Fire Behavior

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.

In other words, Fire Dynamics is the study of how fires start, spread and develop. But what exactly is a fire?

Defining Fire

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

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.

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).

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).

Heat Flux is the rate of heat energy transferred per surface unit area – kW/m².

Temperature vs. Heat Release Rate

One candle vs. ten candles – same flame temperature but 10 times the heat release rate!

HRR: ~ 80 W Temperature:
500 C – 1400 C
(930 F – 2500 F)

HRR: ~ 800 W

Heat Transfer

Heat transfer is a major factor in the ignition, growth, spread, decay and extinction of a fire.

It is important to note that heat is always transferred from the hotter object to the cooler object – heat energy transferred to and object increases the object’s temperature, and heat energy transferred from and object decreases the object’s temperature.

CONDUCTION

Conduction is heat transfer within solids or between contacting solids.

The governing equation for heat transfer by conduction is:

Where T is temperature (in Kelvin), A is the exposure area (meters squared), L is the depth of the solid (meters), and k is a constant that unique for different materials know as the thermal conductivity and has units of (Watts/meters*Kelvin).

Thermal Conductivity of Common Materials

CONVECTION

Convection is heat transfer by the movement of liquids or gasses.

The governing equation for heat transfer by convection is:

Where T is temperature (in Kelvin), A is the area of exposure (in meters squared), and h is a constant that is unique for different materials known as the convective heat transfer coefficient, with units of W/m²*K.

These values are found empirically, or, by experiment.

For free convection, values usually range between 5 and 25. But for forced convection, values can range anywhere from 10 to 500.

RADIATION

Radiation is heat transfer by electromagnetic waves.

The governing equation for heat transfer by radiation is:

Where T is temperature (in Kelvin), A is the area of exposure (in meters squared), α is the thermal diffusivity (a measure of how quickly a material will adjust it’s temperature to the surroundings, in meters squared per second) and ε is the emissivity (a measure of the ability of a materials surface to emit energy by radiation).

Fire Phenomena

Fire Development is a function of many factors including: fuel properties, fuel quantity, ventilation (natural or mechanical), compartment geometry (volume and ceiling height), location of fire, and ambient conditions (temperature, wind, etc).

Traditional Fire Development The Traditional Fire Development curve shows the time history of a fuel limited fire. In other words, the fire growth is not limited by a lack of oxygen. As more fuel becomes involved in the fire, the energy level continues to increase until all of the fuel available is burning (fully developed). Then as the fuel is burned away, the energy level begins to decay. The key is that oxygen is available to mix with the heated gases (fuel) to enable the completion of the fire triangle and the generation of energy.

Watch Windows: Traditional Fire Development in a Compartment Fire Mac: Traditional Fire Development in a Compartment Fire

Fire Behavior in a Structure The Fire Behavior in a Structure curve demonstrates the time history of a ventilation limited fire. In this case the fire starts in a structure which has the doors and windows closed. Early in the fire growth stage there is adequate oxygen to mix with the heated gases, which results in flaming combustion. As the oxygen level within the structure is depleted, the fire decays, the heat release from the fire decreases and as a result the temperature decreases. When a vent is opened, such as when the fire department enters a door, oxygen is introduced. The oxygen mixes with the heated gases in the structure and the energy level begins to increase. This change in ventilation can result in a rapid increase in fire growth potentially leading to a flashover (fully developed compartment fire) condition.

Watch Windows: Fire Behavior in a Structure (Ventilation limited) Mac: Fire Behavior in a Structure (Ventilation limited)

Flashover is the transition phase in the development of a contained fire in which surfaces exposed to the thermal radiation, from fire gases in excess of 600° C,

reach ignition temperature more or less simultaneously and fire spreads rapidly through the space.

This is the most dangerous stage of fire development.

Videos:

• Compartment Fire Flashover
• Flashover Compilation

Reports:

• Backdraft Phenomena
• Fatal Training Fires

Informational Source: The National Institute of Standards and Technology (NIST) is an agency of the U.S. Department of Commerce. (HERE)

Predictability of Performance: Its Occupancy Risk NOT Occupancy Type

Tactical Patience and the New Considerations of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction

Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction

For many of you that have been following my writings and perspectives on building construction, firefighting, command risk management and operational excellence for firefighter safety have long recognized that I have been promoting and advocating the fact the fireground is changining, our stratgies and tactics demand change adn does the demand for increased knowledge within the areas of building construction, fire dynamics, while integrating the art and science of firefighting. The most recent release of the testing report from Underwriters Laboratories; Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction and the accompaning emphirical data further validates assumptions and presmises that many of us shared based upon field obervations and first hand incident operations related to the dramatic changes being witnessed as a result of operational challenges in a wide varity of occupanies and building types.

This material is a must read for all emerging and practicing company and command officers ( for starters) to being grasping the magnitude and extent of quantifiable data that supports the premise that combat fire engagement and suppression operations and the rules of engagement are going to change and that change is fast approaching.

