Jean-Louis Consalvi

A formal averaging procedure for radiation heat transfer in particulate media

Abstract Radiative heat transfer in participating particulate media is modeled using a formal volume averaging procedure. The multiphase medium is composed of emitting–absorbing–scattering phases, i.e., a gas phase and several particle phases. Each particle phase contains large, opaque, gray, diffuse, and spherical particles having locally the same geometrical, thermophysical, and radiative properties. The resulting multiphase radiative transfer equation (MRTE) is solved using the discrete ordinates method. The present computed results are found to be in good agreement with those obtained using the Monte-Carlo theory and with the available experimental results. The coupling effect of the MRTE with the averaged energy equations in a three-dimensional cavity which is differentially heated or which contains a volumetric heat source is studied. A parametric study is performed for particle-phase and gas properties, and wall emissivity.

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 wildland fire behavior and emissions using a fine-scale physical model

ABSTRACT A physical fine-scale two-phase model has been developed for the purpose of determining wildland fire behavior and emissions. The situation modeled corresponds to a spreading wildfire driven by wind through a fuel bed of combustible elements. The numerical model solves a set of time-dependent conservation Equations for both phases (the gas and the vegetation elements) coupled through exchange terms. It accounts for the dynamics, turbulence, soot formation, and radiation. This model has been applied to a prescribed savanna fire. Good qualitative agreement was found between the simulation results and available in situ experimental data on the rate of spread and fuel consumption ratio.

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.

MODELING THERMAL IMPACT OF WILDLAND FIRES ON STRUCTURES IN THE URBAN INTERFACE. PART 1: RADIATIVE AND CONVECTIVE COMPONENTS OF FLAMES REPRESENTATIVE OF VEGETATION FIRES

ABSTRACT A 3-D computational fluid dynamics model is used to estimate the thermal impact on structures exposed to fire in the urban interface. The burning of vegetation is represented by a well-adjusted gas burner diffusion flame. This article examines two situations, depending on which modes of heat transfer from the flame are considered. Model predictions reveal how the presence of the structure modifies the dynamic flow pattern, which in turn may cause the overattachment of the fire plume to the structure and then the enhancement of thermal impact. Numerical results, including flame geometry, are found to be consistent with experimental observations.

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.

DYNAMIC AND RADIATIVE ASPECTS OF FIRE–WATER MIST INTERACTIONS

A two-phase multiclass model is developed to describe the interaction between a compartment fire and a water mist. Turbulent combustion is modeled using the EBU-Ar coupled with the renormalization group k–ϵ turbulence model. A multiphase radiative transfer equation including the contributions of soot particles, combustion products and water droplets is used to model radiation. The model is applied first to a two-dimensional enclosure fire. A parametric study of the influence of water spray flow rate and water droplet diameter on fire mitigation is presented. Gas-phase cooling is found to be the main fire suppression mechanism. The influence of water spray on radiation is studied with special emphasis on the contribution of each radiative phenomenon. Results demonstrated that the attenuation of radiation by the water spray is two-fold: the radiant energy emitted by the flame is reduced and this energy is attenuated by water droplets. The role of droplet scattering in the attenuation of thermal radiation in fire–water mist interactions is clearly demonstrated. For the droplet diameters considered, it is found that there are two distinct regimes: a fire extinction regime and a fire enhanced regime. This finding is consistent with previous experiments. Applied to a three-dimensional fire-sprinkler scenario, the model predictions are in good agreement with experimental data.

A physically based model of the onset of crowning

The objective of this study is to investigate the capability of a physical two-phase model to predict the ignition of crown fuels by a surface fire and then to determine the degree of crowning. The model considers the hydrodynamic aspects of the flow and accounts for the basic physicochemical processes resulting from the thermal degradation of organic matter. Turbulence, soot formation, and its impact on radiation are considered in order to improve the physical insight. Calculations have been performed to investigate the effects of crown base height and aerial fuel moisture content on the onset of crowning. Numerical results are found to be consistent with experimental observations and the widely used Canadian Fire Behavior Prediction System classification by crown fraction burned. This model may be used to extend the domain of application of semiphysical theories, e.g., Van Wagners theory, where fuel and environmental factors are generally determined from empirical observations of previous fires. It provides a means of adjusting these factors in other fire situations without requiring additional experiments.

Modelling thermal radiation in buoyant turbulent diffusion flames

This work focuses on the numerical modelling of radiative heat transfer in laboratory-scale buoyant turbulent diffusion flames. Spectral gas and soot radiation is modelled by using the Full-Spectrum Correlated-k (FSCK) method. Turbulence-Radiation Interactions (TRI) are taken into account by considering the Optically-Thin Fluctuation Approximation (OTFA), the resulting time-averaged Radiative Transfer Equation (RTE) being solved by the Finite Volume Method (FVM). Emission TRIs and the mean absorption coefficient are then closed by using a presumed probability density function (pdf) of the mixture fraction. The mean gas flow field is modelled by the Favre-averaged Navier–Stokes (FANS) equation set closed by a buoyancy-modified k-ϵ model with algebraic stress/flux models (ASM/AFM), the Steady Laminar Flamelet (SLF) model coupled with a presumed pdf approach to account for Turbulence-Chemistry Interactions, and an acetylene-based semi-empirical two-equation soot model. Two sets of experimental pool fire data are used for validation: propane pool fires 0.3 m in diameter with Heat Release Rates (HRR) of 15, 22 and 37 kW and methane pool fires 0.38 m in diameter with HRRs of 34 and 176 kW. Predicted flame structures, radiant fractions, and radiative heat fluxes on surrounding surfaces are found in satisfactory agreement with available experimental data across all the flames. In addition further computations indicate that, for the present flames, the gray approximation can be applied for soot with a minor influence on the results, resulting in a substantial gain in Computer Processing Unit (CPU) time when the FSCK is used to treat gas radiation.

Simulations of sooting turbulent jet flames using a hybrid flamelet/stochastic Eulerian field method

The stochastic Eulerian field method is applied to simulate 12 turbulent C1−C3 hydrocarbon jet diffusion flames covering a wide range of Reynolds numbers and fuel sooting propensities. The joint scalar probability density function (PDF) is a function of the mixture fraction, enthalpy defect, scalar dissipation rate and representative soot properties. Soot production is modelled by a semi-empirical acetylene/benzene-based soot model. Spectral gas and soot radiation is modelled using a wide-band correlated-k model. Emission turbulent radiation interactions (TRIs) are taken into account by means of the PDF method, whereas absorption TRIs are modelled using the optically thin fluctuation approximation. Model predictions are found to be in reasonable agreement with experimental data in terms of flame structure, soot quantities and radiative loss. Mean soot volume fractions are predicted within a factor of two of the experiments whereas radiant fractions and peaks of wall radiative fluxes are within 20%. The study also aims to assess approximate radiative models, namely the optically thin approximation (OTA) and grey medium approximation. These approximations affect significantly the radiative loss and should be avoided if accurate predictions of the radiative flux are desired. At atmospheric pressure, the relative errors that they produced on the peaks of temperature and soot volume fraction are within both experimental and model uncertainties. However, these discrepancies are found to increase with pressure, suggesting that spectral models describing properly the self-absorption should be considered at over-atmospheric pressure.