Fundamentals of Fire Behavior and Building Construction PDF

CHAPTER 3 Fundamentals of Fire Behavior and Building Construction Courtesy...

CHAPTER 3 Fundamentals of Fire Behavior and Building Construction Courtesy of Jamie Novak, Novak Fire Investigations and St. Paul Fire Dept. KEY TERMS ambient, p. 35 flame spread, p. 40 overhaul, p. 40 backdraft, p. 41 flammable liquid, p. 36 plume, p. 35 buoyancy, p. 35 flashback, p. 41 radiation, p. 47 conduction, p. 45 flashover, p. 49 self-heating, p. 40 convection, p. 46 fuel load, p. 54 smoldering combustion, p. 33 entrainment, p. 35 glowing combustion, p. 38 stoichiometry, p. 39 fire-resistive, p. 73 heat release rate, p. 43 vented, p. 64 firestorm, p. 47 heat transfer, p. 45 ventilation, p. 38 flameover, p. 56 incipient, p. 54 watt, p. 43 OBJECTIVES After reading this chapter, you should be able to: Describe the four conditions that must exist for a fire to occur. Recognize the details involved in the typical flaming combustion of organic fuels. Describe several states of combustion. Explain the three basic methods of heat transfer. Calculate the visible flame height given the heat release rate and effective diameter of a fire. Discuss the four phases of fire development. Explain the effects of environmental conditions on fire. Distinguish between Type III (ordinary) construction and Type V (wood frame) construction as seen in single-family residences in the United States. 32  For additional review materials, appendices, and suggested readings, visit www.bradybooks.com and follow the MyBradyKit link to register for book-specific resources. Register for MyFireKit by following directions on the MyFireKit student access card provided with this text. If there is no card, go to www.bradybooks.com and follow the MyFireKit link to Buy Access from there. F ire is an exothermic oxidation reaction that proceeds at such a rate that it generates detectable heat and light. As indicated in the previous chapter, two types of combustion (or fire) are (1) flaming and (2) smoldering. Flaming combustion, as defined in Chapter 2, is a gaseous combustion in which both the fuel and oxidizer are gases. Smoldering combustion smoldering combustion The direct combina- involves the surface of a solid fuel with a gaseous oxidizer (usually the oxygen in air). Nearly tion of a solid fuel with all destructive fires are flaming combustion. The smoldering fire is not uncommon, either atmospheric oxygen to alone or in combination with flames. For example, the nonflaming fire in a mattress or a pile generate heat in the absence of gaseous of sawdust is a good illustration of a smoldering fire (which can do extensive damage), as is the flames; see glowing charcoal fire used for barbecues. Many destructive flaming fires began as small smoldering combustion. fires. The differences between flaming and smoldering combustion are a result of the nature and condition of the fuel, as well as the availability of oxygen, all of which influence the rate at which heat is being produced. If the smoldering mattress or sawdust is stirred up, it may develop into a flaming fire because more fuel is brought into contact with oxygen. On the other hand, the glowing charcoal has little or no flame because the compounds that could be volatilized into gases (and thereby contribute to an open flame) were lost at the time the original fuel was charred or coked. The differences between a wood fire and a fire of charcoal made from wood are discussed later, in the section dealing with the properties of fuels. Basic Combustion For a fire to occur, the following conditions must exist: Combustible fuel must be present. An oxidizer (such as oxygen in air) must be available in sufficient quantity. Energy as some means of ignition (e.g., heat) must be applied. The fuel and oxidizer must interact in a self-sustaining chain reaction. The first three elements listed have long been described as the fire triangle, but the fourth must also be present if the fire created is to be continuous (self-sustaining), thus creating what is called the fire tetrahedron1 (see Figure 3-1). Although these requirements appear obvious, it is true, nevertheless, that statements have been made in court describ- ing fires burning on bare concrete floors devoid of fuel. Absolute absence of any source of heat is sometimes claimed. In general, disregard of the simple requirements just listed is not at all uncommon. It must be remembered that removal of any one of the four ele- ments results in extinguishment, and a careful analysis of all the factors is required to establish what happened, since mechanisms of ignition, combustion, and even sources of fuel may not be obvious at first glance. Some are transient and require careful analysis of possible mechanisms before their presence and effects can be reasoned out. Chapter 3 Fundamentals of Fire Behavior and Building Construction 33  FIGURE 3-1 The fire tetrahedron: fuel, oxidizer, and heat interacting in a self-sustaining chemical reaction. HEAT OX YG EL EN FU CHAIN REACTION Experience with the common gaseous, liquid, and solid fuels is so widespread that misjudgments of their contributions to a fire would not be expected. However, there are many less common materials (e.g., plastics and metals) with which experience is less wide- spread. In addition, the mere presence of a suitable fuel in conjunction with the other con- ditions does not guarantee that a fire will result. The fuel must be present in a suitable physical state to be ignited. For example, common furnace oil is an excellent fuel but when spread on a concrete slab, it generally resists every effort to ignite it, even with a blowtorch. If that same oil is soaked into a cloth wick, it is easily ignited. Another exam- ple of the importance of the fuel’s state is newspaper, which is so helpful in starting fires but can be used to extinguish some small fires. Proper interpretation of a fire situation requires knowledge of the properties of the fuel, its physical state, and the character and dynamics of the fire itself. Thus, the knowledge of availability and suitability of fuels in specific instances may require more than everyday experience. Flaming Fire A fire characterized by flame is the most common type. Here the flame actually is the fire—the production of gaseous reaction products with the evolution of heat and light. The color of the light emitted is determined in part by the elements in the reacting mix- ture. The color emitted by a hydrocarbon gas burning well mixed with air (e.g., a cor- rectly adjusted oxyacetylene torch) is clear blue. The gaseous flame is made more visible when carbon and other solid or liquid by-products resulting from incomplete combustion are raised to incandescent temperatures and glow—red, orange, yellow, or white, depending on their temperature. The reactions of a flaming fire are, under ordinary circumstances, oxidative, with oxygen from the air as the oxidizer. An important fact to remember is that there cannot be a flaming fire unless a gas or vapor is burning. This holds true whether the fuel is a gas to begin with or a vapor evaporated from a liquid or distilled or driven off from a solid. The flame is a totally gaseous reaction. Liquid fuels can produce only flaming fires, since liquids themselves do not burn and do not pyrolyze to leave a solid char (except in unusual circumstances) to support smoldering. Solid fuels often produce predominantly flaming fires, although they are frequently accompanied by glowing fire. Wood decomposes readily under the influence of heat to generate combustible gases; therefore, it tends to burn in flaming combustion, especially in the early stages of the fire. Later, the charcoal formed will continue to combust as a smol- der fire. Coal, in contrast, has less volatile components and tends to give off proportionately 34 Chapter 3 Fundamentals of Fire Behavior and Building Construction  C FIGURE 3-2 Typical H2O Buoyant smoke plume flaming combustion of organic fuel showing HCN CO2 decomposition region HCN where volatized fuel decomposes to simpler H2O species before combustion, CO CO2 and intermediate products are