An optical study of flammability limits

Abstract

Divergent views on the fundamental cause of flammability limits suggest insufficient knowledge of the mechanism of flame failure. Direct and schlieren photography have been applied to a study of flame failure in tubes at the fuel-lean limit of flammability. Results show that a bubble of hot exhaust gas rises in contact with upward propagating flames and continues to rise even after the visible flame has failed. Downward propagating flames show no evidence of such a mechanism. Comparison with fluid dynamical calculations shows that the exhaust bubble and attached flame rise in accordance with a relation involving only the tube diameter and the local gravitational acceleration. It is deduced that the bubble plays a major part in assisting the stability of a rising flame. hence accounting for the difference between flammability limits for upward and downward propagation. Further discussion suggests that this mechanism itself causes flammability limits as conventionally defined and shows how limits for different fuels are related. The final conclusion is that the limit of flammability, as commonly measured, is not a fundamental combustion parameter of a fuel.