| The effects of atmosphere composition and the convection environment (downward (opposed buoyant flow), upward (concurrent buoyant flow), or microgravity (negligible buoyant flow)) on the flame spread rate (Sf) over thin solid fuel beds were measured and compared to theoretical predictions.; For downward and microgravity (μg) flame spread, two modifications to the standard air atmosphere were considered. First, the effect of diluent type on Sf was studied by comparing results using He, N2, Ar, CO2, and SF6 diluents. Like prior studies in N 2 diluent, for He, N2, or Ar diluents it was found that downward Sf was larger than the μg Sf, however, for CO 2 diluent, downward Sf was slightly lower than μg S f and for SF6 diluent, the downward Sf was much lower than μg Sf. Moreover (unlike He, N 2, or Ar), for CO2 and especially SF6 diluents the minimum O2 concentration required to support flame spread at μg was lower than the minimum concentration for downward spread at 1g (for SF 6, the μg limit was even lower than the upward limit). This behavior is proposed to be a result of reabsorption of radiation emitted from the gases. Secondly, the effects of sub-flammability-limit concentrations of a gaseous fuel (CO or CH4) were measured and compared to an existing theoretical model that was extended to μg conditions. The agreement between the model and experiment is reasonable. Notably, both model and experiment show that the effect of added gaseous fuel is greater at μg than for downward spread at 1g.; For upward flame spread, steady spread was found under conditions where heat and momentum losses to the sides of the fuel sample or surface radiative losses were significant. These losses are argued to be unavoidable because the flame length grows until these losses balance the heat generation. By equating heat generation and losses, approximate predictions of spread rates were obtained. Experiments over a large range of Grashof number were performed and generally support the validity of the proposed mechanisms. |
