[Björn Karlsson; James G Quintiere] -- "Enclosure Fire Dynamics provides a complete description of enclosure fires and how the outbreak of a fire in a. The increasing complexity of technological solutions to both fire safety design issues and fire safety Enclosure Fire Dynamics Preview PDF. Smoke Filling. Most fire deaths are due to the inhalation of smoke and toxic gases. for assessing the environmental consequences of a fire in an enclosure.
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enclosure fire dynamics cover Dr. Björn Karlsson has been very nice to share his teaching material which is based on the textbook Enclosure Fire Dynamics. Enclosure Fire Dynamics Enclosure Fire DynamicsBjörn Karlsson James G. QuintiereCRC Press Boca Raton London New Y Size Report. DOWNLOAD PDF. Enclosure Fire Dynamics - Frazer MacDonald Fire Enginee Modelling of Enclosure Fire Dynamics using a Tangent Linear Ap the first quarter of the 21st c.
Reviews Summary The increasing complexity of technological solutions to both fire safety design issues and fire safety regulations demand higher levels of training and continuing education for fire protection engineers. Historical precedents on how to deal with fire hazards in new or unusual buildings are seldom available, and new performance-based building codes often require mathematical or computer fire modeling. Until now, however, there has been no current, truly comprehensive engineering book that builds an in-depth understanding of the scientific aspects of enclosure fires. Enclosure Fire Dynamics fills this void with a complete description of enclosure fires and how the outbreak of a fire in a compartment causes changes in the environment. The authors-both internationally renowned experts in fire safety and protection engineering-offer a clear presentation of the dominant mechanisms controlling enclosure fires and develop simple analytical relationships useful in designing buildings for fire safety. They show readers how to derive engineering equations from first principles, stating the assumptions clearly and showing how the resulting equations compare to experimental data. The details and the approach offered by this text provide readers with a confidence in-and the applicability of-a wide range of commonly used engineering equations and models.
Quintiere, James G. K37 Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use.
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Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. For educational purposes this discipline can be divided into a number of fundamental courses. One of these is enclosure fire dynamics: Professor Sven Erik Magnusson initiated and defined the contents of the course and collected material to be taught from a variety of sources. The course literature until now has consisted of a collection of journal articles, scientific papers, and selected chapters from various handbooks and textbooks.
The lack of completeness and homogeneity in the course literature has been a source of frustration for both teachers and students.
The authors of this textbook have to a considerable extent followed the framework provided by Professor Magnusson but they have added some new topics and expanded others. Fire safety science is a rapidly developing field and fire safety engineering a relatively young engineering discipline. The scientific and engineering communities have therefore not fully standardized the assignments of symbols and units used in the field. Further, the contents of the fundamental courses—including enclosure fire dynamics—have not been standardized and may vary greatly from one educational institution to another.
The purpose of this textbook is not only to act as course literature for fire safety engineering students, but also to offer educators in the field the opportunity to comment on the contents of the course, its organization, and the definitions of dimensions and symbols used. It is the hope of the authors that this book will contribute toward some standardization of both educational material and terminology used in the fire safety engineering discipline.
His research has focused on modeling ignition, flame spread, and fire growth on solids; the development of an expert system for risk analysis in industry; the under-ventilated fire; twozone fire models; performance-based codes for fire safety engineering; and performance-based test methods for the reaction-to-fire of products.
He has initiated and managed several projects in cooperation with the Swedish Rescue Services, most recently on the phenomena of flashover, backdraft, and smoke gas explosion. He has been an invited speaker at a national meeting of the American Chemical Society and has been involved in research projects in Japan, Europe, and the United States. In , Dr. He is currently Senior Lecturer in the Department of Fire Safety Engineering, Lund University, and project manager for a 2-year research project with the aim of establishing a risk index method for assessing the fire safety of timber-frame multistory apartment buildings.
James G. Quintiere, Ph. The author of more than 78 publications, 42 reports, and one book, Dr. Quintiere is the recipient of the Howard W. VII ; and the Harry C. Bigglestone Award, Fire Technology, Vol. Magnusson provided the basic framework, identified the subject matter, and gave guidance on where to strike the balance between basic and applied knowledge.
Without his considerable efforts and patience with respect to the artwork, the preparation of the manuscript would have been a far more onerous task. His invaluable assistance and encouragement is deeply appreciated.
Johan Lundin is gratefully acknowledged for collecting and summarizing information on Internet resources for Fire Safety Engineering, presented in Appendix A. Finally, a number of scholars in the field of fire safety science have read and commented on selected sections of the text. While the responsibility for the material presented is entirely in the hands of the authors, we gratefully acknowledge the assistance of Ove Pettersson, Gunnar Heskestad, Jim Shields, Andy Buchanan, Craig Beyler, and Dougal Drysdale.
The units shown here are those most commonly used; some variations with respect to the SI decimal relationships kilo and mega may occur. It discusses the core curriculum of fire safety engineering and places the material presented in the book into context with other topics within the fire safety engineering discipline.
Enclosure fire models currently used in fire safety engineering design are briefly discussed. Finally, an overview of the contents of the book is presented and general notations are explained. Rapid developments in modern building technology in the last decades often have resulted in unconventional structures and design solutions.
The physical size of buildings increases continually; there is a tendency to build large underground car parks, warehouses, and shopping complexes. The interior design of many buildings—with large light shafts, patios, and covered atriums within buildings connected to horizontal corridors or malls—introduces new risk factors concerning spread of smoke and fire.
