TED-AJ03-327 A COMPUTATIONAL STUDY ON FLAME RADIATION-SURFACE INTERACTION IN FLAME SPREAD OVER THIN SOLID-FUEL

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This article presents a detailed numerical study on the interaction of gas phase radiation with the surface in flame spread phenomena over thin solids in quiescent microgravity (μg) and in normal gravity (1g) environments. The computations are performed using an opposed flow flame spread model in which gravity and flow can be varied as parameters. The numerical model in gas phase consists of two-dimensional full Navier-Stokes equations along with conservation equations of mass, species and energy. These flow and combustion equations are solved using SIMPLER algorithm. The overall gas phase chemistry is single step reaction with second order kinetics. The solid phase is assumed to be a thin cellulosic material (Kimwipes) which is modeled through one dimensional mass and energy equations with a zeroth order Arrhenius type pyrolysis law. The Gas phase and solid phase are coupled through interface boundary conditions. Radiation (gaseous and/or surface) plays an important rolein the μg flames and is a key part in the model. It is responsible for the existence of the low-speed quenching limit and also a heat transfer mechanism besides conduction/ convection in combustion systems. Here, gas-phase thermal radiation is modeled using gray-gas approximation with carbon dioxide and water vapor as participating media and soot neglected. The mean absorption coefficient used in the gray gas model here is a corrected local Planck mean absorption coefficient. The correction is needed to account for radiation self-absorption in varying optical length along the flame. The correction factor is obtained by calibrating against the narrow-band results through a quasi-one-dimensional flame. The solid radiation is also assumed to be gray and diffuse. The radiative transfer equation is solved using S-N discrete ordinates method to obtain the gas phase radiation loss to the ambient and heat feed back to solid. In 1g downward spreading flame, buoyancy induces large flow velocities that results in a short closed tipped flame close to the fuel surface. Conduction/Convection heat transfer dominates the gas-solid heat exchange process and radiation is only a minor contributor to the heat transfer mechanism. The μg self propagating flame on the other hand is characterized by low velocity field. The flame is long with flame tip located away from the surface. Radiative heat transfer here becomes an important mechanism of heat feed back to the solid and heat loss from the system (both solid and gas). The μg flame is very sensitive to the surface radiation properties. For example the flame spread rate at 30% oxygen for μg flame drops rapidly with decrease in emissivity (here ε=α), while it increases marginally for 1g flame. The Limiting Oxygen Index (LOI) also shows rapid drop with emissivity (ε=α) for μg flame. The LOI for μg flame can be higher than for 1g flame for ε=α close to unity but the trend is reversed for lower values ε=α<0.5. The interaction of ags radiation with solid surface is further investigated on μg self propagating flame by varying the radiative properties of solid absorptivity (α) and solid emissivity (ε) independently (taking up values of 0 or 1). The four combinations (ε=α=0,ε=1,α=0,ε=0,α=1 and ε=α=1) from these variations are compared with each other and with a case of adiabatic flame (without gas or surface radiation). It was observed that fuel with high solid absorptivity (α) can absorb substantial flame radiation and flame spreads faster than the corresponding adiabatic case irrespective of value of solid emissivity (ε).
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TED-AJ03-327 A COMPUTATIONAL STUDY ON FLAME RADIATION-SURFACE INTERACTION IN FLAME SPREAD OVER THIN SOLID-FUEL

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