Fire dynamics and growth rate

Fire Growth Rate

• The fire growth rate is defined as the duration of growth that starts with ignition and lasts until all combustible materials in an enclosure are fully burned
• There are four distinct phases of fire growth:
  • Initial
  • Growth
  • Steady combustion
  • Decay

Fire Behavior

• Fire behavior is dependent on the type of fuel available.
• Rapid ignition occurs when the fuel is gaseous.
• Slow combustion arises when the fuel is solid material.
• Smoke detectors are crucial for alerting people in the building of a fire for evacuation purposes.
• Smoke detectors are important to increase the chance of saving lives.

Nature of Fire Stages

• Stage 1 Ignition & growth: The first stage of fire, starting from ignition.
• Stage 2 Development: Stage where pyrolysis rate, heat release rate and temperature will be incresed
• Stage 3 Full Fire: Stage where the fire is fully developed and steady
• Stage 4 Decay: Final stage of the fire

Initial Phase (Ignition)

• Fire behavior mainly depends on the types of fuel available in the building.
• Rapid ignition occurs if the fuel is in gaseous form.
• Slow combustion occurs if the fuel is a solid material, like timber.
• The fire behavior is influenced by the pyrolysis process.
• Only smoke and heat are released during this phase.
• The installation of a smoke detector is crucial to alert people in the building for timely evacuation, before the fire becomes serious and uncontrollable.

Fire Growth (Development)

• Pyrolysis rate, heat release rate (HRR), and temperature will increase.
• Smoke layer temperature and boundaries increase, leading radiated heat fluxes to ignite all fuels.
• Predicting flashover is important to establish the maximum available time for egress since flashover may occur

Factors Influencing the Duration of the Growth Period

• Spacing of combustible materials in the room
• Size and location of ignition sources
• Size and location of openings in the room
• Wind direction and velocity
• Shape and dimension of the room
• Amount and size of combustible materials in the room

Flashover

• Flashover occurs when the hot layer temperature reaches 500°C to 600°C.
• Flashover involves a transition from one combustible item burning to all combustible items burning.
• Before a flashover, fire growth is limited by the pyrolysis rate of fuels.
• After a flashover, the fire size is limited by the available ventilation and is pyrolyzed rather than being burnt using air supply.
• Flashover leads to high temperature and a high concentration of carbon monoxide and smoke, with a lack of oxygen, making survival impossible.
• Flashover doesn't always occur, for example, in a large firecell or a well-sealed firecell.

Preventing Flashover

• Proper ventilation can prevent a flashover.
• Venting allows superheated air and fuel-loaded fire gases to escape the room by horizontal or vertical ventilation.
• Possibility of flashover is reduced by cooling the fire area with a hose stream.

Fully Developed Fire (Full Fire)

• Steady-state combustion has a temperature range between 900°C and 1200°C.
• Duration of a fully developed fire depends on the amount of fire load, surface area, and available ventilation.
• Predicting fire load in a building will assist the engineer to:
  • Estimate the fire severity
  • Provide adequate and cost-effective fire protection
  • Help fire fighters identify the most vulnerable or dangerous area in a fire
  • Provide guidance on fire spread

Decay

• Fire stops completely when there is no more fuel.
• Transition to the decay stage is the time when 80% of the fuel has been consumed.
• Charcoal and dust remain as the final residue.
• This phase is still dangerous and fatal due to exposure to smoke and toxic gases.

Backdraft

• A backdraft is a smoke explosion that occurs when additional air is introduced into a smoldering fire, and heated gases enter their flammable range and ignite with explosive force.
• A backdraft is an air-driven event, unlike a flashover, which is temperature-driven.
• Backdrafts can occur anytime during the decay stage of fire development, if gases have cooled to below their ignition temperature.
• If the room has been left and the gases have cooled before any oxygen was introduced, no backdraft would occur, but the room would have been consumed by fire.

Untenable Condition

• An untenable condition includes:
  • Visibility issues
  • Elevated smoke layer temperature
  • High carbon monoxide concentration
  • Radiation heat flux