Past experiences or historical precedents which form the basis of current prescriptive building codes and regulations rarely provide the guidance necessary to deal with fire hazards in new or unusual buildings. At the same time there have been great strides in the understanding of fire processes and their interrelationship with humans and buildings. Advancement has been particularly rapid in the area of analytical fire modeling.
Several different types of such models, with varying degrees of sophistication, have been developed in recent years and are used by engineers in the design process.
As a result, we have a worldwide movement to replace prescriptive building codes with ones based on performance. Instead of prescribing exactly which protective measures are required such as prescribing a number of exits for evacuation purposes , the performance of the overall system is presented against a specified set of design objectives such as stating that satisfactory escape should be effected in the event of fire.
Fire modeling and evacuation modeling can often be used to assess the effectiveness of the protective measures proposed. The need to take advantage of the new emerging technology, both with regard to design and regulatory purposes, is obvious.
Apart from the book by Drysdale and the one by Shields and Silcock,4 textbooks on fire safety engineering specifically written for engineering students have been scarce.
Design guides and handbooks generally list engineering problems and provide methodologies by which these problems can be solved using specific calculational procedures. The equations used are seldom derived from first principles, and little information is given on the assumptions made or the validity of the approach.
To fully understand the effect these assumptions may have in a specific design situation and to be confident of the validity of the chosen calculational procedure, the engineer at some point must have derived the equations from first principles. The purpose of this textbook is not to act as a design guide or a list of equations that can be applied to specific scenarios, but rather to show how engineering equations for certain applications can be arrived at from first principles, to state the assumptions clearly, and to show how the resulting analytical equations compare to experimental data.
In this way the reader will get a strong feeling for validity and applicability of a wide range of commonly used engineering equations and models. This textbook specifically examines enclosure fire dynamics, the study of how the outbreak of a fire in a compartment causes changes in the environment of the enclosure.
Before introducing the contents of the book we shall discuss the fire safety engineering core curriculum. In Section 1. Finally, we discuss some symbols and units. It is not immediately obvious which of these topics of interest should be addressed in a textbook for students.
The fundamental courses are divided into five modules: The modules, however, are interlinked to a considerable extent, and it is often a question of preference where to include borderline topics and where to present a summarized background.
The book by Drysdale1 is an excellent text for a course on fire fundamentals that emphasizes the basic chemistry and physics of fire, but the book also touches upon several topics within the other modules listed above.
Also, it is not obvious where to strike the balance between material presented in the fundamental modules and material assumed to be prerequisite knowledge from basic courses in physics, chemistry, fluid mechanics, etc. We assume that the student has a basic knowledge of mathematics, physics, and chemistry. This textbook does not attempt to provide an in-depth study of the phenomena, but rather to present the most dominating mechanisms controlling an enclosure fire and to derive some simple analytical relationships that can be used in practice.
In view of the increased use of calculational procedures and computer models in building fire safety engineering design, the main purpose of this textbook is to: The models can be classified as either probabilistic or deterministic. Probabilistic models do not make direct use of the physical and chemical principles involved in fires; rather, they make statistical predictions about the transition from one stage of fire growth to another.
Such models will not be discussed further here. The deterministic models can roughly be divided into three categories: CFD models, zone models, and hand-calculation models. CFD models: The CFD modeling technique is used in a wide range of engineering disciplines. Generally, the volume under consideration is divided into a very large number of subvolumes, and the basic laws of mass, momentum, and energy conservation are applied to each subvolume.
Figure 1. The governing equations contain as further unknowns the viscous stress components in the fluid flow. Substitution of these into the momentum equation yields the so-called Navier—Stokes equations, the solution of which is central to any CFD code.
The myriad engineering problems that can be addressed by CFD models are such that no single CFD code can incorporate all of the physical and chemical processes that are of importance.
There is only a handful of CFD codes that can be used for problems involving fire. These, in turn, use a number of different approaches to the sub-processes that need to be modeled. Some of the most important of these sub-processes can be considered to be turbulence modeling, radiation and soot modeling, pyrolysis and flame spread modeling, and combustion modeling. Cox6 provides an excellent summary of the main issues. A description of the fundamental laws of physics and chemistry contained in CFD models is outside the scope of this textbook.
Use of CFD models requires considerable computational capacity as well as expert knowledge, not only in physics and chemistry, but also in numerical methods and computer science.
In addition, it is a very time consuming and costly process to set up the problem, run it on the computer, extract the relevant output, and present the results, so practical use of this methodology for fire safety engineering design is relatively rare. However, such a modeling methodology can be very useful when dealing with complex geometries, and it may be the only way to proceed with certain design problems. Two-zone models: A second type of deterministic fire model divides the room into a limited number of control volumes or zones.
The equations for mass and energy conservation are solved numerically for both zones for every time step. The momentum equation is not explicitly applied; instead, information needed to calculate velocities and pressures across openings comes from analytically derived expressions where a number of limiting assumptions have been made.
Several other sub-processes, such as plume flows and heat transfer, are modeled in a similar way. The section on hand calculations below lists a number of these processes, and later chapters of this book derive the equations and introduce the assumptions made.
Many two-zone models have been described in the literature. Some of these only simulate a fire in a single compartment; others simulate fires in several compartments, linked by doors, shafts, or mechanical ventilation. Additionally, the degree of verification, documentation, and user-friendliness varies greatly between these models. In recent years there has been an upsurge in the use of two-zone models in fire safety engineering design.
This is partly due to the increasing availability and user-friendliness of computer programs. However, any serious use of such models demands that the user be well acquainted with the assumptions made and the limitations of the models, i.
