B. Porterie

Spectral emission of flames from laboratory-scale vegetation fires

Outdoor experiments were conducted on a laboratory scale to study the infrared radiation emission of vegetation flames. Measurements were made in the spectral range 1000–4500 cm–1, using a compact and portable Fourier-transform infrared spectrometer including an HgCdTe/InSb dual detector. Flame emission was compared with the reference signal emitted by a blackbody surface at 1000 K. We carried out two different series of fire experiments: a series of fires in a 0.45 m-diameter steel tray and a series of wind-tunnel fires. Various types of wildland fuels were used: wood wool, vine branches, dry wood, and Kermes oak branches. From a qualitative observation of emission spectra, it appears that the main contribution comes from the hot gaseous combustion products, with a low-intensity background radiation from soot, as the small-scale flames in these experiments were optically thin. It was also found that, in the flaming combustion zone of the fuel bed, both phases contribute to infrared emission. Our results, in combination with existing data on the absorptivity of vegetation, give a better understanding of radiative transfer in vegetation fires and show how total radiative properties could be deduced from spectral measurements. We believe that this preliminary study provides pilot data for future studies in this area.

Predicting Fire Suppression Efficiency Using Polydisperse Water Sprays

Thermoplastic fire suppression by water sprays is numerically investigated using an Eulerian-Eulerian two-phase approach. The polydisperse spray model is based on the moments of the droplet size distribution function. Turbulent combustion is approached using the Arrhenius/Eddy-Break-up model coupled with the RNG k − ϵ turbulence model. A multiphase radiative transfer equation including the contributions of soot, combustion products and water droplets is used to describe radiation. Pyrolyzate mass flow rate is predicted from a thermal degradation model for poly-methyl-methacrylate. The influence that the main parameters of the water spray system, located directly above the fire source, have on the fire suppression efficiency is examined. Over the wide range of spray conditions tested, it is found that, first, polydisperse sprays are generally more efficient than monodisperse sprays, and, second, for a polydisperse spray, there exists an optimal reference Sauter mean radius, which corresponds to the shortest time for fire extinguishment. Results indicate that the time for fire suppression using polydisperse sprays decreases as the water flow rate increases, but with an asymptotic behavior. Finally, the model is used to determine the minimal water flow rate required for extinction.

METHOD FOR COMPUTING THE INTERACTION OF FIRE ENVIRONMENT AND INTERNAL SOLID REGIONS

A numerical finite-volume procedure for predicting the fire environment in enclosures containing internal solid regions (e.g., internal obstacles, islands, barriers, and partitions) is reported. The blocking-off operation is extended to fire models. In this procedure, the main advantage is to use a calculation domain that includes both the fluid and the solid regions. It consists of modifying the coefficient definitions in the discretized form of the transport equations. A special emphasis is put on the fluid energy equation where the conjugate heat transfer problem at the fluid/solid interface is solved. A modified blocked-off discrete ordinates method is used to solve the radiative transfer equation. Sample examples areshown to prove its efficiency. As an example of the use of the general procedure, thephysical problem solved is that of a large compartment with a partition and a soffit. Forthis simulation, the thermal responses of the partition/soffit structural elements are predicted.

Universal scaling in wildfire fractal propagation

A small-world network model is proposed to predict wildfire behavior near critical propagation conditions. It includes long-range connections due to flame radiation and a weighting procedure based on times of degradation and combustion of vegetation. Scaling laws are found universal near the propagation transition. For isotropic propagation, the critical behavior of the usual percolation theory is recovered, whereas, for anisotropic conditions due to wind and/or topography, propagation is fractal with a dimension of 1.3 and a critical exponent of the correlation length of 1.7, following the extrapolation for Euclidian systems. The exponent value agrees well with that of directed percolation.

A theoretical and numerical evaluation of the steady-state burning rate of vertically oriented PMMA slabs

The steady state burning rate of vertically oriented slabs of poly-methyl-methacrylate (PMMA) is numerically investigated. Model predictions are compared with measurements and results of the laminar boundary layer (LBL) theory. The numerical model provides a solution of the Favre-averaged Navier–Stokes equations coupled with sub-models for turbulence, combustion, soot production and radiation. The modelling of condensed phase processes is based on the one-dimensional heat transfer equation and pyrolysis is treated as a phase change using the latent heat approach. Results show that the pyrolysing region can be divided into three regions. In the laminar part of the flow (Gr x < 4.3 × 107), the predicted normalised burning rate, ṁ″ p x/μ∞, is a power-law function of Gr x with an exponent close to that of the LBL theory, surface re-radiation being the primary source of discrepancies. From the LBL theory for free flow, it is demonstrated that the local burning rate is inversely proportional to the shear velocity gradient. This is globally confirmed by numerical model results. At Gr x = 4.3 × 107 the change in slope of the burning rate observed experimentally, which indicates the end of the laminar flow region, is reproduced numerically. From Gr x = 2.5 × 109 model results show that the surface mass flux of pyrolyzate increases with x, in agreement with experimental data in literature.

Unsteady convection in a cavity due to pyrolysis

Abstract Unsteady convection in a square enclosure produced by pyrolysis of combustible wall is numerically studied. Gas phase processes including mass, momentum and heat transfers are coupled with solid phase processes, heat conduction and thermal degradation (pyrolysis) through conditions at the solid interface. For the gas phase, the unsteady two dimensional Navier-Stokes equations are written in stream-function vorticity formulation under the Boussinesq approximation. Solid phase processes are described by a conduction equation. This study examines the results obtained in the case of a right combustible wall, and compares them with the case of a floor combustible wall.

Solid-propellant fire in an enclosure fitted with a ceiling safety-vent

Abstract The response of an enclosure having a ceiling safety-vent to a fire of solid propellant located on the floor is investigated numerically. The full Navier—Stokes equations are solved along with the species continuity equations. A recent method is used to compute chemical equilibria and the coupling between chemistry and thermodynamics is treated according to a new strategy. The particular boundaries, which are the combustion zone of the propellant and the outflow section, require an original treatment by solving a set of ‘full’ or ‘half’ Riemann problems taking into account the transport of chemical species. The SOLAICE algorithm is successfully developed for the reactive-diffusive case dealing with particular boundaries. A fire of a standard hot homogeneous propellant in a rectangular cavity initially filled with air is simulated for two opening conditions of the safety-vent. They predict the increase in the rates of energy release and CO2/H2O production in the reaction zone caused by afterburning processes involving the air of the enclosure. The course of the compartment fire is described in terms of time evolution of the average gas temperature and pressure, and oxygen depletion for both opening configurations.

Analyse du risque thermique dans une structure multicompartiment

ABSTRACT A field model based upon the fluid mechanics equations is presented. Heat transfers at the walls are predicted which leads to a good fire development risk assessment. Predictions are in good agreement with experiments. They reveal a failure in the use of usual standards (ISO, CNPP).

Gun propellant break-up effects on transient combustion processes

A two-dimensional interior ballistics computer code has been developed to solve the viscous two-phase flow in a combustion chamber of a gun tube during the ignition phase beginning with primer discharge and ending with projectile motion. The motivations are to achieve a better understanding of the physical processes occuring during this phase and to predict overpressures which can occur under certain ignition conditions and configurations of loading. Calculations are presented for axisymmetric two-phase flow in a model combustion chamber assuming grain fracture or not. Predicted results indicate that grain fracture due to high compaction may generate substantial local pressure peaks along the combustion chamber. In extreme cases of severe grain fracture, the result is a catastrophic overpressure which can explain structural gun-damage and spontaneous deflagration-to-detonation transition.