formed. CO2 H2O Entrainment (mixing, diffusing, and cooling) C CO2 CO2 CO H 2O H2 H2 C CH3OH CH CH HCN CO CH3 C CHO Radiant heat CN CH O Radiant heat C 2 CH4 H2 Flame plume C CH CO CH CH3 C CH C C 2 CH CH2 CHO H H CH2 CH3 CH CH2 C CH3 O2 C H CH2O O2 CH Decomposition region H Air N2 N2 Air Vaporized fuel Entrainment Pyrolysis zone (supplying O2) Fuel smaller quantities of combustible gases than wood and a lesser amount of open flame. Figure 3-2 illustrates the dynamics of flaming combustion of a solid or liquid. Note that heat generated by the flame radiates onto the surface of the fuel, raising its temperature, evaporating it if it is a liquid, and pyrolyzing it if it is a solid. [The equilibrium surface tem- perature of a solid fuel is controlled by its chemistry. For most fuels it is on the order of 350°C to 500°C (660°F to 930°F)].2 The molecules of vapor boiled out of the liquid or pyrolyzed out of the solid are dissociated or torn apart by the chemical conditions within the flame (sometimes called reducing conditions). They can then readily combine with oxy- plume The convective gen coming from the outside. The plume of hot gases (and solid [soot] or liquid [aerosols] column of hot gases products from incomplete combustion) rises by buoyancy (being hotter and therefore less generated by a flame. dense than the ambient air in the room). This movement draws surrounding air and its buoyancy Tendency oxygen into the flame, entraining and mixing it with the fuel gases, thereby maintaining or ability to rise or float the reaction. Entrainment is the drawing of air (or other gases) into the buoyant plume. in air or liquid as a result If a flaming fire is confined to a closed room where the oxygen being used in the flames of a difference in density. is not replaced, the oxygen concentration in the surrounding air will fall from its normal ambient Surrounding level of about 20.9 percent. The rate at which the concentration of oxygen drops depends conditions. on the size of the fire, the size of the room, and the amount of fresh air that can leak into the room from any small openings. Generally speaking, when the oxygen concentration entrainment The mixing of two or falls below about 15 percent in the vicinity of the flame, the combustion rate of ordinary more fluids as a result combustibles begins to decrease. Eventually, the concentration reaches a limiting oxygen of laminar flow or concentration that will no longer support flaming combustion, and the flames will die out. movement. Chapter 3 Fundamentals of Fire Behavior and Building Construction 35  The oxygen limit that will actually extinguish the flames is dependent on the nature of the fuels involved and on the temperature of the combustion gases—the higher the tempera- tures produced, the lower the oxygen concentration can drop and still support flames. For instance, oxygen concentrations between 5 and 8 percent have been observed in room tests flammable liquid A with flammable liquids where the ceiling temperatures were on the order of 900°C to liquid having a flash 1,000°C; and in post-flashover room fires, oxygen concentrations between 0 and 5 percent point below 38°C have been observed when temperatures in the hot gas layer were over 1,000°C.3 (100°F). Structure of Flames Most of the flames we encounter in fire investigation are diffusion flames. This means that the gases or vapors that are supporting the flame diffuse outward (or upward) from the surface of the fuel and the oxygen for combustion diffuses toward the fuel from the surrounding air. If the concentration of fuel and the concentration of oxygen are plotted as a function of distance from the fuel surface, a plot like Figure 3-3c results. At some distance, Vertical fuel (solid) Pool fire Oxygen Flames Radiant in air heat Radiant Vapors heat from fuel Fuel Oxygen in air (b) Flames Fuel Vapors generated surface by pyrolysis of fuel surface (a) 21% Saturation Concentration of Concentration of oxygen (%) fuel vapors Combustion zone 0 0 Distance from fuel surface (c) FIGURE 3-3 (a) Diffusion of fuel vapor away from vertical fuel surface as oxygen diffuses toward fuel surface. (b) Diffusion of fuel vapor upward from horizontal fuel surface. (c) Plot of concentration of O2 and fuel vapor as a function of the distance from the fuel surface. 36 Chapter 3 Fundamentals of Fire Behavior and Building Construction the concentrations of fuel vapors and oxygen are correct, the mixture can be ignited, and a flame can be maintained in that zone as long as the rate of fuel generation is maintained.4 (The heat from the flame radiating onto the fuel surface helps maintain that supply from liquid or solid fuels.) The rate at which fuel is delivered to the flame is dependent on the total heat flux reaching the fuel surface and the latent heat of vaporization (for liquids) and latent heat of gasification (for solids). In combustion explosions, the fuel vapors and air are mixed together and then ignited in what is called a premixed flame. In gas appli- ances, this mixing is created by the gas jet. If properly adjusted, the gas is fed into the combustion zone at a rate and at a concentration at which nearly complete (and con- trolled!) combustion occurs in the desired location. A small flame such as that of a candle is called a laminar flame, because the zones of high fuel concentration, dissociation (reducing), mixing, and combustion are well defined in layers surrounding the candle wick. The airflow around the flame is such that an envelope of very high temperatures is maintained around the source of fuel. In a candle, this high-temperature zone allows the combustion of the soot created by the pyrolysis (described in Chapter 2) of the wax to take place at very high temperatures (with the soot particles radiating visible light [incandescence] in the yellow-white colors of very high temperature surfaces). By the time the soot escapes the high-temperature zone it has been (almost) completely consumed and turned into CO2 (as in Figure 3-4). If the flame becomes too large, the smooth flow of the air cannot be maintained, or if the envelope is disturbed by a sudden draft, this, in effect, tears open the high-temperature zone, allowing some of the soot and other intermediate products to escape unburned. The presence of the ragged tips of flames like these are a sign that the combustion has Soot-formation region (luminous) Inner edge of diffusional combustion 1200°C 1400°C 1200°C 800°C 200°C 1000°C Hydrocarbon cracking region (dark) 800°C 10 mm Outer edge of diffusional combustion 500°C Pseudo-premixed combustion Molten wax (blue luminosity) (C20H42) 20 mm (a) (b) FIGURE 3-4 (a) Typical laminar flame of candle showing blue luminosity at base. Courtesy of John D. DeHaan. (b) Temperature distribution in laminar candle flame showing a zone of very high temperature. Source: From Fire, H. Rossotti, St. Anne’s College, Oxford University, United Kingdom. Chapter 3 Fundamentals of Fire Behavior and Building Construction 37 FIGURE 3-5 Gasoline pool fire on plywood produces a typical turbulent flame plume. Courtesy of Jamie Novak, Novak Fire Investigations and St. Paul Fire Dept. become turbulent (as in Figure 3-5). Turbulent combustion dominates nearly all fires, glowing combustion causes the flicker that we associate with most fires, and in some cases can be extremely tur- The rapid oxidation of bulent. a solid fuel directly with atmospheric oxygen creating light and heat Smoldering Fire in the absence of flames. A smoldering fire is characterized by the absence of flame but the presence of very hot ventilation A tech- materials on the surface of which combustion is proceeding. If the temperature of that sur- nique for opening a burning building to face is high enough [500°C (900°F) or higher], a visible glow or incandescence can be seen. allow the escape of As we shall see later, the color of the incandescent glow on the surface is related to its heated gases and smoke temperature, as is also true of the airborne particles in the flame. Some visible colors and to prevent explosive their related temperatures are illustrated in Table 3-1. (Note that these are not the temper concentrations (smoke colors visible on polished metal surfaces after heating and cooling.) “Glowing” and “smol- explosions or backdrafts) and to allow the dering” are often used interchangeably [although Babrauskas describes glowing combustion advancement of hose as a non-self-sustaining condition (usually as a result of forced ventilation), while smolder- lines into the structure. ing is self-sustaining solid gas combustion].5 TABLE 3-1 Visual Color Temperatures of Incandescent Hot Objects APPROXIMATE APPROXIMATE COLOR TEMPERATURE (°C) TEMPERATURE (°F) Dark red (first visible glow) 500–600 930–1,100 Dull red 600–800 1,110–1,470 Bright cherry red 800–1,000 1,470–1,830 Orange 1,000–1,200 1,830–2,200 Bright yellow 1,200–1,400 2,200–2,550 White 1,400–1,600 2,550–2,910 Source: Data taken from C. F. Turner and J. W. McCreery, The Chemistry of Fire and Hazardous Materials (Boston: Allyn and Bacon, 1981), 90. See also D. D. Drysdale, An Introduction to Fire Dynamics, 2nd ed. (Chichester, UK: Wiley, 1999), 53. 