Pierre Joulain

The behavior of pool fires: State of the art and new insights

This review attempts to extract, from the particularly abundant literature devoted to the study of pool fires, some pertinent contributions to some of the issues that should be addressed to meet fire-safety requirements. The author tries to successively address the characteristics of pool fire flame and plume structure, including flame height, the entrainment of air, the pulsation of the flame and the influence of cross-flow, the formation and properties of soot, the heat feedback, and mass burning including radiation transport and radiative energy blockage, by considering experimental approaches and some numerical modeling. It appears that among the various parameters that play an important role in the characterization of pool fires, some have already been satisfactorily taken into account but some have not. Definitively, the full understanding and the realistic modeling of key phenomena like the pulsating nature of air entrainment, the formation and the characterization of soot, or the radiative energy blockage near the surface of the pool require more basic research, especially for establishing a systematic methodology to evaluate the real threat due to large pool fires and to provide solutions to control their impact.

Influence of gravity and pressure on pool fire-type diffusion flames

Experiments are conducted to study the effects of gravity and pressure on the characteristics of diffusion flames of the pool fire type, that is, a diffusion flame stabilized on a burning horizontal fuel surface. In the experiments, the pool fire is simulated by injecting at very low velocity, a gaseous fuel (ethane) through a small-scale, porous, flat, horizontal surface burner and generating a diffusion flame over the burner by the reaction of the gaseous fuel with air. The resulting diffusion flame is characterized by a low Froude number. The diffusion flame characteristics (visual appearance, height, radiant output and temperature, and velocity distribution) are investigated at gravity levels ranging from microgravity (parabolic trajectory of an aircraft) to 12 times normal gravity (centrifuge facility—atmospheric pressure), and at ambient pressures ranging from 0.03 to 0.3 MPa (normal gravity). The results provide information about the effects of these variables on the flame characteristics and data for validation of numerical models of diffusion flames. Furthermore, they also help in understanding some of the limitations of Froude, or pressure, modeling of fires. The experiments indicate that the effect of gravity and pressure on the flame characteristics appears primarily through their effect on the buoyantly induced entrainment of air by the flame plume. Although at elevated pressures the effects are similar on the flame size and shape, important differences are observed on their effect on soot formation. It is found that for pressures above atmospheric, pressure has a major influence in soot formation and, consequently, on the radiant characteristics of the flames, increasing as pressure is increased. It is also found that at pressures below atmospheric pressure and gravity have opposite effects on flame size and soot formation and that consequently their effects on flame radiation also differ.

On the flame structure at the base of a pool fire

A detailed mapping is presented of the temperature, CO, CO 2 and O 2 species concentrations, and monochromatic absorption coefficient at the base region of a kerosene pool fire. Comparative analysis of the isotherm and iso-concentration patterns indicates that the buoyant flame has an intermittent character at its base, contrary to its normally attributed persistent, or anchored, character. The temperature field shows a band of temperature maxima that approximately coincides with the luminous flame boundary. The temperature in this band, however, decreases toward the fire axis and is lower than that expected in a kerosene flame. This, in conjunction with the presence of high oxygen concentration in the fuel side of the temperature maxima band, indicates that the flame fluctuates around a mean position, and that as a consequence the measured temperatures are lower averaged values of the fluctuating temperature field. The CO and CO 2 iso-concentration fields show a region of elevated concentrations at the axis near the liquid surface that is displaced toward the fuel side with respect to the temperature maxima region, rather than coinciding with it. The lines of constant absorption coefficient, which can be associated with lines of constant soot concentration, also show a region of elevated values that approximately coincides in location with that of the CO and CO 2 concentrations. These results indicate the fluctuating character of the flame, and suggest the existence of a gas stagnation, or recirculation, zone at the axis near the liquid surface that permits the accumulation of combustion products in this region. From the observed characteristics of the flame structure it can be concluded that the flame at the pool fire base is better described as a fluctuating, laminar, diffusion flame, which later becomes a turbulent, intermittent one as it evolves upward along the fire plume.

