Fire Simulation for Engineers/FDS/Lagrangian Particles

Lagrangian particles

Third, some properties are set up, as the properties of Lagrangian particles (PART).

Defining Lagrangian particles, PART

Lagrangian particles are used in FDS as water or liquid fuel droplets, flow tracers, and various other objects that are not defined or confined by the numerical mesh. Sometimes the particles have mass, sometimes they do not. Some evaporate, absorb radiation, etc. PART is the namelist group that is used to prescribe parameters associated with Lagrangian particles.

All Lagrangian particles must be explicitly defined via the PART namelist group.

Once a particular type of particle or droplet has been described using a PART line, then the name of that particle or droplet type is invoked elsewhere in the input file via the parameter PART_ID.

There are no reserved PART_ID, all must be defined. For example, an input file may have several PART lines that include the properties of different types of Lagrangian particles:

 &PART ID='my tracer', MASSLESS=.TRUE. /

Then these Lagrangian particles can be introduced in the fluid flow from a solid surface via a boundary condition as explained in Section [sec:Injecting-Lagrangian-particles].

The following table summarizes some PART parameters:

Parameter	Type	Description	Unit	Default
ID	String	Identifier
MASSLESS	Logical	Massless particles		.FALSE.
WATER	Logical	Water droplets		.FALSE.
AGE	Real	Droplet lifetime	s	100000
COLOR	String	Color
RGB(3)	Integer	Color
DT_INSERT	Real	Time between insertions	s	0.01
XB(6)	Real	Volume, initial particle location	m

Massless particles

The simplest use of Lagrangian particles is for visualization, in which case the particles are considered massless tracers. In this case, the particles are defined via the line:

&PART ID='my tracer', MASSLESS=.TRUE. /

Water droplets

WATER=.TRUE. declares that the liquid droplets evaporate into WATER VAPOR, a separate gas phase specie that is automatically added to the calculation by this command. By default, WATER=.FALSE., even though the default properties of droplets are that of water. Setting WATER=.TRUE. instructs FDS to add WATER VAPOR as an explicitly defined specie, and it also invokes appropriate constants related to the absorption of thermal radiation by the water droplets. It also causes the droplets to be colored blue in Smokeview.

For example:

 &PART ID='droplets', WATER=.TRUE. /

When a droplet strikes a solid surface, it sticks and is reassigned a new speed and direction. If the surface is horizontal, the direction is randomly chosen. If vertical, the direction is downwards.