38 Chapter 3 Fundamentals of Fire Behavior and Building Construction  A charcoal or coal fire is a good example of smoldering combustion. In the past it was common to see a blacksmith heating iron objects in a fire made with coal through which a forced-air draft passed. These fires showed very little flame, but the glowing fire induced by the air draft was intensely hot, more so than most flaming fires. Similar situations are sometimes seen in the deep piles of hot coals left on collapse of large wood-frame struc- tures. When there is sufficient draft through the coals and a layer of insulative ash to retain the heat, the temperatures reached can melt copper, steel, silver, and even cast iron, pro- ducing unusual artifacts. The duration of such high temperatures in glowing coals can be considerably longer than the high temperatures reached during the open-flame stages of the fire, making it possible to melt even massive solid objects. Because the smoldering fire involves two phases, a solid and a gas, the ratios of oxidizer and fuel are not of direct concern, although the chemistry of the reaction still determines the total consumption of each. The proportions of fuel, oxygen, and final products are called the stoichiometry of the reaction. Perfectly balanced chemical reactions, which result in the stoichiometry Balance complete reaction of all starting materials with no waste, are called stoichiometric or ideal of chemical reactants reactions. Although glowing/smoldering combustion is often considered the end (decay) and products. stage of a fire, it can occur at any time during a fire depending on the ventilation conditions. Figure 3-6 shows glowing combustion on the surface of a burning wood timber where the incoming draft is so strong it is pushing the vapors generated from the wood aside and forc- ing very rapid glowing combustion of the surfaces. The vapors burn as flames to the sides and rear of the timber, where they can mix with air. Smoldering combustion can continue even in an oxygen-deficient atmosphere (below 5 percent), in part due to the very high temperatures at the oxidizing surface. In the com- bustion of ordinary solid fuels, a transition from flaming to smoldering may be seen as the oxygen level drops in an enclosure, which can revert back to open flames if a fresh supply of oxygen is introduced. In this instance, unlike in a flaming fire, the fuel and oxidizer FIGURE 3-6 Glowing combustion on timber with a very strong draft pushing combustion gases away from the burning surface. Courtesy of Lamont “Monty” McGill, Fire/Explosion Investigator. Chapter 3 Fundamentals of Fire Behavior and Building Construction 39  are not premixed or even mixed at the moment of combustion but react as solid surfaces over which the oxidizing gas passes. Inasmuch as fuels are generally organic materials that are often reduced to charcoal (or coke) as the volatile decomposition products are distilled out and burned, most fires reach a smoldering state in their later phases. It must be remembered that the smoldering material here is not the original fuel but a derived fuel resulting from the earlier portions of the fire. An important feature of the glowing or smoldering fire—especially when there is a forced draft, as in the blacksmith’s forge—is that the limited flame that accompanies it results chiefly from the burning of carbon monoxide (CO) to carbon dioxide (CO2). The carbon monoxide is generated by incomplete combustion of carbon due to insufficient oxygen and is further oxidized with a hot, but rather small, generally bluish, flame. In the investigation of most fires, the smoldering phase generally represents either the incipient stage or the last part of the fire process. As an incipient fire, smoldering may be very prolonged and may do a great deal of damage before flames occur. The investigator and indeed the firefighter may be much more concerned with flaming fire and its effects overhaul The fire- in the earlier stages of the fire. Much of the post-fire cleanup or overhaul by firefighters fighting operation of at fire scenes, however, is directed at seeking out smoldering spots and ensuring that they eliminating hidden do not rekindle into a destructive fire later. The investigator is primarily concerned with flames, glowing embers, or sparks that may the initial small fire, whatever its nature, that grew into the large one. rekindle the fire, usually Some exceptions to these observations must be considered. A fair proportion of flam- accompanied by the ing fires follow a period of smoldering. When the fire is ignited in a suitable organic fuel removal of structural or in an environment in which ventilation is very limited, it does not generate the heat contents. release rate necessary to burst into flames. This slow combustion may proceed for an appreciable time before the flaming fire erupts and yet do a great deal of damage. The rate of spread of a smoldering fire may be on the order of 0.0001 m/s (0.2 to 0.25 in./min) flame spread The as compared with the 0.001 m/s to 0.01 m/s (0.04 to 0.4 in./s) typical of flame spread rate at which flames across solid fuels.6 The conditions for transition from smolder to flame are not well extend across the understood at this time and have not been successfully modeled. surface of a material (usually under specific Smoldering fires may be produced by the ignition of latex rubber or natural fiber conditions). (cotton) mattresses or upholstery padding in residences; hay or grain piles in barns; or coal dust, sawdust, or other finely divided cellulosics in industrial premises. Fires that appear to start at times when there is no obvious source or reason for ignition are often attributed to such prolonged ignition mechanisms. Proof of such mechanisms requires careful consideration of all the following factors: initial source of ignition (there must be some source of heat to trigger the process); evidence of prolonged smoldering (adequate fuel mass); nature of the fuel (its ability to form a rigid porous char that will allow air (oxygen) to percolate into the mass of fuel); and cause (or sequence of events) that caused the fire to reach the open-flame state, that is, “burst into flame.” Stay Safe Even if the fire never reaches flaming combustion, considerable destruction may The extensive produc- result from the slow, long-term burning of closely packed fuels such as paper, grain, cloth, tion of carbon monoxide typical of smoldering or upholstery stuffing. The extensive production of carbon monoxide typical of smolder- fires makes them serious ing fires makes them serious life safety risks. The smoke and heat generated by even mod- life safety risks. est smoldering fires can seriously damage other contents of a room or building. Even in the absence of open flame, a smoldering fire is still a fire because it can accomplish the destruction of the fuel by the thermal processes of pyrolysis and combus- self-heating An tion. All smoldering fires produce high concentrations of CO and visible smoke, since exothermic chemical or such combustion is never complete. The mechanisms of self-heating, by which