Effect of Gap Height on Thin Fuel Opposed Flow Flame Spread in the Narrow Channel Apparatus

The Narrow Channel Apparatus (NCA) is designed to establish a fully developed laminar flow profile above and below a thin, flat, solid fuel or on one side of a thick fuel for flame spread investigations. Due in part to its ability to suppress buoyantly-induced flow, the NCA is under investigation as a possible test method for screening materials for space flight applications. This study focused on characterizing the flame response over a range of channel gap widths and opposed air flow velocities. A key objective is to determine whether the gap width can be chosen to suppress buoyancy without excessive heat loss to the inert top and bottom plates. Tests were conducted with Whatman 44 filter paper for full gap heights between the two inert plates ranging from 6 to 20 mm (with a half-gap below and above the sample). Average opposed flow velocities ranged from 1.0 to 40 cm/s. Flame spread rates measured from video analysis ranged from 0.12 to 0.37 cm/s. Side view images were used to qualitatively assess the role of buoyancy on the flame shape. A scaling analysis shows that flames at low flows become fuel rich when the forced flow becomes of the same magnitude as diffusive flow. Heat loss increases as the quenching distance increases relative to the gap height at low flow, increasing conductive loss to the confining walls. Buoyancy suppression is achieved by making the gap as narrow as possible. For thin fuels, the data suggest that a full gap of 10 mm (half-gap of 5 mm) provides a reasonable compromise between heat loss and buoyancy suppression.