The Evolving Fireground Research-Based Tactics Chapter 1

The importance of understanding fire behavior and applying research-based tactics to firefighting is emphasized. Fire dynamics and fire behavior are explained, along with the difference between temperature and heat release rate. The mechanisms of heat transfer and the concept of pressure in fires are discussed. Legacy fire development and modern fire development are compared, with a focus on flashover and backdraft incidents. The flow path of fire, heat, and smoke is also addressed, highlighting the need to control ventilation openings. Firefighters are urged to recognize and adapt to changing fire behavior. The Evolving Fireground, Research-Based Tactics, Chapter 1, Fire Behavior. We have moved into a time when our education about the fires that we respond to and how those fires react to the tasks that we perform becomes more obtainable through research. The importance of understanding fire behavior is an ever-evolving subject that will allow us to progress with smarter and safer firefighting tactics. In an effort to become smarter firefighters, it's imperative to evaluate the research and apply it to fireground operations. The body of knowledge created by researchers for the fire service only makes us better as a profession. One of our goals is to help close the gap between the researchers and the fire service so we can work harmoniously together as a team in order to evolve the fire service. When given the opportunity to apply knowledge gained, firefighters quickly adapt and overcome obstacles to solve issues. Firefighters are viewed as strong, blue-collar workers who heroically show up to mitigate the emergency. We are a profession that continuously trains and educates ourselves in order to master our craft. In the past, we haven't been focused on fire behavior. Yes, we were taught the basics in recruit school, but did we ever really understand it? It was likely taught by fire academy instructors who didn't fully understand fire behavior themselves, but just clicked through the slideshow and stomped their foot when test questions would show up on the slides. Sound familiar? The purpose of this chapter is not to bore you with graphs and charts, but to give you a small piece of fire behavior that will challenge you to seek more knowledge on the topic. Key terms. Fire dynamics and fire behavior. Let's start with a simple explanation of the difference between fire dynamics and fire behavior. Fire dynamics is the study of how fire starts, spreads, and develops. Fire dynamics has the scientific measurements and calculations needed to explain fire behavior. Fire behavior is exactly what it sounds like. It is the way that fire reacts and behaves to its environments. Temperature and heat release rate. Another set of important terms to understand the difference of is temperature and heat release rate. You may have heard it said, fires burn hotter in today's structures. Well, that's not exactly the case. What is true is that in today's structures, fire temperatures increase at a faster rate, causing fires to reach flashover temperatures sooner. Also, increasingly common in fires, flashover occurs before the fire department has arrived. A candle flame that burned in 1776 was the same temperature as that of a candle flame burning in 2018. But one of the keys in today's fires is the heat release rate rather than the temperature. A visual that is often used is a single candle compared to ten candles. They have the same flame temperatures, but ten times the heat release rate. Heat transfer, the mechanism of heat movement, is critical to understand because heat is always transferred from the hotter object to colder object. There are different types of heat transfer including conduction, convection, and radiation. Conduction is heat transferred to the fire's immediate surrounding area. Convection is heat transferred by means of moving gases or liquids. Radiation is heat transferred by electromagnetic waves the closer you get the hotter it feels. Heat can be transferred by all three mechanisms to the ceiling, walls, and floor of the fire compartment. Pressure. Fires create pressure, and pressure moves from high to low. The high pressure is the fire room and is being created by the fire. The low pressure is any opening that is created at a distance away from the fire. When the pressure gradient is created, we have the making of a flow path. High pressure combustible gases, for example smoke, are forced across the upper parts of a compartment and the fire is drawing in the lower pressure air from a low pressure opening. If we stop or control the air from being drawn into the fire, we reduce the temperatures and slow the combustion process. In slowing the combustion process, fire reduces in size along with the amount of combustible gases being produced. Interior visibility will worsen, unless it's already at zero, but the rest of the interior conditions improve with lower temperature and less or equal combustible gases. The flammable range of combustible gases can be compared to the action of combustion within an automobile engine. Where the correct mixture of fuel and air is needed, smoke burns rich and lean as the fuel-air mixture occurs. The upper explosive limit, UEL, is the rich and the lower explosive limit, LEL, is the lean. The pressure is at its highest point if the fuel-air mixture is allowed to go above the LEL and gets closer to the ideal mixture. At this point, and with an ignition source, it's possible to see a significant force of pressure during a fire event. As the flammable range goes past the ideal mixture and toward the UEL, rich, the pressure drops because it's too rich to burn. One of the interesting things about flammable range is that the fire cannot burn if it's above the UEL or below the LEL, because the mixture would either be too lean or too rich. Legacy and modern