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
The Effect of Gravity on Flame Spread over PMMA Cylinders
Fire safety is a concern in space travel, particularly with the current plans of increasing the length of the manned space missions, and of using spacecraft atmospheres different than in Earth, such as microgravity, low-velocity gas flow, low pressure and elevated oxygen concentration. In this work, the spread of flame over a thermoplastic polymer, polymethyl methacrylate (PMMA), was conducted in the International Space Station and on Earth. The tests consisted of determining the opposed flame spread rate over PMMA cylinders under low-flow velocities ranging from 0.4 to 8 cm/s and oxygen concentrations from 15% to 21%. The data show that as the opposed flow velocity is increased, the flame spread rate first increases, and then decreases, different from that on Earth. The unique data are significant because they have only been predicted theoretically but not been observed experimentally before. Results also show that flame spread in microgravity could be faster and sustained at lower oxygen concentration (17%) than in normal gravity (18%). These findings suggest that under certain environmental conditions there could be a higher fire risk and a more difficult fire suppression in microgravity than on Earth, which would have significant implications for spacecraft fire safety.
A theory of flame spread over a solid fuel including finite-rate chemical kinetics
Abstract A theory for the steady-state flame spread over a thermally thin solid fuel is developed in this study. The model considers a laminar diffusion flame in a uniform opposed flow and includes the two-dimensional, elliptic, gas-phase energy, and species equations with one-step overall chemical reaction and second-order, finite-rate Arrhenius kinetics. The unsteady, solid-fuel equations neglect heat conduction ahead of the flame but include transient heating and Arrhenius pyrolysis kinetics and are coupled to the quasisteady gas phase. The equations are solved in the laboratory coordinate system. In this study the two-dimensional distributions of temperature and species are obtained, including the low reactivity zone in the flame region. The solid-fuel surface profiles indicate a region of almost uniform temperature (vaporization temperature) during pyrolysis for some parameter values; however, the value is not universally constant for the fuel and does depend on the ambient parameters (pressure, oxygen mass fraction, and magnitude of opposed velocity). The dependence of the flame-spread rate on the ambient parameters is investigated, and qualitative agreement is obtained with experimental results in the near-extinction-limit region. Quantitative agreement with experimental data depends on the choice of parameter values, especially the gas-phase kinetics model parameters, which are generally unknown. The flame-extinction limits due to increased opposed velocity, reduced pressure, and reduced ambient oxygen mass fraction are all obtained in the results calculated from this theory.