Experimentally Determined Flame Properties near Flammability Limits Under Gravity and Microgravity Conditions

Abstract

Flammability limits for flames propagating in a rich propane/air mixture under gravity conditions appeared to be 6.3% C₃H₈ for downward propagation and 9.2% C₃H₈ for upward propagation. Different limits might be explained by the action of preferential diffusion of the deficient reactant (Le < 1) on the limit flames, which are in different states of instability. In one of the previous studies, the flammability limits under microgtravity conditions were found to be between the upward and downward limits obtained in a standard flammability tube under normal gravity conditions. It was found in those experiments that there are two limits under microgravity conditions: one indicated by visible flame propagation and another indicated by an increase of pressure without observed flame propagation. These limits were found to be far behind the limit for downward-propagating flame at 1 g (6.3% C₃H₈) and close to the limit for upward-propagating flame at 1 g (9.2% C₃H₈). It was decided in the present work to apply a special schlieren system and instant temperature measuring system for drop tower experiments to observe combustion development during propagation of the flame front. A small cubic closed vessel (inner side, 9 cm × 9 cm × 9 cm) with schlieren quality glass windows were used to study limit flames under gravity and microgravity conditions. Flame development in rich limit mixtures, not visible in previous experiments under microgravity conditions for strait photography, was identified with the use of the schlieren method and instant temperature measuring system. It was found in experiments in a small vessel that there is practically no difference in flammability limits under gravity and microgravity conditions. In this paper, the mechanism of flame propagation under these different conditions is systematically studied and compared and limit burning velocity is estimated.

Keywords:

• Flammability limits
• Limit burning velocity
• Microgravity

ACKNOWLEDGMENTS

The work reported here was sponsored by the Marie Curie ToK project from the 6th FP, contract number MTKD-CT-2004-509847, and by the State Committee for Scientific Research, contract number 4T12D 051 29.