4. Fire Dynamics

Compartment Fire Development

Compartment fire development depends upon whether the fire is fuel-limited or ventilation-limited. When sufficient oxygen is available for flaming combustion, the fire is said to be fuel-limited. Under fuel-limited condi-tions, the fuel's characteristics such as heat release rate and configuration control fire development. As long as the fire can reach more ignitable fuel, it will continue to burn. Conversely, ventilation-limited fires have access to all of the fuel needed to maintain combustion. However, the fire does not have access to enough oxygen to continue to burn or to spread to all available fuels. All compartment fires begin in the incipient stage as fuel-limited fires. Once the fire reaches the growth stage, the fire will either remain fuel-limited, if there is enough oxygen to support continued growth, or the fire will consume all available oxygen and become ventilation-limited. A fuel-limited fire will usually progress through the stages of fire development in order. Ventilation-limited fires tend to enter an early state of decay at the end of the growth stage because there is no longer enough available oxygen for the fire to become fully developed. This section will define the stages of fire development and then describe the progression of a fire in a com-partment. The examples in the information boxes describe fire behavior in a room with one exterior window, an exterior doorway, and typical modern furnishings found in a residential living room.

Fire Science

Fire Science Firefighters should have a scientific understanding of combustion, fire, heat, and temperature. Fire can take vari-ous forms, but all fires involve a heat-producing chemical reaction between some type of fuel and an oxidizer, most commonly oxygen in the air. Oxidizers are not combustible but will support or enhance combustion.

Fire dynamics

Fire dynamics describes the meeting point between fire science, materials science, fluid dynamics of gases, and heat transfer. Understanding the basic physics of these sciences can give firefighters the knowledge needed to fore-cast fire growth at a scene and predict the likely consequences of various tactical options available for controlling a fire. All of the following provide firefighters with pieces of the total picture about a fire's likely behavior during fireground operations: • Fire science • The combustion process • Fire behavior and its relationship to various materials and environments • Classifications of fires and their corresponding extinguishing agents • Recognition of fire behavior indicators, fire development patterns, and the potential for rapid fire development • Various ventilation and suppression tactics used as tools for controlling fires

Fully Developed Stage

Fully Developed Stage The fully developed stage occurs when the heat release rate of the fire has reached its peak, because of a lack of either fuel or oxygen. There are two main types of fully developed fires: ventilation-limited and fuel-limited fires. The factor limiting the peak heat release rate is used to identify which type of fully developed fire exists. Firefighters often misinterpret the term "fully developed" to mean that the fire can no longer grow. A more accurate description would be that the fire has grown as much as it can. New sources of fuel introduced after full development will allow fuel-limited fires to grow. Likewise, new sources of oxygen introduced after full develop-ment will allow ventilation-limited fires to grow. Fuel-Limited Conditions The available fuel limits the peak heat release in a fuel-limited, fully developed fire. The most effective method of increasing the heat release rate is to provide more fuel. A campfire located in a fire ring is a good example of fuel-limited conditions. The fire reaches its peak when all the fuel becomes involved. The fire ring separates the burning fuel from other potential fuel resulting in a fuel-limited, fully developed fire. Adding additional fuel or firewood would increase the energy release of the fire to a new peak heat release rat Technically speaking, most compartment fires, even those that are ventilated and have untenable interior en-vironments, are ventilation-limited. Adding ventilation points to a compartment fire that is already ventilated will add oxygen that will allow the fire to grow. Fuel-limited full development usually occurs when fires are not contained within compartments such as wildland fires, vehicular fires, or fires burning in collapsed structures. Ventilation-Limited Conditions In contrast, a fully developed, ventilation-limited fire lacks the oxygen available to grow because the number and size of openings in the compartment limit the entrainment of air. The fire reaches a peak when it consumes all the available oxygen from the air intake, typically with incomplete combustion. Additional fuel is available and gaseous fuel is leaving the compartment in the smoke; however, the fire cannot release any more energy. Allowing additional air into the compartment via an additional opening or enlarging the existing opening will provide more oxygen, resulting in a higher peak heat release rate. WARNING: Even coordinated tactical ventilation increases the combustion rate in ventilation-limited fires. Ventilation-limited, fully developed fires present a hazardous situation to firefighters. The potential for a window failure to provide fresh oxygen and increase the peak heat release rate can endanger both firefighters and potential victims. To reduce the risk of the unpredictable window failure, firefighters must transition the fire from ventilation-limited to fuel-limited. With the high heat of combustion found in modern furnishings, the only mechanism to transition the fire is to extinguish some of the burning fuel. It is not possible to make enough openings in a compartment to transition a fire from ventilation-limited to fuel-limited conditions. WARNING: Additional ventilation alone will not transition a ventilation-limited fire to a fuel-limited fire.