fire development. Legacy fire development is related to fuel-limited fires. The legacy fire isn't limited by a lack of oxygen. It keeps growing as long as it has fuel to burn, and once all of the fuel is burning, the fire is at its fully developed stage. As the fuel burns down, the legacy fire begins to decay, but it still has enough air to mix with gases and complete its combustion. The legacy fire department graph shows fire that isn't limited by a lack of oxygen. It grows to the point that all of the fuel is burning at the fully developed stage. During decay, there is sufficient air to mix with gases and complete the combustion of the fuel. Modern fire development commonly relates to ventilation-limited fires. Early on, there is enough air to mix with gases and create complete flammable combustion of those gases. Some would consider this flashover, but as the fire starts to run out of air, it starts to decay and temperatures decrease. Introduction of air, which happens when firefighters arrive and open a door to gain access, a door or window fails, or a bystander opens a door to try and evacuate residents, gives the fire the oxygen that it needed to rapidly grow and likely flashover into a fully developed compartment fire. Flashover is a dangerous transition between the growth stage and fully developed stage. It is predictable and will cause serious injury or even kill firefighters if the precursors go unrecognized on the fire ground. When fire gases completely ignite and the radiant heat from the initial growth stage of the fire ignites the room contents, rapid growth spreads within the compartment. Depending on the compartment size and the amount of pressure that builds prior to flashover, there could be a pressure wave seen with the flashover. While this phenomenon has become more common, it's likely been around many years and described as a backdraft in the past. However, the dated definition of backdraft from our Basic Firefighter Certification textbook describes backdraft as an event that occurred at temperatures greater than 1600 degrees Fahrenheit and characterized by smoke-stained windows, yellow-gray smoke, etc. Backdraft. When modern fires become ventilated, become ventilation-limited, the temperature decreases significantly. Thus, the older definition, temperatures greater than 1600 degrees Fahrenheit, causes there to be conflict in the definition of the ventilation-limited blasts that we are seeing in today's fires. An updated definition in NFPA 921-2011 version for backdraft is a deflagration resulting from the sudden introduction of air into a confined space containing oxygen-deficient products of incomplete combustion. The backdrafts that most commonly occur in attic spaces, knee walls, or any other void space found on many documented fires, are the cause of many firefighter injuries across the country. During these incidents, the fire remains hidden and ventilation-limited until a firefighter creates a hole to find the fire and gives it just enough air to rapidly ignite, catching the firefighter off guard. It could be something as simple as poking a small hole in the ceiling to check for fire or smoke spread above the firefighter's head, which is a standard procedure taught in many strategies and tactic books. Modern fire development commonly relates to ventilation-limited fires. Early on, there is enough air to mix with gases and create complete flaming combustion of those gases. For every action, there is an equal and opposite reaction, Sir Isaac Newton's third law of motion. In the above scenario, it is common to see a time delay from the initial opening created by firefighters until the flashover occurs. While this time delay varies, it needs to be recognized by firefighters. The old saying, where there is smoke, there is fire, applies to these situations. If a small opening is made, water should be applied to the smoke and into the compartment as quickly as possible to reduce the temperatures and stop the occurrence of fire growth. Again, firefighters must be aware of the consequences when creating openings within a structure fire. Flow Path The flow path is the space through which fire, heat, and smoke progress, moving from the high-pressure fire area toward the low-pressure oxygen sources, such as door and window openings. Controlling the ventilation openings is critical on the fire ground because of the need to avoid creating a flow path until necessary for rescue or extinguishment. When this occurs, the flow path should be recognized and controlled, if possible. Recognizing fire behavior as it changes and reacts to our tactics is possibly one of the most underutilized skills for firefighters. Not only should we recognize the fire behavior that occurs from ventilation openings that are made by firefighters, but we should also recognize those made by window or door failures caused by fire conditions. These window and door failures have caused firefighter injuries and death. Although the openings are unpredictable, a solid size-up should recognize imminent failure to a window or door. Interrupting the flow path involves tactics that are innovative and sometimes counterintuitive, but fire research shows that these methods can reduce heat within the structure, minimize the potential for flashover, and greatly enhance the safety for firefighters. While this can vary with the location and size of the fires of today's structure fires, the time between tactical ventilation and flashover could be as little as two minutes, compared to eight minutes in a traditional fire. Giving firefighters only a quarter of the time they previously had to recover from poorly timed ventilation, in order to formulate safe and effective ventilation tactics, it is imperative to better understand flow paths. Based on the building's configuration, there may be several flow paths within the structure. Firefighting or rescue operations