Introduction to Fire Dynamics

Fire dynamics is the study of how chemistry, fire science, material science, and the mechanical engineering disciplines of fluid mechanics and heat transfer interact to influence fire behavior. Understanding how fires develop through their various stages is crucial for fire safety, prevention, and firefighting strategies.

The Four Stages of Fire Development

Select Fuel Type:

Select Ventilation Condition:

Oxygen is readily available, allowing the fire to reach its full potential heat release rate.

Heat Release Rate Over Time

Time (minutes)

Heat Release Rate (kW)

Fire Visualization

Ignition/Incipient Stage

The incipient stage begins when all four elements of the fire tetrahedron coalesce, the fuels reach their ignition temperature, and the fire begins. This stage is characterized by:

Small flame size
Localized to initial fuel source
Low heat release rate
Minimal smoke production
Temperatures near ambient in most of the compartment
Oxygen levels normal (around 21%)

This is the crucial stage for fire suppression as it is easiest to control and causes the least damage.

Growth Stage

During the growth stage, the fire increases fuel consumption and creates more heat and smoke. Key characteristics include:

Increasing flame size
Fire spreading to adjacent fuels
Rapidly increasing heat release rate
Increasing smoke production
Rising temperatures
Oxygen levels beginning to decrease
Potential for flashover at the end of this stage

Flashover: A transitional phase where surfaces exposed to thermal radiation reach ignition temperature simultaneously, resulting in rapid fire spread throughout the space. Temperatures can rise to 1,000°F in seconds, creating a life-threatening environment.

Fully Developed Stage

A fully developed fire is the hardest to suppress because the fire is at maximum temperatures and causing the most heat damage. This stage is characterized by:

Maximum heat release rate
Maximum temperatures (can reach 1000°C/1832°F)
Oxygen-limited burning (ventilation-controlled)
Potential for backdraft if new oxygen is introduced
Significant thermal radiation
Structural damage begins

At this stage, the maximum amount of fuel and oxidizers are consumed, producing the highest heat release rate.

Decay Stage

The decay stage is when the fire decreases in intensity until it is either a smolder or non-existent. This phase is characterized by:

Decreasing heat release rate
Decreasing temperatures
Reduced flame size
Increased smoldering
Oxygen levels begin to recover
Potential for re-ignition if new fuel or oxygen is introduced

The decay occurs when the available fuel has been consumed. Different fuels exhibit different decay patterns: hydrocarbon and liquid fuels typically show fast decay, while charring cellulosic fuels (wood) demonstrate slower decay.

Factors Affecting Fire Growth and Decay

Ventilation Conditions

The availability of oxygen significantly impacts fire behavior:

Well-Ventilated Fires: With abundant oxygen, fires are fuel-controlled, meaning the heat release rate is determined primarily by the amount and type of fuel available. These fires burn more efficiently with less smoke production.
Limited-Ventilation Fires: As oxygen becomes restricted, fires transition to ventilation-controlled burning. The heat release rate becomes limited by available oxygen rather than fuel. This can extend the duration of the fire.
Poorly-Ventilated Fires: With severely restricted oxygen, combustion becomes incomplete, leading to increased smoke production, lower temperatures, and potential for dangerous conditions like backdraft if fresh oxygen is suddenly introduced.

Fuel Characteristics

Different materials burn differently, affecting the fire growth curve:

Upholstered Furniture: Modern upholstered furniture often contains polyurethane foam which burns rapidly with high heat release rates (up to 3MW for a single couch). These materials tend to have fast growth rates and can reach peak HRR in 3-5 minutes.
Wood and Cellulosic Materials: Wood-based materials typically have a slower growth rate but can sustain burning for longer periods. They form a char layer that can slow the burning process, resulting in a more gradual decay phase.
Workstations and Electronics: Office workstations combine various materials (plastics, wood, fabrics) and can produce complex burning patterns with multiple peaks in the heat release rate curve as different components become involved.

Compartment Geometry

The size and configuration of the space affects fire development:

Ceiling Height: Higher ceilings allow more smoke accumulation before descending to occupant level, potentially delaying flashover.
Room Size: Larger rooms require more heat to reach flashover conditions.
Opening Factors: The size and position of doors, windows, and other openings determine ventilation patterns and can create localized areas of increased burning.

Thermal Properties of Surrounding Materials

The materials making up walls, ceilings, and floors affect heat transfer:

Thermal Inertia: Materials with low thermal inertia (like gypsum board) heat up quickly at the surface but don't conduct heat well, while high thermal inertia materials (like concrete) absorb more heat but heat up more slowly.
Thermal Feedback: As compartment surfaces heat up, they radiate heat back to the fuel, potentially accelerating fire growth.

Mathematical Models of Fire Growth

T-Squared Fire Growth Model

The t-squared model is commonly used to describe fire growth, where the heat release rate (Q) is proportional to the square of time (t):

Q = α × t²

Where:

Q is the heat release rate (kW)
α is the fire growth coefficient (kW/s²)
t is the time (s)

Fire Growth Coefficients (α):

Slow: 0.003 kW/s² (e.g., thick solid wood products)
Medium: 0.012 kW/s² (e.g., traditional mattresses, wood pallets)
Fast: 0.047 kW/s² (e.g., upholstered furniture, thin plywood)
Ultrafast: 0.19 kW/s² (e.g., flammable liquids, some synthetic materials)

Select Fire Growth Rate:

Heat Transfer in Fires

Heat transfer is a major factor in the ignition, growth, spread, decay, and extinction of a fire. Heat is always transferred from the hotter object to the cooler object.

Conduction

Heat transfer within solids or between contacting solids.

q = kA(T_Hot - T_Cold)/L

Convection

Heat transfer through the movement of fluids (liquids or gases). This is the primary mechanism for heat transfer in the early stages of a fire as hot gases rise and transfer heat to upper surfaces.

Radiation

Heat transfer through electromagnetic waves, requiring no medium. Radiation becomes increasingly important as the fire grows, and is the dominant heat transfer mechanism during flashover and the fully developed stage.