Ahmed Kaiss

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.

Sensitivity Analysis of a Fire Field Model in the Case of a Large-Scale Compartment Fire Scenario

The objective of this work is to show how a sensitivity study based on a fractional factorial design can be helpful to quantify the impact of parameter variations on model predictions. These parameters have been carefully chosen due to their high variability in fire modeling and the analysis is conducted by simulating a compartment fire with a CFD model. Through a rigorous approach, it is demonstrated that this fractional design composed of eight simulations gives the same information as a standard full design of 64 runs. Physically, it is found that some turbulence and combustion parameters are significant for most the responses.

MODELING THERMAL IMPACT OF WILDLAND FIRES ON STRUCTURES IN THE URBAN INTERFACE. PART 2: RADIATIVE IMPACT OF A FIRE FRONT

ABSTRACT A fast model of radiative impact on structures exposed to a fire front in the urban interface is presented. The front is viewed as a collection of turbulent diffusion flames whose properties (composition and temperature) are taken from a database previously created from a three-dimensional computational fluid dynamics model. Using the gray soot assumption, two gas radiative property models, the spectral line-based weighted-sum-of-gray-gases and a simpler gray gas model, are compared in terms of accuracy and computational time. Applied to the curved fire front propagation, the thermal response of structures is estimated as well as fire safety zones. It is found that radiation can lead to pilot ignition under the action of firebrands or flame contact.

Heat transfer components at the surface of burning thick PMMA slabs

Mass pyrolysis rate is the key parameter to predict fire behavior. It is generally deduced from the energy balance at the surface of the solid material. However, due to lack of knowledge, existing pyrolysis models use simplifying assumptions neglecting all or part of in-depth losses into the solid material or the net radiation at its surface. In order to improve the accuracy of pyrolysis models, experiments are conducted to quantitatively evaluate the heat transfer components at the surface of burning thick clear poly-methyl-methacrylate (PMMA) slabs at steady state. The contributions of each transfer mode including radiation and convection from the flame, surface re-radiation, and in-depth losses, to total heat flux are determined from two series of experiments. Pure pyrolysis (non-flaming) cone calorimeter experiments are first carried out to evaluate in-depth losses in horizontally-oriented slabs exposed to an incident heat flux below that of ignition. A specific procedure based on video processing is used to track the position of the PMMA regressing surface with time. The second series of experiments consist in burning vertically-oriented slabs from 2.5 cm to 20 cm in height, 10 cm in width and 3 cm in thickness. It is found that only a small part of flame radiation is transmitted through the virgin solid, most in-depth radiation being absorbed by the bubble surface, which in turn strongly emits radiation inward. An excellent agreement is obtained between the local mass loss rate deduced from the energy balance and literature data.

Transport and combustion of Ponderosa Pine firebrands from isolated burning trees

A numerical model is developed to describe the transport and the combustion of firebrands lofted by a fires buoyant plumes. A preliminary study of the thermal degradation and combustion of woody fuel particle is presented. The comparison with lab-scale experiments on cylinder-shaped limbwood samples of Ponderosa Pine (PP) shows a fairly good agreement. A three-dimensional physics-based is used to predict the steady-state flow and thermal fields induced by a crown fire. Trajectories and burning rates of disk-shaped firebrands lofted by the fire plume, and transported downwind are determined for a fire intensity of 20MW/m and various windspeeds from 10 to 20mi/h. Firebrands of different sizes and densities are launched from a specified location at the top of canopy. Results show that the spotting distance depends to the product rhowood0 timestau (rhowood0 : initial wood density, tau : thickness), and varies almost linearly with wind speed while it is independent of the initial particle diameter.

Identification du terme de production de chaleur dans des élastomères sollicités en cisaillement

Abstract We present a method for the determination of the magnitude of mechanical heat production in dissipative media. It combines temperature measurement, an inverse method and a finite element model. The dissipation behavior of elastomer media has been investigated with this method. The study shows that a different evolution of the heat source team prevails during the first stage of the shear tests. The width of the temperature interval which corresponds to this stage depends on the excitation conditions. We also show that the source term profile in function of the temperature is well represented by an Arrhenius law type.