This is the fourth in a series of posts dealing with flashover, to review prior posts see:

• Understanding Flashover: Myths and Misconceptions
• Understanding Flashover: Myths and Misconceptions: Part 2
• The Ventilation Paradox

As previously discussed flashover requires sufficient heat release rate for the temperature of fuel packages within a compartment to increase sufficiently to ignite and the fire to rapidly transition to the fully developed stage. However, during fire development in a compartment the fire often becomes ventilation controlled, with fire growth and heat release rate limited by the available air supply. In some cases, the fire generates sufficient heat release rate despite being ventilation controlled. In others, there is insufficient oxygen in the air supplied for the fire to reach flashover (unless ventilation is increased). All of this is fairly simple and straightforward if we are examining fire in a single compartment. This simple explanation of flashover is based on fire development in a single compartment, such as that described in the ISO 9705 Fire Tests-Full Scale Room Fire Tests for Surface Products6American Society for Testing and Materials (ASTM) Standard E 603-6 (Figure 1)

Figure 1. Full Scale (Six Sided) Room Fire Test Compartment

Note: Underwriters Laboratory (UL) fire test photo adapted from Fire Behavior in Single Family Dwellings, [PowerPoint Presentation], National Fire Academy.

Things get a bit more complex when a fire occurs in a multi-compartment building as individual compartments are interconnected smoke and flames may extend from compartment to compartment throughout the building.

Ventilation and Air Track

Contrary to the common fire service definition of ventilation as “[planned and] systematic removal of heated air, smoke, and fire gases and replacing them with cooler air (IFSTA, 2008), ventilation is simply the exchange of the atmosphere inside the building with that which is outside. This process is ongoing under normal, non-fire conditions. However, under fire conditions, ventilation also involves movement of smoke and air between compartments as well as discharge of smoke from the building and intake of air from outside the structure.

Remember! If you can see smoke coming from the building, ventilation is occurring (but not necessarily the type or amount of ventilation that you need to effectively control the fire environment and the fire).

The term air track is used to describe the characteristics of air and smoke movement (e.g., direction, velocity). The movement of both air and smoke are important, but the direction and path of smoke movement is particularly significant for several reasons:

• Smoke is fuel
• Hot smoke has energy

Through convection, smoke carries energy away from the fire compartment and transfers this energy to objects having lower temperature (such as other fuel packages or firefighters working inside the building). The rate of heat transfer is substantially dependent on temperature difference and in the case of convection on the velocity of the hot gases. Higher velocity and turbulence results in a higher rate of convective heat transfer (much the same as the increase in wind chill as wind speed increases in a cold environment).

Air Track on a Single Level

Examination of air track on a single level provides a simple way to illustrate the influence of air track on the movement of smoke (think fuel and energy) from compartment to compartment, fire extension, and multi-compartment flashover.

With no significant ventilation (with the exception of slight building leakage) smoke will fill the fire compartment and extend through openings such as doorways to adjacent compartments (see Figure 2). If insufficient oxygen is available from the air within the compartments the fire will become ventilation controlled and growth may slow and the fire may decay (heat release rate lessens)

Figure 2. Limited Ventilation

Note: Unless the building is tightly sealed, there is likely to be some leakage resulting in smoke discharge and inward movement of air.

If an opening is made in the presently uninvolved compartment, smoke will move from the fire to the opening, exiting out the upper area of the opening while cool air moves inward through the bottom of the opening and towards the fire (see Figure 3). This is a bi-directional air track.

Figure 3: Single Opening with Bi-Directional Air Track

As pointed out in The Myth of the Self-Vented Fire and The Ventilation Paradox, providing additional oxygen to a ventilation controlled fire results in increased heat release rate and may result in ventilation induced flashover. However, it is important to consider how this impacts adjacent compartments as well.

Increased heat release rate in a still ventilation controlled fire results in higher hot gas layer temperatures and increased smoke production. Increasing temperature and volume of the hot gas layer will cause it to lower and velocity to increase as the smoke moves through adjacent compartments and out ventilation openings. This increases both radiant and convective heat transfer and potentially speeds progression to flashover in adjacent compartments.

Horizontal tactical ventilation can be accomplished rapidly and may, under some conditions, be a useful approach to improving interior conditions. Increasing the number and size of horizontal openings can raise the level of the hot gas layer (by providing additional exhaust). However, when dealing with a ventilation controlled fire the increased oxygen supplied to the fire will increase heat release rate. In addition, in the absence of wind or application of positive pressure at the entry point, two openings at the same level will result in a bi-directional air track at both openings as illustrated in Figure 4.

Figure 4. Two Openings with a Bi-Directional Air Track

If heat release rate is sufficient, this may result in vent induced flashover in the compartments between the fire and the exhaust openings as illustrated in the following video clip.

Important! Horizontal ventilation is not a bad tactic. However, it is essential to recognize and manage the air track as well as ensuring that ventilation is coordinated with fire attack.

More to Follow

Examination of the flashover phenomenon will continue with a case study involving a 1999 fire in a Washington, DC townhouse that resulted in the line of duty deaths of two firefighters. This incident is particularly important as it is one of the first times that the National Institute of Standards and Technology (NIST) Fire Dynamics Simulator (FDS) and Smokeview were used for forensic fire scene reconstruction. This data, in conjunction with the District of Columbia Fire and EMS Reconstruction Report and National Institute for Occupational Safety and Health (NIOSH) Death in the Line of Duty Report provides a solid basis for understanding the impact of burning regime and air track in multi-compartment, ventilation induced flashover.

Ed Hartin, MS, EFO, MIFireE, CFO

References

International Fire Service Training Association (IFSTA). (2008). Essentials of firefighting (5^th ed.). Stillwater, OK: Fire Protection Publications.

Reading the Fire Heat Indicators briefly examined energy, temperature, and heat in thermodynamic systems, and introduced the two major categories of heat related fire behavior indicators: those that we can see (visual) and others that can be felt (tactile) as illustrated in Figure 1.

Figure 1. Basic Heat Indicator Categories

Heat Indicators Part 2 elaborated on tactile effects. This post will examine visual effects and provide an expanded heat indicators concept map.

Remember that as with each of the B-SAHF (building, smoke, air track, heat, and flame) indicators; it is essential that assessment of heat is integrated with other elements of the B-SAHF scheme to gain a clearer sense of fire conditions and likely fire behavior.

Visual Effects

Air track often provides an early heat indicator. Observation of turbulent smoke pushing from the building at high velocity is a reliable indicator of a tremendous amount of heat energy and high temperatures inside the structure.

Figure 2. Air Track as a Heat Indicator

Note: Photo by Terry Moody, Ranlo NC.

Some visual indicators can be observed from the exterior such as bubbling paint, melting or softening roofing material, crazing glass, and condensation of pyrolyzate on windows. High temperature (and in some cases, not so high temperature) can have a more dramatic effect (see Figure 3). This vinyl frame window failed due to heat resulting from a fully developed fire inside the compartment.