Growth Stage

Growth Stage Within the growth stage, a variety of fire behaviors can occur, depending upon the number of ventilation sources. The fire may consume all of its available oxygen and enter a ventilation-limited state of decay or ventilation may provide enough oxygen for rapid fire development and/or growth to full development. Rapid fire development usually occurs during the growth stage. Understanding fire dynamics is largely an understanding of everything that can happen during the growth stage. NOTE: Keep in mind that if the fire enters ventilation-limited decay, it does not necessarily indicate that the fire is in its final stage of development. As the fire transitions from incipient to growth stage, it begins to influence more of the compartment's environ-ment and has grown large enough for the compartment configuration and amount of ventilation to influence it. The first effect is the amount of air that is entrained into the fire. Unconfined fires draw air from all sides and the entrainment (drawing in) of air cools the plume of hot gases, reducing flame length and vertical extension (Figure 4.32). In a compartment fire, the location of the fuel package in relation to the compartment walls affects the amount of air that is entrained and thus the amount of cooling that takes place. The following tenets describe entrainment based on the positioning of fuel packages: • Fires in fuel packages in the middle of the room can entrain air from all sides. • Fires in fuel packages near walls can only entrain air from three sides. • Fires in fuel packages in corners can only entrain air from two sides. herefore, when the fuel package is not in the middle of the room, the combustion zone (the area where suf-ficient air is available to feed the fire) expands vertically and a higher plume results. A higher plume increases the temperatures in the developing hot gas layer at ceiling level and increases the speed of fire development. In addi-tion, heated surfaces around the fire radiate heat back toward the burning fuel which further increases the speed of fire development. A fire is said to be in the growth stage until the fire's heat release rate has reached its peak, either because of a lack of fuel or a lack of oxygen. In other words, when a fire cannot grow without the introduction of a new fuel source or a new oxygen source, it has left the growth stage and become fully developed. Two common routes to full development are as follows: • Fires that consume all available oxygen and transition to a state of ventilation-limited decay. • Fires that have enough oxygen and move through the growth phase and possibly into rapid fire development. Thermal Layering Once the ceiling jet reaches the walls of the fire compartment, the hot gas layer begins to develop. Thermal layer-ing is the tendency of gases to form into layers according to temperature, gas density, and pressure. Provided that there is no mechanical mixing from a fan or a hose stream, the hottest gases will form the highest layer, while the cooler gases will form the lower layers (Figure 4.34). In addition to the effects of heat transfer through radiation and convection described earlier, radiation from the hot gas layer also acts to heat the interior surfaces of the compartment and its contents. Changes in ventilation and flow path can significantly alter thermal layering. The flow path is defined as the space between the air intake and the exhaust outlet. Multiple openings (intakes and exhausts) create multiple flow paths. he products of combustion from the fire begin to affect the environment within the compartment. As the fire continues to grow, the hot gas layer within the fire compartment gains mass and energy. As the mass and energy of the hot gas layer increases, so does the pressure. Higher pressure causes the hot gas layer to spread downward within the compartment and laterally through any openings such as doors or windows. If there are no openings for lateral movement, the higher pressure gases have no lateral path to follow to an area of lower pressure. As a result, the hot gases will begin to fill the compartment starting at the ceiling and filling down. Isolated or intermittent flames may move through the hot gas layer. Combustion of these hot gases indicates that portions of the hot gas layer are within their flammable range, and that there is sufficient heat to cause igni-tion. As these hot gases circulate to the outer edges of the plume or the lower edges of the hot gas layer, they find sufficient oxygen to ignite. This phenomenon frequently occurs before more substantial involvement of flam-mable products of combustion in the hot gas layer. The appearance of isolated flames is sometimes an immediate indicator of flashover. he interface between the hot gas layers and cooler layer of air is commonly referred to as the neutral plane because the net pressure is zero, or neutral, where the layers meet. The neutral plane exists at openings where hot gases exit and cooler air enters the compartment. At these openings, hot gases at higher than ambient pressure exit through the top of the opening above the neutral plane. Lower pressure air from outside the compartment entrains into the opening below the neutral plane (Figure 4.35). Transition to Ventilation-Limited Decay Most residential fires that develop beyond the incipient stage become ventilation-limited. Even when doors and windows are open, insufficient air entrainment may prohibit the fire from developing based on the available fuel. When windows are intact and doors are closed, the fire may move into a ventilation-limited state of decay even more quickly. While a closed compartment reduces the heat release rate, fuel may continue to pyrolize, creating fuel-rich smoke. As the interface height of the hot gas layer descends toward the floor, the greater volume of smoke begins to in-terrupt the entrainment of fresh air and oxygen to the seat of the fire and into the plume. This interruption causes the fire within the compartment to burn less efficiently. As the efficiency of combustion decreases (incomplete combustion), the heat release rate decreases and the amount of unburned fuel within the hot gas layer increases. he fire is now in a state of ventilation-limited decay because: • There is not enough oxygen to maintain combustion. • The heat release rate has decreased to the point that fuel gases will not ignite. Although the heat release rate decreases when a fire is ventilation-limited, the temperature in the room may remain high. Because there is not enough oxygen to maintain combustion, the fire has a lower heat release rate, but that does not mean that the environment is tenable. The compartment fills with fuel-rich gases that only need more oxygen to ignite because of the higher temperatures in the compartment. Even if temperatures decrease, pyrolysis can continue. Under these conditions, a large volume of flammable products of combustion can accumulate within the compartment. These gases are fuel that can ignite, given a new source of oxygen. If no other source of oxygen exists, the compart-ment will fill with black smoke and slowly cooling fuel gases. The compartment will show no visible flames. The characteristics of the fuel and fuel load in today's typical fires will cause fires to quickly become ventilation-limited. eeds a new supply of oxygen. Ventilation intro-duces outside air to the fire as this new source of oxygen. If windows or doors fail, the sudden in-troduction of fresh air creates a rapid increase in the heat release rate and growth of the fire. This rapid increase can also occur when firefighters open a door or window to enter the compartment for extinguishment, which creates a new flow path (Figure 4.37). WARNING: Even coordinated tactical ventilation increases the combustion rate in ventilation-limited fires. The pressure outside the compartment is lower than the pressure inside the compartment (Figure 4.38). Because of these pressure differences, any ventilation to the outside - opening an interior or exterior door, or breaking or opening a window - provides a flow path along which the hot gases can now move from the high pressure area inside to the low pressure area outside. Rapid Fire Development Rapid fire development refers to the rapid transition from the growth stage or early decay stage to a ventilation-limited, fully developed stage (Figure 4.40, p. 154). Among these events are flashover and backdraft. NOTE: Smoke explosions are also incidents of rapid fire development, but they involve more than just one compartment of a structure. Smoke explosions will be described later in this chapter. Rapid fire development has been responsible for numerous firefighter deaths