Fire Simulation for Engineers/FDS/Output

= Output =

Finally, the user prescribes the output quantities (DEVC, SLCF, BNDF, ISOF).

All output quantities must be specified at the start of the calculation. In most cases, there is no way to retrieve information after the calculation ends if it was not specified from the start.

Prescribing output
FDS computes the temperature, density, pressure, velocity and chemical composition within each numerical grid cell at each discrete time step. There are typically hundreds of thousands to millions of grid cells and thousands to hundreds of thousands of time steps. In addition, FDS computes at solid surfaces the temperature, heat flux, mass loss rate, and various other quantities.

So the user must carefully select what data to save, much like one would do in designing an actual experiment. Even though only a small fraction of the computed information can be saved, the output typically consists of fairly large data files (it can be many GBytes!).

Remember that all output quantities must be specified at the start of the calculation. Before a calculation is started, carefully consider what information should be saved.

Typical output quantities for the gas phase include:

• Gas temperature;

• Gas velocity;

• Gas species concentration (water vapor, CO2, CO, N2);

• Smoke concentration and visibility estimates;

• Pressure;

• Heat release rate per unit volume;

• Mixture fraction (or air/fuel ratio);

• Gas density;

• Water droplet mass per unit volume.

On solid surfaces, FDS predicts additional quantities associated with the energy balance between gas and solid phase, including:

• Surface and interior temperature;

• Heat flux, both radiative and convective;

• Burning rate;

• Water droplet mass per unit area.

Global quantities recorded by the program include:

• Total Heat Release Rate (HRR)Heat Release Rate (HRR);

• Sprinkler and detector activation times;

• Mass and energy fluxes through openings or solids.

Time histories of various quantities at a single point in space or global quantities like the fire’s heat release rate (HRR) are saved in simple, comma-delimited text files that can be plotted using a spreadsheet program, like Openoffice.org Calc or Microsoft Excel.

Most field or surface data are visualized with Smokeview. FDS and Smokeview are used in concert to model and visualize fire phenomena. Smokeview performs this visualization by presenting animated tracer particle flow, animated contour slices of computed gas variables and animated surface data. Smokeview also presents contours and vector plots of static data anywhere within a scene at a fixed time.

Output control parameters, DUMP
The namelist group DUMP contains parameters that control the rate at which output files are written, and various other global parameters associated with output files.

For example, NFRAMES parameter is the number of output dumps per calculation. The default is 1000. Device data, slice data, particle data, isosurface data, 3D smoke data, boundary data, solid phase profile data, and control function data are saved every (T_END - T_BEGIN) / NFRAMES seconds unless otherwise specified.

The following table summarizes some DUMP parameters:

Point measurement devices, DEVC
The same devices DEVC used for control logic (see Chapter [cha:Devices-and-control-logic]) are employed to save a given quantity at a single point in space so that this quantity can be plotted as a function of time, like for example a thermocouple temperature measurement.

The prescribed QUANTITY is recorded as a column in an output file named mycase_devc.csv. This type of file can be imported into any spreadsheet program.

An example:

Record a gas phase quantity
For example, if you just want to record the time history of the temperature at a given point, add:

&DEVC XYZ=6.7,2.9,2.1, QUANTITY='TEMPERATURE', ID='T-1' /

and a column will be added to the output file mycase_devc.csv under the label T-1.

FDS reports the value of the QUANTITY calculated in the cell where the point XYZ is located.

Record a solid phase quantity
When prescribing a solid phase quantity, be sure to position the probe at a solid surface. It is not always obvious where the solid surface is since the mesh does not always align with the input obstruction locations.

To help FDS locate the appropriate surface, suggest the right orientation of the face using the parameter IOR (index of orientation).

There are still instances where FDS cannot determine which solid surface is being designated, in which case an error message appears in the diagnostic output file. Re-position the probe and try again. For example, the line

&DEVC XYZ=0.7,0.9,2.1, QUANTITY='WALL TEMPERATURE', IOR=-2, ID='wt1' /

designates the surface temperature of a wall facing the negative y direction.





Record integrated quantities
In addition to point measurements, the DEVC group can be used to report integrated quantities. For example, you may want to know the mass flow out of a door or window. To report this, add the line:

&DEVC XB=0.3,0.5,2.1,2.5,3.0,3.0, QUANTITY='MASS FLOW', ID='mf1' /

Note that in this case, a face is specified rather than a point.

QUANTITY='HRR'Heat Release Rate (HRR) can be used to compute the total heat release rate within a subset of the domain. In this case, the sextuplet XB ought to define a volume rather than a face.

Remember to avoid faces or volumes that cross multiple mesh boundaries: FDS has to decide which mesh to use in the integration.