Figure 3. Temperature Effects on Building Materials

Note: Photo by Ed Hartin

Fire stream effects such as evaporation of water from a hot surface (such as a door) or lack of return from a temperature check (brief application of water fog into the hot gas layer to check overhead temperature) also provide an indication of temperature (Figure 4)

Figure 4. Checking the Door and Temperature Check

Note: Photo by John McDonough

A thermal imaging camera (TIC) provides a highly effective means for visualizing temperature differences (see Figure 5). Use of a TIC should begin on the exterior and continue during interior operations.

Figure 5. Thermal Image

Note: Photo provided by Stefan Svensson

Despite the tremendous advantage provided by use of a TIC, it is essential to be mindful of the limitations of this technology. Thermal images only identify the temperature differences that are in direct line-of-sight. Insulating materials such as compartment linings can prevent the TIC from identifying fire conditions outside the compartment (e.g., floor below, ceiling void). In tests of floor and roof assemblies conducted by the Underwriters Laboratory (UL), thermal imaging cameras were unable to detect a fully developed fire below a typical wood floor (see Figure 6). The floor in Figure 6 consisted of an engineered system of I-joists, sub-floor, finished floor, carpet padding, and carpet. The floor system was being tested over a 14′ x 17′ (4.27 m x 5.18 m) gas fired furnace in accordance with ASTM E119. While the temperature indicated by the TIC is 80.1o F (27^o C), the highest temperature measured by thermocouples on the structural members was in excess of 1341o F (727^o C). It is essential to integrate thermal image data with direct visual observations to obtain a more complete picture of temperature conditions.

Figure 6. UL Floor Systems Test

Adapted from Underwriters Laboratory Structural Stability of Engineered Lumber in Fire Conditions [on-line training program]

The tests of engineered lumber systems and training program developed by UL provide excellent information on performance of these structural materials under fire conditions. This program as well as on-line training on fire behavior in single family dwellings and fire modeling are available free of charge from UL University.

Technological advancements also include temperature sensing integrated into breathing apparatus, personal alert safety systems (PASS), and even protective clothing (see Figure 7) to assist firefighters in recognizing dangerous elevated temperatures and in some cases telemetry to transmit this information to others, outside the hazardous environment. This could be viewed as a visual indicator (based on visual display of the information) or as augmentation of tactile indicators.

Figure 7. Smart Clothing by Viking Industries

Work in Progress

Hopefully we have been working on this project together and you have been developing or refining the air track segment of your fire behavior indicators concept map. My current map is illustrated in Figure 8.

Figure 8. Heat Indicators Concept Map v5.2.2.1

You can also download a printer friendly version of the Heat Indicators Concept Map v5.2.2.1 As always, should you have any suggestions or feedback, please post a comment!

Ed Hartin, MS, EFO, MIFireE, CFO

References

Underwriters Laboratory. (2009). Structural Stability of Engineered Lumber in Fire Conditions [on-line training program]. Retrieved August 27, 2009 from http://www.ul.com/global/eng/pages/offerings/industries/buildingmaterials/fire/courses/structural/

As discussed in prior Reading the Fire posts and the ongoing series examining fire behavior indicators (FBI) using the B-SAHF (Building, Smoke, Air Track, Heat, and Flame) organizing scheme, developing proficiency requires practice. This post provides an opportunity to exercise your skills using three video segments shot during a commercial fire. In addition to practicing your skill in reading the fire, use these video clips to help develop or refine your smoke indicators concept map (see Reading the Fire: Smoke Indicators).

Commercial Fire

The Lake Station (IN) Fire Department was dispatched to a reported structure fire in the vicinity of the American Legion Hall on Central Avenue. Responding companies found a commercial building with fire and smoke showing at the intersection of Central Avenue and Howard Street.

Download and the B-SAHF Worksheet.

While the video clip of this incident does not allow you to walk around the building and observe fire conditions, Google maps street view allows you to view all sides of the building. If you haven’t used street view, have a look at the following Google Street View Tutorial.

Perform a “walkaround” by clicking on the following link to view the building involved at this incident: 1691 Central Ave, Lake Station, IN. Note: Radio communication in the video clip identifies the Incident Commander as “Howard Command”. However, for this activity, I have identified Central Avenue as the A Side of the involved building. Click on the arrows to move east on Central Avenue and move and adjust the compass rose to look at Side D. Move back along Central Avenue and then go down Howard Street, again adjusting the compass rose to look at Sides B and C. After your “walk around”, complete the Building Factors segment of the B-SAHF Worksheet.

The video clip of this incident begins with the view of Side B from the A/B Corner prior to the arrival of the first engine company. Watch the first 60 seconds of Video Segment 1. Consider the information provided in this segment of the video clip. First, describe what you observe in terms of the Building (add to what you have done so far), Smoke, Air Track, Heat, and Flame Indicators and then answer the following five standard questions:

1. What additional information would you like to have? How could you obtain it?
2. What stage(s) of development is the fire likely to be in (incipient, growth, fully developed, or decay)?
3. What burning regime is the fire in (fuel controlled or ventilation controlled)?
4. What conditions would you expect to find inside this building?
5. How would you expect the fire to develop over the next two to three minutes?

Watch the next three minutes of the video and identify if, and how conditions change from the beginning of the clip until the first line is placed in operation (at approximately 04:00).

Watch the next 2 minutes 30 seconds until the firefighters make entry through the door on Side A (at approximately 06:30).

1. What conditions would you expect to find inside this part of the building?
2. How would you expect the fire to develop over the next two to three minutes?

Watch the remainder of the video clip.

Important: While not related to Reading the Fire, you likely heard the Personal Alert Safety System (PASS) device sounding through much of the incident. While PASS devices can (and often are) accidentally activated, continuous sounding of a PASS indicates a firefighter in distress. While this was not the case in this incident, failure to silence PASS devices that are accidentally activated desensitizes firefighters to this important audible signal.

Remember the Past

August 1994 saw the loss of two company officers and a firefighter in three separate incidents involving extreme fire behavior. Rapidly changing fire conditions are a threat to firefighters working in career staffed, urban fire departments and volunteer departments serving small communities.

August 7, 1994
Captain Wayne Smith
Fire Department of the City of New York, New York

On August 7, Captain Wayne Smith of the New York City Fire Department was critically injured while conducting search and rescue operations on an upper floor of a building when he was trapped by high heat and heavy smoke conditions. Captain Smith was burned over 40 percent of his body and received severe smoke inhalation injuries to his lungs. He died on October 4 from his injuries. Fourteen other firefighters were injured in the blaze. Initial operations were hampered by a faulty fire hydrant across the street from the building.

August 8, 1994
Sergeant Craig Drury
Highview Fire District, Kentucky

On August 8, Sergeant Craig Drury of the Highview (KY) Fire District was caught in a flashover while making entry into a single story house. Sgt. Drury suffered severe burns to his lungs that eventually caused his death. The fire was started by an arsonist.