This textbook aims to provide the necessary background. Hand-calculation models: A third way to analytically describe some basic fire processes is to use simple hand-calculation methods. These are basically a collection of simplified solutions and empirical methods to calculate flame heights, mass flow rates, temperature and velocities in fire plumes, time to sprinkler activation, room overpressure, and many other variables.
The remainder of this section describes the hand-calculation models. The methods discussed below can, for convenience, be divided into three categories: Typical burning rates and the heat of combustion for a range of liquid fuels burning in the open have been experimentally determined and are provided in the literature.
This allows the energy evolved to be calculated if the area of the liquid spill is known.
If the amount of spilled liquid is known then the time to burn-out can also be calculated. Fire growth information for solids and other burning objects is available from several sources.
Energy release rates for many items of furniture, curtains, and different types of materials are available. Such values are also available for species production rates, which allows calculation of species concentrations. The rate of energy evolved in a compartment is also dependent on the rate of supply of oxygen.
Knowledge of the ventilation conditions can therefore be used to evaluate the maximum rate of energy release inside a compartment. Any excess, unburned fuel will then be burned outside the fire compartment, where oxygen is available.
Computer programs with material databases are also available to assist the user in choosing an appropriate energy release rate curve. The application of the conservation laws will lead to a series of differential equations.
By making certain assumptions about the energy and mass transfer in and out of the compartment boundaries, the laws of mass and energy conservation can result in a relatively complete set of equations. The complexity and the large number of equations involved make a complete analytical solution impossible, so one must resort to numerical analysis through computer programs. However, analytical solutions can be derived by using results from experiments and a number of limiting approximations and assumptions.
Such solutions have generated numerous expressions, which may be used to predict a variety of environmental factors in a fire room. Several examples are given below. The buoyant gas stream rising above a burning fuel bed is often referred to as the fire plume. The properties of fire plumes are important in dealing with problems related to fire detection, fire venting, heating of building structures, smoke filling rates, etc.
By using dimensional analysis, the conservation equations and data from experiments, expressions for various plume properties have been developed. These include expressions for plume temperature, mass flow, and gas velocities at a certain height above the fire as well as flame height. Similar expressions have been derived for the jet that results when the plume gases impinge on a ceiling.
Mass flow in and out of compartment openings can be calculated since the pressure differences across the opening can be estimated. The use of classical hydraulics and experimentally determined flow coefficients has resulted in hand calculation expressions for such mass flows. The gas temperature in a naturally or mechanically ventilated compartment can be calculated by hand, using regression formulae based on experimentally measured gas temperatures in a range of fire scenarios and a simplified energy and mass balance.
Such expressions are available for both pre- and post-flashover fires. By using similar expressions, the onset of flashover can be estimated. By combining the expressions for gas temperature, plume flows, and vent flows, the descent of the smoke layer as a function of time can be calculated.
Such solutions usually require an iteration process or the use of precalculated curves or tables. Several other types of hand calculation expressions have been developed, including expressions for mass flow through roof openings, buoyant pressure of hot gases, species concentration, fireinduced room pressures, flame sizes from openings, etc. Some such expressions have been collected in relatively user-friendly computer programs. Classical textbooks on heat transfer provide innumerable hand-calculation expressions for calculating heat fluxes to and from solids, liquids, and gases, as well as expressions for estimating the resulting temperature profiles in a target.
These analytical expressions are usually arrived at by setting up the energy balance, by assuming constant properties and homogeneity in the media involved, and by ignoring the heat transfer mechanisms that seem to be of least importance in each case. The radiative heat flux from flames, hot gases, and heated surfaces impinging on a solid surface can be estimated using classical heat transfer and view factors.
The same applies for convective heat transfer to solids and conductive heat transfer through solids. The surface temperature of a solid subjected to a radiative, convective, or conductive heat flux can be calculated by hand assuming the solid is either semi-infinite or behaves as a thermally thin material.
Numerous types of heat transfer problems can be solved in this way. A few examples are given below. Assuming that a secondary fuel package is subjected to a known heat flux and that it has a certain ignition temperature and constant thermal properties, then the time to ignition can be calculated. Similarly, if the activation temperature of a sprinkler bulb is known, the activation time can be estimated. Several other problems can be addressed in this way, including temperature profiles in building elements, flame spread over flat solids, heat detector activation, spread of fire from one building to another, etc.
Analytical solutions to such problems can be found in standard textbooks on heat transfer. This textbook addresses these issues in the following order: Chapter 2: A qualitative description of enclosure fires. This chapter contains a general, qualitative description of the chemical and physical phenomena associated with fires in enclosures and the environmental conditions that result.
The different stages in enclosure fire development are discussed. Terms essential for the subsequent treatment of the subject are identified and defined. Chapter 3: Energy release rates. In order to calculate the environmental consequences of a fire in an enclosure, the rate at which the fuel releases energy must be known. This chapter outlines the methods commonly used to estimate the energy release rate produced by a burning fuel package.
Chapter 4: Fire plumes and flame heights. The buoyant gas flow above a fire source is called a plume. As the hot gases rise, cold air will be entrained into the plume.
This mixture of combustion products and air then rises to the ceiling of an enclosure and causes formation of a hot upper layer. This chapter discusses the most fundamental properties of fire plumes; gives expressions for calculating variables associated with them; and examines flame heights and analytical expressions for estimating these heights in certain given scenarios.
Chapter 5: Pressure profiles and vent flows for well-ventilated enclosures. In this chapter we derive engineering equations used to calculate pressure differences across openings, as well as equations for calculating the mass flow of gases in and out through vents for several common enclosure fire scenarios. Chapter 6: Gas temperatures in ventilated enclosure fires. Knowledge of the temperature of the hot smoke in an enclosure can be used to assess when hazardous conditions for humans will arise, when flashover may occur, when structural elements are in danger of collapsing, and the thermal feedback to fuel sources or other objects.
