Fire science and fire dynamics
Domain 1 tests quantitative reasoning about a developing fire: burning rate, growth, flashover, smoke movement and what a chosen model can and cannot represent. Expect scenario questions rather than definitions.
Combustion basics
Burning needs fuel, oxidant, heat and the chain reaction. The design-relevant properties are LFL/UFL, flash point, auto-ignition temperature and heat of combustion. A pool fire's steady burning rate per unit area is set by radiative feedback from the flame to the fuel surface, which is why bund diameter and fuel type matter more than the ignition source.
Growth and the t-squared fire
Pre-flashover growth is modelled as Q = αt² with α ≈ 0.0029 (slow), 0.0117 (medium), 0.047 (fast) and 0.1876 kW/s² (ultrafast). Growth is fuel-controlled until demand exceeds the air that openings or mechanical ventilation can supply; the fire then becomes ventilation-controlled and the HRR plateaus, at roughly 1500·A√H kW for a single opening of area A and height H.
Flashover versus backdraught
Flashover is a transition inside the compartment: upper-layer gas around 550–600 °C, radiant flux at floor level near 20 kW/m², flames from the opening. Backdraught is an oxygen-starved compartment admitting air, giving a deflagration in the inflow path with a pressure pulse. They are different phenomena leaving different evidence.
Smoke movement
Plume, then ceiling jet, then a descending layer. The neutral plane sits where internal and external pressures balance; leakage area, opening height and extract rate all move it. Hot smoke expands, roughly threefold in volume from ambient to 600 °C, which drives leakage spread through cracks, shafts and service penetrations.
Modelling choices
Zone models (CFAST, OZone) assume uniform upper and lower layers: fast, good for single compartments, weak for large or multi-room geometries. CFD (FDS) resolves the field but demands grid-independence checks and defensible boundary conditions.
Worked example
A medium-growth office fire is adopted and the design fire is 2.5 MW. From Q = αt², t = √(2500/0.0117) ≈ 462 s, about 7.7 minutes to reach design size. That time becomes the input to the smoke-filling calculation or to the start of sprinkler-controlled decay, so an error here propagates into every downstream tenability result.
Traps
Calling a backdraught a flashover; quoting a fuel-controlled HRR for a fire that is plainly under-ventilated; treating α as a physical constant rather than a statistical envelope; ignoring radiative feedback when a hot layer sits close to the fuel bed.