August 27, 1994
Firefighter Paul MacMurray
Hudson Falls Volunteer Fire Department, New York

On August 27, Firefighter Paul MacMurray of the Hudson Falls (NY) Volunteer Fire Department responded as part of an engine company to a fire on the first floor of in a three story hotel. Assigned to search for and rescue occupants on the second floor, MacMurray and another firefighter successfully evacuated several victims while attempts to extinguish the fire were initiated below them. Upon their return to continue the search, conditions quickly changed from a light haze of smoke to black smoke with high heat conditions. MacMurray and his partner became separated in their attempt to locate the stairwell and get out of the building. The other firefighter made several efforts to locate MacMurray, but was forced to retreat due to untenable conditions. Several rescue efforts were made but heavy fire conditions eventually forced the evacuation of all fire personnel to defensive positions as the entire structure burned. MacMurray’s body was recovered the following day. The fire was of incendiary origin.

Ed Hartin, MS, EFO, MIFIreE, CFO

Reading the Fire Heat Indicators briefly examined energy, temperature, and heat in thermodynamic systems, and introduced the two major categories of heat related fire behavior indicators: those that we can see (visual) and others that can be felt (tactile) as illustrated in Figure 1.

Figure 1. Basic Heat Indicator Categories

As with each of the B-SAHF (building, smoke, air track, heat, and flame) indicators, it is essential that assessment of heat is integrated with other elements of the B-SAHF scheme to gain a clearer sense of fire conditions and likely fire behavior.

The Thermal Environment

The thermal environment that firefighters encounter can be complex, but involves one or more of the following scenarios (Bryner, Madrzykowski, & Stroup, 2005):

• Immersion in a relatively static layer of hot gases (i.e., crawling or crouching in a room full of hot combustion products and smoke)
• Contact with a moving layer of hot gases (i.e., entry through a door or moving down a hallway with a strong air track)
• Exposure to radiant heat (i.e., working in proximity to flames or below a layer of hot gases)

Figure 2 illustrates the variations in temperature that firefighters may encounter during operations in a highly compartmentalized, multi-level structure. It is important to note that temperature varies from compartment to compartment and at different levels within each compartment.

Figure 2. Smokeview Slice

Note: Adapted from National Institute of Standards and Technology (NIST) Visualization techniques, Slice animation of a townhouse kitchen fire.

Firefighters’ personal protective equipment insulates them from the thermal environment. This layer of insulation makes it difficult to accurately assess temperature and heat flux (amount of heat transfer) that they are exposed to during firefighting operations. The thermal insulation provided by personal protective equipment slows, but does not stop heat transfer from the fire environment to the firefighter. Thermal exposure is dependent on gas temperature and radiant heat flux (heat transfer due to radiation)./

Thermal exposure can be divided into four categories: Ordinary, Hazardous, Extreme, and Critical (Foster & Roberts, 1995; Donnelly, Davis, Lawson, J., Selpak, 2006). As illustrated in Figure 3.

Figure 3. Thermal Exposure Limits in the Firefighting Environment

Note: Adapted from Measurements of the firefighting environment. Central Fire Brigades Advisory Council Research Report 61/1994 by J.A. Foster & G.V. Roberts, 1995. London: Department for Communities and Local Government and Thermal Environment for Electronic Equipment Used by First Responders by M.K. Donnelly, W.D. Davis, J.R. Lawson, & M.J. Selepak, 2006, Gaithersburg, MD: National Institute of Standards and Technology.

Several challenges confront firefighters in assessing the thermal environment during firefighting operations. These include:

• Of necessity, firefighters are insulated from their environment, delaying tactile perception of changes in temperature and heat flux.
• Perception of temperature is influenced by a wide range of factors and varies considerably from individual to individual.
• Firefighters focused on the task at hand may not notice subtle changes in temperature and heat flux.
• Temperature and heat flux do not always present obvious visual indicators.
• Conditions can change extremely rapidly, particularly as the fire approaches flashover.
• Firefighters may ignore warning signs of worsening conditions, believing that it is part of the job to tolerate extreme conditions.

Firefighters must have a sound understanding of the thermal environment encountered during firefighting operations and the, at times, subtle indicators of changing thermal conditions.

Tactile Effects

Tactile effects include sensing temperature or temperature change. Firefighters may sense temperature and changes in temperature, but as noted earlier, this is limited by the extent of thermal protection provided by their protective clothing and focus on the task at hand. Firefighters’ protective clothing effectively insulates them from the thermal hazards typically encountered in firefighting. The multiple layers of insulation in the protective ensemble slows (but does not stop) heat transfer. This time lag makes it difficult for the firefighter to appreciate their thermal exposure (Bryner, Madrzykowski, & Stroup, 2005).

Firefighter’s personal alert safety system (PASS) devices may be equipped with a temperature sensing function that provides warning at a specified exposure value when the specified temperature is exceeded for a specified time period (Figure 4). However, National Fire Protection Association 1982 Standard on Personal Alert Safety Systems (PASS) (NFPA, 2007) does not address thermal sensing and there is not standardized test protocol for these types of devices (Bryner, Madrzykowski, & Stroup, 2005). Thermal sensing devices use a temperature response curve to provide warning for long duration exposure to lower temperature and short duration exposure to higher temperature. However, during rapid increases in temperature such as those encountered in flashover or other forms of rapid fire development, adequate early warning to permit egress is unlikely due to limited sensitivity of the sensors (Bryner, Madrzykowski, & Stroup, 2005). While firefighters must be attentive to heat level and temperature change, it is often difficult to perceive these changes quickly enough to react to rapidly developing fire conditions. This reinforces the importance of integrating all the fire behavior indicators in your ongoing size-up and dynamic risk assessment.

Figure 4. PASS Device Temperature Sensor

Next Steps

The next post will conclude this look at Heat Indicators with examination of visual effects. While temperature and heat transfer cannot be observed directly, there are a number of ways in which firefighters can see the effects of temperature and heat.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Bryner, N., Madrzykowski, D., Stroup, D. (2005). Performance of thermal exposure sensors in personal alert safety system (PASS) devices, NISTR 7294. Retrieved August 19, 2009 from http://www.fire.nist.gov/bfrlpubs/NIST_IR_7294.pdf.

Donnelly, M., Davis, W., Lawson, J., & Selpak, M. (2006). Thermal environment for electronic equipment used by first responders, NIST Technical Note 1474. Retrieved August 19, 2009 from http://www.fire.nist.gov/bfrlpubs/fire06/PDF/f06001.pdf

National Institute of Standards and Technology (NIST) Visualization techniques, Slice animation of a townhouse kitchen fire, [digital video file]. Retrieved August 19, 2009 from http://www.fire.nist.gov/fds4/refs/thouse3/thouse3_slice.avi

I originally intended to write this post about the influence of air track on flashover in multiple compartments. However, after several conversations in the last week about the bathtub analogy and ventilation induced flashover, I had a change in plans.

The Bathtub Analogy

In Understanding Flashover: Myths and Misconceptions, I presented the bathtub analogy (Kennedy & Kennedy , 2003)as a simplified way of understanding how flashover occurs when a compartment fire is burning in a fuel controlled regime.