Considerations for Tactical Patience and Adaptive Fireground Management are continued themes I will expand upon in future postings….

Here’s the executive summary of the report and findings from UL. For an download of the entire UL Report, go HERE.

Under the United States Department of Homeland Security (DHS) Assistance to Firefighter Grant Program, Underwriters Laboratories examined fire service ventilation practices as well as the impact of changes in modern house geometries. There has been a steady change in the residential fire environment over the past several decades. These changes include larger homes, more open floor plans and volumes and increased synthetic fuel loads. This series of experiments examine this change in fire behavior and the impact on firefighter ventilation tactics. This fire research project developed the empirical data that is needed to quantify the fire behavior associated with these scenarios and result in immediately developing the necessary firefighting ventilation practices to reduce firefighter death and injury.

Two houses were constructed in the large fire facility of Underwriters Laboratories in Northbrook, IL. The first of two houses constructed was a one-story, 1200 ft², 3 bedroom, 1 bathroom house with 8 total rooms. The second house was a two-story 3200 ft², 4 bedroom, 2.5 bathroom house with 12 total rooms. The second house featured a modern open floor plan, two-story great room and open foyer. Fifteen experiments were conducted varying the ventilation locations and the number of ventilation openings. Ventilation scenarios included ventilating the front door only, opening the front door and a window near and remote from the seat of the fire, opening a window only and ventilating a higher opening in the two-story house. One scenario in each house was conducted in triplicate to examine repeatability.

The results of these experiments provide knowledge for the fire service for them to examine their thought processes, standard operating procedures and training content. Several tactical considerations were developed utilizing the data from the experiments to provide specific examples of changes that can be adopted based on a departments current strategies and tactics.

Under the United States Department of Homeland Security (DHS) Assistance to Firefighter Grant Program, Underwriters Laboratories examined fire service ventilation practices as well as the impact of changes in modern house geometries.

There has been a steady change in the residential fire environment over the past several decades. These changes include larger homes, more open floor plans and volumes and increased synthetic fuel loads. This series of experiments examine this change in fire behavior and the impact on firefighter ventilation tactics.

This fire research project developed the empirical data that is needed to quantify the fire behavior associated with these scenarios and result in immediately developing the necessary firefighting ventilation practices to reduce firefighter death and injury.

• Two houses were constructed in the large fire facility of Underwriters Laboratories in Northbrook, IL.
• The first of two houses constructed was a one-story, 1200 ft2, 3 bedroom, 1 bathroom house with 8 total rooms.
• The second house was a two-story 3200 ft2, 4 bedroom, and 2.5 bathroom house with 12 total rooms.
• The second house featured a modern open floor plan, two story great room and open foyer.

 Fifteen experiments were conducted varying the ventilation locations and the number of ventilation openings. Ventilation scenarios included ventilating the front door only, opening the front door and a window near and remote from the seat of the fire, opening a window only and ventilating a higher opening in the two-story house.

One scenario in each house was conducted in triplicate to examine repeatability. The results of these experiments provide knowledge for the fire service for them to examine their thought processes, standard operating procedures and training content. Several tactical considerations were developed utilizing the data from the experiments to provide specific examples of changes that can be adopted based on a departments current strategies and tactics.

The tactical considerations addressed include:

