Upward Propagation of Very Lean Methane-Air Flames in Vertical Tubes

Abstract

The upward propagation of a lean methane-air flame front in a vertical tube is investigated. The velocity of the flame front has been measured with an array of photodiodes set along the tube wall and, independently, from photographic records. A PIV system triggered by a photodiode signal has been used to measure the velocity of the flow induced by the flame front in a vertical plane through the axis of the tube. The contour of the luminous region of the flame front, assumed cylindrically symmetric, has been extracted from the recorded images. As expected, the shape and velocity of a very lean flame front, and the velocity of the fresh gas relative to the front, are similar to those of a bubble rising in the tube. The flow of the burnt gas features a region of low velocity (relative to the flame front) which enhances radiation losses and seems to play an important role in the extinction of the flame at the flammability limit. This limit is found to depend very sensitively on the temperature of the tube wall. A simple model is proposed of the flow around the axis of the tube and the combustion around the tip of the flame front. This model uses the measured gas velocity at the axis of the tube together with simplified conservation equations to compute the temperature and species concentrations along the axis for a given kinetic scheme and radiation law. The results for a single overall Arrhenius reaction and for a four-step reduced scheme, both in an optically thin gas, are in reasonable agreement with our experimental data and shed some light on the roles played by radiation losses and kinetic effects on the flammability limit measured in the standard flammability tube. The model can also be used to test other kinetic schemes and radiation laws.