## Fire Dynamics Simulation

You can perform fire dynamics simulation either locally using Fenix+ 3 Classic or remotely using Fenix Server.

Fire dynamics simulation for projects created with the demo version of the program is carried out over the Internet on the developer’s server. Such projects shall comply with a number of requirements (for more information, see the email with the download link for the demo version of the program).

To run fire dynamics simulation, Fenix+ 3 Classic uses the FDS (Fire Dynamics Simulator) developed by the National Institute of Standards and Technology (NIST) of the US Department of Commerce with the assistance of the VTT Technical Research Center.

FDS implements a computational hydrodynamic model of heat and mass transfer during combustion. FDS numerically solves the Navier-Stokes equation for low-speed temperature-dependent flows. FDS focuses on smoke distribution and heat transfer during a fire.

Before you perform the fire dynamics simulation, make sure that the FDS is ready for operation (for more information, see General settings).

## Setting Fire Dynamics Simulation Parameters

Time and number of simulation threads

To specify the simulation time and the number of processors for running simulation in the multi-threaded mode, do the following:

1. Click Simulation | Fire simulation parameters.
2. Open the General tab.
3. Select the desired scenario.
4. Specify the desired simulation time.

Enable or disable multi-threaded calculation mode

The purpose of the multi-threaded simulation mode is to reduce the simulation time of the dynamics of fire development.

To use multi-threaded mode to simulate the dynamics of fire development, it is necessary to do the following:

1. Specify the number of processes, which is larger than 1.
2. Select the path to the mpiexec.exe file and, if necessary, add it to the exclusions list of the Windows firewall and the installed anti-virus program (for more information, see General settings).

Not always an increase in the number of threads used to simulate the dynamics of fire development leads to a corresponding decrease in simulation time (an increase in the number of threads by a factor of 2 will not reduce the simulation time by the same factor). This happens due to the fact that the program spends time for processing threads.

Initial conditions

To specify the initial conditions of the fire development, do the following:

1. Click Simulation | Fire simulation parameters.
2. Open the Initial Conditions tab.
3. Select the desired scenario.
4. Specify the desired parameter values.

Temperature is ambient temperature;

Background pressure is pressureС

Relative humidity is humidity;

Temperature gradient is a change in temperature with a change in height;

Room temperature is the initial air temperature inside the premises. The volume of a room is the area of space bounded by the contours of the room and a height equal to the height of the floor on which the room is located. It means that if an area inside the building is not marked as a room, then the initial temperature in this room will be equal to the ambient temperature (the value specified in the Room temperature field). By default, the indoor temperature is equal to the ambient temperature.

Flame extinction in the gaseous phase is the process of flame attenuation simulated with a decrease in the oxygen concentration in the room (this parameter is not used by default).

The Material properties section defines the influence of the thermophysical properties of materials on the initial conditions of the fire development. Possible values: Inert - the thermophysical properties of construction materials do not affect the initial conditions of fire development. The Actual material - the thermophysical properties of materials affect the initial conditions of fire development.

By default, Fenix+ 3 Classic uses the value of the Room temperature parameter for all rooms except those where a different value is specified. This condition is fulfilled only if the Use specified value parameter is selected in the properties of such rooms. For more information, see Room.

Doors and windows behavior

By default, all doors (except those marked as Fire resistant and/or With closer) are treated by FDS as openings. Fire-proof doors and doors with closers are treated as obstacles. Windows are also treated by FDS as obstacles.

Consequently, the dangerous fire factors easily spread through doors. They do not spread through fire-proof doors and doors with door closers, though.

In some cases, this leads to significant difference between the simulation results and the real situation.

To make fire dynamics simulation more realistic, it is possible to determine the behavior of windows and doors on the basis of the dynamics of the distribution of dangerous fire factors and evacuation of people.

To determine the behavior of doors and windows, do the following:

1. Click Simulation | Fire simulation parameters.

2. Open the Doors and Windows tab.

3. Select the desired scenario.

4. Specify desired parameter values.

Doors

The parameters located on this tab affect only doors marked as Fire resistant.

The With closer parameter determines whether doors should open when people pass through them and close after a certain time. The default closing time is 25 seconds. If necessary, you can specify a different value. For more information, see the specification of the door closer.

With this parameter, you can determine the closing time for all fire-proof doors in the scenario. If you need to specify a different value for a certain door, then select the With closer parameter in the door properties and set the desired closing time (for more information, see Door).

