which significantly exceeds the room area. However, this does not mean that it is necessary to place a fire source element in the room that occupies the entire floor area.
If there are occupants in the room, the fire source must be made smaller than the room area to ensure space for people to fit in. Occupants must not be placed close to the fire source; otherwise, they may immediately find themselves trapped, as a fire source with the “Specify Maximum Burning Area” property burns over its entire area. Even though the burning power may be small at the beginning, placing a registering device close to the fire source can rapidly record exceeding dangerous factors, which may not realistically reflect the situation that needs to be simulated.
Also, such a fire source must not be placed close to doors, as it would quickly block the exit.
A fire source with the “Specify Maximum Burning Area” property is not intended for detailed simulating of the spread of gangerous fire factors inside a room with a fire. It provides a law of power change according to circular spread to any given area.
However, if the fire source is placed where a fire is likely to start in the room, the spread of dangerius fire factors at the beginning will also be reasonably accurate, except for the area very close to the fire source.
If the location of the fire source in the room is not known in advance, it makes sense to choose the size and position of the fire source so that it does not hinder the presence of people in the room and is not located right next to doors.
It is best practices to specify the fire source size as large as possible, but so that it does not contradict the above restrictions.
“On Fire” Property
The “On Fire” property is available in Fenix+ 3 scenario for elements that have the “Material” property, if the material is combustible.
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Enabling the “On Fire” property causes the element to be covered with a surface (SURF group) during simulation, featuring a heat release rate per unit area (HRRPUA parameter) equal to the specific heat release rate of the chosen material.
The element will be engulfed in flames over its entire surface. To reduce the impact of simulation instability that occurs due to a rapid fire ignition, as previously mentioned in the “Fire Source” section, the ignition of “On Fire” elements is delayed by 10 seconds. Similar to the “Fire Source” element, the ignition law is set using the RAMP_Q parameter of the SURF parameter group.
&MATL ID='1' FYI='Derevo + oblicovka' CONDUCTIVITY=1 DENSITY=1000 HEAT_OF_COMBUSTION=14400 SPECIFIC_HEAT=1/
&SURF ID='2' HRRPUA=180.792 THICKNESS=0.2 RGB=193,191,180 MATL_ID='1'/
&OBST XB=0.75,0.75,1,1.25,0,3 SURF_ID='2'/
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=10 F=1/
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Note that if the simulation time is less than 20 seconds, the ignition time is reduced by half of the simulation time. However, with a short simulation time, the result may be unreliable due to computational instability associated with the too rapid ignition of the fire source.
Creating a fire source using the “On Fire” property can be applied when a prolonged ignition is not required. That is, in cases where an element is quickly engulfed in flames, and also when the results of the simulation up to the point of full ignition of the element are not of interest. The second case is typical when measuring the heat flux falling on a building during a fire in an adjacent building.
Reducing the Burn Rate by Half
When calculating fire risks, after a fire alarm is triggered, the fire simulation is paused, evacuation simulation is performed, necessary changes are made to the FDS file, and the fire simulation continues.
In particular, if the simulated scenario involves reducing the burn rate by half after the fire-fighting system is activated, changes of the fire source power are entered into the FDS file after the fire alarm is triggered. For details, see the Reduce Burn Rate by Half option. You can configure the delay of the Automatic Fire-Fighting System (AFFS) activation after the fire alarm on the “Fire Source” tab of the Fire Simulation Parameters.
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You can reduce the power of fire sources when modifying the FDS file by altering RAMP parameter groups linked with RAMP_Q elements for “Fire Source” and elements with the “On Fire” property.
Below are a few examples:
a) A fire source flaring up cell-by-cell.
Originally in the FDS file, a RAMP group was linked with the RAMP_Q parameter as follows:
&SURF ID='SURF_1' HRRPUA=180.792 COLOR='RED' RAMP_Q='RAMP_1'/
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=0.5 F=1/
After the fire detector is triggered, the RAMP group is modified to look like this:
&SURF ID='SURF_1' HRRPUA=180.792 COLOR='RED' RAMP_Q='RAMP_1'/
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=0.5 F=0.5/
The new value F=0.5 is applied to the part of the calculation that is performed after the detector is triggered, thus the power of the fire source is reduced from the moment the alarm activates. Below, two graphs are shown: the blue line represents the power of the fire source as it would have been without reduction; the green line represents the power halved starting from the 37th second when the fire detector was triggered.
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The delay in triggering the automatic fire suppression system (AFFS) for fires that flare up cell-by-cell is not accounted for.
b) A fire source with a manually specified area.
