Flame spread along a fuel rod in the absence of gravity
An analysis is carried out of the steady spread of a flame with infinitely fast kinetics over a thermally thin solid rod in the absence of gravity. Due to the radial convergence on the solid surface of the heat flux heating and vaporizing the fuel, the spread rate is found to be higher than for a flame spreading over a slab, and the region of the gas around the flame which is affected by molecular transport is large compared with the radius of the rod. The effects of radiation, of a buoyancy force opposing flame propagation and of axial heat conduction in the solid are discussed briefly
Buoyancy Effect on Downward Flame Spread Over PMMA Cylinders
Understanding material flammability at different gravity levels is important for fire safety applications in space facilities where the environments may include microgravity, low velocity flows, low pressure and elevated oxygen concentration. One possible approach to simulate on-earth the burning behavior inside spacecraft environments, and facilitate testing, is to reduce buoyancy effects by decreasing ambient pressure. The objective of this work is to study the effect of pressure, and consequently buoyancy and indirectly gravity, on downward flame spread rate over cylindrical samples of polymethyl-methacrylate (PMMA), and by comparison with reduced gravity data, observe up to what point low-pressure can be used to replicate flame spread in space facilities. Experiments in normal gravity are conducted using pressures ranging between 100 kPa and 30 kPa and oxygen concentrations between 19% and 23%, with a forced flow velocity of 100 mm/s. The low-pressure data is compared with microgravity data obtained aboard the International Space Station during the BASS-II experiments. Results show that reductions of ambient pressure slow down the flame spread process approaching that expected at low gravity. The normal gravity and microgravity data are correlated in terms of a mixed convection parameter that describes the main controlling mechanisms of heat transferred. Although the correlation works well for the normal gravity data it does not work as well for the microgravity data. However, it provides information about what is to be expected in environments of variable ambient pressure, oxygen concentration, and reduced gravity, providing an insight for future designs when considering fire safety in spacecrafts.
Propagation and extinction of downward burning fires
Abstract Measurements are reported on downward burning of PMMA fuel rods in an apparatus similar to that used in the Limiting Oxygen Index flammability test. As the ambient oxygen concentration is reduced the flame propagation rate decreases. The propagation rate V does not go to zero as the oxygen concentration Y ox is reduced, rather extinction occurs at a finite value of V . Although the propagation rate decreases as Y ox is reduced, the surface regression rate is not reduced. In agreement with this finding, the calculated heat flux per unit area to the pyrolysis region does not decrease as Y ox decreases. Thus, extinction is not caused by reduction of heat transfer from the flame to the burning region. To investigate the cause of extinction a previously developed analysis for the effect of surface heat flux on flame propagation is applied to the present problem. It is found that flame propagation is sensitive to the amount of heat transferred to the fuel surface ahead of the flame, and that extinction occurs (at a finite propagation rate) for sufficiently low values of forward heat flux. Comparisons of predicted and measured values of V are made using heat flux distributions measured at values of Y ox for atmospheric and extinction conditions and reasonable agreement is found. A transient measurement near the extinction limit confirms that the Limiting Oxygen Index is a measure of the critical condition for downward flame propagation rather than a more general limit for burning.