Near-limit flame spread in low-speed opposed and concurrent flows over thick fuels

(1) In the low-velocity flow regime considered, at a given flow velocity, the opposed flame spreads much faster than the concurrent flame. In opposed flows, the flame spread rate also depends on oxygen concentration, while the spread rate of concurrent flame appears less sensitive to the variation of oxygen concentration. (2) For opposed spread, the non-dimensional flame spread rates are correlated well with the Damköhler number. The entire flame spread domain can be divided into three regimes with respect to DaEST. For concurrent spread, the flame spread rates agree with the theoretical prediction in general. However, for both flame modes, the spread rates in low-velocity gas flows may deviate from theoretical predictions due to radiative heat (3) Concurrent flame spread has a wider flammable range than opposed case. Beyond the flammability boundary of opposed spread, there is an additional flammable area for concurrent spread, where the spreading flame is sustainable in concurrent mode only. (4) The present narrow channel tests provide significant information about the flame spread over thick fuels in low-velocity flows. However, more studies, especially microgravity experiments, are clearly needed to validate some of the findings in a quantitative manner.