and injuries. To protect yourself and your crew, you must be able to: • Recognize the indicators of rapid fire development • Know the conditions created by each of these situations • Determine the best action to take before they occur In this section, rapid fire development conditions are described along with their indicators. Flashover. Rapid transition from the growth stage to the fully developed stage is known as flashover. When flashover occurs, the combustible materials and fuel gases in the compartment ignite almost simultaneously; the result is full-room fire involvement. Flashover typically occurs during the fire's growth stage, but may occur dur-ing the fully developed stage as the result of a change in ventilation. Flashover conditions are defined in various ways; however, during flashover, the environment of the room changes from a two-layer condition (hot on top, cooler on the bottom) to a single, well mixed hot gas condition from floor to ceiling. The environment is untenable, even for fully protected firefighters. As flashover occurs, the gas temperatures in the room reach 1,100 °F (593°C) or higher. Rapid Fire Development A significant indicator of flashover is rollover. Rollover describes a condition where the unburned fire gases that have accumulated at the top of a compartment ignite and flames propagate through the hot gas layer or across the ceiling. Rollover may occur during the growth stage as the hot gas layer forms at the ceiling of the compartment. Flames may appear in the layer when the combustible gases reach their ignition temperature. While the flames add to the to-tal heat generated in the compartment, this condition is not flashover. Rollover will generally precede flashover, but it may not always result in flashover. Rollover contributes to flashover conditions because the burning gases at the upper levels of the room generate tremendous amounts of radiant heat which transfers to other fuels in the room. The new fuels begin pyrolysis and release the additional gases necessary for flashover. he transition period between preflashover fire conditions 5 seconds after door is opened (growth stage/ventilation-limited decay) to postflashover (fully developed stage) can occur rapidly. Radiation from the compartment's upper layer heats the compartment's contents until they reach their ignition temperature simultaneously. When the upper layer ignites, the amount of radiation in-creases to levels which rapidly ignite contents in the room, even if they are remote from the fire. During flashover, the volume of burning gases can increase from approximately 1⁄4 to 1⁄2 of the room's upper volume to fill the room's entire vol-ume and extend out of any openings from the room. When flashover occurs, burning gases push out of compartment openings (such as a door to another room) at a substantial velocity. here are four common elements of flashover: • Transition in fire development — Flashover represents a transition from the growth stage to the fully developed stage. • Rapidity — Although it is not an instantaneous event, flashover happens rapidly, often in a matter of seconds, to spread fire completely throughout the compartment. • Compartment — There must be an enclosed space such as a single room or enclosure. • Pyrolysis of all exposed fuel surfaces — Fire gases from all of the combustible surfaces in the enclosed space ignite, provided thhat there is sufficient oxygen to support flaming combustion. Two interrelated factors determine whether a fire within a compartment will progress to flashover. First, there must be sufficient fuel and the heat release rate must be sufficient for flashover conditions to develop. For example, ignition of discarded paper in a small metal wastebasket may not have sufficient heat to develop flashover conditions in a large room lined with gypsum drywall. On the other hand, ig-nition of a sofa with polyurethane foam cushions placed in the same room will likely result in flashover provided the fire has sufficient oxygen. he second factor is ventilation. Regardless of the type, quantity, or configuration of fuel, heat release depends on oxygen. A developing fire must have sufficient oxygen to reach flashover, an amount that a sealed room may not provide. The available air supply limits the heat release. If there is insufficient natural ventilation, the fire may enter the growth stage but not reach the heat release rate or gaseous fuel production to transition through flash-over to a fully involved fire. NOTE: The autoignition temperature of CO, the most abundant fuel gas created in most fires, is approxi-mately 1,100° F (595°C). Survival rates for firefighters are extremely low in a flashover. At the floor level, a heat flux of approximately 20 kW/m2 is also typical of rollover conditions at the start of the flashover. Once flames begin to affect a surface, the heat flux could range from 60 to 200 kW/m2 . For frame of reference on heat flux, consider that NIST testing conducted in 2013 (Purtoti, 2013) has shown that SCBA face pieces begin to fail after 5 minutes of exposure to a heat flux of 15 kW/m2 . You must be aware of the following flashover indicators to protect yourself: • Building indicators — Interior configuration, fuel load, thermal properties, and ventilation • Smoke indicators — Rapidly increasing volume, turbulence, darkening color, optical density, and lowering of the hot gas layer and/or neutral plane • Heat indicators — Rapidly increasing temperature in the compartment, pyrolysis of contents or fuel packages located some distance away from the fire, or hot surfaces • Flame indicators — Isolated flames or rollover in the hot gas layers or near the ceiling Levels of the neutral plane observed from the exterior of the structure are also good indicators of fire behavior within the structure as follows (Figure 4.41): • High neutral plane — May indicate that the fire is in the early stages of development. Remember that high ceilings can hide a fire that has reached a later development stage. A high neutral plane can also indicate a fire above your level. • Mid-level neutral plane — Could indicate that the compartment has not yet ventilated or that flashover is approaching. • Very low-level neutral plane — May indicate that the fire is reaching backdraft conditions. This occurrence could also mean that a fire is below you (basement fire or lower story). Observing the neutral plane from outside a structure can provide indications of the behavior of the fire within. Backdraft. A ventilation-limited compartment fire can produce a large volume of flammable smoke and other gases due to incomplete combustion. While the heat release rate from a ventilation-limited fire decreases, elevated temperatures may still be present within the compartment. An increase in ventilation such as opening a door or window can result in an explosively rapid combustion of the flammable gases, called a backdraft. Backdraft oc-curs in a space containing a high concentration of heated flammable gases that lack sufficient oxygen for flaming combustion. When potential backdraft conditions exist in a compartment, the introduction of a new source of oxygen will return the fire to a fully involved state rapidly (often explosively). A backdraft can occur with the creation of a horizontal or vertical opening. All that is required is the mixing of hot, fuel-rich smoke with air. Backdraft conditions can develop within a room, a void space, or an entire building. time a compartment or space con-tains hot combustion products, firefighters must consider potential for backdraft before creating any openings into the compartment. Backdraft indicators include: • Building indicators — Interior configuration, fuel load, thermal properties, amount of trapped fuel gases, and ventilation • Smoke indicators — Pulsing smoke movement around small openings in the building; smoke-stained windows • Air flow indicators — High velocity air intake • Heat indicators — High heat, crackling or breaking sounds • Flame indicators — Little or no visible flame he effects of a backdraft can vary considerably depending on a number of factors, including: • Volume of smoke • Degree of confinement • Temperature of the environment • Pressure • Speed with which fuel and air mix Do not assume that a backdraft will always occur immediately after an opening is made into the building or involved compartment. You must watch the smoke for indicators of potential rapid fire development including the air currents changing direction, or smoke rushing in or out. To some degree, the violence of a backdraft depends upon the extent to which the fuel/air mixture is confined in the compartment. The more confined, the more vio-lent the backdraft will be.

