Experimental observations of spot radiative ignition and subsequent three-dimensional flame spread over thin cellulose fuels

Non-piloted radiative ignition and transition to flame spread over thin cel lulose fuel samples was studied aboard the USMP-3 STS-75 Space Shuttle miss ion, and in three test series in the 10 second Japan Microgravity Center (J AMIC). A focused beam from a tungsten/halogen lamp was used to ignite the c enter of the fuel sample while an external air flow was varied from 0 to 10 cm/s. Non-piloted radiative ignition of the paper was found to occur more easily in microgravity than in normal gravity. ignition of the sample was a chieved under all conditions studied (shuttle cabin air, 21%-50% O-2 in JAM IC), with transition to flame spread occurring for all but the lowest oxyge n and Bow conditions. Although radiative ignition in a quiescent atmosphere was achieved, the flame quickly extinguished in air. The ignition delay ti me was proportional to the gas-phase mixing time, which is estimated by usi ng the inverse flow rate. The ignition delay was a much stranger function o f Bow at lower oxygen concentrations. After ignition, the flame initially s pread only upstream, in a fan-shaped pattern. The fan angle increased with increasing external flow and oxygen concentration from zero angle (tunnelin g flame spread) at the limiting 0.5 cm/s external air Bow, to 90 degrees (s emicircular flame spread) for external Bows at and above 5 cm/s, and higher oxygen concentrations. The fan angle was shown to be directly related to t he limiting air flow velocity. A surface energy balance reveals chat the he at feedback rate from the upstream flame to the surface decreases with decr easing oxygen mass transport via either imposed flow velocity or ambient ox ygen concentration. Quenching extinction occurs when the heat feedback rate from the flame is no longer sufficient to offset the ongoing surface radia tive heat losses. Despite the convective heating from the upstream flame. t he downstream flame was inhibited due to the 'oxygen shadow' of the upstrea m flame for the air Bow conditions studied. Downstream flame spread rates i n air, measured alter upstream flame spread was complete and extinguished, were slower than upstream flame spread rates at the same Bow. The quench re gime for the transition to flame spread was skewed toward the downstream, b ecause of the augmenting role of diffusion for opposed Bow flame spread, ve rsus the canceling effect of diffusion at very low cocurrent flows. (C) 200 1 by The Combustion Institute.