Table of Contents
Surfaces are used to define the properties of solid objects and vents in your FDS model. The surface can use previously defined materials in mixtures or layers. By default, all solid objects and vents are inert, with a temperature that is fixed at the ambient temperature (set in the Simulation Parameters dialog. In addition to defining heat conduction in a solid, surfaces can also be used to define a burner, specify the ignition temperature for an object, give a vent an supply velocity, and set the many other properties supported by FDS.
To create, modify, and delete surfaces, you can use the Edit Surfaces dialog. To open the surface manager dialog, on the Model menu, click Edit Surface Properties.... The dialog shown in Figure 6.1 shows the dialog being used to edit an upholstery surface.
There are four fundamental or "reserved" surface types: ADIABATIC, INERT, MIRROR, and OPEN. These surfaces cannot be changed and are present in every analysis.
This surface remains fixed at the ambient temperature. There is no heat transfer (radiative or convective) from the gas to an adiabatic solid.
This surface remains fixed at the ambient temperature. Heat transfer does occur from gases to INERT surfaces. This is the default surface in PyroSim.
MIRROR[1]
This surface is used only for vents on the exterior mesh boundary. A MIRROR is a no-flux, free-slip boundary that reverses flow. It is intended to be applied to an entire mesh boundary to symmetrically double the size of the domain.
OPEN[1]
This surface is used only for vents on the exterior mesh boundary. OPEN denotes a passive opening to the outside and is often used to model open doors and windows.
PyroSim aids the user by organizing the surface options into logical types, such as a burner to define a simple fire or a layered surface to represent a solid, heat conducting wall. The available surface types are described below.
This surface type is identical to the built-in ADIABATIC surface type. It allows you to customize the description, color, and texture of the adiabatic surface described in Section .
This surface type is identical to the built-in INERT surface type. It allows you to customize the description, color, and texture of the inert surface described in Section .
This surface type represents a fire with a known heat release rate[2] or mass (fuel) loss rate.
Parameters for burner fires are arranged in two groups: heat release and particle injection. Heat release options:
| Parameter | Description |
|---|---|
| Heat Release | |
| Heat Release Rate (HRR) | The heat release rate per unit area of this burner. |
| Mass Loss Rate | The mass loss rate per unit area of this burner. |
| Ramp-Up Time | At the beginning of the simulation, this surface will not be burning. This field allows you to describe how the heat release ramps up from ambient to the specified value. |
| Extinguishing Coefficient | This parameter governs the suppression of the fire by water. For more information, see section 10.7 of the FDS users guide. |
| Temperature | |
| Surface Temperature | The surface temperature of this burner. The value TMPA represents ambient temperature. |
| Convective Heat Flux | The heat flux per unit area at the surface. |
| Ramp-Up Time | This field allows you to describe how the temperature ramps up from ambient to the specified value. |
| Other | |
| Emissivity | This parameter controls how the surface radiates heat. Using a value of 1.0 makes this surface a black body. Lower values increate the amount of radiated heat. |
Particle injection options:
| Parameter | Description |
|---|---|
| Emit Particles | Enable this option to emit particles from the surface. |
| Particle Type | Select a particle to emit. To create a new particle, click the Edit Particles... button. |
| Number of Particles per Cell | Controls the number of particles inserted per second. A value of 1 will insert one particle per cell per second. |
| Mass Flux | For particles that have mass, this option provides an alternate way to control the number of particles inserted per second. |
This surface type represents a radiative heat source. The options are identical to the options for a burner without the heat release options. If the surface temperature is less than the ambient temperature, the surface will remove heat from the surrounding gases.
This surface represents a vent that injects air into the simulation domain. The parameters for supply surfaces are arranged in 4 groups: air flow, temperature, species injection, and particle injection.