Incipient Stage

Incipient Stage The incipient stage is where a fire begins (Figure 4.30). Once ignition occurs and the combustion process begins, development in the incipient stage depends largely upon the characteristics and configuration of the fuel involved (fuel-limited fire). Air in the compartment provides adequate oxygen to continue fire development. The following describe what occurs when a compartment fire enters the incipient stage: • Radiant heat warms the adjacent fuel and continues the process of pyrolysis. A thin plume of hot gases and flame rises from the fire and mixes with the cooler air in the compartment. • The hot gases in the plume rise until they encounter the ceiling and then begin to spread horizontally. This flow of fire gases is called the ceiling jet. • Hot gases in contact with the surfaces of the compartment and its contents transfer heat to other materials. In this early stage of fire development, the fire has not yet influenced the environment within the compartment to a significant extent. The temperature, while increasing, is only slightly above ambient in areas that the fire, plume, and ceiling jet directly affect. During the incipient stage, occupants can safely escape from the compart-ment, and a portable extinguisher or small hoseline can safely extinguish the fire. he transition from incipient to growth stage can occur quickly (in some cases in seconds), depending on the type and configuration of fuel involved. A visual indicator that a fire is leaving the incipient stage is flame height. When flames reach 2.5 feet (750 mm) high, radiated heat begins to transfer more heat than convection. The fire will then enter the growth stage. CAUTION: Transition from the incipient to growth stages can occur in a matter of seconds depending upon the type and configuration of fuel.

Thermal layer-ing

Once the ceiling jet reaches the walls of the fire compartment, the hot gas layer begins to develop. Thermal layer-ing is the tendency of gases to form into layers according to temperature, gas density, and pressure. Provided that there is no mechanical mixing from a fan or a hose stream, the hottest gases will form the highest layer, while the cooler gases will form the lower layers (Figure 4.34). In addition to the effects of heat transfer through radiation and convection described earlier, radiation from the hot gas layer also acts to heat the interior surfaces of the compartment and its contents. Changes in ventilation and flow path can significantly alter thermal layering he interface between the hot gas layers and cooler layer of air is commonly referred to as the neutral plane because the net pressure is zero, or neutral, where the layers meet. The neutral plane exists at openings where hot gases exit and cooler air enters the compartment. At these openings, hot gases at higher than ambient pressure exit through the top of the opening above the neutral plane. Lower pressure air from outside the compartment entrains into the opening below the neutral plane (Figure 4.35).

