Patrick J. Pagni

Firebrands and spotting ignition in large-scale fires

Spotting ignition by lofted firebrands is a significant mechanism of fire spread, as observed in many large-scale fires. The role of firebrands in fire propagation and the important parameters involved in spot fire development are studied. Historical large-scale fires, including wind-driven urban and wildland conflagrations and post-earthquake fires are given as examples. In addition, research on firebrand behaviour is reviewed. The phenomenon of spotting fires comprises three sequential mechanisms: generation, transport and ignition of recipient fuel. In order to understand these mechanisms, many experiments have been performed, such as measuring drag on firebrands, analysing the flow fields of flame and plume structures, collecting firebrands from burning materials, houses and wildfires, and observing firebrand burning characteristics in wind tunnels under the terminal velocity condition and ignition characteristics of fuel beds. The knowledge obtained from the experiments was used to develop firebrand models. Since Tarifa developed a firebrand model based on the terminal velocity approximation, many firebrand transport models have been developed to predict maximum spot fire distance. Combustion models of a firebrand were developed empirically and the maximum spot fire distance was found at the burnout limit. Recommendations for future research and development are provided.

Flame spread through porous fuels

A quantitative analysis of the steady-state propagation of a quasi-one-dimensional flame through a thermally thin, porous layer of fuel is presented. The rate of energy transfer from the combustion zone to the fuel is assumed to control the rate of flame propagation. Energy-transfer mechanisms considered are: flame and ember radiation, surface and internal convection, turbulent diffusion of flame eddies, and gas-phase conduction. The effects of ambient flow, fuel moisture, fuel-bed slope, and endothermic pyrolysis are included. A nondimensional flame-spread velocity is obtained as a function of nondimensional fuel, flame, and ambient flow properties. It is found that, in the case of ambient flow in the direction of flame propagation, the flame-spread velocity increases rapidly as the ambient flow velocity increases. From model predictions for a moderately porous fuel, typical of pine-needle beds, the following conclusions are drawn: (1) with no ambient flow, the dominant preheating mechanism is flame radiation, with contributions from ember radiation and gas-phase conduction; (2) for most monzero ambient flow velocities, the primary mechanism is convection with a significant contribution from flame radiation; (3) energy transferred by turbulent flame eddies appears to be generally negligible; and (4) energy absorbed by pyrolysis, prior to ignition, is negligible. Excellent quantitative agreement with experiment has been obtained. More-rigorous analytical models may be formulated, based on the relative import of the energy-transfer mechanisms indicated here. Applications are discussed.

Forced cocurrent smoldering combustion

Abstract An analytic model of the propagation of smoldering combustion through a very porous solid fuel is presented. Here smoldering is initiated at the top of a long, radially insulated, uniform fuel cylinder, so that the smolder wave propagates downward, opposing an upward forced flow of oxidizer. Because the solid fuel and the gaseous oxidizer enter the reaction zone from the same direction, this configuration is referred to as cocurrent (or premixed-flame-like). It is assumed that the propagation of the smolder wave is one-dimensional and steady in a frame of reference moving with the wave. Buoyancy is included and shown to be negligible in the proposed application of a smoldering combustion experiment for use on the Space Shuttle. Radiation heat transfer is incorporated using the diffusion approximation and smoldering combustion is modeled by a finite rate, one-step reaction mechanism. Because the solid and the gas move at different velocities, both the downstream temperature, Tf, and the smolder velocity, ν, are eigenvalues. The dimensionless equations are very similar to those governing the propagation of a laminar premixed flame. A straightforward extension of the activation energy asymptotics analysis presented by Williams for premixed flames yields an expression for a dimensionless eigenvalue determining Tf. A global energy balance provides a relation for the smolder velocity, ν. Predictions are compared with the experimental findings of Rogers and Ohlemiller and with the numerical results of Ohlemiller, Bellan, and Rogers. Key results include (1) for a given solid fuel, Tf depends only on the initial oxygen mass flux, m oi ″ , and increases logarithmically with m oi ″ ; (2) ν increases linearly with m oi ″ and at fixed m oi ″ , increasing the initial oxygen mass fraction, Yoi, increases ν; (3) steady smolder propagation is possible only for Y oi ≥ c eff (T f − T i ) Q , with extinction occurring when all of the energy released in the reaction zone is used to heat the incoming gas. General explicit expressions for Tf and ν are presented.

Fire-induced thermal fields in window glass. I—theory

Window glass breaking plays an important role in compartment fire dynamics as the window acts as a wall before breaking and as a vent after breaking. Previous work suggested a model for the time to breakage of a window glass exposed to a particular fire. In this paper, the glass thermal fields obtained using that model are examined in detail. The temperature field dependence on heat transfer coefficients, radiative decay length and flame radiation is explored. The results show that the glass surface temperature increases with a decrease in the decay length and increases with an increase in flame radiation heat flux. Early in the fire, the glass temperature may be higher than the hot layer temperature due to direct impingement of flame radiation. Later the glass temperature lags the hot layer temperature. The variation of the time to breakage as function of the shading width and decay length is also presented and the results indicate that the breaking time decreases with an increase in the shading width and decreases with a decrease in decay length. Heat flux maps for typical conditions indicate that most of the heat influx is stored in the glass, increasing its temperature.