Convective and radiative transport in pool and wall fires : 20 years of research in Poitiers

This paper is intended to present the main features of our approach, at different stages, to the study of pool and wall fires during the past 20 years in Poitiers. The main experimental, theoretical and numerical approaches developed to obtain a proper evaluation of the different transports sustaining the burning phenomena are presented here. First, the combustion of a horizontal solid in a gaseous stream flowing parallel to its surface is presented. Then, a vertical burning wall is investigated. The structure of the reaction flows was studied in parallel, using simulation burners or water-cooled porous wall burners. The last part of the paper is related to the influence of gravity on wall and pool fires, and of cross-wind on the structure of the diffusion flames, and to phenomena involved in the generation and movement of smoke and in liquid pool fires producing boilover. Special emphasis has been given to the evaluation of convective and radiative transports.

On the numerical modeling of buoyancy-dominated turbulent vertical diffusion flames

This work concerns the modeling of vertical turbulent diffusion flames representative of fires. In these flames, ambient air is entrained just above the burning surface into the reactive zone at large mass flow rates and a recirculation zone develops. To describe the flow and especially this important region, a suitable mathematical model has been developed and integrated into an elliptic code. The model, based on a flame sheet assumption and including direct effects of buoyancy into the k and e balance equations, has been validated using experimental data from two (one axisymmetric and one rectangular) turbulent pool fires. It successfully predicted the variation of the mean velocity and temperature on the flame axis and the entrainment velocity at the base of the fires. Important features of highly complex inhomogeneous flows (recirculation zone, narrowing and broadening of the flame) have also been correctly reproduced by the model. In general terms, the comparison with experiments is reasonably good, although the flame spread is clearly underestimated.

Laminar diffusion flame in microgravity: The results of the minitexus 6 sounding rocket experiment

A flat plate of poly(methyl methacrylate) (PMMA) exposed on one surface to a laminar oxidizer flow (40% oxygen and 60% nitrogen) was ignited at the downstream edge, allowing for propagation of a diffusion flame opposing the flow. The flame front is flat, so edge effects can be neglected and the flame can be considered two-dimensional. The experiment was conducted on board a sounding rocket (MiniTexus 6) that provided 180 seconds of microgravity (10 −5 g 0 ). Two charge-coupled device (CCD) cameras provided a lateral and a top view of the flame, and an infrared camera with a narrowband filter centered at 4.28 μ m served to estimate the temperature of the PMMA surface. A green-light sheet perpendicular to the fuel surface, and a third CDD camera (side view) with a narrowband filter centered on the light served to record illuminated silver-coated glass beads. The glass beads follow the flow, allowing the determination of streamlines and the estimation of the distortion of the flow field introduced by the flame. The experimental results are presented in the context of other experimental results obtained in drop towers and parabolic flights. Experiments were conducted for three different flow velocities, 150 mm/s, 100 mm/s, and 50 mm/s, corresponding to strong propagation, transitional regime, and extinction regime, respectively. The experimental results provide evidence on the role of flame radiation, in-depth conduction, and surface reradiation in the low-velocity extinction process. The illuminated particles show that the flame modifies the flow structure upstream of the flame beyond the leading edge region. The effect of the flame on the flow decreases with the oxidizer velocity.

The effect of gravity on a laminar diffusion flame established over a horizontal flat plate