Oxygen

Oxygen Oxygen in the air is the primary oxidizing agent in most fires. Normally, air consists of about 21 percent oxygen. The energy release in fire is directly proportional to the amount of oxygen available for combustion. When a fire ignites in an open area where air is plentiful, the fire will release energy based on the given surface area. In con-trast, when a fire ignites within a compartment with limited air supply the fire can only react with oxygen from the compartment's air and any additional oxygen supplied through openings. Thus, in most compartment fires, the energy released is proportional to the limited amount of oxygen available, not the amount of fuel available to burn. At normal ambient temperatures (68°F [20°C]), materials can ignite and burn at oxygen concentrations as low as 15 percent. When oxygen concentration is limited, the flaming combustion will diminish, causing combus-tion to continue in the nonflaming mode. Nonflaming or smoldering combustion can continue at extremely low oxygen concentrations even when the surrounding environment's temperature is relatively low. However, at high ambient temperatures, flaming combustion may continue at considerably lower oxygen concentrations. As the surface-to-mass ratio of a fuel becomes higher (increases), the energy required for ignition is lower (reduced). Effects of Oxygen Concentration Oxygen concentration in the atmosphere has a significant effect on both fire behavior and our ability to survive. Typically, an atmosphere having less than 19.5 percent oxygen is considered oxygen deficient and presents a hazard to persons not wearing respiratory protection, such as SCBA, to provide fresh air. Even if oxygen levels are not low enough to trigger an alarm, reduced levels of oxygen potentially represent a significant hazard in the form of toxic contaminants. Responders should wear SCBA in these circumstances even if oxygen levels are above 19.5 percent. When the oxygen concentration in the atmosphere exceeds 23.5 percent, the atmosphere is considered oxygen enriched and presents an increased fire risk. When the oxygen concentration is higher than normal, materials exhibit different burning characteristics. Materials that burn at normal oxygen levels will burn more intensely and may ignite more readily in oxygen-enriched atmospheres. Some petroleum-based materials will autoignite in oxygen-enriched atmospheres. Many materials that do not burn at normal oxygen levels will burn in oxygen-enriched atmospheres. One such material is Nomex® fire-resistant fabric, which is used in many types of protective clothing. At normal oxygen lev-els, Nomex® does not burn. When placed in an oxygen-enriched atmosphere of approximately 31 percent oxygen, Nomex® ignites and burns vigorously. Fires in oxygen-enriched atmospheres are more difficult to extinguish and present a potential safety hazard. Firefighters may find these conditions in hospitals and other healthcare facilities, some industrial occupancies, and even private homes where occupants use breathing equipment containing pure oxygen. For combustion to occur after a fuel converts into a gaseous state, the fuel must be mixed with air (an oxidizer) in the proper ratio. The range of concentrations of the fuel vapor and air is called the flammable (explosive) range. The fuel's flammable range is reported using the percent by volume of gas or vapor in air for the lower ex-plosive (flammable) limit (LEL) and for the upper explosive (flammable) limit (UEL). The LEL is the minimum concentration of fuel vapor and air that supports combustion. Concentrations below the LEL are said to be too lean to burn. The UEL is the concentration above which combustion cannot take place. Concentrations above the UEL are said to be too rich to burn. Within the flammable range, there is an ideal concentration at which there is exactly the correct amount of fuel and oxygen required for combustion (Figure 4.27). The flammable range is a relatively narrow band of conditions at which a mixture of fuel vapors and air will burn. Table 4.12 presents the flammable ranges for some common materials. Chemical handbooks and documents such as the National Fire Protection Association (NFPA) Fire Protection Guide to Hazardous Materials present the flammable limits for combustible gases. The Guide and other sources normally reports the limits at standard temperature and atmospheric pressures. Variations in temperature and pressure can cause the flammable range to vary considerably.

Self-Sustained Chemical Reaction

Self-Sustained Chemical Reaction The self-sustained chemical reaction involved in flaming combustion is complex. As flaming combustion occurs, the molecules of a fuel gas and oxygen (O2 parts of molecules). Free radicals combine with oxygen or with the elements released from the fuel gas to form new substances (molecules) and even more free radicals. The process also increases the speed of the oxidation reaction. he combustion of a simple fuel such as methane and oxygen provides a good example. Complete oxidation of methane releases the elements needed to create carbon dioxide and water as well as release energy in the form of heat and light. The elements released when methane molecules break down (carbon and hydrogen) recombine with oxygen in the air to form CO2 and H2O (carbon dioxide and water) (Figure 4.28). At various points in the combustion of methane, this process results in production of carbon monoxide and formaldehyde, which are both flammable and toxic. When more chemically complex fuels burn, their combustion creates different types of free radicals and intermediate combustion products, many of which are also flammable and toxic. Flaming combustion is one example of a chemical chain reaction. Sufficient heat will cause fuel and oxygen to form free radicals and initiate the self-sustained chemical reaction. The fire will continue to burn until it consumes the fuel or oxygen or an extinguishing agent, applied in sufficient quantity, interferes with the ongoing reaction. Chemical flame inhibition occurs when an extinguishing agent, such as dry chemical or Halon-replacement agent, interferes with this chemical reaction, forms a stable product, and terminates the combustion reaction.

