Oscillatory extinction of spherical diffusion flames: Micro-buoyancy experiment and computation

Abstract

The transient extinction behavior of near-limit spherical diffusion flames under microbuoyancy conditions was experimentally investigated. An inverse flame configuration was employed with the oxidizer being ejected from a porous sphere burner into a fuel (H 2 ) environment in a low-pressure (0.096 atm) chamber to effectively minimize buoyancy. Various inert gasses (N 2 , CO 2 and He) were used to dilute the oxidizer to change the Lewis number and radiative properties of the mixture so that both the transport-induced and the radiative-induced limit could be achieved. Extinction was triggered by gradually decreasing the H 2 mole fraction in the ambient. At the transport-induced limit, extinction is characterized by sudden quenching of the flame as demonstrated by a rapid decrease of the radiometer signal voltage. However, at the radiation-induced limit extinction is preceded by oscillations in the flame luminosity that grows in amplitude before extinction. Computational simulation of the experiment was performed by using a onedimensional spherically symmetric domain with detailed chemistry and transport and radiatively thin heat loss. Oscillations were computationally observed at the radiative limit with approximately the same frequency as the experiment (∼2 Hz). However, the critical ambient H 2 mole fraction that triggered oscillation was much larger in the simulation than in the experiment, possibly due to the neglect of radiation reabsorption in the simulation.