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In this tutorial you will learn how to model a Radially Spreading Fire, both on a flat plane and among obstructions.

All of this will be accomplished without modeling the pyrolysis of the solid fuels directly.

This function is commonly used to emulate a realistic fire growth/spread.

It allows the user to mimic a t-squared fire growth rate in a more natural way.

Let’s first model a normal combustion on a flat plane using the VENT and SURF line:

`&VENT XB= 1, 9, 1, 9, 0, 0, SURF_ID='fire' / &SURF ID='fire', HRRPUA=500 /`

These lines tell FDS to model a fire on the ground (z, z’ = 0) that burns at 500 kW/m2.

The total heat released during the combustion is equal to the area of the vent times the HRRPUA value:

(8 m * 8 m) * 500 kW/m2 = 32000 kW

To change this phenomenon into a radially spreading fire we need to add 2 parameters on the VENT line:

• XYZ – which indicates the origin of the spreading fire (if unspecified, default is the center point of the VENT)
• SPREAD_RATE – which indicates the speed of the fire spread in m/s

`&VENT XB= 1, 9, 1, 9, 0, 0, SURF_ID='fire', XYZ= 4, 4, 0, SPREAD_RATE=0.1 / &SURF ID='fire', HRRPUA=500 /`

In this example , by setting XYZ= 2, 2, 0, and SPREAD_RATE=0.1, we instructed FDS to ignite in the x=4 and y=4 coordinates of the ground (z=0) and that the speed of the spread should be 0,1 m/s.

In this way we have emulated a t-squared fire growth in a natural and realistic way.

If we want to emulate also the descending part of the curve, we will also need to tell FDS how much time every cell of the vent should burn with the command RAMP_Q.

RAMP_Q parameter recap:
This function enable you to specify for each second of the simulation the multiplying factor “F”, which increases/decreases the HRRPUA value over time. By setting it to 0 and 1 you are able to switch on and off the combustion for each cell involved in the fire model.

`&VENT XB= 1, 9, 1, 9, 0, 0, SURF_ID='fire', XYZ= 4, 4, 0, SPREAD_RATE=0.1 / &SURF ID='fire', HRRPUA=500, RAMP_Q='fireramp' / &RAMP ID='fireramp', T= 0, F=0 / &RAMP ID='fireramp', T= 1, F=1 / &RAMP ID='fireramp', T=200, F=1 / &RAMP ID='fireramp', T=201, F=0 /`

In this example the descending part of the HRR curve will start at 200 s because the first cell lightened by the radially spreading fire will stop burning at 200 s.

Every cell of the vent will stop burning after 200 second from its first ignition.

This picture shows the Heat Release Rate over time measured by the “_hrr.csv” file generated by FDS:

`&OBST XB=0.8,2,0.8,2,0.4,0, SURF_ID='fire1' / &OBST XB=2.6,3.8,0.8,2,0.8,0, SURF_ID='fire1' / &OBST XB=4.4,5.6,0.8,2,1,0, SURF_ID='fire1' / &OBST XB=6.2,7.4,0.8,2,0.2,0, SURF_ID='fire1' / &OBST XB=8,9.2,0.8,2,0.6,0, SURF_ID='fire1' /&SURF ID='fire1', HRRPUA=250, SPREAD_RATE=0.1, XYZ=5,5,0 `/