conducted in a flow path can place firefighters at significant risk. The UL-NIST Governor's Island Experiments advanced the understanding of fire behavior relative to flow paths and helped validate new, more effective tactics to enhance survivability. We discovered that the increasing ventilation by opening doors, clearing windows, or cutting the roof in a ventilation-limited structure fire may lead to a rapid transition to flashover. In addition, anyone in the exhaust path between the fire and its direction of travel is in danger. Conversely, interrupting the flow path by closing doors or windows can limit fire growth and reduce temperatures. It's not the first fire that kills firefighters, it's the second. It's important to understand that the smoke and fire gases are what cause fire spread. Many times we get focused on the flames, but the flames are just a light show. The smoke is what we should be keeping our eyes on. Smoke volume, velocity, density, and color are essential components to understanding fire behavior. Recognizing smoke behavior can help firefighters recognize imminent flashover and predict fire spread. Controlling the ventilation openings is vital to stopping smoke and fire spread. If the fire has not been controlled with water. While we have defined some hazardous fire behavior situations that firefighters could face, it is important to understand more of the components of fire dynamics, following our other important terms to review to help us understand new age fire behavior. Ambient. Someone's or something's surroundings, especially as they pertain to the local environment. For example, ambient air and ambient temperature. Backdraft. A deflagration resulting from the sudden introduction of air into a confined space containing oxygen deficient products of incomplete combustion, a.k.a. smoke explosion. Blast pressure front. The expanding leading edge of an explosion reaction that separates a major difference in pressure between normal ambient pressure ahead of the front and potentially damaging high pressure at and behind the front. Ceiling layer. A buoyant layer of hot gases and smoke produced by a fire in a compartment. Conduction. Heat transfer to another body or within a body by direct contact. Convection. Heat transfer by circulation within a medium such as a gas or a liquid. Fire dynamics. The detailed study of how chemistry, fire science, and the engineering disciplines of fluid mechanics and heat transfer interact to influence fire behavior. Fire spread. The movement of fire from one place to another. Flame. A body or stream of gaseous material involved in the combustion process and emitting radiant energy at specific wavelength bands determined by the combustion chemistry of the fuel. In most cases, some portion of the emitted radiant energy is visible to the human eye. Flashover. A transition phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperatures more or less simultaneously and fire spreads rapidly throughout the space, resulting in full room involvement or total involvement of the compartment or enclosed space. Fuel load. The total quantity of combustible contents of a building, space, or fire area, including interior finish and trim, expressed in heat units of the equivalent weight in wood. Fuel-controlled fire. A fire in which the heat release rate and growth rate are controlled by the characteristics of the fuel, such as quantity and geometry, in which adequate air for combustion is available. Gas. The physical state of a substance that has no shape or volume of its own and will expand to take the shape and volume of the container or enclosure it occupies. Heat. A form of energy characterized by vibration of molecules and capable of initiating and supporting chemical changes of state. Heat flux. A measure of the rate of heat transfer to a surface expressed in kilowatts, kilojoules, or BTUs per second. Heat release rate, HRR. The rate at which heat energy is generated by burning. Ignition. The process of initiating self-sustained combustion. Kilowatt. A measurement of energy release rate. Oxygen deficiency. Insufficiency of oxygen to support combustion, also known as ventilation limited. Plume. The column of gases, flames, and smoke rising above a fire. Pyrolysis. A process in which material is decomposed or broken down into simpler molecular compounds by the effects of heat alone. Pyrolysis often precedes combustion. Radiation. Heat transfer by way of electromagnetic energy. Rollover. The condition where unburned fuel, pyrolysate, from the originating fire has accumulated in the sealing layer to a sufficient concentration, for example, at or above the lower flammable limit, that it ignites and burns. Rollover can occur without ignition of, or prior to, the ignition of other fuels separate of the origin, for example, flameover. Scientific method. The systematic pursuit of knowledge involving the recognition and formulation of a problem, the collection of data through observation and experiment, and the formulation and testing of a hypothesis. Smoke. The airborne solid and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion, together with the quantity of air that is entrained or otherwise mixed in the mass. Smoldering. Combustion without flame, usually with incandescence and smoke. Temperature. The degree of sensible heat of a body as measured by a thermometer or similar instrument. Ventilation. Circulation of air in any space by natural wind or convection or by fans blowing air into or exhausting air out of a building. A firefighting operation of removing smoke and heat from the structure by opening windows and doors or making holes in the roof. Ventilation-limited fire. A fire in which the heat release rate, or growth, is controlled by the amount of air available to the fire, also known as ventilation-controlled fire. Venting. The escape of smoke and heat through openings in a building.