Flashover has been analogously compared to the filling of a bathtub with the drain open. In this practical, though not perfect, analogy water represents the heat energy. The quantity of water available is the total heat of combustion of the available fuels (fuel load). The size of the spigot and the water pressure control the amount of water flow that is the heat release rate. The volume of the bathtub is analogous to the volume of the compartment and its ability to contain the heat energy. The size and location of the bathtub drain controlling the rate of water loss is the loss of heat energy through venting and conductance. In this analogy, if the bathtub becomes full and overflows, flashover occurs. (Kennedy & Kennedy, 2003, p. 7)

Figure 1. The Bathtub Analogy-Fuel Controlled Burning Regime

Note: Adapted from Flashover and fire analysis: A discussion of the practical use of flashover in fire investigation, p. 7, by Patrick Kennedy & Kathryn Kennedy, 2003. Sarasota, FL: Kennedy and Associates, Inc.

All Models are Wrong

While the bathtub model provides a simple explanation and makes it easy to understand how flashover might occur, it is inaccurate. However, as Box and Draper (1987) stated: “Essentially, all models are wrong, but some are useful” p. 424).

Models or analogies provide a way of understanding based on simplification. This is useful, but this simplification, while providing a starting point for understanding can overlook important concepts or elements of a complex system. In the case of the bathtub analogy, simplification overlooks the criticality of oxygen to the combustion process.

Ventilation is the exchange of the atmosphere inside a compartment with that which is outside. This process is necessary and ongoing in any space designed for human habitation. In a compartment fire, ventilation involves the exhaust of smoke and intake of air from outside the compartment.  Note that this is different than tactical ventilation, which is the planned and systematic removal of hot smoke and fire gases and their replacement with fresh air. However, both normal and tactical ventilation involve exhaust of the compartment atmosphere and replacement with fresh air.

While the bathtub analogy is simple, and provides a useful starting point, it fails to address the air side of the ventilation equation. As ventilation is increased, the compartment looses energy through convection. However, if the fire is ventilation controlled (heat release rate (HRR)is limited by the available oxygen), increased ventilation will also increase HRR.

Revised Bathtub Analogy

For many years, firefighters have been taught tactical ventilation prevents or slows progression to flashover. Somewhat less commonly, firefighters have been taught to close the door to the fire compartment, limiting inward air flow and slowing fire growth (tactical anti-ventilation). My friend and colleague Inspector John McDonough of the New South Wales (AU) Fire Brigades refers to this as the Ventilation Paradox. Increased ventilation increases the HRR required for flashover to occur and may prevent or slow progression to flashover or it may (and often does) result in flashover. Reduction in ventilation may prevent or slow progress to flashover, but also reduces the HRR required for flashover to occur and (less commonly) may result in flashover. It depends! Not the answer that firefighters want to hear.

Making the bathtub analogy a bit more complex may provide a starting point for understanding the ventilation paradox. At the root of this apparent paradox is the impact of ventilation on the thermodynamic system and the relationship between oxygen and release of energy from fuel (Thornton’s Rule). See Fuel and Ventilation [LINK) for more information on Thornton’s Rule and the relationship between oxygen, fuel, and energy.

As illustrated in Figure 2, the revised bathtub analogy incorporates several changes. The inlet pipe has been enlarged (making it larger than the drain) and valves have been added to both the inlet and drain pipes. Most importantly, control of the valves is interconnected (but this is not shown visually as it makes the drawing even more complicated). Changing the position of either the inlet or drain, results in a corresponding change in the other valve.

Figure 2. Revised Bathtub Analogy-Ventilation Controlled Burning Regime

This analogy provides a reasonable (but still overly simplified and thus somewhat inaccurate) representation of a ventilation controlled compartment fire when normal building openings (e.g., doors, windows) serve as ventilation openings.

As illustrated in Figure 2, opening the drain also results in an increase in flow from the (larger) inlet, which without intervention is likely to result in the tub overflowing. In a compartment fire, increasing ventilation to a when the fire is burning in a ventilation controlled regime, increases convective heat loss, but HRR will also increase, potentially resulting in flashover.

Resolving the Paradox

Resolution of the problems presented by the paradox involve recognition of what burning regime the fire is in (fuel or ventilation controlled), understanding the influence of the location and size of ventilation openings on convective heat loss, understanding the influence of increased air intake on HRR, and coordination of ventilation and fire control tactics. On the surface, this all sounds quite simple, but is considerably more complex in practice.

Feedback

I would like to thank my friend and colleague Lieutenant Chris Baird, Gresham Fire & Emergency Services and my wife Sue for serving as my sounding board as I worked through the process of revising the bathtub analogy. As always your feedback and suggestions will be greatly appreciated.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Box, G.& Draper, N. (1987). Empirical Model-Building and Response Surfaces, San Francisco: Wiley & Sons.

Kennedy, P. & Kennedy, K. (2003). Flashover and fire analysis: A discussion of the practical use of flashover in fire investigation. Retrieved July 30, 2009 from http://www.kennedy-fire.com/Flashover.pdf

In Reading the Fire: How to Improve Your Skills, I discussed building a concept map of fire behavior indicators as a method to increase competence in reading the fire. Construction of a concept map increases awareness of key indicators and understanding their interrelationships. I am working through this process along with you, with the latest revision to my concept map. Thus far, I have examined Building Factors, Smoke Indicators, and Air Track Indicators, the first three categories in the B-SAHF (Building, Smoke, Air Track, Heat, and Flame) organizing scheme. For review of the discussion of the work done so far, see the following Reading the Fire posts:

• Building Factors
• Building Factors Part 2
• Building Factors Part 3
• Smoke Indicators
• Smoke Indicators Part 2
• Air Track Indicators
• Air Track Indicators Part 2

Focus Question

The process of developing or refining a concept map identifying fire behavior indicators (FBI) and their interrelationships starts with the following focus question:

What building, smoke, air track, heat, and flame indicators
provide clues to current and potential fire behavior?

As you develop the heat indicators concept map it is likely that you will uncover potential additions to the Building Factors, Smoke, or Air Track Indicators concept maps. You may also identify interrelationships that you may not have thought of previously. Don’t forget to go back and capture these thoughts by adding them to your other maps or placing them in a staging area for further consideration.

Heat and Temperature

Firefighters, like everyone else, have a commonsense understanding of heat and temperature. This is likely where many of our challenges in really understanding thermodynamics begins. The way in which we use the concepts of heat and temperature on a daily basis are likely to be considerably different than they are used in science.

Thermodynamics is a branch of physics that describes processes that involve changes in temperature, transformation of energy, and the relationships between heat and work. Fires and firefighting also involves changes in temperature, transformation of energy, heat and work. “Thermodynamics, like much of the rest of science, takes terms with an everyday meaning and sharpens them – some would say, hijacks them – so that they take on an exact an unambiguous meaning” (Atkins, 2007, p. 3).

Thermodynamics deals with systems. A thermodynamic system is one that interacts and exchanges energy with the area around it. A system could be as simple as a block of metal or as complex as a compartment fire. Outside the system are its surroundings. The system and its surroundings comprise the universe. For example we might consider a burning fuel package as the system and the compartment as the surroundings. On a larger scale we might consider the building containing the fire as the system and the exterior environment as the surroundings.

