J. C. Loraud

A multiphase formulation for fire propagation in heterogeneous combustible media

A general formulation based on a weighting average procedure is developed for describing the fire-induced behaviour of a multiphase, reactive and radiative medium. The complete set of the resulting equations should be used as the basic one for later studies, especially in the framework of wildland fires. For the moment, in order to demonstrate the capability of the general formulation, a simplified model, named zeroth-order model, in which some physical phenomena (such as char combustion, second-order terms, particle motion) are neglected is presented. In the frame of this simplified model, reverse and forward one-dimensional fire propagations through an heterogeneous medium composed of fixed fuel particles are studied numerically.

Firespread through fuel beds: Modeling of wind-aided fires and induced hydrodynamics

The propagation of wind-aided line fires through fuel beds is simulated by using a multiphase approach. In this approach, a gas phase flows through N solid phases which constitute an idealized reproduction of the heterogeneous combustible medium. A set of time-dependent equations is obtained for each phase and the coupling between the gas phase and the solid phases is rendered through exchange terms of mass, momentum, and energy. Turbulence is approached by using a RNG k−e statistical model constructed from the Favre averaging method. The radiative transfer equation extended to multiphase media is solved using the discrete ordinates method (DOM). Soot formation is taken into account for the evaluation of the absorption coefficient of the soot/fuel particles/combustion products mixture using the gray gas assumption. First-order kinetics is incorporated to describe water vaporization, pyrolysis, and char combustion processes. The solution is performed numerically by a finite-volume method including a high-o...

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.

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.

Numerical simulation of turbulent diffusion flame in cross flow

The effects of cross wind on the behaviour of a turbulent diffusion flame have been studied numerically. The multicomponent turbulent reacting flow is approached using a two-equation (κ - ϵ) statistical model constructed from the Favre averaging method. To improve the representation of turbulent fields in fully developed and weak turbulent regions, RNG (Renormalization Group) κ - ϵ turbulence modelling is adopted. Infinitely fast reaction is assumed and the combustion rate is fully determined by the turbulent mixing rate of fuel and oxygen using the Eddy Dissipation Concept (EDC). Energy lost by radiation is taken into account and soot formation is computed for the evaluation of the absorption coefficient of the soot-gas mixture using the gray gas assumption. Calculations have been performed both with and without wind. The numerical results show that the flow is characterized by oscillations which affect significantly the flame behaviour. The development of shear buoyancy-driven instabilities (Kelvin-Helm...

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.

On the Prediction of Firebreak Efficiency

Abstract The firebreak problem is studied by adopting a multiphase model based on a continuum formulation of the conservation equations for each of the gas and the N solid phases composing the heterogeneous combustible medium. The strong coupling between the gas and each solid phase is rendered through exchange terms of mass, momentum, and energy. Sub-models for turbulence, soot formation and radiation are incorporated, along with first-order kinetics to describe drying, pyrolysis, and combustion of the organic matter. The model is applied to predict the firebreak efficiency for various conditions of wind and terrain slope. It is found that radiation is responsible for the fire crossing the firebreak. The effects of other parameters investigated and reported include the drag and heat transfer coefficients, the initial fuel volume fraction, and the fuel moisture content.