Flame-spread over solid fuels facing an opposing flow: experimental investigation of the aerodynamic effects on spread rate and extinction

Flame-spread over solid fuels in presence of an opposed flow field has received increasing attention over the last forty years, due to its implications in fire safety and combustion of solid propellants. The opposing flow can be due to a buoyancy-induced flow, forced flow, or a combination of them. A vertical wind tunnel was used to generate flow velocities in the range of 0-100 cm/s, against which a laminar flame spread along the fuel sample. The influence of the boundary layer over the sample on the flame-spread rate has been experimentally investigated using cellulosic fuel (filter paper) and PMMA; results show that the actual velocity seen by the flame embedded in the boundary layer (equivalent velocity) leads to a decreasing spread rate even for constant free stream velocities. This behavior was justified relating the residence time, and therefore the Damköhler number, to the equivalent velocity; thus the flame entered in the kinetic regime before it was expected, eventually causing local blow-offs and the complete extinction of the flame. The distance from the sample edge to the point where extinction occurred increased for higher flow velocities, suggesting that a critical equivalent velocity could determine extinction; an easy-to-use formula has been proposed accordingly with results for both fuels