This chapter derives and reviews a few analytical methods that have been developed to predict temperatures in both the pre- and post-flashover phases of well-ventilated enclosure fires. Chapter 7: Heat transfer in compartment fires. The enclosure energy balance is greatly affected by transfer of heat from the flames and the hot gases to the enclosure surfaces and out through the enclosure openings. The heat transferred from these sources toward a fuel package will control, to a considerable extent, the rate at which fuel evaporates and heat is released.
This chapter focuses on radiative heat transfer in enclosures and briefly discusses convection heat transfer as applied to enclosure fires. Chapter 8: Conservation equations and smoke filling. This chapter states the conservation laws for mass and energy and introduces some commonly applied assumptions that allow the derivation of analytical solutions and iterative methods, which can be applied to problems related to the smoke filling process.
Two types of ventilation conditions are considered: The conservation equations are applied to calculate smoke filling time and derive smoke control methodologies for several cases. Chapter 9: Combustion products. The ability to estimate the toxic hazards of combustion gases in a fire compartment enables us to estimate the toxic hazard to humans. This chapter discusses methods for estimating the amount of each toxic species produced per unit fuel burnt, i. Once the production term is known, the concentration in the fire gases can be estimated.
The generation of combustion products is a very complex issue, and the engineer must rely on measurement and approximate methods for estimating the yield of a product. This chapter introduces some methods available to the engineer for estimating the yield of a species and discusses methods for calculating species concentrations. Chapter Computer modeling of enclosure fires. This chapter summarizes how the methods discussed in the previous chapters are used in compartment fire modeling set up on a computer.
CFD models are also discussed. The final section of the chapter lists some Internet addresses from where computer models can be downloaded. Appendix A: Fire safety engineering resources on the Internet.
Appendix A provides a list of Internet addresses of special interest fire safety engineering professionals. Appendix B: Suggestions for experiments and computer labs. Appendix C: A physical quantity may be characterized by dimensions. The arbitrary magnitudes assigned to the dimensions are called units.
For example, mass is a dimension often assigned the symbol m, and often expressed in kilograms, which is assigned the symbol kg. Two sets of unit systems are in common use today: The SI, a simple and logical system based on a decimal relationship between the various units, is used for engineering work in most industrialized nations, including Britain.
The U. Since this textbook uses the SI system, we shall discuss the basis of the SI and provide some conversion factors to the British system. Table 1. There are seven fundamental dimensions in the SI, and all other dimensions in the SI can be expressed in terms of these fundamental dimensions. Units and symbols used in fire safety engineering: Fire safety science is a rapidly developing field, and fire safety engineering is a relatively young engineering discipline.
The scientific and engineering communities have therefore not yet fully standardized the assignments of symbols and units used in this field. The variation in symbols and units used is often a source of confusion and irritation to the engineering student. In fire safety engineering, energy release rates are commonly given in kilowatts kW , whereas according to SI they should be given in watts W.
We shall follow the fire safety engineering tradition and express energy release rates in kW. For example, the equations given in this book for calculating plume mass flow rates require that the energy release rate used in the expressions be given in kW.
Units and symbols used in this book: Below we list some of the most commonly used units and symbols in this book. A symbol with a dot above it denotes a quantity per unit time. A symbol followed by a double prime sign denotes a quantity per unit area. Most other symbols and units used in this book are commonly known and widely accepted. The symbols are also defined in the text. Temperature conversions: Pressure, P: Drysdale, D. Klote, J. Shields, T.
Magnusson, S. Cox, G. Quintiere, J. It lays groundwork for the more calculation-intensive chapters that follow. We introduce and define a number of terms necessary for our subsequent treatment of the subject of enclosure fires.
The chapter begins with a general description of the process of combustion and a description of a typical development of a fire in an enclosure. The fire development is commonly divided into different stages; these are identified and discussed. Finally, we discuss a number of factors that influence enclosure fire development. When an opening is suddenly introduced, the inflowing air may mix with these, creating a combustible mixture of gases in some part of the enclosure.
Any ignition sources, such as a glowing ember, can ignite this flammable mixture, resulting in an extremely rapid burning of the gases.
Expansion due to the heat created by the combustion will expel the burning gases out through the opening and cause a fireball outside the enclosure. The phenomenon can be extremely hazardous. Flashover — The transition from the fire growth period to the fully developed stage in the enclosure fire development. The fire can also be fuel-controlled in later stages. Fully developed fire — Synonymous with the post-flashover fire, which lasts from the occurrence of flashover through the decay stage to extinction.
During most of this period the fire is ventilation-controlled; at some point during the decay period it becomes fuelcontrolled again. Post-flashover fire — When the objective of fire safety engineering design is to ensure structural stability and safety of fire fighters, the post-flashover fire is of greatest concern.
The design load in this case is characterized by the temperature—time curve assumed for the fully developed fire stage. Pre-flashover fire — The growth stage of a fire, where the emphasis in fire safety engineering design is on the safety of humans.
The design load is in this case characterized by an energy release rate curve, where the growth phase of the fire is of most importance.
Ventilation-controlled fire — As the fire grows it may become ventilation-controlled, when there is not enough oxygen available to combust most of the pyrolyzing fuel.
The energy release rate of the fire is then determined by the amount of oxygen that enters the enclosure openings. The interactions between the flame, its fuel, and the surroundings can be strongly nonlinear, and quantitative estimation of the processes involved is often complex. The processes of interest in an enclosure fire mainly involve mass fluxes and heat fluxes to and from the fuel and the surroundings. Figure 2. In order to introduce the most dominant of these processes, this section includes a general qualitative description of the chemical and physical phenomena associated with fire.
