Lewis number effects on flame spreading over thin solid fuels

The propagation rates and thermal characteristics of flames spreading downward over thin solid fuel samples are measured. Effects of diluent type, oxygen concentration, pressure, and fuel bed thickness are examined. Pressure and fuel bed thickness effects showed the trends expected based on existing flame spread theory. However, comparison of observed and predicted spread rates in various O 2diluent atmospheres indicate a Lewis number (Le) effect which has not been considered previously. An approximate extension of previous flame spread theory to include Le effects is proposed and found to compare favorably with the experimental results. A new phenomenon is observed in O 2SF 6 and O 2CO 2 atmospheres near the limit: cellular flame front propagation. An explanation based on Le effects and finite-rate chemistry is proposed. Application of these results to the assessment of fire hazards and fire suppression in air and nonstandard atmospheres is discussed.