Air flow options:
| Parameter | Description |
|---|---|
| Specify Velocity | Use a constant velocity to define air movement through the vent. |
| Specify Volume Flux | Use a constant volume flux to define air movement through the vent. |
| Specify Mass Flux | Use a constant mass flux to define air movement through the vent. |
| Specify ... Individual Species | Define air movement through the vent using a table of extra species and their mass fluxes. This method requires a model that includes extra (non-reactive) species. Flux data is specified on the Species Injection tab. |
| Tangential Velocity | The tangential velocity of the air flow. The first parameter is the velocity in the x or y direction and the second parameter is in the y or z direction, depending on the normal direction of the vent. An example of tangential velocity is shown in Figure 6.2. |
| Slip Factor | The slip factor affects the calculation of velocity at the walls. To specify a no-slip boundary condition, use -1. To specify a free slip boundary condition, use 1. Values between -1 and 1 represent partial slip conditions. |
| Ramp-Up Time | At the beginning of the simulation, vents with this surface will not be blowing. This parameter controls the time it takes to ramp the air flow up to the specified amount. |
| Wind Profile | The default wind profile is constant (Top Hat), to model wind conditions outdoors there are two additional options: parabolic and atmospheric. Parabolic produces wind with a parabolic profile whose maximum is the specified velocity. Atmospheric produces a wind profile of the form u=u0(z/z0)^p. |
| Atmospheric Profile Exponent | The term p in the atmospheric profile equation. This option is only available when atmospheric profile is selected. |
| Atmospheric Profile Origin | The term z0 in the atmospheric profile equation. This option is only available when atmospheric profile is selected. |
Figure 6.2. A simulation demonstrating affect of the normal axis on the direction of tangential velocity.
The temperature of the air injected by supply vents can be controlled using the following options:
| Parameter | Description |
|---|---|
| Surface Temperature | The temperature of the injected air. The value TMPA represents ambient temperature. |
| Convective Heat Flux | The heat flux per unit area at the surface. |
| Ramp-Up Time | This field allows you to describe how the temperature ramps up from ambient to the specified value. |
| Other | |
| Emissivity | This parameter is not used for supply surfaces. |
The species injection options are available if the Specify Mass Flux of Individual Species option in the Air Flow group is selected and there are extra, non-reactive species present in the simulation.
Particle injection options:
| Parameter | Description |
|---|---|
| Emit Particles | Enable this option to emit particles from the surface. |
| Particle Type | Select a particle to emit. To create a new particle, click the Edit Particles... button. |
| Number of Particles per Cell | Controls the number of particles inserted per second. A value of 1 will insert one particle per cell per second. |
| Mass Flux | For particles that have mass, this option provides an alternate way to control the number of particles inserted per second. |
Exhaust surfaces can be used to remove gas from the simulation domain. The specification of their air movement parameters is identical to that of a supply surface, but instead of the velocity or flux driving air into the domain, they are pulling air out.
A fan is a special type of surface that more accurately simulates a fan than a supply surface. Fans must be attached to a thin obstruction that separates two pressure zones. For more information about fans, please refer to section 8.3.2 of the FDS users guide.
Layered surfaces are composed of one or more material definitions. Materials include solid and liquid substances such as concrete, pine, and ethanol. For more information about materials and how they can be specified in PyroSim, please refer to Chapter 5. This type of surface is ideal for walls and other objects that are composed of real-world materials. This surface type can also be used to inject extra (non-reactive) species into the simulation.