The discussion is divided into two parts: In fire safety engineering, a course on fire fundamentals deals extensively with the subject of combustion. In our introductory discussion on combustion we will use a candle as our fuel; the discussion is therefore limited to a laminar, steady flame on a solid substrate.
The study of a burning candle, however, is very illustrative of the natural processes in which we are interested. Consider Figure 2. An ignition source, a match for example, heats up the wick and starts melting the solid wax. The wax in the wick vaporizes, and the gases move, by the process of diffusion, out into a region where oxygen is found. The gases are oxidized in a complex series of chemical reactions, in regions where the oxygen—fuel mixture is flammable.
The candle flame is then stable; it radiates energy to the solid wax, which melts. Since the wax vaporizes and is removed from the wick, the melted wax moves up the wick, vaporizes, burns, and the result is a steady combustion process.
Adapted from Friedman1. With permission. From Lyons3. The processes occurring in the flame involve the flow of energy and the flow of mass. The flow of energy occurs by the processes of radiation, convection, and conduction.
The dominant process is that of radiation; it is mainly the soot particles produced by combustion that glow and radiate heat in all directions.
The radiation down toward the solid is the main heat transfer mode, which melts the solid, but convection also plays a role. The convective heat flux is mainly upward, transferring heat up and away from the combustion zone. The larger and more luminous the flame, the quicker the melting process. The wick is therefore introduced as a way to transport the melted wax up into the hot gases, where the combined processes of radiation, convection, and conduction supply sufficient energy to vaporize the melted wax.
The mass transfer and the phase transformations are also exemplified by the burning candle. The fuel transforms from solid to liquid state. The mass balance requires that the mass that disappears from the wick by vaporization be replaced, and thus the liquid is drawn up into the wick by capillary action.
Once there, the heat transfer from the flame causes it to vaporize, and the gases move away from the wick by the process of diffusion. The inner portion of the flame contains insufficient oxygen for full combustion, but some incomplete chemical reactions occur, producing soot and other products of incomplete combustion. These products move upward in the flame due to the convective flow and react there with oxygen. At the top of the flame nearly all the fuel has combusted to produce water and carbon dioxide; the efficiency of the combustion can be seen by observing the absence of smoke emanating from the top of the candle flame.
This self-sustained combustion process can most easily be changed by altering the dimensions and properties of the wick, and thereby the shape and size of the flame. A longer and thicker wick will allow more molten wax to vaporize, resulting in a larger flame and increased heat transfer to the solid.
The mass and heat flows will quickly enter a balanced state, with steady burning as a result. Other solid fuels: Without the wick, the candle will not sustain a flame, as is true for many other solid fuels. Factors such as the ignition source, the type of fuel, the amount and surface area of the fuel package determine whether the fuel can sustain a flame.
A pile of wooden sticks may sustain a flame, while a thick log of wood may not do so. Once these factors are given, the processes of mass and energy transport will determine whether the combustion process will decelerate, remain steady, or accelerate.
Also, the phase transformations of other solid fuels may be much more complicated than the melting and vaporizing of the candle wax. The solid fuel may have to go through the process of decomposition before melting or vaporizing, and this process may require considerable energy.
The chemical structure of the fuel may therefore determine whether the burning is sustained. For fuels more complex than the candle, it is difficult to predict fire growth. The difficulty is not only due to the complexity of the physical and chemical processes involved, but also due to the dependence of these processes on the geometric and other fuel factors mentioned above and the great variability in these.
When the fuel package is burnt in an enclosure, the fire-generated environment and the enclosure boundaries will interact with the fuel, as seen in Figure 2. It is currently beyond the state of the art of fire technology to predict the fire growth in an enclosure fire with any generality, but reasonable engineering estimates of fire growths in buildings are frequently obtained using experimental data and approximate methods.
The following is a general description of the various phenomena that may arise during the development of a typical fire in an enclosure. After ignition, the fire grows and produces increasing amounts of energy, mostly due to flame spread. In the early stages the enclosure has no effect on the fire, which then is fuelcontrolled.
Besides releasing energy, a variety of toxic and nontoxic gases and solids are produced. From Cooper. The issue of energy release rate is examined in Chapter 3 and the issue of combustion product yields is discussed in Chapter 9.
The hot gases in the flame are surrounded by cold gases, and the hotter, less dense mass will rise upward due to the density difference, or rather, due to buoyancy. The buoyant flow, including any flames, is referred to as a fire plume. This mixture of combustion products and air will impinge on the ceiling of the fire compartment and cause a layer of hot gases to be formed. Only a small portion of the mass impinging on the ceiling originates from the fuel; the greatest portion of this mass originates from the cool air entrained laterally into the plume as it continues to move the gases toward the ceiling.
As a result of this entrainment, the total mass flow in the plume increases, and the average temperature and concentration of combustion products decreases with height. Methods for estimating the mass flow rate in the plume as well as other plume properties are discussed in Chapter 4.
Ceiling jet: When the plume flow impinges on the ceiling, the gases spread across it as a momentum-driven circular jet. The velocity and temperature of this jet is of importance, since quantitative knowledge of these variables will allow estimates to be made on the response of any smoke and heat detectors and sprinkler links in the vicinity of the ceiling.
Methods for estimating the velocity and temperature of the ceiling jet are discussed in Chapter 4. The ceiling jet eventually reaches the walls of the enclosure and is forced to move downward along the wall, as shown in Figure 2. However, the gases in the jet are still warmer than the surrounding ambient air, and the flow will turn upward due to buoyancy. A layer of hot gases will thus be formed under the ceiling.