a mass of biological process that can generate enough fuel can reach its ignition conditions in the absence of any outside ignition source will be heat to become an considered in Chapter 6. A destructive smoldering fire need not be accidental. Some ignition source. incendiary fires are started in a suitable fuel but in a restricted space where the oxygen 40 Chapter 3 Fundamentals of Fire Behavior and Building Construction supply is deficient and flames are suppressed. Enough air may be available to sustain a smoldering fire until the fuel is exhausted or more oxygen becomes available. Another consequence of smoldering results from the discarding of previously flaming, but supposedly extinguished, materials that are still combusting. Discarded matches that are not specially treated will continue to smolder for some time after the flame is extin- guished and can initiate a fire in a suitable fuel. The smoldering cigarette certainly falls into this category (see later section). Hot coals, spilled from fireplaces or removed from barbecues or other similar sources, will start larger fires if they contact suitable fuel. These will be discussed in Chapter 6 on sources of ignition. Explosive Combustion Explosive combustion may not be recognized as a fire. It requires consideration here because this type of combustion does accompany fire, often as an initiating factor and some- times during a conventional fire when favorable circumstances develop. Because this type of combustion is considered in a later chapter, it will be discussed only briefly at this point. Explosive combustion can occur when vapors, dusts, or gases, premixed with an appropriate amount of air, are ignited. Under these circumstances, the combustion that results is not different from that which happens in the burning of these materials, except that the premixing allows the entire combustion to occur in a very short period of time. Thus, all the heat generation, creation of combustion products, and the expansion of those products—which normally would require an appreciable time—become an almost instantaneous event that is recognized as an explosion by its mechanical effects. The event may be very forceful and produce great damage, including the blowing apart of an entire building, or it may be so small as to produce only an audible pop. Sometimes, the first phase of a fire is an explosion, as flammable gases or vapors have accumulated from some source, mixed with appropriate amounts of air, and become ignited. Such explosions are sometimes followed by flaming fire, especially if there is residual unburned fuel after the initial combustion. As we shall see later, the brief dura- tion of such events precludes ignition of all but the thinnest, lightest fuels to create a fol- lowing fire. In such instances, it is the cause of the initial explosion that is the concern of the investigator, since it is known that flammable gases were generated by some source and that they were later ignited. It remains for the investigator to identify the source of the fuel vapor, a suitable ignition source, and the conditions that brought them together. When a vapor explosion occurs during a fire, it is usually an indication that a new source of fuel was made available locally and in some quantity. The most common source of this material is a sealed can, bottle, or drum of an ignitable liquid that breaks or bursts because the heat of the fire has expanded the contents beyond the strength of the con- tainer. Being heated above its normal boiling point, this fuel vaporizes immediately upon release and forms an explosive mixture, which is then ignited when an ignition source backdraft A defla- such as a flame is present. From the investigative standpoint, this event has two values. grative explosion of gases and smoke from First, it proves that the container was sealed initially and was therefore not involved in an established fire that starting the fire. Second, it proves that an extensive fire and much heat surrounded the has depleted the oxygen container before it burst, and this situation was the result of fire progression from else- content of a structure, where, an unusual accident, or the work of an arsonist. most often initiated by introducing oxygen These facts can be of importance in the interpretation of many fires. Another cause through ventilation or of an explosion during a fire is a smoke explosion, also called a backdraft or flashback. structural failure. In energetic fires in confined spaces, a great deal of fuel may be pyrolyzed into com- bustible gases and vapors that cannot burn because there is insufficient oxygen. If fresh flashback The ignition air is suddenly admitted to the compartment (by the failure of a window or the opening of a gas or vapor from an ignition source back of a door) and mixed with this hot, fuel-rich mixture, a destructive combustion explosion to a fuel source (often can occur. The dynamics of explosive combustion of gases and vapors are discussed in seen with flammable Chapters 4 and 12. Explosions involving detonating (high) explosives are not combustion liquids). Chapter 3 Fundamentals of Fire Behavior and Building Construction 41  based, as they do not involve the combination of a fuel with atmospheric or chemical oxy- gen. They occur when the explosive material itself dissociates (comes apart) and then recombines with an extremely high rate of reaction and heat release rate. These chemical reactions will be discussed in more detail in Chapter 12. Heat Aside from the significance of the chemical reactions that produce a fire, the most funda- mental and important property of the fire is heat, which is a measure of energy. Heat ini- tially starts a fire, and the fire produces heat. As will be discussed in the chapter on ignition, every method by which a fire may be ignited involves the application or genera- tion of heat, and nothing but heat is ever necessary to start the fire when the fuel and its environment are suitable. However, most of the destructiveness of a fire is the direct result of the heat gener- ated. Heat produces the damage to the structure, intensifies the fire, is the means by which the fire spreads and enlarges, and provides the greatest barrier to the extinguishment of the fire. When the heat can be properly studied and understood in connection with a fire, the sequence and cause of that fire will generally be very clear. There are several special considerations of heat, as applied to a fire investigation, that require understanding: 1. Heat as it applies to igniting the fire (This topic is developed further in Chapter 6 on ignition.) 2. Heat as it applies to the increase of the rate of chemical reactions (including fires) as they develop 3. The transfer of heat as the factor controlling the spread of fire when the additional conditions of available fuel and oxygen are met 4. The effects of heat on materials that survive the fire to bear indicators of the fire’s intensity, duration, or direction of spread HEAT AND THE RATE OF REACTION The rate of all chemical reactions is dependent on temperature. With rare exceptions, all reactions have a higher rate at greater temperatures. The Q10 value is the increase in rate of the reaction that results from raising the temperature 10°C (18°F). For most oxidation reactions, this value is two or more. Therefore, it can be generalized that the rate of a combustion reaction usually doubles with every increase of 10°C. The great importance of temperature stems, at least in part, from the fact that the fire generates much heat and raises the temperature of the reacting components, thus increasing the rate of reaction. This in turn generates more heat, thus again increasing the rate of reaction. This is the key to the fourth element of the fire tetrahedron—the chain reaction. If it were not for the diminishing availability of fuel and oxygen, combined with loss of heat to the larger sur- roundings, that brings this chain reaction under control, every fire would become a vio- lent and rapid holocaust. Because these factors do act to control fire, the most immediate significance of the concept of rate of reaction is