According to the U.S. National Institute of Standards and Technology, “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. ” Stated more simply, fire dynamics is the study of how fires start, spread and develop.
As defined further below, by the Pennsylvania Office of the State Fire Commissioner, fire dynamics is “the detailed study of how chemistry, fire science, material science, and the engineering disciplines of fluid mechanics and heat transfer, interact to influence fire behavior.”
In practical terms, fire dynamics can provide a firefighter with the knowledge to understand how a fire will grow and spread within a structure and how best to control that growth.
Every fire is different. Yet from a science perspective, most fires share some basic similarities.
There’s much that can be known about fire dynamics, and a foundational place to start is to understand the basic definitions and language of fire dynamics.
FIRE DYNAMICS TERMINOLOGY
BACKDRAFT: A deflagration resulting from the sudden introduction of air into a confined space containing oxygen-deficient products of incomplete combustion
BI-DIRECTIONAL FLOW / NEUTRAL PLANE: Where smoke/hot gases (HIGHER PRESSURE) are exhausting out of the top of the vent opening and it is drawing a current of cooler/fresh air (LOWER PRESSURE) in through the bottom of the opening which creates a neutral plane.
CONDUCTION: The transfer of heat from the direct contact of a solid of higher temperature to a solid of lower temperature
CONVECTION: The transfer of heat through the movement of hot smoke and gases to solid surfaces of lower temperature
COMBUSTIBLE: Capable of burning, generally in air under normal conditions of ambient temperature and pressure, unless otherwise specified. Combustion can occur in cases where an oxidizer other than oxygen in air is present (e.g., chlorine, fluorine, or chemicals containing oxygen in their structure).
DOOR CONTROL: The process of ensuring the entrance door providing access to the fire area is controlled and closed as much as possible after teams enter the structure. Steps must be taken to prevent the door from locking behind the entering members. By controlling the door, we are controlling the flow path of the fire conditions from the high pressure of the fire area towards the low-pressure area on the other side of the door. Door control also limits fire development by controlling the flow path of fresh air at the lower level of the open door towards the seat of the fire.
DICERS-VO: a fire extinguishment model utilizing interior tactics to extinguish fires
- Detect the location of the fire
- Isolate the fire area
- Confine the fire
- Extinguish the fire
- Rescue occupants
- Search of the fire area and adjoining spaces
- Ventilation coordinated with fire attack
- Overhaul of the fire area for extension
FIRE DYNAMICS: The detailed study of how chemistry, fire science, material science, and the engineering disciplines of fluid mechanics and heat transfer, interact to influence fire behavior.
- INCIPIENT: The early stage of fire development where the fire’s progression is limited to a fuel source and the thermal hazard is localized to the area of burning material.
- GROWTH: The stage of fire development when the heat release rate from an incipient fire has increased to the point where heat transferred from the fire and the combustion products are pyrolyzing adjacent fuel sources. The fire begins to spread across the ceiling of the fire compartment (rollover).
- FULLY DEVELOPED STAGE: Fire development, within a compartment, has reached its peak heat release rate. This usually occurs after flashover, resulting in floor to ceiling burning within the compartment, creating heat conditions untenable for members.
- DECAY STAGE: The stage of fire development within a structure characterized by either a decrease in the fuel load or available oxygen to support combustion, resulting in lower temperatures and lower pressure in the fire area.
FLAMEOVER (ROLLOVER): The condition in which unburned fuel (pyrolysate) from the originating fire has accumulated in the ceiling layer to a sufficient concentration (i.e., at or above the lower flammable limit) that it ignites and burns. Flameover can occur without ignition of or prior to the ignition of other fuels separate from the origin.
FLASHOVER: A transition in the development of a compartment fire when surfaces exposed to thermal radiation from fire gases reach ignition temperature simultaneously. This causes the fire to spread rapidly throughout the space, resulting in fire involvement of the entire compartment.
FLOW PATH: The movement of heat and smoke from the fire area (HIGHER PRESSURE) to towards the lower pressure areas accessible via doors and window openings. Based on varying building configurations, there may be several flow paths within a structure. Operations conducted in the flow path, between the fire and where the fire wants to go, will place members at significant risk due to the increased flow of fire, heat and smoke toward their position.
FLOW PATH CONTROL: The tactic of controlling or closing ventilation points which will:
- Limit additional oxygen into the space, thereby: limiting fire development, heat release rate and smoke production.