Gas temperatures: Experiments have shown, for a wide range of compartment fires, that it is reasonable to assume that the room becomes divided into two distinct layers: Further, the properties of the gases in each layer change with time but are assumed to be uniform throughout each layer.
For example, it is commonly assumed when using engineering methods that the temperature is the same throughout the hot layer at any given time. Methods for estimating the upper layer temperature as a function of time are discussed in Chapter 6. The hot layer: The plume continues to entrain air from the lower layer and transport it toward the ceiling. The smoke-filling process and methods for calculating smoke-filling times are discussed in Chapter 8.
Heat transfer: As the hot layer descends and increases in temperature, the heat transfer processes are augmented. Heat is transferred by radiation and convection from the hot gas layer to the ceiling and walls that are in contact with the hot gases.
Heat from the hot layer is also radiated toward the floor and the lower walls, and some of the heat will be absorbed by the air in the lower layer. Additionally, heat is transferred to the fuel bed, not only by the flame, but to an increasing extent by radiation from the hot layer and the hot enclosure boundaries.
This leads to an increase in the burning rate of the fuel and the heating up of other fuel packages in the enclosure. These heat transfer processes are discussed in Chapter 7. Vent flows: If there is an opening to the adjacent room or out to the atmosphere, the smoke will flow out through it as soon as the hot layer reaches the top of the opening.
Often, the increasing heat in the enclosure will cause the breakage of windows and thereby create an opening. Methods for calculating the mass flow rates through vents are discussed in Chapter 5. The fire may continue to grow, either by increased burning rate, by flame spread over the first ignited item, or by ignition of secondary fuel packages.
The upper layer increases in temperature and may become very hot. As a result of radiation from the hot layer toward other combustible material in the enclosure, there may be a stage where all the combustible material in the enclosure is ignited, with a very rapid increase in energy release rates.
This very rapid and sudden transition from a growing fire to a fully developed fire is called flashover. The fire can thus suddenly jump from a relatively benign state to a state of awesome power and destruction. The solid line in Figure 2. Once Point B has been reached the fully developed fire , flashover is said to have taken place. Flashover is further described in Section 2.
The fully developed fire: At the fully developed stage, flames extend out through the opening and all the combustible material in the enclosure is involved in the fire. The fully developed fire can burn for a number of hours, as long as there is sufficient fuel and oxygen available for combustion.
Section 2. Oxygen starvation: For the case where there are no openings in the enclosure or only small leakage areas, the hot layer will soon descend toward the flame region and eventually cover the flame. The air entrained into the combustion zone now contains little oxygen and the fire may die out due to oxygen starvation. The dotted line in Figure 2. Even though the energy release rate decreases, the pyrolysis may continue at a relatively high rate, causing the accumulation of unburnt gases in the enclosure.
If a window breaks at this point, or if the fire service create an opening, the hot gases will flow out through the top of the opening and cold and fresh air will flow in through its lower part.
This may diminish the thermal load in the enclosure, but the fresh air may cause an increase in the energy release rate. The fire may then grow toward flashover, as shown by the dotted line in Figure 2. As a worst case, the inflowing air may mix up with the unburnt pyrolysis products from the oxygen-starved fire.
Any ignition sources, such as a glowing ember, can ignite the resulting flammable mixture. This leads to an explosive or very rapid burning of the gases.
Expansion due to the heat created by the combustion will expel the burning gases out through the opening. This phenomenon, termed backdraft, can be extremely hazardous, and many firefighters have lost their lives due to this very rapidly occurring event. In Figure 2. Usually, a backdraft will only last for a very short time, in the order of seconds backdrafts lasting for minutes have, however, been observed.
A backdraft will usually be followed by flashover, since the thermal insult will ignite all combustible fuel in the enclosure, leading to a fully developed enclosure fire. Adapted from Bengtsson Smoke gas explosion: When unburnt gases from an underventilated fire flow through leakages into an closed space connected to the fire room, the gases there can mix very well with air to form a combustible gas mixture.
A small spark is then enough to cause a smoke gas explosion, which can have very serious consequences. This phenomenon is, however, very seldom observed in enclosure fires. This can be done in terms of several environmental variables; we mainly discuss this in terms of enclosure temperatures and in terms of mass flows and pressure differences across the enclosure openings.
Ignition can be considered as a process that produces an exothermic reaction characterized by an increase in temperature greatly above the ambient. It can occur either by piloted ignition by flaming match, spark, or other pilot source or by spontaneous ignition through accumulation of heat in the fuel.
The accompanying combustion process can be either flaming combustion or smoldering combustion. Following ignition, the fire may grow at a slow or a fast rate, depending on the type of combustion, the type of fuel, interaction with the surroundings, and access to oxygen. A smoldering fire can produce hazardous amounts of toxic gases while the energy release rate may be relatively low.
The growth period of such a fire may be very long, and it may die out before subsequent stages are reached. The growth stage can also occur very rapidly, especially with flaming combustion, where the fuel is flammable enough to allow rapid flame spread over its surface, where heat flux from the first burning fuel package is sufficient to ignite adjacent fuel packages, and where sufficient oxygen and fuel are available for rapid fire growth.
Fires with sufficient oxygen available for combustion are said to be fuel-controlled. Flashover is the transition from the growth period to the fully developed stage in fire development. Flashover is not a precise term: These occurrences may all be due to different mechanisms resulting from the fuel properties, fuel orientation, fuel position, enclosure geometry, and conditions in the upper layer.
