Enclosure Fires Research Articles

The Pressure Equations in Zone-Fire Modeling.

The nonadiabatic nature of low-speed combustion and fire, in which strongly exothermic reactions produce large temperature variations but only mild pressure variations, can cause difficulty when integrating zone models of enclosure fires. Examples of simple zone fire models are examined to illustrate the analytical nature of the problems encountered. These difficulties arise in the solution of the equations for the pressure in general enclosures because the pressure equilibrates much more rapidly than other dynamical variables. Singular perturbation methods and phase plane analyses, together with numerical integration of the nondimensionalized equations, are employed to study the stiff nature of the equations. We conclude that many of the difficulties associated with numerical integration of zone fire models may be circumvented by appropriate analysis of the zone fire model equations.

Ventilation-controlled combustion of polymers

Ventilation-controlled combustion in buoyant turbulent diffusion flames of polymers with CH, CHO, and CHON aliphatic and aromatic structures has been examined. Correlations have been established between equivalence ratio and combustion efficiencies (chemical, convective, and radiative), generation efficiencies of combustion products (CO, CO 2, hydrocarbons, soot, and other products) and consumption efficiency of O 2. Relationships resembling flammability limit diagrams have been developed between the generation efficiencies of CO and CO 2, and soot and CO. With increase in the equivalence ratio, chemical and convective efficiencies of combustion, generation efficiency of CO 2, and consumption efficiency of O 2 decrease, whereas radiative efficiency of combustion and generation efficiencies of CO, hydrocarbons, soot, and other carbon containing products increase. These changes in the combustion properties are due to deviations from thermodynamic equilibrium and each change is correlated in terms of the equilibrium fraction of the combustion property as a function of the equivalence ratio. For similar magnitude of increase in equivalence ratio, fuels with CHO and CHON structures show rapid increase in the generation efficiencies of CO and hydrocarbons compared to the generation efficiency of soot, whereas fuels with CH structures show rapid increase in the generation efficienty of soot, compared with the generation efficiencies of CO and hydrocarbon. This is consistent with the general understanding of the formation of soot and CO. For equivalence ratio of four or greater, flames are extinguished, but nonflaming combustion continues as long as the fuel continues to be generated as a result of external heat flux exposure. Correlations have been developed as inputs to models for enclosure fires to assess thermal and nonthermal hazards due to generation of heat, soot, CO, hydrocarbons, and corrosive and toxic products and prevention and protection from such hazards.

Time‐dependent mass loss rate behavior of wall materials under external radiation

AbstractExternal radiation in enclosure fires can significantly enhance flame spread and fire growth. One of the effects of external radiation is to increase the mass loss rate of the fuel, which in turn produces larger flames. In this work, a measurement of mass loss with and without applied radiation was made as a function of time for three types of materials: a plastic (polymethylmethacrylate), wood‐based products (particle board and hardboard), and a paper‐based product (cardboard). The levels of applied radiation ranged from 0 to nearly 12 kWm−2. The purpose of the investigation was to (1) quantitatively determine the effect of external radiation on the mass loss of various materials,(2) measure various parameters which may be used to characterize the mass loss rate history of the materials and (3) determine a method for expressing the mass loss rate as a function of time for input into numerical models. Higher levels of external radiation resulted in higher peak mass loss rates and quicker consumption of the material. Quantities which are useful for ranking the flammability of materials have been measured for the samples tested and are tabulated.

Extinguishment of Diffusion Flames of Polymeric Materials by Halon 1301

Halon 1301 flame extinction results are discussed for the com bustion of polymethylmethacrylate (PMMA), eight composite materials, and carbon in the gas phase. Two types of combustion and flame extinction experi ments were performed: (1) in the Factory Mutual Research Corporation (FMRC) flammability apparatus (50 kW scale) for PMMA and composite materials, and (2) in the FMRC electrical arc apparatus for carbon in the gas phase. For char forming composite materials, mass transfer from the surface was low, turbulent diffusion flames were not generated, and flame extinction oc curred between 3 to 4.5% of Halon 1301, close to the value reported for the lam inar diffusion flames of polymers. For non-charring PMMA, mass transfer from the surface was high, flames were turbulent, and flame extinction was found at about 6% of Halon 1301, contrary to the accepted value of about 4% for the lam inar diffusion flames of polymers. With Halon 1301 the conditions for flame in stability and extinction for combustion efficiency less than about 0.40, with sig nificant increase in the amounts of products of incomplete combustion (such as CO and hydrocarbon), were in agreement with flame instability and extinction found for fuel-rich conditions inside well-ventilated laminar and turbulent diffusion flames, in ceiling layers of combustion products, in enclosure fires, in ventilation-controlled buoyant diffusion flames of polymers, and for flame ex tinction of heptane flames by water. Experiments in the FMRC electrical arc apparatus showed that in the gas phase combustion of carbon vapors generated in high energy arc, chemical heat release rate and combustion efficiency decreased with increase in Halon 1301. At about 7.5% of Halon 1301, conditions were close to flame extinction and at 9.0%, oxidative pyrolysis of carbon was indicated. Concentrations of Br- and F- ions, generated from the decomposition of Halon 1301, were also measured. Concentration of Br- ions was higher than the concentration of F- ions, al though there are three F atoms and only one Br atom in Halon 1301. There was brown deposit on the walls of the apparatus with extensive corrosion of rubber gaskets, electrical fan, and other components. The techniques discussed in this article appear to be attractive for the assess ment of flame extinguishability and corrosive characteristics of fire suppres sants to replace ozone layer depleting Halons.

