Free tool · runs in your browser
t² Fire Generator for FDS
Choose the fire growth rate, the peak heat release rate and the burner area. The generator works out the time to peak and the HRRPUA, plots the HRR curve and writes ready FDS lines — either with TAU_Q or with a &RAMP table.
How it is calculated
A t² fire grows as Q(t) = α · t², where α is the growth coefficient. It reaches the peak Q after t = √(Q/α) and then — in this generator — stays constant. The α values for slow, medium, fast and ultra-fast fires are the classic values from NFPA 204 and NFPA 72 (time to 1055 kW: 600, 300, 150 and 75 s).
In FDS this curve can be set in two ways. A negative TAU_Q on the &SURF line gives a t² curve that reaches full power after |TAU_Q| seconds and then stays constant (FDS User Guide, section 11.1). RAMP_Q with a &RAMP table gives full control — you can add a decay phase, ventilation limits or sprinkler activation. TAU_Q and RAMP_Q are mutually exclusive.
HRRPUA is the heat release rate per unit area: the peak HRR divided by the burner area. The burner area has to be represented on the mesh — with a small burner on a coarse mesh FDS snaps its dimensions to the cells and the power changes. Check it in the .out file or in the HRR chart.
Things to watch
- Simulation time. T_END should cover the time to peak and the period in which you assess conditions (e.g. evacuation or fire service arrival).
- Mesh resolution. Size the cell for the peak HRR — the FDS mesh size calculator gives D*/δx. More in the guide A t-squared fire in FDS: TAU_Q, RAMP_Q and checking the HRR.
- The result in the model. FDS writes the actual HRR to
_hrr.csv. Compare it with the design curve — differences usually come from the burner area on the mesh or from oxygen limitation.
Frequently asked questions
What does a negative TAU_Q mean in FDS?
A negative TAU_Q on the SURF line means the heat release rate grows as t² and reaches the full HRRPUA after |TAU_Q| seconds, then stays constant. A positive TAU_Q gives a tanh ramp-up.
Which α should I use?
It depends on the fuel load and the guidance used. As a rough guide (e.g. PD 7974-1): offices — medium, shops — fast, warehouses and industrial buildings — ultra-fast. Use the growth rate required by the guidance applied in your project.
How do I add a decay phase?
Use the &RAMP option and add points with decreasing F at the end (e.g. linearly to zero). The TAU_Q option does not allow decay.
Why does the HRR in FDS differ from the input?
Most often because the burner area on the mesh differs from the intended one (snapping to cells), or the fire is limited by the oxygen supply. Check the HRR column in _hrr.csv.
FDS and Smokeview are software of the National Institute of Standards and Technology; NIST is not affiliated with fdsrun.