Flashover cannot be said to be a mechanism, but rather a phenomenon associated with a thermal instability. Fully developed fire: At this stage the energy released in the enclosure is at its greatest and is very often limited by the availability of oxygen.
This is called ventilation-controlled burning as opposed to fuel-controlled burning , since the oxygen needed for the combustion is assumed to enter through the openings. In ventilation-controlled fires, unburnt gases can collect at the ceiling level, and as these gases leave through the openings they burn, causing flames to stick out through the openings.
As the fuel becomes consumed, the energy release rate diminishes and thus the average gas temperature in the compartment declines. The fire may go from ventilation-controlled to fuelcontrolled in this period. The mass flows in turn depend on the pressure differences across the opening. The pressure for the outside atmosphere is represented by a straight line, sloping because the weight of the column of air increases as we approach the ground level.
Assuming that the temperature of the air in the lower layer of the compartment equals the temperature outside, we know that the pressure in the lower layer is given by a straight line with the same slope as that for the outside.
There is a kink in the pressure line inside the compartment when we move into the upper layer; the hot air is lighter than the cold air. The pressure of the outside atmosphere is the same for all four cases in Figure 2.
The pressure inside the compartment is denoted Pi. We shall discuss these pressure profiles in greater detail in Chapter 5. Stage A: In the first stage of the fire the pressure inside is higher than the pressure outside the compartment. This is due to the expansion of the hot gases, which have a greater volume than the cold gases. If the opening is not at ceiling level, the cold gases will be forced out through the opening due to the hot gas expansion.
As a result, the pressure difference across the opening is positive with respect to the compartment , and there will be no inflow through the opening, only outflow of cold gases. Stage B: Stage B only lasts a few seconds and is often ignored. The smoke layer has just reached the top of the opening and the hot gases have started to flow out. The pressure inside is still higher than the pressure outside and both hot and cold gases flow out through the opening.
There is no mass flow into the compartment. Stage C: In this stage the hot gases flow out through the top of the opening and the mass balance demands that cold gases of equal mass flow in through the lower part of the opening.
This stage can last for a considerable time, until the room is filled entirely with smoke or until flashover occurs. Stages A, B, and C are associated with the growth stage mentioned above and the preflashover stage. Stage D: This stage is associated with the fully developed fire mentioned above. In many cases flashover has occurred between stages C and D.
The pressure profiles are discussed further in Chapter 5, where we derive equations for calculating the mass flow rates out of and into the compartment, and in Chapter 8, where we derive methods for calculating the smoke filling rate in an enclosure. One has to do with the pre-flashover fire, where the emphasis is on the safety of humans.
The design load in this case is characterized by an energy release rate curve, where the growth phase of the fire is of most importance. We discuss this further in Chapter 3. A very different design situation arises when the objective is to ensure structural stability and safety of firefighters.
Here the post-flashover fire is of greatest concern, and the design load is characterized by a temperature—time curve where the fully developed fire stage is of greatest concern. We discuss this further in Chapter 6. The fire development is therefore sometimes simply divided into the pre-flashover stage and the post-flashover stage. After ignition and during the initial fire growth stage, the fire is said to be fuel-controlled, since, in the initial stages, there is sufficient oxygen available for combustion and the growth of the fire entirely depends on the characteristics of the fuel and its geometry.
When the fire grows toward flashover it may become ventilation-controlled, when there is not enough oxygen available to combust most of the pyrolyzing fuel. The energy release rate of the fire is then determined by the amount of oxygen that enters the enclosure openings, and the fire is therefore termed ventilation-controlled. During the decay stage the fire will eventually return to being fuel-controlled. The growth stage of the fire and the pre-flashover fire is therefore often associated with the fuel-controlled fire, and the fully developed stage and the post-flashover fire is often associated with the ventilation-controlled fire.
An ignition source can consist of a spark with a very low energy content, a heated surface or a large pilot flame, for example. The source of energy is either chemical, electrical, or mechanical.
The greater the energy of the source, the quicker the subsequent fire growth on the fuel source. A spark or a glowing cigarette may initiate smoldering combustion, which may continue for a long time before flaming occurs, often producing low heat but considerable amounts of toxic gases. A pilot flame usually produces flaming combustion directly, resulting in flame spread and fire growth. The location of the ignition source is also of great importance.
A pilot flame positioned at the lower end of, say, a window curtain may cause rapid upward flame spread and fire growth. The same pilot flame would cause much slower fire growth were it placed at the top of the curtain, resulting in slow, creeping, downward flame spread. The type and amount of combustible material is, of course, one of the main factors determining the fire development in an enclosure.
In building fires the fuel usually consists of solid materials, such as the furniture and fittings normally seen in the enclosure interior; in some industrial applications the fuel source may also be in liquid state. Heavy, wood-based furniture usually causes slow fire growth but can burn for a long time.
Some modern interior materials include porous lightweight plastics, which cause more rapid fire growth but burn for a shorter time. A high fire load therefore does not necessarily constitute a greater hazard; a rapid fire growth is more hazardous in terms of human lives. The position of the fuel package can have a marked effect on the fire development. If the fuel package is burning away from walls, the cool air is entrained into the plume from all directions.
When placed close to a wall, the entrainment of cold air is limited. This not only causes higher temperatures but also higher flames since combustion must take place over a greater distance. Curve B is for a similar stack near a wall, and curve C is for a stack in a corner. The spacing and orientation of the fuel packages are also of importance. The spacing in the compartment determines to a considerable extent how quickly the fire spreads between the fuel packages.
