Effects of Oxygen Concentration, Diluents, and Pressure on Ignition and Flame-Spread Rates of Nonmetals: A Review Paper

The literature reviewed indicates that oxygen concentration, diluents, and pressure in combustion-test environments can influence the ignition and flame-spread rates of nonmetals. Increased oxygen concentrations result in easier ignition and higher flame-spread rates. The flame-spread rate is generally a non-linear function of oxygen concentration, indicating that different processes are controlling combustion at different oxygen concentrations. The effect of oxygen concentration is higher at the minimum conditions required for ignition and at near-extinction conditions. Diluents affect the ignition and flame-spread rates in oxygen/diluent mixtures due to their effects on the thermal conductivity, specific heat, and diffusivity of the mixtures. Helium, which has a higher thermal conductivity than nitrogen, was shown to be a better inhibitor of ignition processes. Nitrogen/oxygen environments, which have higher heat capacity and lower diffusivity than helium/oxygen mixtures, generally result in lower flame-spread rates. The data also indicates that increased test pressures generally result in easier ignition and higher flame-spread rates. However, higher pressures can affect ignition and flamespread in opposing ways by affecting the gas-phase kinetics of the combustion process. The effects of test pressure are more pronounced at minimum conditions for ignition or at near-extinction conditions.