If a fire resistant door is also marked as With closer, then the closing time value specified in its properties is used and the closing time value specified in the fire dynamics simulation parameters is ignored.

To process opening and closing of doors during the fire dynamics simulation, the program needs to know when people pass through them. So, before running the fire dynamics simulation you should perform the evacuation simulation.

If you perform the fire dynamics simulation before the evacuation simulation, then fire-proof doors and doors with closers will be closed all the time.

Windows

The Closed parameter determines how FDS treats windows: as openings or obstacles.

The Allow destruction at critical temperature parameter determines whether windows should be destroyed when temperature reaches the critical value in the place where they are located.

By default, the destruction temperature is 250 °C.

Temperature is measured in the center of the top edge of the window.

Fire source

The Flame spread over whole surface parameter allows to increase the maximum combustion area without increasing the area occupied by the fire load.

Flame spreads only in case the fire source contacts with the side surface of the fire load.

Flame spreads only to those side surfaces of the fire load that contact with the fire source and are located in such a way that there is at least one calculation area cell between them and other objects on the scene. If the side surface touches the other object on the scene (for example, a the wall), then flame will not spread to this surface.

Flame does not spread to the rear surface in the following image.

{width=50%}

In the report there is a table with a description of the scenarios, where two values are indicated:

1. Possible maximum burning area is the maximum burning area, which can possibly be created in the scenario during fure dynamics simulation. This area is determined by the area of the fire source and the area of the side surfaces to which flame can spread.
2. Actual maximum burning area is the maximum burning area, which is actually created in the scenario during the fire dynamics simulation.

Calculation Areas

All calculation areas in the scenario represent one or several MESH groups in the FDS source file (for more information, see Calculation area). FDS makes a lot of requirements to the MESH groups and their mutual arrangement on the scene.

By default, the program processes calculation areas so that they comply with all necessary requirements (the Adjust automatically parameter is checked).

The program processes calculation areas in the following way:

1. First, the program attempts to merge all calculation areas into larger ones with regard to their cell size.

• Intersections of areas are eliminated (this may lead to incorrect results in the areas of intersection);

• Calculation areas are increasing in directions where the number of cells is less than 3 (this case is very rare and in practice can occur due to an error when you place a calculation area on the scene).

1. If the number of threads that you want to use for fire dynamics simulation is less than the number of resulting MESH groups, then calculation areas with the largest number of cells are split in half in the direction of the largest number of cells. This operation is interrupted in the following cases:

• The boundary of the resulting MESH groups falls on one of the VENT groups (i.e., on the fire source or ventilation)

• The number of meshes is equal to the number of threads

• There are no more meshes that can be broken.

As a result, the MESH groups may be slightly larger than the original calculation areas.

If you select the Transfer calculation areas unchanged parameter, then calculation areas are not merged into larger ones. If necessary, the program can devide calculation areas.

In this case, the resulting MESH groups will completely match the location of the original calculation areas. The height of the calculation areas will be equal to the height of the floor.

If you are not sure about the correct location of calculation areas, it is not recommended to use the Do not adjust calculation areas parameter. If calculation areas are not arranged correctly, the fire dynamics simulationd results in an error.

Calculation area cell size

Possible values ​​for the calculation area cell size are determined by the parameters displayed in the image above.

The image displays the default parameters. If these parameters are used, the following set of cell sizes is possible: 0.5, 0.25, 0.125, 0.0625 and 0.03125 m. Each value is obtained by dividing 1 (BaseSize) by the factor of 2. The power to which two is raised - any integer in the range of 1 (MinRate) to 5 (MaxRate). That is, the number of valid values ​​is determined by MinRate and MaxRate.

For example, if during the fire dynamics simulation simulation it is necessary to use a cell size that equals to 0.3 m, then you only need to set the value of the BaseSize parameter to 0.6 m. This will allow to use the cell sizes that belong to the following range: 0.6, 0.3, 0.15, 0.075 and 0.0375 m.

You can specify parameters that define the possible set of calculation area cell sizes only when there are no calculation areas in the scenario.

Wind speed and direction

Fenix+ 3 Classic can consider the action of the wind during the fire dynamics simulation.

The Speed parameter defines the speed of the wind. If its value not greater than 0, then the program considers the action of the wind during the fire dynamics simulation.

The Scene direction parameter (m.u.: degree) defines the angle between the north and the Y axis in the scene editor. You can use dropdown lists to specify fixed values in the range from 0 to 315. To specify an arbitrary angle, select the Custom option and enter a desired value.

