Surface regression in opposed flow flame spread: Transitions, correlations and partition factors

Surface regression was studied during opposed flow flame spread over thermally thick polymethyl-methacrylate (2.54 cm) under variable forced flows in a horizontal Narrow Channel Apparatus (NCA). The sample nominal thickness was 2.54 cm and opposed flow velocities varied from 8 - 45 cm/s. The post-burn samples were examined for the regressed surface shape. A formula was derived whose primary input is the regression profile: either the spread rate or the mass flux can be calculated from other inputs. The experimental video record gives the flame spread rate. The measured surface profile, along with the flame spread rate, gives the mass flux from the surface. The regression depth increases with increased mass average flow velocity. For mass average flow velocities below 12 cm/s, when the surface barely regresses (2 mm over an ≈ 12 cm length), the average power law exponent for mass loss rate approximates to -1/2. For mass average flow velocities greater than 25 cm/s, the average power law dependency approximates to -1/4. The transition from the -1/2 to -1/4 power occurs over a narrow range of opposed flow stretch rates and mass mean flow velocities. For the higher opposed flow velocities the regression depth is a prominent feature of the burned sample. Based on the hypothesis of partitioning of physical influences, the contributions to flame spread of the flow field, finite rate chemistry and surface regression are evaluated from the experimental data. A semi-empirical flame spread formula is developed.