Animated planar slices, SLCF
The SLCF namelist group parameters allows you to record various gas phase quantities at more than a single point. A slice refers to a subset of the whole domain. It can be a line, face, or volume, depending on the values of XB.

Animated vectors can be created in Smokeview if a given SLCF line has the attribute VECTOR=.TRUE. If two SLCF entries are in the same plane, then only one of the lines needs to have VECTOR=.TRUE. Otherwise, a redundant set of velocity component slices will be created.

For example, the line:

&SLCF PBX=0.8, QUANTITY='TEMPERATURE', VECTOR=.TRUE. /

records the temperature and animated velocity vectors on the plane x~=~0.8

The following table summarizes some SLCF parameters:

Animated boundary quantities, BNDF
The BNDF namelist group parameters allows you to record surface quantities at all solid obstructions. As with the SLCF group, each quantity is prescribed with a separate BNDF line. No physical coordinates need be specified, however, just QUANTITY.

For example, the line:

&BNDF QUANTITY='WALL_TEMPERATURE' /

records WALL_TEMPERATURE for all solid obstructions faces.

For certain output quantities, additional parameters need to be specified via the PROP namelist group. In such cases, add the character string, PROP_ID, to the BNDF line to tell FDS where to find the necessary extra information.

Note that BNDF files can become very large, so be careful in prescribing the time interval.

One way to reduce the size of the output file is to turn off the recording of boundary information on desired obstructions. On any given OBST line, if the string BNDF_OBST=.FALSE. is included, the obstruction is not considered for boundary quantities output. To turn off all boundary recording, set BNDF_DEFAULT=.FALSE. on the MISC line. Then individual obstructions can be turned back on with BNDF_OBST=.TRUE. on the appropriate OBST line. Individual faces of a given obstruction can be controlled via BNDF_FACE(IOR)=.TRUE., where IOR is the index of orientation.



The following table summarizes some BNDF parameters:

Animated isosurfaces, ISOF
The ISOF namelist group is used to specify the output of gas phase scalar quantities, as three dimensional animated contours. For example, a 300 °C temperature isosurface shows where the gas temperature is 300 °C. Three different values of the temperature can be saved via the line:

&ISOF QUANTITY='TEMPERATURE', VALUE(1)=50., VALUE(2)=200., VALUE(3)=500. /

where the values are in °C.



Any gas phase quantity can animated via isosurfaces, but use caution. To render an isosurface, the desired quantity must be computed in every mesh cell at every output time step. For quantities like TEMPERATURE, this is not a problem, as FDS computes it and saves it anyway. However, soot density or oxygen demand substantial amounts of time to compute at each mesh cell.

The following table summarizes some ISOF parameters:

Realistic smoke and fire
When you do a fire simulation using the default mixture fraction combustion model, FDS automatically creates two output files that are rendered by Smokeview as realistic looking smoke and fire.



By default, the output quantities MASS FRACTION of soot and HRRPUV (heat release rate per unit volume) are used in the visualization.

Heat Release Rate (HRR)
Quantities associated with the overall energy budget are reported in the comma delimited file mycase_hrr.csv. This file is automatically generated.

The file consists of six columns. The first column contains the time in seconds. The second through fifth columns contain integrated energy gains and losses, all in units of kW. The second column contains the total heat release rate, the third contains the radiative heat loss to all the boundaries (solid and open), the fourth contains the convective and radiative heat loss to the boundaries (i.e. the energy flowing out of or into the domain), and the fifth contains the energy conducted into the solid surfaces. The sixth column contains the total burning rate of fuel, in units of kg/s. It is included merely as a check of the total heat release rate.

An example:

Visibility
If you are performing a fire calculation using the mixture fraction approach, the smoke is tracked along with all other major products of combustion.

The intensity of monochromatic light I passing a distance L through smoke is attenuated according to:

$$\mathrm{I}/\mathrm{I}_{0}=e^{-\mathrm{KL}}$$

The light extinction coefficientLight extinction coefficient K, is a product of the density of smoke particulate, $$\rho\mathrm{Y_{S}}$$, and a mass specific extinction coefficient Km. Mass extinction coefficient that is fuel dependent:

$$\mathrm{K}=\mathrm{K_{m}\rho\mathrm{Y_{S}}}$$

As showed by [Mulholland 2002], an estimate of visibility through smoke can be made by using the equation:

$$\mathrm{S=C/K}$$ where CVisibility factor is a non-dimensional constant characteristic of the type of object being viewed through the smoke.

Since K varies from point to point in the domain, as it depends on smoke density, the visibility S does as well.

Keep in mind that FDS can only track smoke whose production rate and composition are specified. Predicting either is beyond the capability of the present version of the model. Three parameter control smoke production and visibility; each parameter is input on the REAC line:

• SOOT_YIELD is the fraction of fuel mass that is converted to soot.

