The Mechanisms of Lean Limit Extinguishment of an Upward and Downward Propagating Flame in a Stand

The Mechanisms of Lean Limit Extinguishment of an Upward and Downward Propagating Flame in a Stand

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
Nineteenth Symposium (International) on Combustion/The Combustion Institute, 1982/pp. 1549-1557 THE MECHANISMS OF LEAN LIMIT EXTINGUISHMENT OF AN UPWARD AND DOWNWARD PROPAGATING FLAME IN A STANDARD FLAMMABILITY TUBE J. JAROSINSKI,* R. A. STREHLOW AND A. AZARBARZINt Department of Aeronautical and Astronautical Engineering University of Illinois at Urbana-Champaign** The mechanisms by which lean limit upward and downward propagating flames extinguish in a standard flammability tube (51mm ID, 1.8m long) were deduced using schlieren and direct light photography and temperature measurements and by observing the behavior of the flame under transient loading. The lean methane-air system was studied. The upward propagating flame takes the shape of a stable hemispherical cap attached to a trailing skirt. This flame always extinguishes first at the holding region on the axis of the tube and a failure wave subsequently washes down the flame until extinguishment is complete. The extinguish- ment occurs because the flame is stretched at the tip and heat loss to the walls is not im- portant to this extinguishment. On the other hand the extinguishment of a downward prop- agating flame is a multistep process. First heat loss to the walls causes the flame to extinguish near the walls and a smaller residual flame is left in the center of the tube. Once this happens, differential buoyancy of the hotter central gases and the cooler surrounding gases forces the cooler gases to move ahead of the flame. Finally, the downward propagating flame is observed to rise slightly. At this point the flame extinguishes completely. Introduction Flammability limits were first recognized by Humbolt and Gay Lussac (1) and first determined by Davy (2) over 160 years ago. The empirical ob- servation that flammability limits are apparatus de- pendent was well summarized by Coward and Jones (3) almost 30 years ago. At that time they proposed that the standard apparatus for determining flam- mability limits be a 51mm diameter, 1.8m long ver- tical tube closed at the upper end and open to the atmosphere at the bottom. Using this tube, if a mixture, when ignited at the bottom propagates all the way to the top the mixture is said to be flam- mable. However, if the flame extinguishes part way up the tube the mixture is said to be non- flammable. Prior to 25 years ago there was very little the- oretical effort directed toward extinction mecha- nisms and the effect of boundary and initial con- *Current address: Dr. Jozef Jarosinski, Institute of Aeronautics, Warsaw, Poland. tCurrent address: Ardeshir Azarbarzin, Sun- strand Aviation, Department 715, Rockford, Illinois 61101. **A portion of the work reported herein was per- formed at the Institute of Aeronautics, Warsaw, Poland. ditions on the actual extinction process. In the intervening years a number of simple theories for limits have been proposed. Each of these has fo- cused on a single mechanism of extinction such as heat loss from a one dimensional flame (4-8) con- vection and/or buoyancy (9-10) or chemical kinet- ics (11). In more empirical approaches, flame stretch
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 9

The Mechanisms of Lean Limit Extinguishment of an Upward and Downward Propagating Flame in a Stand

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online