Layered surfaces have five groups of options: material layers, surface props, reaction, species injection, and particle injection. The material layers group contains the following options:
| Parameter | Description |
|---|---|
| Initial Internal Temperature | The initial temperature within this surface. |
| Backing | The backing of a surface is the boundary condition behind the surface. The default value, Air Gap represents an air gap, Exposed will allow the surface to transfer heat into the space behind the wall, and Insulated prevents any heat loss from the back of the material. |
| Gap Temperature | The temperature of air in the air gap. This option is only available when the Air Gap backing type is selected. |
| Temperature Ramp | This field allows you to describe how the temperature ramps up from ambient to the specified value. |
| Material Layers | |
| Thickness | The thickness of this material layer. |
| Material Composition | Within a layer (row), you can specify multiple materials based on mass fraction. For example, to specify a layer that is just brick, type 1.0 BRICK (assuming you have created a material called BRICK). To specify a layer of wet brick, you could enter 0.95 BRICK; 0.05 WATER. Each material is separated by a semi-colon. |
| Edit | Click to specify the materials in this layer using an alternate table UI. |
The Surface Props tab contains the following options:
| Parameter | Description |
|---|---|
| Geometry | The effective geometry for heat transfer calculations. |
| Porous | The property is generally used to create a fan using a thin obstruction. |
| Enable Leakage | This option allows you to select two pressure zones for leakage across the surface. |
| Surface Density | The mass per unit area for the solid surface, calculated using the wall thickness. |
| Initial Internal Temperature | Starting temperature inside the solid. |
| Backing | Defines the boundary condition on the back (internal) surface of the solid face. |
| Gap Temperature | Temperature of air gap behind the solid face. |
| Temperature Ramp | Specifies the surface temperature ramp from ambient, to the specified surface temperature. |
The reaction used to model a given surface can either be taken from the material specifications, or given explicitly by the surface. Manually specifying the parameters will produce a surface similar to a burner. You can edit this behavior using the reaction options:
| Parameter | Description |
|---|---|
| Governed by Material | This surface's reaction will be controlled by the materials that it is constructed from. |
| Governed Manually | Override the default reaction behavior for this surface and specify the following parameters. |
| Heat Release Rate | The heat release rate per unit area of this surface. |
| Mass Loss Rate | The mass loss rate per unit area of this surface. |
| Ramp-Up Time | This field allows you to describe how the heat release ramps up from ambient to the specified value. |
| Extinguishing Coefficient | This parameter governs the suppression of the fire by water. For more information, see section 10.7 of the FDS users guide. |
| Burn Immediately | Select this option to create a surface that is initially burning. |
| Ignite at | Select this option to create a surface that will begin burning at a specified temperature. |
| Heat of Vaporization | Heat yield when this fuel is converted to gas. |
| Allow ... burn away | Surfaces of this type can be removed from the simulation after expending all available fuel. |
You can inject extra (non-reactive) species into the simulation using the species injection options. To use these options, you must first specify extra species using the Edit Extra Species dialog. You can edit the following extra species options:
| Parameter | Description |
|---|---|
| Inject by Mass Fraction | Select this option to specify species injection using mass fractions. |
| Inject by Mass Flux | Select this option to specify species injection using mass flux. |
| Mass Flux of Background Species | The background species (default=AIR) mass flux into the domain. |
| Background Species Ramp | This field allows you to describe how the injection rate ramps up from zero to the specified value. |
| Species | This value cannot be edited. It displays the name of one of the species selected in the Edit Extra Species dialog. |
| Mass Fraction | The mass fraction of an extra species to inject. This option is only available if Inject by Mass Fraction is selected. |
| Mass Flux | The mass flux of an extra species to inject. This option is only available if Inject by Mass Flux is selected. |
| Ramp-Up Type | This field allows you to describe the function used to ramp up the injection rate from zero to the specified value. |
| Ramp Value | The time it takes to achieve the specified injection rate. |
The particle injection parameters for layered surfaces are identical to those for burners.
You can add textures to surfaces to increase the realism of your model. Some default textures are provided or you can import your own. The Room Fire example demonstrates using a wood texture for a pine floor and hanging a picture on a wall. Your textures will be automatically displayed in PryoSim; to display textures in Smokeview, select Textures on the Show/Hide menu.
To define a texture:
On the Model menu, click Edit Surfaces....
Either create or edit the surface to which you want to add a texture.
Click on the Texture box.
Either select a pre-defined texture or click the Import... button and select your own image file.
The image you selected will be displayed. Under the image, click the Details tab. Set the Width and the Height values to correspond to size to be used in the PyroSim model.
Click OK to close the Textures dialog.
The textured surface can now be used in either obstructions or vents.