Thermal Energy

Thermal Energy (Heat) A working knowledge of fire dynamics requires an understanding of temperature, energy, and power or heat release rate. Firefighters often use these terms interchangeably because the differences between the terms are not always understood. Difference between Heat Release Rate and Temperature Heat is the thermal kinetic energy needed to release the potential chemical energy in a fuel. As heat begins to vibrate the molecules in a fuel, the fuel begins a physical change from a solid or liquid to a gas. The fuel emits flammable vapors which can ignite and release thermal energy. This new source of thermal energy begins to heat other, uninvolved fuels converting their energy and spreading the fire. Temperature is the measurement of heat. More specifically, temperature is the measurement of the average ki-netic energy in the particles of a sample of matter. A block of wood at room temperature has stable molecules and is in no danger of ignition. When thermal energy transfers to the wood, the wood is heated, and the temperature of the wood rises because the molecules have begun to vibrate and move more freely and rapidly. Temperature can be measured using several different scales. The most common are the Celsius scale, used in the International System of Units (SI) (metric system), and the Fahrenheit scale, used in the customary system. The freezing and boiling points of water provide a simple way to compare these two scales (Figure 4.12, p. 128) A dangerous misconception is that temperature is an accurate predictor or measurement of heat transfer. It is not. For example, one candle burns at the same temperature as ten candles. However, the heat release rate (kW) of the ten candles is ten times greater than one candle at the same temperature. The increased heat release rate results in an increased heat transfer rate to an object. This energy flow to a unit area (heat flux) is measured in kilowatts per square meter. Translated to an interior fire environment, the temperature in the structure may be within tolerances for personal protective equipment however, the heat flux to the PPE from the fire indicates the real measurement of how long the PPE will protect you. In other words, the temperature tells you it is safe to go in, but the heat transfer rate - not the temperature - tells you how long you can stay in. The two common scales used to measure temperature are the Celsius scale (International System of Units [SI or metric system]) and the Fahrenheit scale used in the Customary System. Sources of Thermal Energy Chemical, electrical, and mechanical energy are common sources of heat that result in the ignition of a fuel. They can all transfer heat, cause the temperature of a substance to increase, and are most frequently the ignition sources of structure fires. Chemical Energy Chemical energy is the most common source of heat in combustion reactions. The potential for oxidation exists when any combustible fuel is in contact with oxygen. The oxidation process almost always results in the produc-tion of thermal energy (Figure 4.13 Self-heating, a form of oxidation, is a chemical reaction that increases the temperature of a material without the addition of external heat. Self-heating can lead to spontaneous ignition which is ignition without the addition of external heat. Oxidation normally produces thermal energy slowly. The energy dissipates almost as fast as it is generated. An external heat source such as sunshine can initiate or accelerate the process. For self-heating to progress to spon-taneous ignition, the following factors are required: • The insulation properties of the material immediately surrounding the fuel must be such that the heat cannot dissipate as fast as it is generated. • The rate of heat production must be great enough to raise the temperature of the material to its autoignition temperature. • The available air supply in and around the heated material must be adequate to support combustion. Rags soaked in linseed oil, rolled into a ball, and thrown into a corner have the potential for spontaneous igni-tion. The natural oxidation of this vegetable oil and the cloth will generate heat if some method of heat transfer such as air movement around the rags does not dissipate the heat. The cloth could eventually increase in tempera-ture enough to cause ignition. he rate of most chemical reactions increases as the temperature of the reacting materials increases. The oxida-tion reaction that causes heat generation accelerates as the fuel generates and absorbs more heat. When the heat generated exceeds the heat being lost, the material may reach its autoignition temperature and ignite spontane-ously. Table 4.5 lists some common materials that are subject to self-heating. Electrical Energy Electrical energy can generate temperatures high enough to ignite any combustible materials near the heated area. Electrical heating can occur in several ways, including the following (Figure 4.14): • Resistance heating — Electric current flowing through a conductor produces heat. Some electrical appliances, such as incandescent lamps, ranges, ovens, or portable heaters, are designed to make use of resistance heating. Other electrical equipment is designed to limit resistance heating under normal operating conditions. • Overcurrent or overload — When the current flowing through a conductor exceeds its design limits, the con-ductor may overheat and present an ignition hazard. Overcurrent or overload is unintended resistance heating. • Arcing — In general, an arc is a high-temperature luminous electric discharge across a gap or through a me-dium such as charred insulation. Arcs may be generated when there is a gap in a conductor such as a cut or frayed wire or when there is high voltage, static electricity, or lightning. • Sparking — When an electric arc occurs, luminous (glowing) particles can form and splatter away from the point of arcing. Mechanical Energy Friction and compression generate mechanical energy (Figure 4.15). The movement of two surfaces against each other creates heat of friction that generates heat and/or sparks. Heat is generated when a gas is compressed. Diesel engines use this principle to ignite fuel vapors without spark plugs. This principle is also the reason that SCBA cylinders feel warm to the touch after they are filled. When a compressed gas expands, the gas absorbs heat. This absorption accounts for the way the cylinder cools when a CO2 extinguisher is discharged. Heat Transfer The transfer of heat from one point or object to another is part of the study of thermodynamics. Heat transfer from the initial fuel package (burning object) to other fuels in and beyond the area of fire origin affects the growth of any fire and is part of the study of fire dynamics. Heat transfers from warmer objects to cooler objects because heated materials will naturally return to a state of thermal equilibrium in which all areas of an object are a uniform temperature. Objects at the same temperature do not transfer heat. he rate at which heat transfers is related to the temperature differential of the bodies and the thermal conductivity of the materials involved. The greater the temperature differences between the bodies, the greater the transfer rate. A material with higher thermal con-ductivity will transfer heat more quickly than other materials. Heat transfers from one body to another by three mechanisms: conduc-tion, convection, and radiation. Conduction Conduction is the transfer of heat through and between solids. Conduction occurs when a material is heated as a result of direct contact with a heat source (Figure 4.16). Conduction results from increased molecular motion and collisions between a substance's