The Wind direction parameter (m.u.: degree) defines the angle between the north and the wind direction. You can use dropdown lists to specify fixed values in the range from 0 to 315. To specify an arbitrary angle, select the Custom option and enter a desired value.

Setting critical values for dangerous fire factors

If necessary, you can specify a different critical value for any dangerous fire factor in the program. The program will use the specified value to determine the blocking time. To restore the default value, use the Default button.

## Starting and Stopping Fire Dynamics Simulation

To start fire dynamics simulation, switch to the Simulation Control tab, select Fire Dynamics for a desired scenario and click the button.

To quicly start evacuation simulation, click the button and then click Evacuation simulatio.

You can run fire dynamics simulation for multiple scenarious simultaneously.

To pause fire dynamics simulation, click the button. You can resume it any time (even after closing and then reopening the project).

To resume fire dynamics simulation, click the button.

To stop fire dynamics simulation, click the button.

Before starting the simulation, the scenario will be checked for errors that may affect the evacuation simulation (see Checking a Project for Errors).

If critical errors are detected in the scenario, an error panel will be displayed at the bottom of the main program window.

If there are critical errors in the scenario, evacuation simulation is not possible.

To continue working, it is necessary to eliminate all critical errors. Only after that it will be possible to carry out a simulation of evacuation.

Errors can also be detected in the scenario, in the presence of which it is possible to simulate the fire dynamics, but the simulation results may turn out to be unreliable. If such errors are detected in the scenario, a corresponding message will appear.

In this case, you can ignore the warning and try to simulate the scenario without eliminating errors, or look at the detected errors, make corrections if necessary, and only after that carry out the simulation.

If there are no errors in the scenario, or the user ignores them, then the Fenix+ 3 Classic determines the approximate amount of RAM that is required for the simulation (for more details see Calculation area).

If the required amount of RAM exceeds the amount of free memory (or the amount of free memory could not be determined), the corresponding information message is displayed.

You can refuse to conduct the simulation to make adjustments to the project (No answer), or try to carry out the simulation, leaving the scenario unchanged (Yes answer).

In the Fenix+ 3 application, the evacuation simulation process has the following statutes:

Not calculated — simulation for this scenario has never been done, or changes have been made to the scenario that require repeated simulation.

Running — Simulation is being performed for this scenario.

Done — The simulation for this scenario was successful.

Error — an error occurred during the simulation that did not allow to complete the calculation. The cause of the error may be one of the ignored program warnings. When simulation fire dynamics using the Fenix Server, the following additional statuses are possible:

Being sent — the project is sent to the server.

Verification is in progress — the project is checked for restrictions on the server.

Number in queue — the project is waiting for the server to start simulation.

Archiving — the simulation results are archived on the server before being sent to the local computer.

## Recommendations For Reducing Fire Dynamics Simulation Time

There is a great number of factors which affect the fire dynamics simulation time. Among them you can choose the following main groups of factors:

1. Simulation volume (size and characteristics of calculation area, fire spread time, during which simulation should be performed).
2. Physical actions, which should be considered during simulation (ventilation, leaks through doorways and openings in windows, presence of wind and etc.)
3. Usage of multithread simulation tools.

Simulation volume

A simulation object is regarded as a physical volume where you should perform fire dynamics simulation (i.e. a number of cells this object is devided into) as well as fire spread time, during which simulation should be performed.

In both cases the dependency is almost directly proportional. That is, if you encrease a number of cells or fire spread time by 2 times, simulation time is also encreased by 2 times.

Simulation time depending on a number of cells and fire spread time brings forth a number of cells and fire spread time. It is clear that to save simulation time you should try to keep a balance in order to get valid simulation results, which are suitable for further use.

Number of cells in the calculation area is defined as a relation of calculation area volume with a cell volume. Therefore, to decrease a total amount of cells you should decrease calculation area size and encrease calculation area cell size. In case a scenario contains several calculation areas, then a total number of cells equals to a sum of cells in every calculation area.

Recommendations for positioning calculation areas

• Place the calculation area only in those places of the building where the distribution of the dangerous fire factors is possible.

• Position the calculation area so that it covers a minimum of empty space outside the building.

• Avoid overlapping calculation areas.

• In case ventilation size or fire source (or the size of the object on which they are located) is less than the size of the cell of the calculation area, position an additional calculation area with a smaller cell size covering only ventilation or fire source.