Figure 1. Thermodynamic Systems

In a compartment fire, energy is exchanged within the thermodynamic system and between the system and its surroundings.

Energy is the ability to do mechanical work or transfer thermal energy from one object to another. Energy can only be measured on the basis of its effects. There are basically two kinds of energy, kinetic and potential. Potential energy is that which is stored and may be released at a later time. The chemical energy contained in fuel that can be released during combustion is one example of potential energy. Kinetic energy is associated with motion of an object. Movement of molecules when heated during combustion is a good example of kinetic energy. Temperature is a measure of average kinetic energy.

The word flow is often used in discussing heat transfer (e.g., energy flows from materials with higher temperature to those with lower temperature). This helps visualize patterns of movement, but it is important to remember that neither energy nor heat is a fluid. Heat is the process of energy transfer due to temperature differences.

It is important to remember that we cannot see energy, temperature, or heat. However, we can see and feel the impact of increases in temperature as a result of heat (energy transfer). Use of the word heat to describe this category of indicators is appropriate as these indicators are all related to transfer of energy within and out of the compartment fires thermodynamic system.

Getting Started

When reading the fire it is important not to focus on a single indicator or category of indicators. In the case of heat, there are many interrelationships with air track and flame indicators. In some cases, it is arguable whether an indicator belongs in one category or the other (likely it is not important as long as you recognize the interrelationships).

As always in developing a concept map it is important to move from general concepts to those that are more specific. Heat Indicators can be divided into two basic categories, those that you can see (visual effects) and those that you can feel (tactile effects) as illustrated in Figure 2. However, you may choose to approach this somewhat differently.

Figure 2. Basic Heat Indicators

Developing the Detail

Expanding the map requires identification of additional detail for each of the fundamental concepts. If an idea appears to be obviously related to one of the concepts already on the map, go ahead and add it. If you are unsure of where it might go, but it seems important, list it off to the side in a staging area for possible additions. Download a printer friendly version of Heat Indicators to use as a starting point for this process.

Next Steps

Remember that the process of contracting your own map is likely as important as the (never quite) finished product. The following steps may help you expand and refine the building factors segment of the map:

• Look at each of the subcategories individually and brainstorm additional detail. This works best if you collaborate with others.
• Have a look at the following video clip using your partially completed map and notes as a guide to identifying important heat indicators. While this video clip is of conditions inside a compartment, also think about how this fire would present if viewed from the exterior.

The following video clip illustrates a recreation of the Station Night Club Fire that occurred in Rhode Island in 2003 that was conducted at the National Institute of Standards and Technology (NIST) laboratory in Gaithersburg, MD

The next post in this series will discuss visual and tactile heat indicators in greater depth and examine the increasing influence of technology in our perception (and misperception) of developing fire conditions.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Atkins, P. (2000). Four laws that drive the universe. Oxford, UK: Oxford University Press

Just over a year ago I had the idea to develop a blog focused on compartment fire behavior and firefighting. A bit of work on the technology side and I made my introductory post on 8 August 2008. That month the CFBT-US web site had 2900 page views, this past July the page view count was in excess of 24,000 with 4400 unique readers. While this is not a huge readership in terms of the total number of firefighters in the world who have English as a language, it shows significant growth.

Accomplishments

At the start of this adventure, I set a goal to post twice weekly (Monday and Thursday mornings) and for the most part have managed to keep this schedule. Dominant themes have included:

• Reviews of books, training programs, magazine/journal articles, and conference presentations
• Case studies based on National Institute for Occupational Safety and Health (NIOSH) and agency reports on significant incidents, injuries, and fatalities
• An ongoing series of posts examining the B-SAHF (building, smoke, air track, heat, and flame) organizing scheme for fire behavior indicators and reading the fire
• B-SAHF video and photo based exercises in reading and interpreting B-SAHF indicators to predict likely fire behavior and the impact of tactical operations
• Examination of extreme fire behavior phenomena such as flashover, backdraft, smoke explosion, and flash fire with an emphasis on understanding the underlying causes and influence of tactical operations on fire dynamics
• Discussion of research on positive pressure ventilation and wind driven fires conducted by the National Institute for Standards and Technology
• Identification of the potential learning opportunity presented by systematic investigation of near miss, injury, and fatality incidents
• Discussion of the importance of deliberate practice and the concept of the need for 10,000 hours to master your craft

Hopefully you have found these posts useful in developing your understanding of compartment fire behavior or have motivated you to take action and share your knowledge of our profession with others. I have benefited greatly from the thought process and effort of writing on a regular and systematic basis.

As a reference, I have prepared a printer friendly Compartment Fire Behavior Blog Index in portable document format (PDF) which includes the date, title, URL, and brief synopsis of post content.

I Need Your Help

Your comments and feedback are important to making the Compartment Fire Behavior Blog better. If I write something that you do not agree with or think that a concept could be expressed more clearly, please comment or question!

The Way Forward

I am currently working on a loose editorial calendar to help guide my writing over the next year. Several important themes will continue:

• Case studies and lessons learned
• Reading the fire and B-SAHF exercises
• Practical fire dynamics
• Review of books, magazine/journal articles
• Fire control and tactical ventilation

If there are topics you think should be on the list, please provide your input as a comment on this post.

My next several posts will get back to study of the B-SAHF scheme with a look at Heat Indicators and continuing examination of flashover. As I have been looking back over the last year, I find that I have taken two distinctly different approaches to sequencing posts. Some topics have been addressed in successive posts (e.g., case studies and discussion of wind driven fires) and others have alternated between several different topics (e.g., B-SAHF and flashover). From my perspective, each has its advantages and disadvantages. If you have a preference or opinion, please let me know!

Thanks for your readership and participation,

Ed Hartin, MS, EFO, MIFireE, CFO

A Quick Review

The first post in this series, Understanding Flashover: Myths & Misconceptions provided a definition of flashover and examined this extreme fire behavior phenomenon in the context of fire development in a compartment.

Flashover is the sudden transition to fully developed fire. This phenomenon involves a rapid transition to a state of total surface involvement of all combustible material within the compartment….Flashover may occur as the fire develops in a compartment or additional air is provided to a ventilation-controlled fire (that has insufficient fuel in the gas phase and/or temperature to backdraft).

Burning Regime

In the incipient and early growth stages of a compartment fire, the speed of fire growth is fuel controlled as fire development substantially influenced by the chemical and physical characteristics of the fuel. However, oxygen is required for the fuel to burn and release thermal energy. As a compartment fire develops, the available air supply for combustion becomes a more important factor. Increased combustion requires more oxygen and as smoke fills the compartment while the lowering neutral plane at compartment openings restricts the introduction of air into the compartment (see Figure 1).

The neutral plane is the level at a compartment opening where the difference in pressure exerted by expansion and buoyancy of hot smoke flowing out of the opening and the inward pressure of cooler, ambient temperature air flowing in through the opening is equal (Karlsson & Quintiere, 2000).