Diffusion flame analyses

Nine classic diffusion flames, i.e., combustion systems with initially separate fuel and oxidizer, are synthesized in a search for common dimensionless parameters which may serve as indice of fire hazard. The problems examined are: planar and cylindrical Burke-Schumann; droplet burning; planar and cylindrical stagnant film; and forced, free, mixed, and stagnation point combusting boundary layers. Similarity solutions in pyrolysis regions permit identification of flame locations and quantification of excess pyrolyzate, i.e., fuel which is not consumed locally. Numerical solutions in downstream regions give explicit expressions for flame extensions in terms of the number and the B number, with the former dominating. This key parameter, the physically available oxygen-to-fuel ratio divided by the chemically required oxygen-to-fuel ratio or inverse equivalence r ratio, emerges as indicative of the fire hazard associated with flame extension. Full scale tests have shown that flame extension is related to fire spread beyond the compartment of origin. Small r means large flame. Polystyrene with r = 0.12, for example, has an order of magnitude longer flame than wood with r = 0.6. Other polymers fall between these extremes. Much progress remains to be made in the areas of flame soot, radiation, turbulence, and compartment interactions.

Causes of the 20 October 1991 Oakland Hills conflagration

The meteorologic and topologic causes of the 20 October 1991 Oakland Hills conflagration are described here qualitatively. A 3 GW example fire, 100 m in diameter, is used to show the impact of the 10 m/s wind and strong inversion layer at 600 m that existed on 19 October. It is concluded that the dry, high speed NE wind, coupled with the inversion layer and the local topography channeled the hot products of pyrolysis and combustion, along with flaming debris, through a high fuel load region downwind and downslope of the initial fire, thus causing the unusually rapid initial fire spread and consequent conflagration.

Thermal Breakage of Double-Pane Glazing By Fire:

A model for double-pane window breakage due to heating by fire is developed that applies to both compartment fires and to urban/wildland intermix fires. This work builds on the model and computer c...

Compartment fire experiments : Comparison with models

Abstract Twenty full-scale compartment fire experiments suitable for model comparison were conducted. Ceiling jet temperatures, surface heat fluxes and heat transfer coefficients which have not been previously reported are discussed. The ceiling jet temperatures 0·10 m below the ceiling show the effects of compartment ventilation, near-field entrainment conditions and burner location on the ceiling jet. Net and radiant incident heat fluxes to the upper and lower-walls and the floor are estimated. Combined (radiation and convection) interior heat transfer coefficients for the three surfaces are reported. As compartment fire models such as CFAST and FIRST continue to develop in sophistication, it is important that they be compared to experimental data. Data at three heat release rates: 330, 630 and 980 kW, are used to evaluate these comprehensive compartment fire models and two simpler models for the upper-layer gas temperature. CFAST predicts upper-layer gas temperatures 150–260°C hotter than the measured bulk outflow gas temperatures. The increased temperatures appear to be due to insufficient heat transfer through the compartment surfaces. FIRST predicts upper-layer gas temperatures that are slightly cooler (on average, 20C) than the measured bulk outflow gas temperatures. The two simpler models are within 40°C, on average, of the measured upper-layer gas temperatures

Glass Breaking In Fires

Glass breaking in compartment fires is an important practical problem since a window acts as a wall before breaking and as a vent after breaking. If sufficient excess pyrolyzates have accumulated in the hot layer, this sudden geometric change can lead to backdraft and flashover. As Emmons explained at the First Symposium, windows break in fires due to thermal stress from the differential heating of the central portion and the shaded edge. The focus of this paper is on quantifying the connection between the compartment fire and the glass temperature to predict the window breaking time, tb. Techniques are presented for accurately calculating the history of the central glass temperature profile, T (x .r), for any fire exposure. Two-dimensional temperature histories, T (x ,y ,t), where x is depth and y is toward the center, and mean stress histories, azz (y ,t), are also calculated. It is determined here that breaking occurs when the mean glass temperature difference is ~T = (ablE ~)g, where ablE is the maximum glass tensile strain, ~ is the thermal coefficient of linear expansion and g is a geometry factorpf order one. Calculations suggest that the edge remains at its initial temperature, T; , so that ~T = T (tb) Ti, when the shading is large, sIL ~2, and the heating is fast, atb Is2::;1, where L is the glass thickness, s is the shaded edge width and «is the glass thermal diffusivity,

Quantitative Backdraft Experiments

This paper focuses on 17 experiments in a 1.2 m by 1.2 m by 2.4 m compartment. A methane burner, flowing at either 70 kW or 200 kW, was ignited inside a closed compartment and burned until the initially available oxygen was consumed. After the fire self-extinguished, the burner was left on allowing the unburned fuel mass fraction in the compartment to increase. After removing a hatch, covering a 1 . 1 m wide by 0.4 m high slot opening, a gravity current entered the compartment. It traveled across the floor, mixed with the unburned fuel, and was ignited by a spark near the burner. After mixture ignition, a backdraft occurred as a deflagration ripped through the compartment culminating in a large external fireball. Histories recorded prior to backdrafl included: fuel flow rates, upper layer temperatures, lower layer temperatures, upper layer species concentrations for 0 2 , CO2, CO, and HC. Data collected to quantify the backdraft included opening gas flow velocities and compartment pressures. Results indicate that unburned fuel mass fractions >lo% are necessary for a backdrafl to occur.