in connection with those processes that lead to spontaneous combustion. This subject is treated separately in a later section; how- ever, it can be briefly considered in the following light. Assume that a system of fuel and oxygen is properly contained, insulated, and pro- portioned, and a very limited exothermic reaction is proceeding. This might be a slow oxi- dation, bacterial activity, or some other type of chemical transformation in which heat is generated. If the heat cannot escape as fast as it is generated, the temperature will rise. As it rises it speeds up the very reaction that is generating the heat, thus leading to an increas- ing rate, which results in greater and greater quantities of heat. This heat will finally be sufficient to raise the temperature of the system to the ignition temperature of at least part of the fuel. Naturally, when this temperature is reached, the fuel ignites to a free-burning 42 Chapter 3 Fundamentals of Fire Behavior and Building Construction fire, and we say that it resulted from spontaneous combustion. If the heat can escape (due to convective, radiative, or conductive losses), the reaction rate does not increase as rap- idly, and the reaction may continue at a reduced rate until the temperature stabilizes at some point and will not increase further. HEAT AND TEMPERATURE Heat is energy in a kinetic form, or energy of molecular motion. Except at absolute zero ( 273°C, 460°F, 0 K) all matter contains heat because its molecules are in motion. Temperature is merely an expression of the relative amount of this energy that a body pos- sesses. Heat or temperature is described using four units of measure: Celsius (°C), Kelvin (K), Fahrenheit (°F), or Rankine (°R). The conversions are as follows: TC (TF 32) 5>9 TK (TF 32) 5>9 273 TF TC (9>5) 32 TR TF 460 Table 3-2 presents selected temperatures in those four units of measure, as well as conversions. The total amount of heat contained in a body is determined by the mass of that body and by its thermal capacity. The thermal or heat capacity of material is a measure of how much heat must be added to a given mass of material to increase its temperature. The value is given in joules/kilogram kelvin (J/kg · K) (since it is dependent on the temperature of the mass as well). Thermal capacity is usually represented by the symbol cp. The thermal capacities of some materials are shown in Table 3-3 (see also Appendix C). The fundamen- tal properties of a material that influence its ignitability are thermal capacity, thermal con- ductivity, and density. These properties will be explored in later sections of this text. For the purposes of fire investigation, it must be remembered that heat is a measure of energy, watt Unit of heat release; 1 watt while temperature is merely a means of comparing the heat content of two objects. 1 joule/second; unit of power or work (in elec- HEAT RELEASE RATE trical circuits, equivalent to voltage multiplied The amount of heat produced by a fire or any heat source is measured in calories or, more by amperes). commonly, in British thermal units (Btu) or joules (or kilojoules, kJ) (1 Btu 252 cal 1,055 J 1.055 kJ). The rate at which that heat is being produced by the heat source is measured heat release rate The most conveniently in joules per second, Btu per second, or calories per second. Because 1 J/s rate at which heat is generated by a source, 1 watt, the rate is measured in watts (W) or Btu/s (0.95 Btu/s 1000 # W 1 kilowatt, or usually measured in 1 kW). This# value, the heat release rate, is usually represented by Q (the dot meaning per watts, joules per second, unit time). Q is the quantity that characterizes the size of a fire. Another way of looking at or Btu per second. TABLE 3-2 Temperatures Measured in Different Units K °C °F R 773 500 932 1392 373 100 212 672 298 25 77 537 273 0 32 492 255 18 0 460 233 40 40 420 0 273 460 0 Kelvin Celsius Fahrenheit Rankine Chapter 3 Fundamentals of Fire Behavior and Building Construction 43  TABLE 3-3 Heat Capacities and Thermal Conductivities of Some Common Materials HEAT CAPACITY (cP) THERMAL CONDUCTIVITY THERMAL INERTIA MATERIAL (@20°C) (J/kg · K) (k) (W/m · K) (k c)a (W2/m4K2)/s Copper 380 387 1.3 109 Aluminum 900* 273* — Steel (mild) 460 45.8 1.6 108 Glass (window) 840 0.76 1.7 107 Brick (common) 840 0.69 9.3 105 Gypsum 840 0.48 5.8 105 Wood (oak) 2,850 0.17 3.2 105 Polyurethane foam 1,400 0.034 9.5 102 Cotton 1,300* 0.06* — Air 1,040 0.026 — Water 4,180† 0.60 2.5 106 Sources: D. D. Drysdale, An Introduction to Fire Dynamics, 2nd ed. (Chichester, UK: Wiley,1999), 33. * R. H. Perry and D. Green, Perry’s Chemical Engineers’ Handbook (New York: McGraw-Hill, 1984); SFPE Handbook of Fire Prevention Engineering, 2nd ed. (Quincy, MA: NFPA, 1995), table B-7. † SFPE Handbook of Fire Protection Engineering, table B-4, p. A-30. a ρ density of the material in kg/m3 the heat release rate of a fire is to consider the amount of fuel being consumed in it per second and multiplying that mass burning rate by the heat of combustion. In a real fire, there is a certain inefficiency due to incomplete combustion, so the relationship is not perfect. The heat release rate is a most useful way of comparing fires or predicting their behavior and their influ- ence on other fuels nearby. The heat release rates of some typical fires are shown in Table 3-4. TABLE 3-4 Heat Release Rates of Some Typical Fires Smoldering cigarette 5W Wooden kitchen match or cigarette lighter 50 W Candle 50–80 W Office wastebasket with paper 50–150 kW Small chair (some padding) 150–250 kW Armchair (modern) 350–750 kW (typical)—up to 1.2 MW Recliner (synthetic padding and covering) 500–1,000 kW (1 MW) Sofa (synthetic padding and covering) 1–3 MW Pool of gasoline (2 qt, on concrete) 1 MW Christmas tree (dry, 6–7 ft) 1–2 MW (typical)—up to 5 MW Living or bedroom (fully involved) 3–10 MWa Sources: J. Quintiere, Principles of Fire Behavior (Albany, NY: Delmar, 1998), 114–22; J. Krasny, W. Parker, and V. Babrauskas, Fire Behavior of Upholstered Furniture and Mattresses (Park Ridge, NJ: Noyes, 2001). a Depending on ventilation. 44 Chapter 3 Fundamentals of Fire Behavior and Building Construction HEAT TRANSFER AND HEAT FLUX An integral part of every fire is the transfer of heat, both to the fuel (which is critical to continuity of the fire) and away from regions of combustion, whatever their type. The principles of heat transfer are relatively simple and are vital to the understanding of the heat transfer Spread fire itself. The rate at which heat is falling on a surface (or passing through an area) is of thermal energy by called the heat flux and is measured in watts per square centimeter (W/cm2) or kilowatts convection, conduction, 2 2 or radiation. per square # meter (kW/m ) (1 W/cm 10 kW/m2). The heat flux is represented by the symbol q – (the double prime being notation for “per unit area”). Heat is transferred in three fundamental ways: (1) conduction, (2) convection, and (3) radiation. All these processes play a part in fires; however, the relative importance of each will vary with the intensity and size of the fire, as well as with the shape and content of the environmental system that is burning. Each will be considered separately. Conduction Conduction is the transfer of heat energy through a material by contact between its mov- conduction Process ing molecules. Because actual contact of the vibrating molecules is necessary, conduction is of transferring heat limited, for the most part, to localized action. Its effects are most noticeable in solid mate- through a material or between materials by rials where molecular contact is at its highest and where convection (large-scale, physical direct physical contact. movement of the molecules) does not occur. Heat always travels from hot areas of a solid to cold ones by conduction. This can be illustrated readily by heating one end of a metal rod and observing the temperature at the other. Heat traveling through the rod will cause a rise in temperature at the other end, but with considerable lag. The amount of heat flow- ing through the rod is proportional to the time, the cross-sectional area, and