Figure 1. Lowering Neutral Plane

Note: Photos adapted from National Institute of Standards and Technology (NIST) ISO-Room/Living Room Flashover.

The distinction between fuel controlled and ventilation controlled is critical to understanding compartment fire behavior. Compartment fires are generally fuel controlled while in the incipient and early growth stage and again as the fire decays and the demand for oxygen is reduced (see Figure 2).

Figure 3. Fire Development with Limited Ventilation

While a fire is fuel controlled, the rate of heat release and speed of development is limited by fuel characteristics as air within the compartment and the existing ventilation profile provide sufficient oxygen for fire development. However, as the fire grows the demand for oxygen increases, and at some point (based on the vent profile) will exceed what is available. At this point the fire transitions to ventilation control. As illustrated in Figure 1, a ventilation controlled fire may reach flashover, all that is necessary is that sufficient oxygen be available for the fire to achieve a sufficient heat release rate for flashover to occur.

Heat Release and Oxygen

Combustion, as an oxidation reaction requires sufficient oxygen to react with the available fuel. Heat of combustion (energy released) and oxygen required for complete combustion are directly related (Thornton, 1917).The energy released per gram of oxygen consumed during complete combustion of natural and synthetic organic fuels is fairly consistent, averaging 13.1 kJ/g (±0.5%) (Huggett, 1980).

Release of chemical potential energy from fuel is dependent on availability of adequate oxygen for the combustion reaction to occur. Interestingly, while the heat of combustion of various types of organic (carbon based) fuel varies widely, the amount of oxygen required for release of a given amount of energy remains remarkably consistent.

In the early 1900s, British scientist W.M. Thornton (1917) discovered that the amount of oxygen required per unit of energy released from many common hydrocarbons and hydrocarbon derivatives is fairly constant. In the 1970’s, researchers at the National Bureau of Standards independently discovered the same thing and extended this work to include many other types of organic materials and examined both complete and incomplete combustion (Huggett, 1980; Parker, 1977).

Each kilogram of oxygen used in the combustion of common organic materials results in release of 13.1 MJ of energy. This is referred to as Thornton’s Rule. See Fuel and Ventilation for a more detailed discussion of the application of Thornton’s Rule to compartment fires and ventilation.

Failure to Reach Flashover

Ventilation controlled compartment fires may reach flashover and fully developed compartment fires are generally ventilation controlled (IAAI, 2009). However, lack of ventilation may prevent a compartment fire from generating sufficient heat release rate to reach flashover. In some cases, ventilation controlled fires to not become fully developed, but decay and self-extinguish due to lack of oxygen.

In late 2007 an engine and truck company responded to a report of an odor of smoke in a three-story, wood-frame, apartment building. They discovered a ground floor apartment was smoke logged. They requested a first alarm assignment, forced entry, and initiated fire attack and primary search. Smoke was cool and to the floor, the fire was confined to an upholstered chair and miscellaneous items in the living room and at the time of entry was simply smoldering (see Figure 3). A rapid search discovered a deceased occupant in a bedroom remote from the fire.

Figure 3. Self-Extinguished Compartment Fire

Note: Gresham Fire & Emergency Services Photo

While a fire involving an upholstered chair typically results in sufficient heat release rate for the fire to extend to other nearby fuel packages and ultimately reach flashover, this fire did not as evidenced by the condition of the Christmas tree on the opposite side of the living room from the point of origin (see Figure 4). The Christmas tree, like many other fuel packages in the apartment showed evidence of pyrolysis, but did not ignite.

Figure 4. Condition of Other Fuel Packages

Note: Gresham Fire & Emergency Services Photo

Why didn’t this fire reach flashover? The fire occurred in early winter and the apartment’s energy efficient windows and doors were tightly closed. The developing fire consumed the oxygen available within the apartment and absent significant ventilation, decayed, and the temperature inside the apartment which had been increasing as the fire developed, dropped to a temperature slightly higher than would normally be expected inside an occupied apartment.

How might the development of this fire been different if it had been discovered earlier? What if a neighbor had opened a door or window in an effort to rescue the occupant? What if the fire department had opened the door without recognizing that the fire was significantly ventilation controlled?

When fire development is limited by the ventilation profile of the compartment, changes in ventilation will directly influence fire behavior. Reducing ventilation (i.e. by closing a door) will reduce the rate of heat release and slow fire development. Increasing ventilation (i.e. by opening a door or window) will increase the rate of heat release and speed fire development. Changes in ventilation profile may be fire caused (failure of glass in a window), occupants (leaving a door open), or tactical action by firefighters; but all will have an influence on fire behavior!

Figure 5. Ventilation Induced Flashover

For many years firefighters have been taught that ventilation reduces the potential for flashover. While this is sometimes true, it is only part of the story. Increasing ventilation to a fuel controlled fire will allow hot gases to exit, transferring thermal energy out of the compartment and replacing the hot gases with cooler air (which increases heat release rate). The combined influence of these two factors slows progression towards flashover and increases the heat release rate required to reach flashover. The bathtub analogy presented in Understanding Flashover: Myths and Misconceptions [LINK], does not apply in this case, because when a fire is ventilation controlled, heat release rate is limited by the available oxygen. Under ventilation controlled conditions; increasing air supply by creating opening results in increased heat release rate. This increased heat release rate may result in flashover (see Figure 5). For more information see Hazards of Ventilation Controlled Fires.

Two Paths to Flashover

With adequate fuel and oxygen, a growth stage compartment fire may flashover and rapidly transition to the fully developed stage. Given adequate fuel, but lacking adequate oxygen (due to limited ventilation), a growth stage compartment fire may begin to decay before becoming fully developed. However, this can quickly change if ventilation is increased, potentially resulting in ventilation induced flashover.

Understanding these two paths to flashover is essential, but still does not provide a complete picture of the flashover phenomena. The next post in this series will will use several case studies to examine the influence of air track on flashover in multiple compartments the threat that rapid fire progression presents to firefighters.

Ed Hartin, MS, EFO, MIFIreE, CFO

References

Karlsson, B. & Quintiere, J. (2000). Enclosure fire dynamics. New York: CRC Press.

National Institute of Standards and Technology. (2005). ISO-room/living room flashover [digital video disk]. Gaithersburg, MD: Author.

Thornton, W. (1917). The relation of oxygen to the heat of combustion of organic compounds. The Philosophical Magazine,33(6), 196-203.

Parker, W. (1977). An investigation of the Fire Environment in the ASTM E 84 Tunnel Test, NBS Technical Note945. Gaithersburg, MD: U.S. Department of Commerce/National Bureau of Standards.

International Association of Arson Investigators (IAAI). (2009). Post flashover fires. On-Line Training Program, Downloaded August 6, 2009 from http://www.cfitrainer.net.

Air track includes factors related to the movement of smoke out of the compartment or building and the movement of air into the fire. Air track is caused by pressure differentials inside and outside the compartment and by gravity current (differences in density between the hot smoke and cooler air). Air track indicators include velocity, turbulence, direction, and movement of the hot gas layer.