In case the size of the ventilation size or the fire source (or the size of the object on which they are located) is less than the size of the calculation area cell, then it may be impossible to run simulation or simulation results may be incorrect.

• Do not create multiple small calculation areas. It is better to create one large calculation area.

This recommendation in some way comes in conflict with the assertion, that the less the calculation area is, the less time the simulation takes. But you should consider the fact that when you encrease a number of calcualtion areas, the program will spend time for processing their results and this time can prevail the time which is spent for processing a single calculation area.

Recommendations for selection of the calculation area cell size

To select the cell size of the calculation area, you can use the following formula:

{width=20%}

where:

D* — the characteristic diameter of the fire source

Q* — the capacity of the fire source

p∞ — air density

Cp — heat capacity of air

T∞ — air temperature (m.u.: K)

g — the acceleration of gravity

The ratio of the characteristic diameter of the fire source to the size of the mesh does not have a definite value. During the validation, FDS developers used values from the range of several to one hundred units (see the Mesh Resolution section of the FDS user manual). The most commonly used values belong to the range between 4 and 16. In other words, the following expression is true:

{width=15%}

where “a” is the mesh size.

For example, with a fire capacity of 1,500 kW (vehicle ignition over an area of 2 m²) with a mesh size of 0.25 m, the ratio of the characteristic diameter to the mesh size is 4.5. This value belongs to the range of the most commonly used values in FDS validation.

Recommendations for selection of the fire spread time for simulation

You should not perform the fire dynamics simulation longer than neceasary.

To define a fire risk value, the fire spread time for simulation shall be specified with regard to the safety factor (i.e. fire spread time shall exceed evacuation simulation time by 1.25 times). Do not enter a great value because it will not affect the validity of fire risk calculation results but fire dynamics simulation will take more time.

Phisical conditions which are regarded during simulation

Phisical conditions (ventilation, leaks through doorways and openings in windows, presence of wind and etc.) which are regarded during the fire dynamics simulation increase simulation time.

The greater value you use for the above-mentioned factors, the more it affects the simulation time. For example, a high capacity smoke exhaust system will increase the simulation time in a greater way than a low capaсity one.

Fenix+ 3 Classic can perform fire dynamics simulation in the multi-thread mode. In this mode all processing load is devided among several processes (threads) which are run in parallel. In addition, every calculation area (in fact, a mesh group in the FDS source file) is processed in a single thread.

As a rule, a total simulation time in the multi-thread mode should be less than when all calculations are performed in a single thread.

But you should consider that the more a number of threads is, the more time the program will need to process the results. If a number of threads is large, the deciding factor is not the time the program spends on processing calculation areas in several threads but the time it spends on adjusting the results. If a number of threads is too large, then the simulation can require more time than when it is run with a fewer threads.

By default, fire dynamics simulation is performed with a single thread.

The number of threads should not exceed the number of logical cores.

The number of threads, which will allow to perform the simulation with minimum time depends on the size of the calculation area, location of several calculation areas and the number of cores. Therefore, it is impossible to create rules for selection of optimal number of threads for simulation.

How to calculate RAM required for fire dynamics simulation

Memory size (m.u.: Gb), which is required for running fire dynamics simulation in one calculation area can be defined using the following formula:

{width=30%}

where:

width, length and height are the sizes of the calculation area and a is the mesh size.

In case a scenario has several calculation areas, you should define the total memory required for all of them.

Total memory required for simulation defined using the above formula is approximate. In fact, a real amount of required memory can be more or less. As a rule, in practice the real amount of required memory is less than the calculated value.

## Fire Dynamics Simulation Results

Fire dynamics simulation results are stored in the following folder:

..\Project directory\Results\Scenario Id\fds

Where:

• Project directory — the directory where the project file is located;

• Scenario Id — the identifier of the scenario in the project.

The scenario identifier cannot be found using the program tools. Therefore, in order to open the folder with the fire dynamics simulation results, you should double-click fire dynamics simulation for the desired scenario on the evacuation simulation control panel. The scenario identifier is not displayed anywhere in the program. To open the folder with the fire dynamics simulation results, you should double click Fire for a desired scenario on the Simulation tab.

When the fire dynamics simulation is performed, FDS creates many different files. Here is a description of some of them.

The label XXXXXXXX in the file name indicates the identifier of the scenario.