• Stages of fire development: The stages of fire development change when a fire becomes ventilation limited.
  • It is common with today’s fire environment to have a decay period prior to flashover which emphasizes the importance of ventilatio
• Forcing the front door is ventilation: Forcing entry has to be thought of as ventilation as well.
• • While forcing entry is necessary to fight the fire it must also trigger the thought that air is being fed to the fire and the clock is ticking before either the fire gets extinguished or it grows until an untenable condition exists jeopardizing the safety of everyone in the structure.
• No smoke showing: A common event during the experiments was that once the fire became ventilation limited the smoke being forced out of the gaps of the houses greatly diminished or stopped all together.
  • No some showing during size-up should increase awareness of the potential conditions inside.
• Coordination: If you add air to the fire and don’t apply water in the appropriate time frame the fire gets larger and safety decreases.
  • Examining the times to untenability gives the best case scenario of how coordinated the attack needs to be.
  • Taking the average time for every experiment from the time of ventilation to the time of the onset of firefighter untenability conditions yields 100 seconds for the one-story house and 200 seconds for the two-story house
  • In many of the experiments from the onset of firefighter untenability until flashover was less than 10 seconds.
  • These times should be treated as being very conservative. If a vent location already exists because the homeowner left a window or door open then the fire is going to respond faster to additional ventilation opening because the temperatures in the house are going to be higher.
  • Coordination of fire attack crew is essential for a positive outcome in today’s fire environment.
• Smoke tunneling and rapid air movement through the front door: Once the front door is opened attention should be given to the flow through the front door.
  • A rapid in rush of air or a tunneling effect could indicate a ventilation limited fire.
• Vent Enter Search (VES): During a VES operation, primary importance should be given to closing the door to the room.
  • This eliminates the impact of the open vent and increases tenability for potential occupants and firefighters while the smoke ventilates from the now isolated room.
• Flow paths: Every new ventilation opening provides a new flow path to the fire and vice versa.
  • This could create very dangerous conditions when there is a ventilation limited fire.
• Can you vent enough?: In the experiments where multiple ventilation locations were made it was not possible to create fuel limited fires.
  • The fire responded to all the additional air provided.
  • That means that even with a ventilation location open the fire is still ventilation limited and will respond just as fast or faster to any additional air.
  • It is more likely that the fire will respond faster because the already open ventilation location is allowing the fire to maintain a higher temperature than if everything was closed. In these cases rapid fire progression if highly probable and coordination of fire attack with ventilation is paramount.
• Impact of shut door on occupant tenability and firefighter tenability: Conditions in every experiment for the closed bedroom remained tenable for temperature and oxygen concentration thresholds.
  • This means that the act of closing a door between the occupant and the fire or a firefighter and the fire can increase the chance of survivability.
  • During firefighter operations if a firefighter is searching ahead of a hoseline or becomes separated from his crew and conditions deteriorate then a good choice of actions would be to get in a room with a closed door until the fire is knocked down or escape out of the room’s window with more time provided by the closed door
• Potential impact of open vent already on flashover time: All of these experiments were designed to examine the first ventilation actions by an arriving crew when there are no ventilation openings.
  • It is possible that the fire will fail a window prior to fire department arrival or that a door or window was left open by the occupant while exiting.
  • It is important to understand that an already open ventilation location is providing air to the fire, allowing it to sustain or grow.
• Pushing fire: There were no temperature spikes in any of the rooms, especially the rooms adjacent to the fire room when water was applied from the outside. It appears that in most cases the fire was slowed down by the water application and that external water application had no negative impacts to occupant survivability.
  • While the fog stream “pushed” steam along the flow path there was no fire “pushed”.
• No damage to surrounding rooms: Just as the fire triangle depicts, fire needs oxygen to burn.
  • A condition that existed in every experiment was that the fire (living room or family room) grew until oxygen was reduced below levels to sustain it.
  • This means that it decreased the oxygen in the entire house by lowering the oxygen in surrounding rooms and the more remote bedrooms until combustion was not possible.
  • In most cases surrounding rooms such as the dining room and kitchen had no fire in them even when the fire room was fully involved in flames and was ventilating out of the structure.

Online Training Program

In order to make the results of this study more user friendly for the fire service to examine, UL developed an online interactive training module that can be viewed by clicking here. The program includes a professionally narrated description of all of the experiments, their results and the tactical considerations. Experimental video is used and graphical data is explained in a way that brings science to the street level firefighter.

Comparison of Modern and Legacy Home Furnishings

An experiment was conducted with two side by side living room fires. The purpose was to gain knowledge on the difference between modern and legacy furnishings. The rooms measured 12 ft by 12 ft, with an 8 ft ceiling and had an 8 ft wide by 7 ft tall opening on the front wall. Both rooms contained similar amounts of like furnishings.

The modern room was lined with a layer of ½ inch painted gypsum board and the floor was covered with carpet and padding.

• The furnishings included a microfiber covered polyurethane foam filled sectional sofa, engineered wood coffee table, end table, television stand and book case.
• The sofa had a polyester throw placed on its right side. The end table had a lamp with polyester shade on top of it and a wicker basket inside it.
• The coffee table had six color magazines, a television remote and a synthetic plant on it.
• The television stand had a color magazine and a 37 inch flat panel television.
• The book case had two small plastic bins, two picture frames and two glass vases on it.
• The right rear corner of the room had a plastic toy bin, a plastic toy tub and four stuffed toys.
• The rear wall had polyester curtains hanging from a metal rod and the side walls had wood framed pictures hung on them.

The legacy room was lined with a layer of ½ inch painted cement board and the floor was covered with unfinished hardwood flooring.

• The furnishings included a cotton covered, cotton batting filled sectional sofa, solid wood coffee table, two end tables, and television stand.
• The sofa had a cotton throw placed on its right side.
• Both end tables had a lamp with polyester shade on top of them.
• The one on the left side of the sofa had two paperback books on it.
• A wicker basket was located on the floor in front of the right side of the sofa at the floor level.
• The coffee table had three hard-covered books, a television remote and a synthetic plant on it.
• The television stand had a 27 inch tube television.
• The right front corner of the room had a wood toy bin, and multiple wood toys.
• The rear wall had cotton curtains hanging from a metal rod and the side walls had wood framed pictures hung on them.

Both rooms were ignited by placing a lit stick candle on the right side of the sofa. The fires were allowed to grow until flashover. The modern room transitioned to flashover in 3 minutes and 30 seconds and the legacy room at 29 minutes and 30 seconds.

View the entire video, or you may also download the video:

• • Download high-resolution (718MB).
    High download time expected – 90 minutes (speed: DSL)
  • Download low-resolution (34MB).