Penetrative convection of a plane turbulent wall jet in a two-layer thermally stable environment: a problem in enclosure fires

An experimental investigation has been carried out on the penetrative characteristics of a heated, two-dimensional, turbulent wall jet discharged downward into a two-layer thermally stable environment. Such flows, with opposing buoyancy effects, are frequently encountered in heat rejection processes and in enclosure fires. The discharge temperature of the jet is taken as higher than the upper layer temperature so that the jet is negatively buoyant in both layers. Of particular interest is the penetration of the jet into the lower layer. The conditions for which it fails to penetrate the interface between the two layers are also determined. The penetration distance of the jet is measured and related to the inflow conditions, particularly to the temperature and the velocity at the discharge. The thermal field is studied in detail to determine the basic characteristics of such flows. The mass flow rate penetrating downward as well as that rising upward due to thermal buoyancy are obtained and compared with the jet inlet mass flow rate. The heat transfer to the surface is measured for several wall temperatures and considered in terms of the penetrative flow. Flow visualization with smoke is also undertaken in order to obtain further insight into the basic nature of the flow.

Study of Force - Ventilated Fires in Closed Space

Gas analysis and measurements of heat flux and temperature were made using a 50cm cubic model box with methanol as the fuel for grasping a phenomena that occur during force-ventilated fires in closed spaces. The criticality for heavy CO production (1-5 vol. %) can be described as a function of stoichiometric fuel to air ratio, and the CO concentration changes dramatically when the stoichiometric fuel to air ratio is approximately equal to 1.0. The combustion characteristics within the force-ventilated fire enclosure change due to the inlet and outlet conditions even if the other ventilation conditions (air supply rate, tray size) are the same. Further, gas analysis and measurements of temperature were made using a large model box (L=3.6m, W=2.4m, H=2.4m) with methane and propane as heat sources with controllable heat release rate for grasping the characteristics of smoke layer formation within the forced-ventilated fire enclosure which is highly air tight. Smoke layer formation within the force-ventilated fire enclosure is influenced by the air flow within the enclosure and temperature difference between the supply air and exhaust gas.

Wall and Corner Flows Driven by a Ceiling Jet in an Enclosure Fire

Abstract This paper presents the results from an experimental simulation of the wall and corner flows generated by a fire-plume driven ceiling jet in a compartment fire. An experimental arrangement is employed to determine the transport processes resulting from these flows in typical full-scale fires in rooms. Of particular interest were the flow and heat transfer in the corner, the heat transfer rate to the wall and the ceiling, the downward penetration distance of the wall flow and the entrainment into the flow. The results obtained are presented in terms of dimensionless variables in order to simulate the conditions for a wide range of fires. The chosen range of the governing parameters corresponds to the values encountered in typical room and enclosure fires. Some results are also presented on a jet impinging at an inclination, other than normal, with the wall to simulate the radial spread of the ceiling jet in an actual fire. It is found that the ceiling-jet-driven wall and corner flows are very impo...

Reactivity of product gases generated in idealized enclosure fire environments

Previous experiments have demonstrated that the mole fractions of major product gases trapped in a hood located above a fire can be correlated in terms of the global equivalence ratio. Temperatures in the hood experiments have generally been low. Full-kinetic calculations are employed to characterize the reactivity and reaction behavior for the product gases observed in a hood experiment burning natural gas as fuel. A range of temperatures (700-1300 K) typical of enclosure fires is considered. Mixing is assumed to be infinitely fast (perfectly-stirred reactor) or infinitely slow (plug-flow reactor). Both isothermal and adiabatic cases are treated. Calculations are reported for a range of residence times (0–20 s) and global equivalence ratios (0.5–2.83). The dominant variable for reaction behavior is found to be temperature. Effects due to mixing and heat transfer assumptions are less important. The results indicate that the hood product gases are reactive for temperatures greater than 800 K. For rich mixtures, reaction generates primarily carbon monoxide as opposed to carbon dioxide. At higher temperatures the formation of hydrogen is favored over water while water is favored in the 800–1000 K range. In the lower temperature range HO 2 is the dominant free radical. Uncertainties in rates for reactions involving this specie introduce considerable uncertainty into the calculated behaviors. At higher temperatures (1100-1300 K) the important free radicals are H atom and OH. Reactions involving these radicals are better characterized than those involving HO 2 . The findings suggest that the results of the hood experiments cannot be used directly for the modeling of species production in enclosure fires.

