Mechanisms of concurrent-flow flame spread over solid fuel beds

It is proposed that concurrent-flow flame spread over solid fuel beds will be steady under conditions where heat and momentum losses to the sides of the fuel samples and/or surface radiative losses are significant. These losses are argued to be unavoidable because the flame length will grow until these losses balance the heat and momentum generation rates. Approximate relations (with no adjustable parameters or necessity for supplemental measured quantities such as heat fluxes or pyrolysis times) are derived for steady spread rates in the presence of these losses for laminar and turbulent flow, buoyant and forced convection, and thin and thick fuels. Experimental tests of these relations were conducted for upward flame spread over thermally thin fuels. Varying pressures, oxygen mole fractions, and diluents were employed to cover a seven-decade range of the Grashof number. These experiments generally support the validity of the proposed mechanisms.