Study on Effects of Ambient Pressure on Flame Spread over Solid Combustibles
Student thesis: Doctoral Thesis
Related Research Unit(s)
As one of the most important research topics in fire science, flame spread over solid combustibles has received great attention from fire researchers. Through experimental investigations, theoretical analysis, and numerical simulations, we have obtained an increasing understanding on the controlling mechanisms and influencing factors of the flame spread process for different types of materials and under different spreading conditions. Many factors influence the process, including the solid fuel properties and the environmental conditions. One of the vital factors is the ambient pressure. A thorough literature review has indicated significant limitations in studies on the effects of ambient pressure on solid fuel flame spread, namely, 1) comparatively limited work has been conducted on concurrent flow spread; 2) experiments have mostly been performed using materials with small scales, of which the findings’ applicability to a relatively large-scale fire needs further investigation; 3) distinct conclusions have been presented, but the reasons causing these discrepancies remain un-discussed. This dissertation presents the results of a series of experiments conducted in Hefei (with an altitude of 29.8m and a pressure level of 100.1kPa) and Lhasa (with an altitude of 3658.0m and a pressure level of 65.2kPa). The experiments were conducted using thick poly (methyl methacrylate) slabs with a relatively large dimension of 105cm × 30cm. The effects of ambient pressure on flame spread over flat fuel surfaces and along vertical corner walls were studied. The flame spread rates in Lhasa are significantly slower than those in Hefei. For upward flame spread over a vertically oriented fuel slab and along a corner, the spread rates in Lhasa are approximately one half of the ones in Hefei. A further examination on the preheating range and the flame heat feedback intensity to the solid fuel revealed that the major cause of the slower spread rates in Lhasa is the lower heat flux level in a lower ambient pressure environment. At reduced ambient pressures, the convective heat transfer coefficient on the fuel surface decreases. The soot particle concentration in the flame zone also drops which causes a reduction in the flame emissivity, thereby decreasing the flame radiant heat flux to the fuel surface. The special behavior of corner flame spread was also studied. The M-shaped pyrolysis front is a consequence of an insufficient oxygen supply near the corner centerline, and a sustained combustion cannot be achieved there. In the following part, the Fire Dynamics Simulator with the direct numerical simulation mode was used to simulate two-dimensional upward flame spread over vertical fuel surfaces under different ambient pressures. The spread rate is roughly proportional to the one-half power of the pressure level for thermally thin fuels and changes roughly linearly with the pressure for thermally thick fuels with a small fuel scale and at pressures P ≥ 55k Pa.Based on the theoretical models formulated by pre-researchers, we established quantitative relations between the spread rates and the ambient pressure level for different fuel thickness regimes and spreading conditions. Although simple, these relations show a potential applicability in spread rate prediction with changing pressure level, as evidenced by the comparison with experimental and numerical data, both from the literature and the present work.
- flame spread, poly (methyl methacrylate) slabs, altitude, ambient pressure, inclination angle, corner angle, direct numerical simulation, power law relation