Mixed Convection Heat Transfer From Thermal Sources Mounted on Horizontal and Vertical Surfaces

An experimental study is carried out on the fundamental aspects of the conjugate, mixed convective heat transfer from two finite width heat sources, which are of negligible thickness, have a uniform heat flux input at the surface, and are located on a flat plate in the horizontal or the vertical orientation. The heat sources are wide in the transverse direction and, therefore, a two-dimensional flow circumstance is simulated. The mixed convection parameter is varied over a fairly wide range to include the buoyancy-dominated and the mixed convection regimes. The circumstances of pure natural convection are also investigated. The convective mechanisms have been studied in detail by measuring the surface temperatures and determining the heat transfer coefficients for the two heated strips, which represent isolated thermal sources. Experimental results indicate that a stronger upstream heat source causes an increase in the surface temperature of a relatively weaker heat source, located downstream, by reducing its convective heat transfer coefficient. The influence of the upstream source is found to be strongly dependent on the surface orientation, especially in the pure natural convection and the buoyancy dominated regimes. The two heat sources are found to be essentially independent of each other, in terms of thermal effects, at a separation distance of more than about three strip widths for both the orientations. The results obtained are relevant to many engineering applications, such as the cooling of electronic systems, positioning of heating elements in furnaces, and safety considerations in enclosure fires.

Thermal Analysis of Hydrogen Combustion in a 1/4-Scale Boiling Water Reactor Containment Building

A mathematical model has been developed for the analysis of hydrogen combustion in boiling water reactor containment buildings. The overall phenomenon is similar to that observed in enclosure fires with the major differences being: (1) The containment building is hermetically sealed, and (2) diffusion flames form above a water pool at the bottom of the containment, through which hydrogen is released. The calculated results are compared with data obtained in tests performed in a 1/4-scale model of the containment building. The zone modeling approach, which was used in these calculations, has been demonstrated to be suitable for application to the combustion dynamics produced by the hydrogen flames in containment volumes.

Thermal radiation properties of galvanized steel and its importance in enclosure fire scenarios

Measurements of the emissivity of hot dip galvanized steel have been performed as a function of temperature from about 200 to 650°C. These results have been used to calculate the temperature increase of a fire-exposed galvanized steel surface according to the European Recommendations for Fire Safety of Steel Structures. It can be concluded that the radiative heat transfer dominates over the convective heat transfer after some minutes. Using a constant estimated value for the emissivity can lead to discrepancies with a real situation. Under certain circumstances a step function for ε(T) seems useful.

Recirculating mixed convection flows in rectangular cavities

A numerical investigation is carried out to determine the flow and thermal structures of mixed convective flow in a rectangular cavity. The effect of aiding and opposing buoyancy on the flow in a vertical downward laminar jet is studied for Re < 200, -1.0 < 'Ri < 0.5, and 0.5 < A < 5.0. Such flow situations occur in enclosure fires, energy storage, and ventilation systems. Results of the velocity profile, temperature distribution, local friction factor, and heat-transfer coefficient are presented for both hot and cold jets. The flow configuration is found to be strongly affected by the positive and negative buoyancy and also by the aspect ratio of the cavity.

Scaling applications in fire research

The principles for scaling fire phenomena are examined from the dimensionless groups derived from the governing differential equations. A review of the literature shows examples of where correlations have been successfully developed for a wide range of fire phenomena in terms of the significant dimensionless groups. Scaling techniques based on Froude modeling, pressure modeling and analog modeling are described and illustrated. The use of small geometric models ranging from fire plumes to enclosure fires are illustrated by many examples.

On the Field Modelling Approach to the Simulation of Enclosure Fires

This paper considers the mathematical field modeling approach to the description of enclosed fires. Field modeling is emerging as the "new technology" for the simulation of the fire in enclosure scenario. A number of field modeling packages are discussed as well as their application to aircraft cabin fires, fire-sprinkler interaction, shopping mall fires, atria fires and fires in sports stadia.

