Ventilation Effects on Fire Patterns during Post Flashover Burning By Matthew Obach, M.A.Sc., EIT, CFEI In order to determine the origin of a fire, fire investigators analyze fire patterns that remain once the fire is extinguished. In some cases, fire patterns can be easily identified and used to determine the point of origin, however, this is not always the case. Most scene investigators and adjusters at one point in their career or another have attended a scene where the entire room (or multiple rooms) and their contents have been involved in the fire. In these instances, the degree of burning and the dynamics of the fire make origin determination more difficult, which is the case following flashover. Flashover occurs when all of the combustible materials in the room reach their ignition temperature, and everything in the room starts to burn. Flashover is often characterized by temperatures greater than 600 C near the ceiling, or continuous flames coming out of the room through an open door or window 1. Post flashover burning can mask initial fire patterns, and may also create patterns which can mislead an investigator into thinking a fire was incendiary in origin, which may not necessarily be the case. As the science behind flashover and its impact on fire investigations evolves, it is becoming increasingly more important for both the fire investigator and the insurance adjuster to understand the signs of flashover, and the effect it can have on fire patterns. This article focuses on the effect of post flashover burning on new and existing fire patterns in a room, and outlines how air moving into a room (ventilation) during post flashover burning can have a marked effect on fire pattern development. Before discussing the effect of flashover, some basic fire dynamics theory is first introduced, and two commonly interpreted patterns created by a fire, the V pattern and inverted V pattern, are outlined. Finally, the importance of ventilation on post flashover fire pattern development is discussed by showing the results from research conducted by the Bureau of Alcohol, Tobacco, Firearms, and Explosives. Fire Dynamics In order for a fire to start, three factors need to come together; fuel, oxygen, and an ignition source. Once the fire has started, the fire itself provides the heat for subsequent ignition. Therefore, once a fire is burning, the factors limiting fire growth are the available fuel and oxygen (air). As a fire burns, the flames and hot gases it produces rise vertically due to buoyance, because they are hotter than the surrounding air. This can be easily demonstrated by lighting a match and holding it in the open. No matter how you change the orientation of the match, the flames will always point upwards. As the hot air rises vertically away from the fire, cooler air fills that space, providing oxygen for the fire.
When a fire burns in a room, the hot air and smoke rise to the top of the room. There the gases collect, and a hot smoky layer builds in the room. Once the bottom of this layer reaches an opening such as an open door, it will flow out. As the hot air flows out of the opening, cooler air will be entrained into the lower portion of the room to fill the space left by the hot air, providing oxygen for the fire. In the early stages of growth, the fire grows as it ignites unburned fuel that it comes in contact with. In this case, the fire is said to be fuel controlled, as the amount of unburned fuel in the vicinity of the fire limits the growth. As the fire continues to produce hot gases that stay in the upper layer of the room, the gases will begin to heat other available fuels via radiation. When the gas layer is hot enough, approximately 600 C, flashover can occur in the room 1. After flashover occurs, most if not all of the available fuel in the room starts to burn, which increases the smoke production from the fire, and increases the amount of oxygen required to sustain the fire. When this happens, the fire is said to be ventilation controlled, as the amount of ventilation, and therefore available oxygen, limits the fire. Fire Patterns Before considering the effect that ventilation has on fire patterns, it is important to understand a few basic fire patterns, how they are created, and how they are interpreted. As mentioned previously, all fires start at a single point where sufficient amounts of heat, fuel, and oxygen come into contact, and the heat and flames rise vertically from that point. Once the fire grows large enough that the flames and hot gases come in contact with the ceiling, they begin to spread out along the ceiling in all possible directions. The following figure shows a sketch of a fire in three different stages of growth. Figure 2: Simple sketch of a fire in three different stages of growth.
If this fire was burning close to a wall, there would be a different pattern created on the wall at each stage. The same sketch of the fire can be seen in the following figure with a simplified version of the pattern they would create on a vertical surface they were in close proximity to. Figure 3: Same sketch with created pattern outlined (simplified). If the fire is suppressed in any of these three stages, a different, yet distinguishable, pattern will be observed on the wall. If there was no other fire in the room, the origin of the fire will typically be at the base of the observed pattern. In Figure 3, the three patterns outlined are typically referred to as the inverted V pattern, the columnar pattern, and the V pattern (from left to right). The transition from one stage to the next depends on a number of factors, and as such, the duration of burning in each phase will vary for different fires. The duration of burning in the first stage is dependent on the fuel that is burning, and how easily the initial fire will grow. For instance, if the burning fuel is a thick piece of wood, the fire will stay in this initial stage for much longer than if it were a combustible couch cushion. The fire may also die out in the early stages, if there is not enough fuel in the vicinity to sustain combustion. If the fire did continue burning and grow into the columnar stage, it would continue on to the last stage fairly quickly. This is because the columnar stage is more of a transition stage, and this pattern will typically only be observed if the fire is suppressed during the transition from the initial to final stage. Once the fire does transition into the final stage, it may continue to grow and expand to other areas of the room, but will not transition back to the initial stage. As the fire continues to burn and spread out along the ceiling, lines of demarcation in a V shape will remain to some degree, making identification of the origin possible. The line of demarcation may be a burnt or sooty outline, or may be a clean patch, depending on the surface material and fire situation. Three pictures of V patterns created by a fire can be seen in Figure 4. A line has been added on each picture to make the pattern easier to identify.
