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Bidimensional Numerical Model for Polyurethane Smoldering in a Fixed Bed

  • Author(s): Ghabi, Chekib
  • Rein, Guillermo
  • Ben Ticha, Hmaied
  • Sassi, Mohamed
  • et al.
Abstract

Smoldering combustion is described as an exothermic superficial heterogeneous-reaction that can propagate in the interior of porous fuels. Smoldering is generally an incomplete combustion reaction, which leaves behind a porous char that contains significant amounts of unburned fuel. If compared to flaming combustion, the heat release and the temperature characteristics of smoldering are low and its propagation is a slow process. Besides its characteristics of a weak combustion process, smoldering poses serious risk to fire safety; it is a common fire initiation scenario, because it is difficult to detect as, it can go unnoticed for long periods of time, It yields a high conversion of fuel to toxic products, and it can suddenly switch to flaming combustion. The propagation of the smoldering front is usually controlled by two factors: oxygen availability and heat losses. However it’s the result of several interacting mechanisms, such as chemical reactions (pyrolysis and oxidation), convection and diffusion of heat and oxygen, heat transfer between the gas and the solid phases, heat losses to the surrounding and flow in a porous media. The developed axisymmetric two-dimensional model solves the governing equations for forward propagation of smoldering in a porous fuel bed. The conservation equations for energy in the solid and the gas phases are considered separately but interact through an interfacial heat exchange. Species conservation in the solid and the gas phases, and overall mass conservation are numerically solved. The heterogeneous chemical kinetics include pyrolysis of the fuel, and oxidation of the fuel and of the carbonaceous residual. The second oxidation is accompanied by the formation of solid ash and gas products, which pose the potential risk of transition from smoldering to flaming combustion. The obtained numerical results feature the gas and solid temperature evolutions, the char mass fraction evolution, and the smoldering propagation velocity. The finite volume method was used for the spatial discretization, and the implicit one for time. The obtained equations were solved by the Bi-Conjugate Gradient Stabilized (BI-CGSTAB) technique

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