• MASS_EXTINCTION_COEFFICIENT, the Km of Equation [eq:Km]. The default value is 8700 m2/kg, a value suggested for flaming combustion of wood and plastics.

• VISIBILITY_FACTOR, the C of Equation [eq:S]. C=8 for a light-emitting sign and C=3 for a light-reflecting sign. C is 3 by default.

The visibility S is output via the QUANTITY keyword VISIBILITY.

Upper and lower layers
Fire protection engineers often need to estimate the location of the interface between the hot, smoke-laden upper layer and the cooler lower layer in a burning compartment. Relatively simple fire models, often referred to as two-zone modelsZone models, compute this quantity directly, along with the average temperature of the upper and lower layers. In a computational fluid dynamics (CFD) model like FDS, there are not two distinct zones, but rather a continuous profile of temperature. Nevertheless, there are methods that have been developed to estimate layer height and average temperatures from a continuous vertical profile of temperature.

The quantities LAYER HEIGHT, UPPER TEMPERATURE and LOWER TEMPERATURE can be designated via DEVC lines in the input file. For example, the entry:

&DEVC XB=2.0,2.0,3.0,3.0,0.0,3.0, QUANTITY='LAYER HEIGHT', ID='whatever' /

produces a time history of the smoke layer height at x~=~2 and y~=~3 between z~=~0 and z~=~3. If multiple meshes are being used, the vertical path cannot cross mesh boundaries.

Heat fluxes and thermal radiation
There are various ways of recording the heat flux at a solid boundary. If you want to record the net heat flux to the surface, qconvective + qradiative, use the QUANTITY='HEAT FLUX'. The individual components, the net convective and radiative fluxes, are CONVECTIVE HEAT FLUX and RADIATIVE HEAT FLUX, respectively.

If you want to compare predicted heat flux with a measurement, you often need to use GAUGE HEAT FLUX. The difference between NET HEAT FLUX and GAUGE HEAT FLUX is that the former is the rate at which energy is absorbed by the solid surface; the latter is the amount of energy that would be absorbed if the surface were cold or some temperature specified with GAUGE_TEMPERATURE.

All of the above heat flux output quantities are defined at a solid surface.

Interfacing with structural models
FDS solves a one-dimensional heat conduction equation for each boundary cell marking the interface between gas and solid, assuming that material properties for the material layers are provided. The results can be transferred (via either DEVC or BNDF output) to other models that predict the mechanical response of the walls or structure. For many applications, the one-dimensional solution of the heat conduction equation is adequate, but in situations where lateral heat conduction within the solid is significant, another approach can be followed. FDS includes a calculation of the Adiabatic Surface Temperature (AST)Adiabatic Surface Temperature (AST), a quantity that is representative of the heat flux to a solid surface, following the idea proposed by [Wickstrom 2007].

Obviously, the objective in passing information to a more detailed model is to get a better prediction of the solid temperature (and ultimately its mechanical response) than FDS can provide.

Visualizing Lagrangian particles
Tracer particles can be injected into the flow field from vents or obstacles, as shown in Section [sec:Injecting-Lagrangian-particles], and then observed in Smokeview. Streak lines can be drawn, too.

Frequently used output quantities
The following table shows an organized list of frequently used output quantities. The column “Namelist” details where the concerned QUANTITY can be specified: B boundary file (BNDF), D device (DEVC), I isosurface file (ISOF), S slice file (SLCF).

∗These quantities require the specification of a gas specie using SPEC_ID.

Here are some examples of output quantities:

&DEVC XYZ=0.1,0,2.39, QUANTITY='THERMOCOUPLE', ID='2.4' / &DEVC XB=0.1,0.1,0,0,0.0,2.4, QUANTITY='LAYER HEIGHT', ID='layer_h' / &ISOF QUANTITY='TEMPERATURE', VALUE(1)=60.0 / &SLCF PBY= -3., QUANTITY='TEMPERATURE', VECTOR=.TRUE. / &BNDF QUANTITY='ADIABATIC SURFACE TEMPERATURE' /

There are some output quantities that require a specie name via SPEC_ID, as for example the MASS FRACTION quantity. The SPEC_ID can select one of the species implicitly defined when doing a mixture fraction calculation (fuel, oxygen, nitrogen, water vapor, carbon dioxide, carbon monoxide, hydrogen, soot, other), or an extra gas specie from those defined via a SPEC line:

&SLCF PBX= 0.1, QUANTITY='VOLUME FRACTION', SPEC_ID='carbon dioxide' / volume fraction of carbon dioxide issued from mixture fraction model gas phase combustion model &SLCF PBX= 0.1, QUANTITY='VOLUME FRACTION', SPEC_ID='CARBON DIOXIDE' / volume fraction of carbon dioxide defined in a SPEC namelist group and injected separately in the flow domain