My prior post, Reading the Fire: Air Track Indicators began the process of developing or refining an existing concept map of air track indicators. It is important to evaluate air track at openings and on the interior of the structure. As a starting point, I have identified direction, velocity & flow, and wind as basic air track indicator categories (see Figure 1). However, you may choose to approach this somewhat differently.

Figure 1. Basic Air Track Indicators

Air track indicators provide critical cues related to stages of fire development, burning regime, and potential for fire spread. However, it is essential that assessment of air track be integrated with other categories of indicators in the B-SAHF scheme to gain a clearer sense of fire conditions and likely fire behavior. Remember that looking at air track alone may be misleading.

Air Track at Openings and on the Interior

Discharge of smoke at openings and potential openings (Building Factors) is likely the most obvious indicator of air track while lack of smoke discharge may be a less obvious, but potentially important sign of inward movement of air.

Observation and interpretation of smoke and air movement at openings is an essential part of air track assessment, but it must not stop there. Movement of smoke and air on the interior can also provide important information regarding fire behavior.

The path taken by the air track will define the direction of fire spread and may present a significant hazard to firefighters operating between inlet and exhaust openings. This necessitates ongoing assessment of air track from both the exterior and interior of the building.

Figure 2. Air Track

Direction

Consider the following observations. You arrive at a fire in a commercial building and observe smoke showing from a door on floor 1 (Figure 2).

The smoke discharge fills the upper half of the door while it appears that air is moving in the bottom half of the door. What can you infer from this? What would you infer if the smoke discharge completely filled the door?

The direction of the air track can also provide valuable cues to fire behavior. When air moves in an opening (inlet) without any smoke discharge, it is likely that smoke is exiting from another opening (exhaust). When this condition is reversed, and smoke comes out with not inward movement of air, it is likely that another opening is serving as an inlet. When the air track is bi-directional and air moves in at the bottom and smoke moves out at the top, this may be the only opening in the compartment or ventilation from other exhaust openings may be inadequate. In any case where smoke is discharging through an opening, the fire is likely moving in that direction.

Mixing of smoke and air occurs at the interface between the hot gas layer and cooler air below. This is a critical factor in creating the conditions required for backdraft and many types of fire gas ignitions. Pulsing air track, outward movement of smoke followed by an inward movement of air is indicative of an underventlated fire and potential backdraft conditions (consider other indicators in determining if backdraft conditions are likely to exist). It is critical to remember that these pulsations can vary in duration and that backdraft does not generally occur immediately upon making an opening. The time between making an opening and occurrence of a backdraft is dependent on many factors including distance of the compartment with backdraft conditions from the opening. Air track is an extremely useful indicator, but it must be integrated with a big picture evaluation of fire behavior indicators.

Location of inlet and exhaust openings (particularly if they are on different levels or if impacted by wind) is an important Building Factor that directly impacts air track. This is an excellent example of why each of the categories of fire behavior indicators (FBI) must be considered together when reading the fire.

Velocity & Flow

Velocity and flow are two interrelated air track factors. Velocity refers to the speed of smoke and air movement. However, the speed with which smoke is traveling (either out of an opening in the compartment or building or within a compartment) must be considered in relation to the size of an opening or conduit. Flow may be either smooth (laminar) or turbulent. This is dependent to a large extent on velocity. High velocity generally results in turbulent flow through a compartment (such as a hallway) or out an opening (e.g., doorway or window). For a given volume, velocity and turbulence will be higher through smaller openings). High velocity smoke discharge and turbulent flow is generally indicative of high temperature within the compartment (another connection, in this case between air track and heat).

Wind

Wind can influence smoke movement on the exterior of a building (in some cases masking exterior air track indicators) or it can have a more direct influence on air track. As discussed in a number of earlier posts, wind can have a significant influence on compartment fire behavior.

• Wind Driven Fires
• NIST Wind Driven Fire Experiments: Establishing a Baseline
• NIST Wind Driven Fire Experiments: Anti-Ventilation Wind-Control Devices
• NIST Wind Driven Fire Experiments: Wind-Control Devices and Fire Suppression
• Wind Driven Fires: Tactical Problem

Understanding the potential influence of wind on fire behavior, provides a basis to read and interpret air track indicators. Wind exerts pressure on structural surfaces (see Figure 3), which under fire conditions can have a significant influence on movement of both smoke and air.

Figure 3. Distribution of Pressure due to Wind

Note. Adapted from Fire Ventilation (p. 34-35) by Stefan Svensson, 2000, Karlstad,Sweden: Räddnings Verket. Copyright 2000 by Stefan Svensson & Räddnings Verket.

Wind on an inlet opening can act much the same as a supercharger, dramatically increasing heat release rate, fire intensity, and rate of spread (see Figure 4).

Figure 4. Wind Effects

Movement of the Hot Gas Layer

Horizontal movement of the hot gas layer and turbulence at the interface between smoke and clear air below indicate air track direction. As discussed in Reading the Fire: Smoke Indicators height of the hot gas layer is a significant indicator of fire conditions. Even more important than the height of the hot gas layer, are changes in height. A sudden rise could indicate that ventilation has occurred (either performed by firefighters or caused by the fire). Gradual lowering of the hot gas layer could indicate worsening conditions and increased potential for flashover. However, inappropriate or excessive application of water can also cause lowering of the hot gas layer. Sudden lowering could indicate worsening conditions caused by flashover in an adjacent compartment. While not commonly known as a backdraft indicator, raising and lowing of the hot gas layer is similar to a pulsing air track observed at an opening (however in this case the compartment is not fully smoke logged, so the expanding and contracting gases cause the bottom of the hot gas layer to move up and down).

Height and more importantly vertical movement of the hot gas layer may be considered as Smoke or Air Track Indicators (a good argument can be made in either case). For now, I have chosen to position these two types of indicator under Smoke, but with linkage to Air Track, but I am considering moving them to Air Track (while maintaining linkage to Smoke Indicators).

Work in Progress

Hopefully we have been working on this project together and you have been developing or refining the air track segment of your fire behavior indicators concept map. My current map is illustrated in Figure 5.

Figure 5. Air Track Indicators Concept Map v5.2.2.1

You can also download a printer friendly version of the Air Track Indicators Concept Map v5.2.2.1 (including notes made during development). As indicated by the significant number of notes in the Staging Area of the printer friendly version, a bit more work remains to be done before integrating the Smoke and Air Track indicators in the complete version of the Fire Behavior Indicators Concept Map. Should you have any suggestions or feedback, please post a comment!

Ed Hartin, MS, EFO, MIFireE, CFO

References

Svensson, S. (2000). Fire ventilation. Karlstad, Sweden: Räddnings Verket.

Flashover is likely the most common type of extreme fire behavior encountered in structural firefighting. As my friends and colleagues from Sweden frequently observe, this is not really extreme fire behavior, its normal fire behavior. I think it is both. The term extreme “is framed within the context of our perception with ‘extreme’ defining our limited ability to control it and its potential impact on firefighter safety” (Close, 2005). However, occurrence of flashover is not abnormal or random; it is a simple matter of the chemistry and physics involved in a compartment fire.