A B C Figure 4: V Pattern created (A) in a compartment; (B) on the side of a house; (C) on the back of a stove. Post Flashover Fire Patterns Research has been conducted by a number of organizations which considers the effect that ventilation has on fire pattern development, including the United States Fire Administration 2, and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). The following pictures were obtained from Steve Carmen, a retired ATF agent who conducted some of this research while with the ATF. In their research, the ATF set up three identical compartments (rooms) with the same furniture in each. In the first test, the fire was suppressed 40 seconds after the transition to ventilation controlled burning occurred, and in the third test, the fire was suppressed 2 minutes 30 seconds after the transition occurred 3. Photos of the compartment after suppression for the first and third tests can be seen in Figure 5. 1 3 Figure 5: Pictures of the V patterns created in the same room with different durations of post flashover burning (Courtesy Steve Carmen, retired ATF agent). The photos in the figure are taken with the photographer standing in the only doorway into the compartment looking diagonally across the room into the opposite corner. The photo from the first test is on the left, and the one for the third test is on the right. Lines have been added to the photos to better highlight the V patterns created.
In the above two pictures, a few different V patterns created by the fire can be discerned. In the photograph on the left (first test), a well defined V pattern can be observed with its base originating from the top of the nightstand (to the right of the chair). A less defined but still visible V pattern can also be seen originating between the bed and chair. In the photograph on the right, a very clear V pattern is observed to the right of the nightstand, extending almost down to the floor. In both of these tests, the fire was started in the exact same location, which was between the bed and the chair, in a wastebasket on the floor 3. Only one of the three visible V patterns on the compartment wall points to this location. At the base of the other two patterns, no ignition source is present, and at the base of the clearly visible pattern on the wall in the second picture, there is no evidence of a fuel load. During the tests conducted by the ATF, video was taken inside the compartment. This video showed that the two V patterns visible on the back wall of the compartment were created during post flashover burning 3. In the first test, the hot upper layer ignited the surface of the nightstand, and the combination of this surface burning and the burning of the gases in the upper layer caused the pattern above it. In the third test, the flames that produce the V pattern visible on the wall occur as a result of the unburnt fuel in the smoke layer burning when it came into contact with available oxygen. This pattern was created directly opposite the open doorway. The air flowing into the compartment at floor level moved towards this wall and then upwards as it encountered the wall. The boundary between the incoming air and the upper layer gases is where the fire occurred, creating the V pattern on the wall. It should also be noted that the burning of the upper layer gases at this position obscured the V pattern produced by the initial fire, a pattern that would have been visible pre flashover. Another interesting result from the research conducted by the ATF was the formation of an inverted V pattern in the test with the longest duration of post flashover burning. An inverted V pattern is depicted by the triangle (smallest fire) shown in the sketch of the three different flame sizes and the respective patterns they create. This pattern was created near the open door, but only in test three, as seen in the following two pictures. 1 3 Figure 6: Pictures of the fire patterns created near the open door in two rooms with post flashover burning (Courtesy Steve Carmen, retired ATF agent).
These two pictures are looking at the front of the dresser, with the open door on the right. During the fire, air would be flowing in through the door and across the front of the dresser. In the first test, there is fire damage to the top and front of the dresser, but this damage does not reach the floor. There is also damage to the wall behind the dresser, but again, the damage does not go down to the floor. In the third test, there is fire damage all the way to the floor on both the dresser and the wall, and there is the inverted V pattern visible on the wall by the door. This pattern shows up white because the fire burned away any soot deposits on the wall, and heated up the surface enough that no more would deposit 3. The fire damage in the first test is easy to interpret, and can be attributed to the heat from the smoke layer in the compartment radiating down to the dresser. The very bottom is not burnt because the cool air flowing into the compartment did not mix with any fuel until it reached the wall above the nightstand, where it ignited and created the V pattern shown on the right side of Figure 5(1). In the third test, the fire pattern is more easily misinterpreted, as an inverted V pattern often indicates fire origin. In this case, there would be multiple origin indicating patterns in the room, a clear V pattern and a clear inverted V pattern, both without a readily available fuel or ignition source at the base. This could quite possibly mislead the investigator into thinking the fire was incendiary in origin, as it was started in more than one location. This is not the case in the third test, however, as the inverted V pattern is the result of the air entering the room during post flashover burning. At the time this pattern was created, there was so much smoke and unburnt gaseous fuel in the compartment that the fresh air entering the room was consumed almost instantly. As fresh air entered through the door, some of it hit the side of the dresser and swirled in the corner, where it mixed with the unburnt fuel and ignited, creating the inverted V pattern. Implication As the research conducted by the ATF shows, post flashover burning in a room can have a marked effect on fire patterns developed pre flashover. Not only can initial lines of demarcation be obscured from the soot production post flashover, but burning of the gases in the smoke layer can create patterns which can easily be misconstrued as origin indicating V patterns. This is not to say that it is impossible to determine the origin of a fire in a room post flashover, it just makes it clear that an understanding of compartment fire dynamics and ventilation effects is important when analyzing the post flashover compartment, and that all of the available fire pattern data must to be considered in order to make an accurate origin determination.
About the Author Matt Obach has conducted thesis research in post flashover compartment fires while completing his Masters of Applied Science (M.A.Sc.) in Fire Safety Engineering at the University of Waterloo. He currently works as a fire investigator with EFI Global in Ottawa. References: 1 Drysdale, Dougal. An Introduction to Fire Dynamics. 1998, 2nd Edition. John Wiley & Sons, Chichester, West Sussex. 2 FEMA. USFA Fire burn Pattern Tests. 1997. Emmitsburg, Md.: Federal Emergency Management Agency, U.S. Fire Administration. 3 Carman, Steven W. Clean Burn Fire Patterns a New Perspective for Investigators, 2012 Proceedings of Interflam, Nottingham, England