Originally in the FDS file, a RAMP group is associated with the RAMP_Q parameter as follows:
&SURF ID='SURF_1' HRRPUA=356.748 COLOR='RED' RAMP_Q='RAMP_1'/
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=7.68 F=0.001875/
&RAMP ID='RAMP_1' T=15.36 F=0.0075/
&RAMP ID='RAMP_1' T=23.03 F=0.016875/
&RAMP ID='RAMP_1' T=30.71 F=0.03/
&RAMP ID='RAMP_1' T=38.39 F=0.046875/
&RAMP ID='RAMP_1' T=46.07 F=0.0675/
&RAMP ID='RAMP_1' T=53.74 F=0.091875/
&RAMP ID='RAMP_1' T=61.42 F=0.12/
&RAMP ID='RAMP_1' T=69.1 F=0.151875/
&RAMP ID='RAMP_1' T=76.78 F=0.1875/
&RAMP ID='RAMP_1' T=84.45 F=0.226875/
&RAMP ID='RAMP_1' T=92.13 F=0.27/
&RAMP ID='RAMP_1' T=99.81 F=0.316875/
&RAMP ID='RAMP_1' T=107.49 F=0.3675/
&RAMP ID='RAMP_1' T=115.16 F=0.421875/
&RAMP ID='RAMP_1' T=122.84 F=0.48/
&RAMP ID='RAMP_1' T=130.52 F=0.541875/
&RAMP ID='RAMP_1' T=138.2 F=0.6075/
&RAMP ID='RAMP_1' T=145.88 F=0.676875/
&RAMP ID='RAMP_1' T=153.55 F=0.75/
&RAMP ID='RAMP_1' T=161.23 F=0.826875/
&RAMP ID='RAMP_1' T=168.91 F=0.9075/
&RAMP ID='RAMP_1' T=176.59 F=0.991875/
&RAMP ID='RAMP_1' T=177.31 F=1/
After the fire detector is triggered, the RAMP group is modified and takes the following form:
&SURF ID='SURF_1' HRRPUA=356.748 COLOR='RED' RAMP_Q='RAMP_1'/
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=7.68 F=0.001875/
&RAMP ID='RAMP_1' T=15.36 F=0.0075/
&RAMP ID='RAMP_1' T=23.03 F=0.016875/
&RAMP ID='RAMP_1' T=30.71 F=0.03/
&RAMP ID='RAMP_1' T=38.39 F=0.046875/
&RAMP ID='RAMP_1' T=46.07 F=0.0675/
&RAMP ID='RAMP_1' T=53.74 F=0.091875/
&RAMP ID='RAMP_1' T=61.42 F=0.12/
&RAMP ID='RAMP_1' T=69.1 F=0.151875/
&RAMP ID='RAMP_1' T=71.0103 F=0.160736254882812/
&RAMP ID='RAMP_1' T=72.0103 F=0.0803681274414062/
&RAMP ID='RAMP_1' T=76.78 F=0.09375/
&RAMP ID='RAMP_1' T=84.45 F=0.1134375/
&RAMP ID='RAMP_1' T=92.13 F=0.135/
&RAMP ID='RAMP_1' T=99.81 F=0.1584375/
&RAMP ID='RAMP_1' T=107.49 F=0.18375/
&RAMP ID='RAMP_1' T=115.16 F=0.2109375/
&RAMP ID='RAMP_1' T=122.84 F=0.24/
&RAMP ID='RAMP_1' T=130.52 F=0.2709375/
&RAMP ID='RAMP_1' T=138.2 F=0.30375/
&RAMP ID='RAMP_1' T=145.88 F=0.3384375/
&RAMP ID='RAMP_1' T=153.55 F=0.375/
&RAMP ID='RAMP_1' T=161.23 F=0.4134375/
&RAMP ID='RAMP_1' T=168.91 F=0.45375/
&RAMP ID='RAMP_1' T=176.59 F=0.4959375/
&RAMP ID='RAMP_1' T=177.31 F=0.5/
At 72 seconds, the power of the fire source is halved. This time includes the delay in triggering the AFSS. If the maximum burning area has not yet been reached by this time, the growth in power continues, but at a rate that is half as fast. The picture below shows the moment of power reduction after the fire detector is triggered (including the AFSS delay) during the circular ignition.
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c) Element with the “On Fire” property.