the difference in temperature between the ends, and is inversely proportional to the length. The rate at which heat is transmitted through a material by conduction is measured as the thermal conductivity (k), in units of W>m K (when the heat impinging on a surface is measured in W/m2, distances or thicknesses in meters, and temperatures in °C or K). Scientists and engineers find it convenient to express such relationships in the form of an equation, because it shows the way in which factors interrelate and allows for prediction of results. The relationship for conductivity is given by # # q kA(T2 T1 )>l or by rearranging, k q l>A(T2 T1 ) Where: # q the rate at which heat is being conducted (in kJ/s or kW) k thermal conductivity of the material A the area through which the heat is being conducted T2 T1 the difference in temperature between the “hot” and “cold” zones l the length (or distance) through which the heat is being conducted7 # Plugging different values into this equation shows that heat is conducted faster (q gets larger) if T2 is much larger than T1, if the area is larger, or if the distance through which the conduction is taking place is smaller. The thermal conductivity of a material (k), is usually specified at 0°C or 20°C, since it varies with temperature. It also varies dramati- cally among materials. The thermal conductivities of some common materials were shown in Table 3-3 (see also Appendix C). An illustration of the importance of thermal conduction might involve the relative behavior of a piece of metal—for example, a length of wire or a galvanized metal sheet— as compared with the behavior of a piece of wood in the same fire. The thermal conduc- tivity of metals is high, so that if the metal piece is heated, the heat rapidly spreads to unheated areas. If the temperature in these portions rises above the ignition temperature of any fuel material in contact with the metal, this fuel may be ignited at a distance from the initial source of heat. A piece of wood, in contrast, being a poor conductor of heat, may be intensely heated and even burn at one location for a long interval. The heat, however, Chapter 3 Fundamentals of Fire Behavior and Building Construction 45  does not spread through it well, and the wood will not be expected to ignite at a distance from the point at which the heat is applied. One side of a simple board will show deep charring in a fire, and the opposite side, unless exposed to flame from another source, remains uncharred and apparently normal wood. The importance of the thermal conductivity of the material through which the heat is being conducted in the development and consequence of fire is considerable. Not only is conductivity involved in the ignition of materials, in which the heat must flow from the heat source to the fuel and to some degree into it before a fire will start, but it also has a marked effect on the degree of fire damage, as we shall see in later sections. The thermal conductivity of easily ignitable fuels such as wood, plastic foam, or paper is very low, which means that heat applied to a surface tends to accumulate there, raising the local temperature, possibly to the fuel’s ignition temperature. The conductivity of metals is very high, meaning that heat applied is quickly dissipated into the bulk of the metal, and the local temperature does not approach ignition temperatures as readily. Copper conducts heat almost 2,000 times more efficiently than does wood. After a fire, thermal damage to the insulation on a copper wire may be seen to extend for some distance from the actual point of heat application or the location of a heat-producing electrical fault. Wood may be damaged very heavily on only one side, and even then only in the area of direct appli- cation of heat. Other areas not directly exposed to fire may be undamaged. Poor thermal conductors such as felt, plastic foam, and the like are used as insulation. They also tend to be poor conductors of electricity as well as heat. The high thermal conductivity of some metals means that it is more difficult to melt them by application of localized heat. For example, the melting point of copper (1,082°C; 1,981°F) is often exceeded in the com- bustion zone of a wax candle, but only the finest gauge copper wire is likely to be melted in a candle flame because larger cross sections permit the heat to be conducted away so quickly that the temperature of the bulk metal never exceeds the melting temperature. The high thermal conductivity of metals contributes to their rapid loss of heat when taken out of a flame. A hot metal particle may contain a great deal of heat due to its high thermal capacity but loses its heat to its surroundings much more quickly than will wood of the same mass. Convection convection Process Convection, as a means of transferring heat, is extremely important in fires. It can be of transferring heat by defined as the distribution of heat by means of a circulating medium or the transfer of movement of a fluid. heat to or from a moving medium. As such, it will occur in gases and liquids but obvi- In convective flow, the warm fluid becomes ously not within solids. Convection is, however, responsible for transferring heat from less dense than the solids to liquids or gases (and vice versa) by convective heat transfer. In most fires it is surrounding fluid and driven by differences in density caused by temperature variations (buoyant flows). Such rises, inducing a buoyant flows are usually referred to as convective or convection flows. circulation. When water is heated, its temperature can rapidly be raised despite its very low heat conductivity. The temperature rises rapidly because the heated water on the bottom of a container expands, becomes less dense, and is displaced by the heavier cold water, which is heated in turn. Another very familiar example is the heating of buildings by furnaces or heaters. Here the fire is small and controlled, but the heated air coming from it is often circulated exclusively by convection. Forced-draft furnaces have a fan or blower to sup- plement this convection. In fires the moving masses of hot materials are the gaseous products of combustion, along with surrounding air, which is also heated. These expand and become lighter and move upward at a rapid rate. This buoyancy is the form of heat transfer that accounts for most heat movement in a normal fire. It also largely determines the fundamental proper- ties of fire with respect to its movement, spread, and ultimately its pattern. Convective heat transfer is represented by the equation # q h(T2 T1 )A (W) 46 Chapter 3 Fundamentals of Fire Behavior and Building Construction  # where, once again, q represents the rate at which the heat moves, A is the area through which it is being transferred, T2 and T1 are the temperatures of the two materials, and h is a value that represents the efficiency of the heat exchange between those two materials. Because convective heat transfer varies with different combinations of materials, surface textures, speeds of movement, and so on, it is a value that one looks up in a reference. For a buoyant flame plume in air, h is between 5 and 10 W/m2 · °C. Application of the preceding equation, with a typical flame temperature of 800°C, would give a convective heat transfer to a nearby surface on the order of 4 to 8 kW/m2.8 This is considerably less than the radiative heat transfer of 40 or 50 kW/m2 one would expect from the same flame at near contact. In very large fires, especially outdoors, the upward movement of hot gases may be so great as to contribute to the formation of what is called a firestorm. In such cases, drafts firestorm Overwhelm- are created that can suck lighter fuels at ground level into the fire, and the buoyant cur- ing progression of fire rent lifts masses of burning gases and solid debris hundreds of feet into the air, debris that through structures or wildlands caused by a falls as burning brands among other fuels downwind. The burning gases can form a sep- combination of convec- arate mass of flames called a fireball or firewhirl, which can ignite fuels that come into tive and