An experimental study is conducted on a laminar diffusion flame established over a horizontal flat porousburner. The objective of this study is to provide further understanding on the transport mechanisms controlling a diffusion flame, with particular interest on the role of buoyancy. Fuel is injected through the burner and the oxidiser is provided by a forced flow parallel to the surface. The fuel used is ethane and the oxidiser used is air. Experiments are conducted in normal and microgravity conditions, and information about the flame is obtained from video records, temperature measurements, and gas analysing of the global combustion products. The parameters varied are the air-forced flow velocity and the fuel injection velocity. In normal gravity, the flame can be divided in two well-determined regions: a boundary layer region (where inertia dominates) and a plume region (where buoyancy dominates). In the boundary layer region, air entrainment, induced by the plume, adds to the forced flow, resulting in flames closer to the burner and premature extinction. When gravity is eliminated, the plume region disappears; therefore, the overall air flow velocity is reduced to that of the forced flow, and thus, the standoff distance increases and higher airforced flow velocities are necessary for extinction to occur. The flame is found to establish, both in normal and microgravity, in a line where fuel and oxidiser rates of delivery are in stoichiometric proportions. Experimental results and a scaling analysis of the region close to the flame show that in microgravity, assumptions typical to boundary layer flow, such as stationary regime or neglectable mass transport in the direction perpendicular to the forced flow plane are no longer valid. The good agreement between theory and experiments confirms the important role of gravity in the determination of the characteristics of a diffusion flame.

Soot Volume Fraction Measurements in a Three-Dimensional Laminar Diffusion Flame established in Microgravity

ABSTRACT A methodology for the estimation of the soot volume fraction in a three-dimensional laminar diffusion flame is presented. All experiments are conducted in microgravity and have as objective producing quantitative data that can serve to estimate radiative heat transfer in flames representative of fires in spacecraft. The competitive nature of formation and oxidation of soot and its direct coupling with the streamlines (source of oxygen) require for these measurements to be conducted within the exact configuration. Thus three-dimensional measurements are needed. Ethylene is injected through a square porous burner and the oxidizer flows parallel to its surface. The methodology uses CH* chemiluminescence measurements to correct for three-dimensional effects affecting light attenuation measurements. Corrected local soot concentrations are thus obtained. All experiments are conducted during parabolic flights and the parameters varied are fuel and oxidizer flow rates.

Microgravity Laminar Diffusion Flame In a Perpendicular Fuel and Oxidizer Stream Configuration

Fuel is injected through a porous flat plate perpendicular to a stream of oxidizer flowing parallel to the surface of the burner for regimes corresponding to a fire scenario in spacecrafts. Particle image velocimetry is used to characterize the flow structure in nonreactive conditions. The influence of fuel injection on the flow structure is evaluated, and a detailed description of the flow structure is presented. For combustion experiments, a minimum fuel injection velocity is shown to be necessary for a stable flame. The influence of thermal expansion on the flame is evaluated. A complementary numerical study is used to support the preceding experimental observations.

Numerical evaluation of boundary-layer assumptions used for the prediction of the standoff distance of a laminar diffusion flame

Numerical simulations of a diffusion flame established over a flat plate with a flow of oxidizer parallelto its surface are presented. All simulations are intended to describe experiments conducted in microgravity and using gaseous fuel injection. The numerical tool uses direct numerical simulation and a combustion model based on infinite chemistry and a mixture fraction approach. The purpose of this study is to better understand the effect of the flame on the flow and to validate the use of boundary-layer approximations in the analytical modeling of the stand off distance. This study does not attempt description of the leading edge of the flame, but concentrates on the description of the flame geometry downstream of this region. Special attention is given to velocity overshoots previously reported in experimental studies. The study of the different contributions to the local acceleration provides insight on the origins and mechanisms leading to these overshoots. It was observed that the acceleration is mainly due to an injection-induced pressure gradient at the leading edge of the porous burner that is significantly amplified by the flame due to the local decrease in density. The fuel injection velocity defines two different flame regimes. For low injection velocity, the flow conforms to the boundary-layer assumptions. The streamlines show that for this regime, soot will be formed far from the flame, and therefore soot oxidation is small. Experiments have shown that blue flames, negligible radiative feedback, and unstable flames characterize this regime. Separation of the flow follows an increase in injection velocity leading to flow conditions that cannot be described by a boundary-layer approach. The streamlines demonstrate that soot will be convected toward the flame leading to high oxidation rates. Experiments have shown that yellow flames, significant radiative feedback, and a stable flame characterize this regime.