molecules, resulting in the transfer of energy through the substance. The more closely packed the molecules of a substance are, the more readily it will conduct heat. For example, if a fire heats a metal pipe on one side of a wall, heat conducted through the pipe can ignite wooden framing components in the wall or nearby combustibles on the other side of the wall. The friction of the match head rubbing across the box's striker generates the heat needed to ignite the match. When gas is compressed, it generates hea Conduction occurs when heat is transferred between solid objects Heat transfer due to conduction is dependent upon three factors: • Area being heated • Temperature difference between the heat source and the material being heated • Thermal conductivity of the heated material Table 4.6 shows the thermal conductivity of various common materials at the same ambient temperature (68°F [20°C]). For example, copper will conduct heat more than seven times faster than steel. Likewise, steel is nearly forty times as thermally conductive as concrete. Air is the least able to conduct heat of most substances, so it is a very good insulator. Insulating materials slow the conduction of heat from one solid to another. Good insulators are materials that do not conduct heat well because their physical makeup disrupts the point-to-point transfer of heat or thermal energy. The best commercial insulators used in building construction are those made of fine particles or fibers with void spaces between them filled with a gas such as air. Gases do not conduct heat very well because their molecules are relatively far apart. Convection Convection is the transfer of thermal energy by the circulation or movement of a fluid (liquid or gas) (Figure 4.17). In the fire environment, convection usually involves transfer of heat through the movement of hot smoke and fire gases. As with all heat transfer, the heat flows from the hot fire gases to the cooler structural surfaces, building contents, and air. Convection may occur in any direction. Vertical movement is due to the buoyancy of smoke and fire gases. Lateral movement is usually the result of pressure differences (movement from high to low pressure) Heat transfer due to convection is dependent upon three factors: • Area being heated • Temperature difference between the hot fluid or gas and the material being heated • Turbulence and velocity of moving gases Convection is the transfer of heat by the circulation of liquids or gases Radiation Radiation is the transmission of energy as electromagnetic waves, such as light waves, radio waves, or X-rays, without an intervening medium (Figure 4.18). Radiant heat can become the dominant mode of heat transfer as the fire grows in size and can have a significant effect on the ignition of objects located some distance from the fire. Radiant heat transfer is also a significant factor in fire development and spread in compartments. Radiation is the transfer of heat by electromagnetic waves without another medium to transfer the heat energy. The effects of radiant heat diminish as the distance between the origin point and an exposure increase As the heat release rate or temperature of the source increases, the thermal radiation given off will also increase Numerous factors influence radiant heat transfer, including: • Nature of the exposed surfaces — Dark-colored materials emit and absorb heat more effectively than light-colored materials; smooth or highly-polished surfaces reflect more radiant heat than rough surfaces. • Distance between the heat source and the exposed sur-faces — Increasing distance reduces the effect of radiant heat (Figure 4.19). • Temperature of the heat source — Unlike other methods of heat transfer that depend on the temperature of both the heat source and exposed surface, radiant heat transfer primarily depends on the temperature of the heat source. As the temperature and heat release rate of the heat source increases, the radiant energy also increases (Figure 4.20, p. 134). As an electromagnetic wave, radiated heat energy travels in a straight line at the speed of light. The heat of the sun is the best example of radiated heat transfer. The energy travels at the speed of light from the sun through space (a vacuum) until it strikes and warms the surface of the earth. Radiation is a common cause of exposure fires. As a fire grows, it radiates more energy which other objects absorb as heat. In large fires, it is possible for the radiated heat to ignite buildings or other fuel packages a con-siderable distance away. Radiated heat travels through vacuums and air spaces that would normally disrupt conduction or convection. However, materials that reflect, absorb, or scatter radiated energy will disrupt the heat transmission. While flames have high temperature resulting in significant radiant energy emission, hot smoke or flames in the upper layer can also radiate significant heat. The Importance of Understanding Temperature and Heat Transfer Rate ) from radiated heat emitted from flames or hot surfaces such as the walls and ceil-ing may cause PPE failure even when the temperature of the gases within a compartment are within acceptable limits. Traditionally, firefighters have focused on the gas temperature, stated in degrees on the Fahrenheit or Celsius scale, within a compartment that is on fire as being the best indicator of the thermal hazard. However, National Institute of Standards and Technology (NIST) laboratory tests show that these temperature measurements may not accurately account for radiated heat. SCBA facepieces, especially, are susceptible to radiated heat flux (Putorti, 2013). Personal protective equipment (PPE) is designed to insulate the wearer from a specified amount of heat long enough to extinguish the fire or exit the compartment under a limited set of conditions. PPE will not protect you indefinitely. While temperature measurements are a useful tool, relying upon per-sonal, situational awareness "in the moment" is still essential for monitoring PPE's performance during operations. Interaction among the Methods of Heat Transfer The methods of heat transfer rarely occur individually during a fire. The fire radiates heat, causes convection of heat through hot fuel gases, and conducts heat through burning materials or metals that are involved in the fire. Convected heat and radiated heat that reaches walls and ceilings heats those surfaces which, in turn, begin to conduct heat to whatever extent possible based upon the material's thermal conductivity. One side of the object is warm and slowly warms through the object until the opposite side is of equal temperature with the heated side. A heated surface will then, in turn, begin to radiate heat which could lead to ignition, combustion, convection, and so on. This cycle continues until interrupted. A good example of this interaction is how your PPE absorbs heat during interior operations. Convected and radiated heat will begin to heat the exterior of your PPE. The longer you are in the heated environment, the more heat that surface will absorb. The PPE has low thermal conductivity, so it will conduct heat slowly. However, even-tually the interior surface of the PPE will heat to the same level as the exterior. Wherever the gear is compressed against skin or underclothing, heat will be conducted faster. Table 4.7 shows various responses of human skin and PPE as they are heated. Where the PPE is not in contact, it will radiate heat to the insulating air layer between your body and the inte-rior surface of the gear. This transferred heat can cause heat stress and will eventually cause PPE to fail. The heat absorption and build-up in PPE is a direct result of all of the heat transfer methods acting at the same time.

