Controlling mechanisms and limiting conditions for the gas-phase ignition of solid combustible material

Studies have been conducted to investigate the controlling mechanisms of the gas-phase ignition of solid materials in a forced oxidizing flow. In order to explore different aspects of ignition phenomena, two modes, namely auto-ignition and piloted ignition, are examined. A theoretical model for the auto-ignition of a flat slab of solid fuel in a high temperature forced flow of oxidizer is formulated, and an analytical solution for the auto-ignition delay is obtained. Subsequently, an experimental study of the piloted ignition of a radiatively heated flat slab of fuel in an oxidizer flow is performed; a model is formulated and numerically solved, and its predictions compared to the experimental results.;The model for the gas-phase auto-ignition considers that the solid ignition is controlled by two primary mechanisms, one being the solid heating and gasification, and the other the onset of the gas phase reaction. Based on this assumption, an explicit formula is obtained that correlates the ignition delay to the flow residence time, but more fundamentally, to the gas-phase Damkohler number. It is shown that there is a critical value of the Damkohler number below which ignition cannot occur, and that for conditions favoring the evolution of the gas-phase reaction, the ignition delay may be approximated by the pyrolysis time alone. The predictions of the model are in qualitative and order of magnitude agreement with experimental observations available in the literature.;The experimental study of piloted ignition is conducted using vertically oriented PMMA samples as fuel. The material is exposed to radiant heat flux intensities ranging from 10 to 40 kW/m$sp2,$ and to a forced air flow of velocities varying from 0.2 m/s (mixed convection regime) to 2.5 m/s. The gaseous fuel mixture resulting from the solid decomposition is ignited with an electrically heated wire, and the ignition delay is measured. The results show that the relative magnitude of the rate of gas-phase convective transport, and of the rate of heating and gasification of the fuel, affect the values of the critical radiant heat flux for ignition and of the solid surface temperature at ignition. Limiting conditions for ignition due to insufficient heating (low radiant flux) or blow-off (high flow velocity) are analyzed. Additionally, different regimes of self-sustained and intermittent ignition are observed. A transient model of the solid heating and gasification, together with the evolution of the gas-phase reaction is formulated, and solved numerically. The numerical predictions obtained for the ignition delay, critical heat flux for ignition, and solid surface temperature at ignition are in agreement with the experimental results presented.