radiative contact with it. The intensity of the fire produces radiative heat of such magnitude that processes. fuels some distance away are heated to their ignition temperatures and burst into flames. The firestorm is the mechanism responsible for the terrible damage in infamous fires such as those in Dresden, Germany, and Tokyo, Japan, (both as a result of air raids during World War II), and in Chicago, Illinois, and Peshtigo, Wisconsin (accidental fires, one urban, the other wildland in origin, both in October 1871). Radiation Transfer of heat by radiation is less commonly appreciated than transfer by conduction radiation Transfer of or convection, and yet it plays the most critical role in fire growth and spread, particu- heat by electromagnetic larly in larger fires. Radiation aids fire spread across a surface, promotes ignition of other waves. fuels, and may produce burn patterns that survive the fire. All objects having a tempera- ture above absolute zero (0 K, 273°C, 460°F) radiate energy in the form of electromag- netic energy. Radiated heat energy can be transferred to another body without any contact or circulating medium. As we shall see later, radiation plays a critical role in the spread of a fire later in its development. At low temperatures, this radiation is solely in the form of infrared emission. At temperatures above 500°C (900°F), some of the radia- tion is in the visible portion of the spectrum, and the color perceived depends on the tem- perature of the body, as described in Table 3-1. Infrared radiation behaves exactly like visible light except that it is not detectable by the unaided eye. The infrared radiation from the sun is the primary source of heat on Earth. As soon as the sun is visible, the heat it radiates can be felt, just as an observer feels the radiated warmth of an open fire. Such warmth is felt because it is absorbed by the body, and the absorption of heat is greater than the radiation of heat by the body under these circumstances. In the same way, a per- son in a cold environment is chilled because his or her body radiates heat faster than it receives heat from the environment. In surroundings where everything is at the same tem- perature, all objects are both radiating and absorbing radiant heat at the same rate. The result is no change in temperature. If the temperature is measured in K (°C 273), the intensity of the total emitted radiation (E) is related to the temperature (in K) raised to the fourth power (T 4), as in the Stefan-Boltzmann relationship: E T4 (W/m2) where is the emissivity of the source and is the Stefan-Boltzmann constant.9 This means that a modest increase in the temperature of a surface, say, doubling it from a room temperature of 300 K (27°C; 81°F) to 600 K (327°C; 621°F), increases the intensity of radi- ant heat emitted by that surface by a factor of 16 (2 to the 4th power 16). The effects of radiant heat are determined by its magnitude and duration. Table 3-5 gives some examples Chapter 3 Fundamentals of Fire Behavior and Building Construction 47  TABLE 3-5 Effects of Thermal Radiation on Thick Solids (Wood, Plastic, Human Tissue) in Still Air @ 20°C RADIANT # HEAT EQUILIBRIUM SURFACE SOURCE FLUX (q – ) (kW/m2) TEMPERATURE* OBSERVED EFFECT Direct summer sun 1 40°C (100°F) None Distance from fireplace 2–4 45°C (120°F) Pain after 30 s Proximity to fireplace 4–6 54°C (130°F) Pain after 8–10 s 2nd degree burns to skin in 20–30 s Near proximity to fireplace 10 100°C–150°C Scorching of some materials (200°F–300°F) Melting of some thermoplastics Face of fireplace 20 150°C–250°C Some cellulosics and synthetics ignite in (300°F–500°F) 60 s Inside of fireplace 30 300°C–400°C Autoignition of many fuels in 0–30 s (600 °F–800°F) (wood in 60 s) Adjacent to flames 50 400°C (800°F) Autoignition of nearly all materials in 5s Post-flashover 120–150 500°C ( 800°F) Rapid combustion *Estimated from various sources including ASTM E1321-97: Standard Test Method for Determining Material Ignition and Flame Spread Properties. of the effects of thermal radiation. These effects are useful in estimating the intensity of heat exposure on surfaces after a fire as an aid to reconstructing the positions of heat sources and “target” (receiving) surfaces. The heat flux is not the only factor that determines the transfer of radiant heat. There is also a dependency on the view factor; that is, does the receiving surface sit at right angles to the path of the radiation, or is it exposed uniformly to radiation from the source? The intensity of the radiation being received from a point source of heat falls off with the square of distance.10 In other words, if an object is 1 m (3 ft) away from a small wastebasket fire, directly facing it, and is receiving 6 kW/m2 radiant heat, and the object is moved twice as far away (to 2 m; 6 ft), the radiant heat intensity (heat flux) drops to 1/4 of 6 kW/m2, or 1.5 kW/m2. The heat flux on a surface some distance r away from a small, localized fire is represented by # # q – Xrad Q >4 r2 # where Q is the total heat release rate of the wastebasket fire, and Xrad is the fraction of 11 heat being released# as radiant heat (for a trash fire, that might be 0.4). One can see that as r gets larger, q – falls off very rapidly (the inverse-square relationship). If an object is very close to a large fire or hot surface, the fire can no longer be rep- resented as a single point source, and the summed contributions from various parts of the large source overwhelm the inverse-square law. The relationship becomes # q –1 T 24F12 where F12 represents the view factor between # the source (at temperature T2) and the receiv- ing or target surface (surface 1), and q –1 is the heat flux falling on that target surface.12 Calculation of the view factor F12 requires an estimation of dimensions of the radiant sur- face and distance of separation, as in Figure 3-7. The value is calculated as the ratio a/c, and the appropriate curve is selected in Figure 3-8. The y-value is calculated from ratio b/c. The intersection of curve x with the vertical line corresponding to y yields the factor F12 on the vertical axis.13 48 Chapter 3 Fundamentals of Fire Behavior and Building Construction  Hot surface FIGURE 3-7 A small target facing a large heat b source (a by b in size) a distance c away is subject to radiant heating from a wide angle of view Field of view Target a c.3 FIGURE 3-8 The config- x=∞ uration factor is taken 2.2 from this nomograph, 1 where x a/c and y.7 b/c. The closest curve for.5 x is selected. Where that.1.4 curve intersects with the y.3 value determines F12..07 From Hamilton and Morgan. Configuration factor, F12.2 NACA Tech. Note 2836,.05 December 1952..15.03 0.1.02.01.007.005 NACA.003.05.1.2.5 1.0 2 3 5 7 10 20 The calculation of the view factor for such exposures is much more complex, but notice that distance alone is no longer a factor. For instance, if the hot gas layer in a room ignites, the entire ceiling layer represents a radiant heat source. If we measure the heat flux on an object 1.2 m (4 ft) below the ceiling layer and the heat flux on an object 2.4 m (8 ft) below the ceiling layer, the heat fluxes will be very similar. That is one reason why radiant heat from a large heat source seems to bring so much of a room’s contents to igni- tion at about the same time. These factors will control how much radiant heat is being received on the target surface. The “edge” or “corner” effect is one reason why the tar- gets in the center of a room ignite long before the carpet at the edges as a room approaches flashover. flashover The final The effect of the heat flux is also subject to the nature of the exposed surfaces and stage of the process of fire growth; when all especially to the degree to which they reflect heat or infrared rays. Heat (or visible light) exposed surfaces of falling on a surface may be absorbed or reflected. Objects that radiate heat readily also combustible fuels absorb it readily. In general, dark-colored objects both absorb and radiate heat better within a compartment than light-colored ones, but some light-colored materials absorb infrared (heat) energy are ignited, the room is