Thermal Layering

Thermal Layering Once the ceiling jet reaches the walls of the fire compartment, the hot gas layer begins to develop. Thermal layer-ing is the tendency of gases to form into layers according to temperature, gas density, and pressure. Provided that there is no mechanical mixing from a fan or a hose stream, the hottest gases will form the highest layer, while the cooler gases will form the lower layers (Figure 4.34). In addition to the effects of heat transfer through radiation and convection described earlier, radiation from the hot gas layer also acts to heat the interior surfaces of the compartment and its contents. Changes in ventilation and flow path can significantly alter thermal layering. The flow path is defined as the space between the air intake and the exhaust outlet. Multiple openings (intakes and exhausts) create multiple flow paths. he products of combustion from the fire begin to affect the environment within the compartment. As the fire continues to grow, the hot gas layer within the fire compartment gains mass and energy. As the mass and energy of the hot gas layer increases, so does the pressure. Higher pressure causes the hot gas layer to spread downward within the compartment and laterally through any openings such as doors or windows. If there are no openings for lateral movement, the higher pressure gases have no lateral path to follow to an area of lower pressure. As a result, the hot gases will begin to fill the compartment starting at the ceiling and filling down. Isolated or intermittent flames may move through the hot gas layer. Combustion of these hot gases indicates that portions of the hot gas layer are within their flammable range, and that there is sufficient heat to cause igni-tion. As these hot gases circulate to the outer edges of the plume or the lower edges of the hot gas layer, they find sufficient oxygen to ignite. This phenomenon frequently occurs before more substantial involvement of flam-mable products of combustion in the hot gas layer. The appearance of isolated flames is sometimes an immediate indicator of flashover. he interface between the hot gas layers and cooler layer of air is commonly referred to as the neutral plane because the net pressure is zero, or neutral, where the layers meet. The neutral plane exists at openings where hot gases exit and cooler air enters the compartment. At these openings, hot gases at higher than ambient pressure exit through the top of the opening above the neutral plane. Lower pressure air from outside the compartment entrains into the opening below the neutral plane (Figure 4.35).

(heat flux)

dangerous misconception is that temperature is an accurate predictor or measurement of heat transfer. It is not. For example, one candle burns at the same temperature as ten candles. However, the heat release rate (kW) of the ten candles is ten times greater than one candle at the same temperature. The increased heat release rate results in an increased heat transfer rate to an object. This energy flow to a unit area (heat flux) is measured in kilowatts per square meter. Translated to an interior fire environment, the temperature in the structure may be within tolerances for personal protective equipment however, the heat flux to the PPE from the fire indicates the real measurement of how long the PPE will protect you. In other words, the temperature tells you it is safe to go in, but the heat transfer rate - not the temperature - tells you how long you can stay in.