Theory 33/Lewis Number/Fire spread
Fire Spread Is a Lewis Problem
The claim
Spread rate through a porous fuel bed is set by the Lewis number of the gas mixture, so fuel moisture changes spread differently than temperature alone predicts.
01Mechanism
Heat must preheat the fuel while vapour must escape; the ratio determines whether the bed can be brought to ignition.
02Prediction
Spread should track Lewis at equal moisture.
03Refutation
Spread tracking moisture alone would remove the ratio.