Experimental study of the aerodynamic and convective effects on flame spread and extinction over solid fuels facing an opposing flow.

Flame spread over solid fuels immersed in an opposing-flow field has been subject of countless studies throughout the last 4 decades. The implications connected to the problem are multiple and disparate, from fire-safety to rocket propulsion. In condition of normal gravity, on Earth, an opposing-flow due to buoyancy is always present. The present work is focused on the interaction between this natural convective flow and the aerodynamic boundary layer generated by a forced-flow blown over thin cellulosic or PMMA fuel sheets. A wind tunnel was used to generate flow velocities in the range of 0-100 cm/s and blow it against the spread of a laminar flame over the solid fuel samples. Experiment were ran in a vertical downward and in a horizontal configuration to change the angle of gravity respect to the spread direction and hence the importance of the buoyancy-generated flow. Results showed, for constant nominal flow velocity, a decreasing spread rate during the flame motion along the sample. This can be related to a flow residence time reduction due to the increasing velocity actually seen by the flame embedded in the boundary layer. From the spread rate results also an approximate evaluation of the buoyant flow velocity inside the wind tunnel was performed. In both configurations the distance from the sample leading edge at which the flame experienced extinction was found to increase with the free-stream velocity. Comparing horizontal and downward spread, extinction distance was found to be generally larger in the second case but the difference was appreciable only for high nominal flow velocities