Misconceptions

For some time, I have been collecting comments and statements related to extreme fire behavior phenomena in the press, fire service publications, and training materials. While it is quite possible to find accurate information on the phenomena of flashover, misconceptions and erroneous information are also common.

• There may have been a flashover at the home when gas in the air ignited
• Firefighters who responded to the blaze nearly got caught inside when an explosion or dangerous condition called a “flashover” occurred
• A flashover occurs when the air temperature reaches between 900 and 1,000, which is hot enough to ignite any gases that are in the air… The result is potentially deadly explosive conditions
• It’s what we call a flashover, were you have a combustible gas or even dust in the area and then all of a sudden, almost explosive-like, that vapour cloud will ignite
• Firefighters were caught in a rare “flashover,” an instance in which superheated gases and combustible materials simultaneously ignite
• When the room bursts into flame, flashover has occurred

Each of the preceding statements was made (or at least reported to have been made) by experienced fire officers. Failure to recognize and mitigate conditions that may result in flashover during firefighting operations results in significant risk to firefighters. At the core of recognition and mitigation is understanding what flashover is, what causes it, and the conditions necessary for it to occur.

What is Flashover?

Flashover is the sudden transition to fully developed fire. This phenomenon involves a rapid transition to a state of total surface involvement of all combustible material within the compartment. If flashover occurs, the rate of heat release in the compartment as well as the temperature in the compartment increases rapidly. Flashover may occur as the fire develops in a compartment or additional air is provided to a ventilation-controlled fire (that has insufficient fuel in the gas phase and/or temperature to backdraft).

Indicators of flashover include a radiant heat flux at the floor of 15-20 kW/m² (radiant heat transfer sufficient to quickly raise ordinary combustibles to their ignition temperature) and average upper layer temperature of 500^o-600^o C (932^o-1112^o F) (Drysdale, 1998). More observable indicators include rapid flame spread and extension of flames out of compartment openings. Compartment windows may also fail due to rapid temperature increases on the inner surface of window glazing (Gorbett & Hopkins (2007).

Figure 1.Flashover

Note: Photo by William Cobb, Cornelius – Lemley Fire Rescue

Flashover and Fire Development

There are a number of definitions or ways to describe flashover, but most importantly, it is a rapid transition to a fully developed fire.

A fuel package such as a couch or upholstered chair burning in open air progresses through four phases. In the incipient stage the fire involves only a small amount of fuel, as the fire moves into the growth stage, more fuel becomes involved and the speed of the combustion reaction increases. Eventually the entire object becomes involved and the fire is fully developed. As the fuel is consumed the fire begins to decay. Throughout this process, fire development is fuel controlled; the speed of fire development and energy released is dependent on the characteristics and configuration of the fuel. As combustion is taking place in the open, there is adequate oxygen to support combustion as the fire progresses through each of the four stages.

Heat of combustion is the energy released when a specific mass of fuel is completely burned. The total energy released when an object burns is dependent on the heat of combustion and the amount (mass) of fuel burned. Heat of combustion is measured in Joules (J). However, this only provides part of the picture. Heat release rate (HRR) is the amount energy released per unit of time. HRR is measured in Watts (W). A Watt is a Joule (unit of energy) per second (unit of time).  The fire service in the United States has traditionally used the British thermal unit (Btu) as a unit of energy. Using this unit of measure, HRR could be expressed in Btu/s. All of this is very interesting, but what does this have to do with flashover? As it turns out, heat release rate has everything to do with flashover!

When a fire is unconfined, much of the heat produced by the burning fuel escapes through radiation and convection. What changes when the fire occurs in a compartment? Fire development becomes influenced by the characteristics of the compartment. Other materials in the compartment as well as the walls, ceiling and floor absorb some of the energy released by the fire.  Some of the energy is not absorbed, but radiates back to the burning fuel continuing and accelerating the combustion process.

Hot smoke and air heated by the fire become more buoyant and rise, on contact with cooler materials such as the ceiling and walls of the compartment; heat is transferred to the cooler materials, raising their temperature. This heat transfer process raises the temperature of all materials in the compartment. As nearby fuel is heated, it begins to pyrolize. Eventually the rate of pyrolysis may reach a point where flaming combustion can be supported and the fire extends to other fuel packages.

However, the most significant difference with fire in a compartment is the compartment’s ventilation profile. The size, location, and configuration of openings in the compartment influence both the oxygen available for combustion and the retention or escape of thermal energy contained in the hot gases and smoke produced by the fire.

While the “stages of fire” have been described differently in fire service textbooks the phenomenon of fire development is the same. For our purposes, the stages of fire development in a compartment will be described as incipient, growth, fully developed and decay (see Figure 2). Despite dividing fire development into four “stages” the actual process is continuous with “stages” flowing from one to the next. While it may be possible to clearly define these transitions in the laboratory, in the field it is often difficult to tell when one ends and the next begins.

Figure 2. Stages of Fire Development

If the fire releases energy faster than it can escape from the compartment, temperature will increase and if sufficient energy is released, flashover will occur and the fire will transition rapidly from the growth to fully developed stage (see Figure 2). As this occurs, the fire will spread across all combustible surfaces in the compartment and flames will exit through compartment openings.

The bathtub analogy (see Figure 3) provides a simple way to explain the relationship between ventilation and flashover in a fuel controlled compartment fire.

Flashover has been analogously compared to the filling of a bathtub with the drain open. In this practical, though not perfect, analogy water represents the heat energy. The quantity of water available is the total heat of combustion of the available fuels (fuel load). The size of the spigot and the water pressure control the amount of water flow that is the heat release rate. The volume of the bathtub is analogous to the volume of the compartment and its ability to contain the heat energy. The size and location of the bathtub drain controlling the rate of water loss is the loss of heat energy through venting and conductance. In this analogy, if the bathtub becomes full and overflows, flashover occurs. (Kennedy & Kennedy, 2003, p. 7)

Figure 3. The Bathtub Analogy

More to Follow

Posts over the next few weeks will continue to examine the process of reading the fire with further exploration of air track, heat, and flame indicators. In addition, I will be continuing this look at the flashover phenomena with a particular emphasis on the relationship between heat release rate, ventilation, and flashover.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Close, K. (2005) Fire behavior vs. human behavior: Why the lessons from Cramer matter. Paper presented at the Eighth International Wildland Fir e Safety Summit, Missoula,MT. Retrieved May 13, 2008 from http://www.myfirecommunity.net/documents/Close.pdf

Drysdale, D. (1998). An introduction to fire dynamics. New York: John Wiley & Sons.

Gorbet, G & Hopkins, R. (2007) The current knowledge & training regarding backdraft, flashover, and other rapid fire progression phenomena. Paper presented at the annual meeting of the National Fire Protection Association, Boston, MA.

Kennedy, P. & Kennedy, K. (2003). Flashover and fire analysis: A discussion of the practical use of flashover in fire investigation. Retrieved July 30, 2009 from http://www.kennedy-fire.com/Flashover.pdf