A numerical model of opposed flow flame spread over thin solid fuels.

摘要

A model of opposed-flow flame spread over a thin solid in laminar flow field has been formulated in two and three dimensions and solved numerically. The gas-phase combustion model includes the full Navier-Stokes momentum equations along with conservation equations of mass, energy and species. The solid is assumed to be a thermally thin, non-charring cellulosic sheet; the solid model consists of continuity and energy equations whose solution provides boundary condition for the gas phase. The gas-phase reaction is represented by a one-step, second-order finite-rate Arrhenius kinetics and the solid pyrolysis is approximated by a one-step, zeroth-order decomposition obeying an Arrhenius law. Solid radiative loss is included in the model and gas phase radiation is included only in the two-dimensional model.; In the first part of the work two-dimensional model is used to study flammability limits, spread rates and gas radiation effects in purely forced and purely buoyant opposed-flow flame spread process in an open domain. The flammability limits of purely forced opposed-flow spread are affected by the length fresh fuel (entrance length) ahead of flame. Shorter entrance length results in higher spread rates and lower oxygen extinction limit in low free-stream flow velocities; but lower spread rates and higher oxygen extinction limit in high free-stream velocities. The flammability limits and flame spreading rates in opposed flow spread are also compared with concurrent spreading flame using consistent models with identical assumptions and properties. At a given free stream velocity, the limiting oxygen limits are lower for concurrent spread except in the very low free stream velocity regime where only opposed flow flame spreading may be possible.; In the second part of the work the three-dimensional model is used to study downward spreading flame over a film type fuel in the mixed flow environment prevailing in a Limiting Oxygen Index (LOI) test apparatus. LOI was found to increase almost linearly with increase in inlet velocity. This trend in LOI was not due to ambient air entrainment into the column but because increased velocity pushed the flame closer to blow-off extinction by reduced residence time for reaction in the flame. Additional results are presented to show three-dimensional effects and flame spread on fuel specimen with free-uninhibited edges.