Opposed flow flame spread over thermally thick solid fuels: buoyant flow suppression, stretch rate theory, and the regressive burning regime

The Michigan State University Narrow Channel Apparatus (MSU-NCA) was used to investigate opposed flow flame spread over samples of thermally thick Polymethylmethacrylate (PMMA). Three different fuel thicknesses were tested for mean airflow velocities 8-58 cm/s. The sample thicknesses were 6.6 mm, 12.1 mm and 24.5 mm, respectively. The measured flame position versus time determined the spread rate. Flame spread rates ranged between 0.02 - 0.07 mm/s depending on fuel thickness and mean opposed flow. Complete sample burnout occurred for the 6.6 mm and 12.1 mm samples at the critical flow velocity of 30 cm/s ± 5 cm/s and higher. The flame spread results appeared to be independent of flow velocities for this range (>30 cm/s): this plateau regime is identified as the regressive burning regime. The 24.5 mm thick samples never completely burned through, but they entered the regressive burning regime at 41.4 cm/s flow velocity. The nature of surface regression and its influence on the spread mechanism in this regime at high flow velocities was discussed for completely burned through samples (6.6 mm and 12.1 mm) and partially burned through samples (24.5 mm). For 12.1 mm thick samples, the flame spread results were compared with the same material (PMMA) and similar thickness (12.7 mm) results from the 1981 Fernandez-Pello et al. study. Their tests used a wind tunnel having a different length and cross-section than the MSU-NCA. The comparison was favorable when employing the stretch rate theory of flame spread incorporating the appropriate numerically computed stretch rate. Since buoyancy was an important factor in the 1981 study, when the buoyant stretch was also included, excellent agreement was obtained between the Fernandez-Pello et al. data and the current NCA data. The results demonstrated the effectiveness of the stretch rate theory for markedly different experimental configurations.