Limiting Length, Steady Spread, and Nongrowing Flames in Concurrent Flow Over Solids

A detailed two-dimensional transient model has been formulated and numerically solved for concurrent flames over thick and thin solids in low-speed forced flows. The processes of flame growth leading to steady states are numerically simulated. For a thick solid, the steady state is a nongrowing stationary flame with a limiting length. For a thin solid, the steady state is a spreading flame with a constant spread rate and a constant flame length. The reason for a nongrowing limiting flame for the thick solid is the balance between the flame heat feedback and the surface radiative heat loss at the pyrolysis front, as first suggested by Honda and Ronney. The reason for achieving a steady spread for thin solids is the balance between the solid burnout rate and the flame tip advancing rate. Detailed transient flame and thermal profiles are presented to illustrate the different flame growth features between the thick- and thin-solid fuel samples.