Fundamentals of Enclosure Fire "Zone" Models

The conservation laws are presented in control volume form and applied to the behavior of fire in enclosures. The behavior of enclosure fires is discussed and the assumptions for justifying the use of the control volume of "zone" modeling approach are presented. The governing equations are derived and special solutions are given. Flow through wall vents, room filling, and growing fires are analyzed. This paper is not a review of the litera ture. It is a theoretical presentation of compartment fire zone modeling, much of which has not been explicitly presented before or integrated into one presentation. The analysis for species in terms of mixture fraction is a distinct new contribution to zone modeling.

An experimental study of the generation and characteristics of a two-layer thermally stable environment

An experimental system has been developed to generate a thermally stratified, two-layer, environment in an enclosure, with air as the fluid. Such a stably stratified circumstance, in which an essentially isothermal heated layer overlies a relatively cooler isothermal layer, frequently arises in several practical problems such as those related to heat rejection, heat transfer from isolated thermal sources in confined regions, and enclosure fires. A detailed study of the thermal field is carried out to determine the basic characteristics of the stratified environment and the dependence of the physical parameters such as interface location and temperature level on the governing variables. The location of the interface, between the two layers can be controlled by varying the temperature and the velocity of the heated fluid discharged at the top of the enclosure for stratifying the enclosed region. It is found that the interface moves down as the Richardson number Ri, based on the conditions of the heated air entering at the top of enclosure, is increased. The stratification can be maintained essentially constant with time. Also, the temperature levels in the two zones can be varied over fairly wide ranges to simulate practical circumstances. The nature of such a stably stratified region is studied, particularly the effect of the inflow and outlet conditions.

Importance of Wall Flows at the Early Stages of Fire Growth in the Mathematical Modeling of Enclosure Fires

This paper considers the negatively buoyant, two-dimensional wall flows that arise during the growth of a compartment fire. Such flows affect the distribution of mass, momentum and energy in the enclosure. The enclosed environment is generally assumed to be comprised of two stably stratified zones in the mathematical modeling of enclosure fires and the transport between these zones is affected by these wall flows. An experimental study is carried out to determine the important thermal characteristics of such negatively buoyant wall flows, particularly the entrainment into the flow, the penetration distance and the heat transfer to the walls of the enclosure. Employing analytical experimental results of buoyancy driven wall and plume flows, the resulting effects in compartment fires are determined. It is shown that the wall flows are very important at the early stages of fire growth,particularly during the establishment of the two zones in the enclosure. Also, the additional transport generated by buoyancy...

One‐dimensional mathematical modelling of enclosure fire dynamics

AbstractThis paper presents a one‐dimensional mathematical model of enclosure fire dynamics. The model is based on the fundamental equations of mass, momentum and energy and uses the top hat velocity and temperature profiles for the movement of plume gases. A computational algorithm is also developed to solve the set of transient model partial differential equations. In addition, the explicit equations have been derived for computing the steady state velocity and temperature profiles. These derivations are based on the assumption that the plume width follows the relationship given by Eqn (12). Further, these relationships help to identify a dimensionless parameter, Frs, known as source Froude number, which has a significant effect on the behaviour of the plume gases. It may be noted that this assumption is only applicable for the case of Frs = 1. The plume behaviour is studied with reference to three cases of source Froude numbers, i.e. Frs &lt; 1, Frs = 1, Frs &gt; 1.

Buoyancy-driven wall flows in enclosure fires

Buoyancy-induced flows frequently arise adjacent to the walls of an enclosure with a fire. Such flows are important since they may lead to a substantial redistribution of mass and energy between the two zones that idealize the environment in an enclosure fire. In this paper, an analytical study of wall flows generated in enclosure fires is carried out, in order to determine the mass, momentum and energy transported by such flows. Employing integral analysis, the resulting flow rates in the boundary layer are computed and compared with those in the corresponding fire plumes. It is found that, for most enclosure fires, the wall flows are important and should be included in zone model analysis in order to accurately predict the changing environment in the enclosed space. The procedure to take these effects into account is outlined.

Thermal Response of Unconfined Ceilings Above Growing Fires and the Importance of Convective Heat Transfer

A procedure is developed to calculate the thermal response of unconfined nonburning ceilings above growing fires. The procedure uses an algorithm for conduction into the ceiling material. It takes account of heat transfer due to radiation from the combustion zone to the ceiling surface, and due to reradiation from the ceiling to the floor and furnishings. Finally, the procedure uses a previously developed algorithm for convective heat transfer to the ceiling from the fire-plume-driven ceiling jet. The procedure is used to predict radial-dependent surface temperature histories of typical ceiling materials under a variety of different realistic levels of hazardous fire energy generation rates and combustion zone-ceiling separation distances. The results give an indication of the influence of convective heat transfer on peak ceiling thermal response, losses from fire plume gases, and radial variations and peak values of ceiling-to-floor irradiation during enclosure fires.