Originally in the FDS file with the RAMP_Q parameter, a RAMP group is associated as follows:
&SURF ID='SURF_1' HRRPUA=178.374 THICKNESS=0.25 RGB=255,0,0 RAMP_Q='RAMP_1'/
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=10 F=1/
After the fire detector is triggered, the RAMP group is modified and takes the following form:
&SURF ID='SURF_1' HRRPUA=178.374 THICKNESS=0.25 RGB=255,0,0 RAMP_Q='RAMP_1'/
&RAMP ID='RAMP_1' T=0 F=0/
&RAMP ID='RAMP_1' T=10 F=1/
&RAMP ID='RAMP_1' T=33.7443 F=1/
&RAMP ID='RAMP_1' T=34.7443 F=0.5/
Here, the power is reduced at 33.7 seconds, considering the delay in the activation of the AFSS. The graph shows the power dynamics. Initially, the power increased linearly for the first ten seconds, as is typical for elements “On Fire”, but after the detector is triggered (and considering the AFSS delay), it is halved.
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Required Simulation Time
The fire simulation duration must primarily be determined by the goals of the simulation. Specifically, it may be regulated by standards and guidelines.
However, it is important to note that the simulation time must not be too short. As mentioned in the sections Fire Source Element and On Fire Property, increases in burning power that are too rapid can lead to instability in simulations and may produce unrealistic effects, such as significant spikes in power. Therefore, for fire sources with zero linear flame spread rate and for elements “On Fire,” an artificial delay in ignition of 5 seconds and 10 seconds respectively has been introduced. “On Fire” elements are often used in fire simulations to justify fire separation distances and have large sizes and high total burning power. Therefore, to enhance the stability of simulations for them, a longer delay time is used.
Instantaneous ignition cannot be simulated. The ignition delay must be considered when configuring the simulation time. As mentioned in the referenced sections, the delay in ignition is no more than half of the user-specified simulation time. Therefore, if the simulation time is short, the fire source will technically reach the expected power, but the faster we try to ignite the source, the greater the instability at the beginning of the simulation and the spike in burning power.
Simulation Time for Justifying Fire Separation Distances
When using elements with the “On Fire” option enabled for burning simulations, which ignite relatively quickly, a few dozens of seconds of simulation time may be sufficient for a rough estimate of the heat flux magnitude. It takes some time for the burning power to stabilize, so a time interval of less than 20 seconds must not be used even for a rough preliminary estimate.
The magnitude of the heat flux is also affected by the temperature of surrounding objects. These objects may heat up during the entire duration of the fire. Thus, a longer simulation time must be used for a more accurate assessment of the heat flux.
The time it takes for the heat flux to reach a steady state depends on the placement of objects in the specific scenario, the ambient temperature, wind, etc. There is no universal amount of time sufficient for simulation.
For instance, you can use the time when fire suppression efforts by fire departments begin.
The adequacy of the simulation time can be assessed based on heat flux graphs for the elements of interest. At the initial stage of simulation, the graph always shows a sharp increase. Subsequently, the growth in the graph usually slows down. The simulation time can be considered sufficient if there is only a slight increase in heat flux density during the second half of the simulation time compared to the value reached during the first half. Moreover, if the achieved heat flux value is close to the maximum allowable limit, the simulation time must be increased to evaluate further dynamics.
Note that surface temperatures usually rise throughout the simulation period. Therefore, if the purpose of the simulation is to assess the amount of heating of structures during a fire, the simulation time must be chosen based on the time during which these structures must withstand heating.
Impact of Computational Domain Dimensions
Note that combustion beyond the computational domain (see Calculation Area) is not simulated. Heat, radiation, and combustion products that exit the boundaries of the computational domain no longer affect the development of the fire within the domain. Therefore, it is not recommended to place Fire Sources or elements with the “On Fire” property near the open boundary of the computational domain.
This is especially true for the upper boundary of the computational domain. Hot combustion products and flames rise upwards. If there is an open boundary of the computational domain above a fire source, the height of the smoke and flame column is limited by the height of the domain. A significant limitation on the height of the flame plume above a Fire Source or an “On Fire” element may lead to incorrect estimates of the heat flux magnitude or temperature.
The height of the flame plume fitting within the computational domain is specifically important for calculations to justify fire separation distances. The plume generates thermal radiation, and excessively limiting its height during simulation can lead to gross errors in estimating the heat flux impacting the walls and windows of the fire-protected object. This issue is relevant in the case of a roof fire when all the flames are at the top of the scenario. If the height of the computational domain is not sufficient, the fire will exit it, almost without affecting the simulation results.
You must allow for several extra meters of height for computational domain above the burning building when justifying fire separation distances. The extra height depends on the specifics of the particular object. Sufficient extra height is either theoretically justified or it is shown through the simulation that further increase does affect the heat flux magnitude.