Defining Fire

Fire can be described in many ways - here are a few:

NFPA 921: "A rapid oxidation process, which is a chemical reaction resulting in the evolution of light and heat in varying intensities."

Webster's Dictionary: "A fire is an exothermic chemical reaction that emits heat and light" Fire can also be explained in terms of the Fire Tetrahedron - a geometric representation of what is required for fire to exist, namely, fuel, an oxidizing agent, heat, and an uninhibited chemical reaction.

Combustion

Combustion is a chemical reaction which basically involves three substances: fuel, oxygen and combustion products. It is a fast oxidation process, the creation of combustion products out of oxygen and fuel, typically based on exothermic processes, but not exclusively.

In order to have a sustained fire, multiple criteria must be satisfied. They are illustrated by the fire tetrahedron as indicated in the following figure. Thus a fire needs, the fuel, an oxydant, here oxygen, which reacts with the fuel and converts chemically bound energy into heat, a supply of heat, which is needed to overcome the activation energy of the chemical reaction, and sufficiently high reaction rates to keep up a chain reaction.

Fire Triangle

Measuring Fire

Heat Energy is a form of energy characterized by vibration of molecules and capable of initiating and supporting chemical changes and changes of state (NFPA 921). In other words, it is the energy needed to change the temperature of an object - add heat, temperature increases; remove heat, temperature decreases. Heat energy is measured in units of Joules (J), however it can also be measured in Calories (1 Calorie = 4.184 J) and BTU's (1 BTU = 1055 J).

Temperature is a measure of the degree of molecular activity of a material compared to a reference point. Temperature is measured in degrees Farenheit (melting point of ice = 32 º F, boiling point of water = 212 º F) or degrees Celsius (melting point of ice = 0 º C, boiling point of water = 100 º C).

Fire Sources Creation in FRI3D

Based on the concepts above, FRI3D's user interface has been designed so as to be able for the user to enter those values to be picked up by the simulation.

Fire Sources are created in FRI3D from the Compartment Inspector by

Simulation Items -> New Fire Source

Fire Properties in FRI3D

FRI3D has fire sources with an assigned fire material. The fire source has the spatial information and the fire material defines properties such as the heat release rate as shown. A fire material can be assigned to multiple fire sources. These properties are used to assign fires and fire material specifications for the simulation (FDS/CFAST or other)

The fire source properties are created and edited by

Fire Source -> Properties & Reactions

The following sections list the properties of fire as modeled by FRI3D. The specifics of each term however is dependent on the exact simulation system used (ie: CFAST/FDS etc). The details pertaining to simulation specific are in subsequent sections.

Heat Release Rate

Heat Release Rate (HRR) is the rate at which fire releases energy - this is also known as power. HRR is measured in units of Watts (W), which is an International System unit equal to one Joule per second. Depending on the size of the fire, HRR is also measured in Kilowatts (equal to 1,000 Watts) or Megawatts (equal 1,000,000 Watts).

Typically, the heat release rate (heat energy evolving on a per unit time basis) of a fire 0 (kW) changes as the size of the fire changes, as a function of time t (seconds) after fire ignition.

There are four stages of fire development: ignition, growth, fully developed, and decay. The ignition stage is when all four elements of the fire tetrahedron coalesce, the fuels reach their ignition temperature, and the fire begins.

Heat of Combusion

During combustion, not only chemical bounds are rearranged, but heat is released. This is quantified as heat of combustion.

This is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat.

Radiative Fraction

Reaction (Stoichiometry C, N, H, Cl, O)

[C + H (fuel)] + [O2 + N2 (Air)] -> (Combustion Process) -> [CO2 + H2O + N2 (Heat)]

where

• C = Carbon
• H = Hydrogen
• O = Oxygen
• N = Nitrogen