very effectively. However, polished surfaces, such as chrome plate, resist both absorption said to have undergone flashover. and emission. The same mirror that reflects visible light also reflects the infrared or heat rays in the same manner, but the glass from which it is made absorbs the infrared, and its Chapter 3 Fundamentals of Fire Behavior and Building Construction 49 FIGURE 3-9A Massive fire in condominium building igniting build- FIGURE 3-9B Structure fire igniting vehicle (and ground cover) by ing across the street by radiant heat (and fire balls). Courtesy of Calvin radiant heat. Courtesy of Greg Lampkin, Knox County, TN, Fire Investigation Bonenberger, Fire Marshall, Lafayette Hill, PA, FD. Unit. temperature rises. Many of the post-fire indicators on which investigators rely to indicate fire travel and intensity are the result of radiant heating, so an understanding of the fac- tors that contribute to it is very important. Radiant heat is a major factor that makes firefighting difficult, because close prox- imity to a large fire heats up equipment and firefighters, making operations difficult if not impossible to carry out safely. When a fire is burning in a portion of a structure, all surfaces that face the fire will be heated by radiant heat, and when this surface temper- ature reaches the autoignition point of the material itself, it will burst into flame. In very large fires, it is not uncommon for adjacent buildings and nearby vehicles to be ignited solely by the radiant heat (as in Figures 3-9a and b). This radiative ignition is a major factor in fire growth in a compartment, as will be discussed shortly. Charred areas inconsistent with fire spread from item to item are commonly due to radiant heat from the room fire and should be so recognized by the fire investigator. Generally, they are not as highly localized as char damage caused by direct flame impingement but can exhibit “shadow” effects where there are intervening solid objects to block some of the radiation. When there are no intervening or overlying solids, or prolonged post- flashover burning, radiation char patterns tend to be uniform in degree of charring. They can complicate the search for a fire origin because radiant heat can produce char- ring at low levels in a room even when the fire generating the heat is not as low in the room (see Figure 3-10). However, the ignition of floor coverings by radiant heating and their burning can result in char damage to the floor that is not as uniform as direct radi- ant heat char (and can be very irregular). These effects will be discussed in more detail in Chapter 7. DIRECT FLAME IMPINGEMENT In a real fire, flames are spread not only by the three basic mechanisms of heat transfer described previously but also by direct flame impingement, and its role cannot be ignored. On a microscopic scale, direct flame contact or impingement is a combination of both convective and radiative mechanisms. If the plume of hot gases from the established flame rises by buoyant flow into contact with more fuel, heat is transferred from the plume by convective heat transfer and radiation until the new fuel pyrolyzes and generates gaseous fuels that are ignited by the flames of the plume. The flames then spread upward and, 50 Chapter 3 Fundamentals of Fire Behavior and Building Construction  FIGURE 3-10A As fire Ceiling extends across the ceiling, heat radiating downward Hot gases from the ceiling layer and flames chars and ignites exposed surfaces; areas under Radiant chair and table are pro- heat tected until later in the fire. When flashover is completed, all fuels in the room will be on fire, and the undersides of tables, chairs, and shelves will be exposed to flames from floor-level burning. Floor Lightly charred Heavily in protected scorched and areas charred areas FIGURE 3-10B A struc- ture fire demonstrating ignition by radiation. Intense localized fire on the floor and railing of the porch was ignited by radiation coming from fire plume exiting the top portion of the door. Courtesy of John D. DeHaan. more slowly, outward. Because fresh fuel is continually being added to the reaction with little loss of heat, the fire can spread quickly through the fuel presented. As more fire is created, the faster fuel is involved and the temperature of the room or enclosure rises, thereby increasing the rate of oxidation of other fuels, and the fire spreads at a faster and faster rate as long as the supply of oxygen is maintained. Flame Plume A fire burning in the middle of a room or out in the open creates a rising column of hot gases (plume) by the buoyant flow just described. The temperature in a simple fire is at a maximum in the well-mixed combustion zone just above the fuel package. As the gases (and solids and liquid aerosols mixed therein) rise they lose their heat, some by mixing with room air (entrainment) and convective transfer to the surrounding air, and some by radiant Chapter 3 Fundamentals of Fire Behavior and Building Construction 51  FIGURE 3-11 Temperature distribution in Temperature distribution buoyant flame and smoke plume in buoyant flame and smoke plume along verti- Height cal centerline. above fuel Smoke plume Tip of flame Flame plume Mixing zone 0 25 800 Ambient Temperature (°C) loss (particularly from soot particles and aerosol droplets of pyrolysis products from incomplete combustion). As a result, if we measured the temperature of the flames, we would see a predictable pattern to the temperature as a function of height above the fuel package. Temperatures decrease from 700°C to 900°C (1,500°F to 1,800°F) just above the fuel to about 500°C (930°F) at the tip of the flame, where it becomes very turbulent and intermittent.14 As the gases continue to rise they continue to lose heat, and the tempera- ture continues to decrease in a predictable pattern until the gases have lost so much heat that they are no longer buoyant. (Remember that most of the combustion products—CO2, SO2—are heavier than air at the same temperature; CO is nearly the same density; only water vapor (H2O) is lighter.) A temperature plot of flame temperature versus height would look something like Figure 3-11. The relationship between the flame height and the heat release rate of the fire is quite predictable. One form of the relationship for a symmet- rical pool fire burning with no nearby walls or ceiling is the Heskestad equation: # Zf 0.23Q 2>5 1.02D # where Zf is the visible flame height in meters, Q is the heat release rate in kilowatts, and D is the diameter of the pool (in meters).15 Entrainment draws fresh air into the base of the fire to keep the reaction going and diffuses and cools the rising gases in the plume, as in Figures 3-12a and b. If the fire is against a noncombustible wall, the entrainment of cooling air is reduced by 50 percent, as in Figure 3-12c. If fire gases have more time to rise before cooling to the point they are no longer incandescent, the visible flame plume is taller. If the fire is in a corner, the entrained air is reduced even more (as in Figure 3-12d),# and the flame plume is even taller. Radiant heat from nearby walls further increases Q. This situation is dramatically illus- trated in Figure 3-13. These fires can be represented by the following equation developed by Alpert and Ward: # Zf 0.17(kQ) 2>5 where k is called the wall factor. The wall factor is 1 for no wall, 2 for a single wall, and 4 for a corner.16 These relationships allow us to predict how big a fire will look to an observer (or what kind of effect its gases will have on materials that come into contact with it at various heights) given the heat release rate of the fire. Due to the turbulence of flames, the height of the flame tip will oscillate up and down, and the tip will sometimes separate from the main body of the flame. The human eye tends to average these varia- tions, and the height is normally fairly close to the predicted Zf. 52 Chapter 3 Fundamentals of Fire Behavior and Building Construction  Fire Entrained air FIGURE 3-12B Top view of FIGURE 3-12A Side view of entrainment around periphery of entrainment—unrestricted fire. unrestricted fire. Fire