Upward flame spread on a vertically oriented fuel surface will occur much more rapidly than lateral spread along a horizontally oriented fuel surface. With material mounted on the ceiling, the flame spreads with the flow of gases concurrent-flow flame spread , causing rapid growth. A, B, and C indicate burning away from walls, by a wall, and in a corner, respectively.
Adapted from Alpert and Ward 7. Flashover occurs when the energy release rate is kW in this compartment. As a result, the time to flashover is 4 minutes in the former case and 12 minutes in the latter case. A fuel package of a large surface area will burn more rapidly than an otherwise equivalent fuel package with a small surface area.
A pile of wooden sticks, for example, will burn more rapidly than a single log of wood of the same mass. Enclosure geometry: The hot smoke layer and the upper bounding surfaces of the enclosure will radiate toward the burning fuel and increase its burning rate. The temperature and thickness of the hot layer and the temperature of the upper bounding surfaces thus have a considerable impact on the fire growth.
A fuel package burning in a small room will cause relatively high temperatures and rapid fire growth. In a large compartment, the same burning fuel will cause lower gas temperatures, longer smoke-filling times, less feedback to the fuel, and slower fire growth. The fire plume entrains cold air as the mixture of combustion products and air move upward toward the ceiling.
The amount of cold air entrained depends on the distance between the fuel source and the hot layer interface. In an enclosure with a high ceiling this causes relatively low gas temperatures, but due to the large amount of air entrained, the smoke-filling process occurs relatively rapidly. The smaller the floor area, the faster the smoke-filling process.
With a low ceiling the heat transfer to the fuel will be greater. Additionally, the flames may reach the ceiling and spread horizontally under it. This results in a considerable increase in the feedback to the fuel and to other combustibles, and a very rapid fire growth is imminent.
For enclosures with a high ceiling and a large floor area, the flames may not reach the ceiling and the feedback to the fuel is modest. The fire growth rather occurs through direct radiation from the flame to nearby objects, where the spacing of the combustibles becomes important. In buildings with a large floor area but a low ceiling height, the feedback from the hot layer and ceiling flames can be very intensive near the fire source.
Further away, the hot layer has entrained cold air and has lost heat to the extensive ceiling surfaces, and the heat flux to the combustible materials, in the early stage of the fire, is therefore lower than the heat flux closer to the fire source.
We can conclude that the proximity of ceilings and walls can greatly enhance the fire growth. Even in large spaces, the hot gases trapped under the ceiling can heat up the combustibles beneath and result in extremely rapid fire spread over a large area. The fire that so tragically engulfed the Bradford City Football Stadium in England in is a vivid reminder of this. Once flaming combustion is established the fire must have access to oxygen for continued development.
In compartments of moderate volume that are closed or have very small leakage areas, the fire soon becomes oxygen-starved and may self-extinguish or continue to burn at a very slow rate depending on the availability of oxygen.
For compartments with ventilation openings, the size, shape, and position of such openings become important for the fire development under certain circumstances. During the growth phase of the fire, before it becomes ventilation-controlled, the opening may act as an exhaust for the hot gases, if its height or position is such that the hot gases are effectively removed from the enclosure.
This will diminish the thermal feedback to the fuel and cause slower fire growth. For other circumstances the geometry of the opening does not have a very significant effect on fire growth during the fuel-controlled regime.
It is when the fire becomes controlled by the availability of oxygen that the opening size and shape first become all-important. We shall derive this result in Chapter 5.
It is thereby shown that the rate of burning is controlled by the rate at which air can flow into the compartment. An increase in the factor Ao Ho will lead to an equal increase in the burning rate. This is valid up to a certain limit when the burning rate becomes independent of the ventilation factor and the burning becomes fuel-controlled. Properties of bounding surfaces: The material in the bounding surfaces of the enclosure can affect the hot gas temperature considerably, and thereby the heat flux to the burning fuel and other combustible objects.
Certain bounding materials designed to conserve energy, such as mineral wool, will limit the amount of heat flow to the surfaces so that the hot gases will retain most of their energy. Insulating materials have a low thermal inertia; materials with relatively high thermal inertia, such as brick and concrete, allow more heat to be conducted into the construction, thereby lowering the hot gas temperatures.
A fire in a compartment can develop in an infinite number of ways; we have here attempted to give a general description of the most significant enclosure fire stages. Until now, however, there has been no current, truly comprehensive engineering book that builds an in-depth understanding of the scientific aspects of enclosure fires. Enclosure Fire Dynamics fills this void with a complete description of enclosure fires and how the outbreak of a fire in a compartment causes changes in the environment.
The authors-both internationally renowned experts in fire safety and protection engineering-offer a clear presentation of the dominant mechanisms controlling enclosure fires and develop simple analytical relationships useful in designing buildings for fire safety. They show readers how to derive engineering equations from first principles, stating the assumptions clearly and showing how the resulting equations compare to experimental data.
The details and the approach offered by this text provide readers with a confidence in-and the applicability of-a wide range of commonly used engineering equations and models. Enclosure Fire Dynamics will enhance the knowledge of professional fire protection engineers, researchers, and investigators and help build a strong foundation for engineering students.
Author s Bio Author. Reviews "…The ability of a textbook to provide this 'positive feedback' from educational materials to the scientific base of the discipline can be regarded as a clear indication of the highest quality of the book…Enclosure Fire Dynamics by internationally renowned scientists Bjorn Karlsson and James G.
Quintier, who is one of the founders of fire safety science, clearly demonstrates this valuable feedback feature…a landmark in the field of fire safety engineering…highly recommended to a wide range of readers from researchers in fire safety science to fire engineers, fire safety professionals, and all those who are involved to some extent in fire related activities…" -Fire Safety Journal.
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