- Control the movement of the heat and smoke conditions out of the fire area to the exterior and to the other areas within the building
FUEL LIMITED FIRE: A fire in which the heat release rate and fire growth are controlled by the quantity and characteristics of the fuel
FUEL LOAD: The total quantity of combustible contents of a building, space, or fire area, including interior finish and trim, expressed in terms of heat release rate. Fires that involve hydrocarbon and synthetic-based contents such as foam plastics, polyesters, polypropylenes, and nylons have a relatively high heat release rate when compared to natural fiber products. Structure fires fueled with common household furnishings composed of synthetic materials have been shown to increase in size and heat release rate due to increased ventilation and the in-flow of additional oxygen.
FUEL RICH: An environment with high heat, smoke and products of combustion but not enough oxygen to sustain combustion
HEAT RELEASE RATE: The rate at which energy is generated by the burning of a fuel and oxygen mixture. As the heat release rate increases, the heat, smoke production and pressure within the area will increase and spread along available flow paths toward low-pressure areas (open doors, windows and roof openings).
NEUTRAL PLANE: The interface at a Bi-Directional Flow where the hot smoke and gases are moving out of the compartment over the top of cooler air, flowing moving into the compartment
PATH OF LEAST RESISTANCE: The path that smoke and heat travel as they move from HIGHER PRESSURE to LOWER PRESSURE
PYROLYSIS: The breakdown of fuels such as wood and foam plastics, into their basic compound when subjected to heat. Fuels will continue to off-gas (produce pyrolysate) and add to the flammable fuel load within the compartment as long as the material is subjected to elevated temperatures.
RADIATION: The transmission of heat energy by electromagnetic waves through the space between a body at a higher temperature to a body at a lower temperature
TRANSITIONAL ATTACK / HITTING IT HARD FROM THE YARD / SOFTEN THE TARGET: The tactic of applying water on the fire, from the exterior, as quickly as possible – regardless of where it is emitting from – to cool the fire and improve conditions in the interior and then transitioning to an interior fire attack
SITUATIONAL AWARENESS: The ongoing activity of assessing what is going on around you during the complex and dynamic environment of a fire incident. Operations will be more effective and safer by continually observing your surroundings, communicating conditions to other members and monitoring radio transmissions.
SIZE-UP: The process of gathering and analyzing information that will influence decisions fire officers make regarding the deployment of resources and the implementation of tactics.
SMOKE: The combination of airborne solid particulates, liquid particulates and gases emitted when a material undergoes pyrolysis or combustion. Smoke is a heated fuel source. Smoke is a toxic mixture that contains numerous poisonous gases such as carbon monoxide, hydrogen cyanide and phosgene.
SLICERS: Structural Fire Tactical Goals reduce temperatures inside a building prior to entry by fire personnel for extinguishment or rescue
- Size Up
- Locate the Fire
- Identify and Control the Flow Path
- Cool the Space from the Safest Location
- Extinguish the Fire
UNI-DIRECTIONAL FLOW: A vent opening with only one way of smoke and hot gases venting outward or fresh air being drawn into the vent opening
VENTILATION: The controlled and coordinated removal of heat and smoke from a structure, replacing the escaping gases with fresh air. This exchange is bi-directional with heat and smoke exhausting at the top and air flowing in towards the fire at the bottom of horizontal vents. The fire will pull the additional airflow into the building towards the fire which can increase the fire size and heat release rate. This exchange can occur by opening doors, windows or roof structures. Coordinated and controlled ventilation will facilitate quicker extinguishment and limit fire spread.
VENT-ENTER-ISOLATE-SEARCH (VEIS): A search tactic where you enter a window or door to search for the location of fire or to locate victims where the priority upon entering via a window is to close the door to that room or area to isolate that area being searched from the fire area (Flow Path Control). When entering a fire area through a door, the door needs to be closed (Door Control) until the fire area is further isolated or a charged hose line is advancing on the fire. By isolating the area, we are controlling the flow path of the fire, heat and smoke towards the ventilation point as well as controlling the airflow from the ventilation point towards the fire.
VENTILATION-CONTROLLED/LIMITED FIRE: A fire with limited ventilation in which the heat release rate or growth is controlled by the amount of oxygen available to the fire.
VENTILATION INDUCED FLASHOVER (VIF): A flashover initiated by the introduction of oxygen into a pre-heated, fuel-rich (smoke-filled), oxygen-deficient area. This phenomenon has become more prevalent in well-insulted homes. Synthetic fuel loads rapidly consume more of the available oxygen within the fire area and produce fuel-rich combustion products creating conditions favorable to a possible ventilation-induced flashover.
VENTILATION PROFILE: The appearance of the fire building’s ventilation points showing the flow paths of heat and smoke out of the structure as well as any air movement into the structure.
VENTILATION TACTICS: The coordinated and controlled opening of ventilation points in a structure to facilitate fire operations.