“XXXXXXXX.fds” is the FDS source file

“XXXXXXXX.out” is the file, which contains statistics of the simulation process

“XXXXXXXX.smv” is the data file for visualizing simulation results in Smokeview

“XXXXXXXX_01.s3d” are files containing information on the distribution of smoke and fire

“XXXXXXXX_devc.csv” are files containing information on how danferous fire factors change

“XXXXXXXX_0001.restart” contains information, which is necessary to resume simulation after it has been stopped

“XXXXXXXX_xx.sf” are files containing information on the fields of dangerous fire factors

“PROJECT NAME.fnx” is a copy of the project file, which includes only one scenario, whose results are stored in this folder. The project extension may be different (for more information, see Logical Structure of Project Folder).

The program stores files with fire dynamics simulation results for a scenario untill you run a new simulation in this scenario. Changing fire dynamics simulation parameters, editing the topology of the building or changing the fire component of the scenario does not modify or delete files with evacuation simulation results. If you make changes to the scenario and close the project without running a new simulation, when you open the project again the program will display the fire dynamics results from the previous simulation that do not reflect the changes you have made to the scenario since that time.

Viewing fire dynamics simulation results

There are several ways to view fire dynamics simulation results:

1. Using Fenix+ 3 Classic.

2. Using Smokeview.

You can use Fenix+ 3 Classic to view fire dynamics simulation results in the form of tables and graphs. For this, open the Results tab and then switch to the Fire simulation results Tab.

If the length of the recording device or the width of the door with the recording device is less than 1 m, then the recording device has only one control point and it is not displayed as an additional line in the table.

In the graph below, the red line shows the maximum allowed (critical) value for the selected dangerous fire factor.

The maximum allowed values for all dangerous fire factors are provided below:

• Temperature - 70°C
• Temperature - 20 m
• Oxygen concentration $O_2$ – 0,226 kg/m³
• Carbon dioxide concentration $CO_2$ – 0,11 kg/m³
• Carbon monoxide concentration $CO$ – 1,16·10-3 kg/m³
• Hydrogen chloride concentration $HCl$ – 23·10-6 kg/m³

The graph can display how a desired dangerous fire factor changes in a desired control point. For example, the figure above displays how all dangerous fire factors change in the place where door is located.

To select a dangerous fire factor and a control point, click a corresponding cell. To select multiple cel, use the Shift and Ctrl keys.

To display all dangerous fire factors for one control point, click the left mouse button on the cell with the name of the desired control point.

If the registering device has several control points, you can view how a certain dangerous fire factor changes in all control points. For this, click on the cell that corresponds to the desired registering device and the desired dangerous fire factor.

If different dangerous fire factors are selected on the graph, then the value of each dangerous fire factor is reduced to the critical value (marked with the symbol “*” on the graph). As a result, the critical value for all dangerous fire factors on the graph equals to 1.

If Visibility is selected for several control points with different critical values, then all values will be reduced to the critical value in the corresponding control point.

You can configure the program settings so that values of dangerous fire factors are always reduced to the critical value (for more information, see General settings).

To visualize fire dynamics simulation results, click button on the additional toolbar. As a result, the visualization panel will open.

The visualization panel is used to display evacuation simulation results as well as fire dynamics simulation results. To display fire dynamics simulation results, select the Fire dynamics option and click .

As a result, the visualization of the selected dangerous fire factor for the selected registering device will be displayed in the scene editor.

{width=50%}

Fenix+ 3 Classic allows to display visualization of the following dangerous fire factors:

• Temperature
• Visibility
• Oxygen concentration $O_2$
• Concentration of hydrogen chloride $CO_2$
• Carbon dioxide concentration $CO$
• Carbon monoxide concentration $HCl$

Since the default height at which dangerous fire factors are measured is 1.7 m from the level of the floor, some objects on the scene (including people) may appear under the field of the dangerous fire factor. Thus, it may be inconvinient to display visualization of evacuation and fire dynamics simulation results at the same time. Therefore, you can change the level at which fields of dangerous fire factors are displayed. For this, change the value in the Display Level field.

To display the fire dynamics simulation results in Smokeview, click Show results in 3D on the visualization panel. As a result, a program window will open displaying all the objects located in the calculation area.

{width=60%}

In Smokeview, click the right mouse button and select “SOOT DENSITY”.

{width=60%}

To display how the fire spreads, right click anywhere in the program window and select “HRRPUV”.

{width=30%}

There are other options you can use when you view simulation results in Smokeview. For more information refer to the Smokeview user guide (Fire Dynamics Simulator (FDS) and Smokeview (SMV) manuals).