Vortex-Coupled Oscillations of Edge Diffusion Flames in Coflowing Air With Dilution.
Vortex-Coupled Oscillations of Edge Diffusion Flames in
Coflowing Air With Dilution.
(1461 K)
Takahashi, F.; Linteris, G. T.; Katta, V. R.
Volume 31; Part 1;
Combustion Institute, Symposium (International) on
Combustion, 31st. Proceedings. Volume 31. Part 1.
August 5-11, 2006, Heidelberg, Germany, Combustion
Institute, Pittsburgh, PA, Barlow, R. S.; Sick, V.;
Glarborg, P.; Yetter, R. A., Editor(s)(s), 1575-1582 pp,
2007.
Keywords:
combustion; diffusion flames; dilution; vortex
interaction; methane; gravity; flame structure; flame
flicker; burners; carbon dioxide; experiments
Abstract:
The unsteady characteristics of oscillating methane
diffusion flames in coflowing air diluted with CO2 in
earth gravity have been studied experimentally and
computationally. The measured frequency of flame
flickering due to buoyancy-driven large-scale vortices
was bi-modal. As CO2 was added into coflowing air
gradually, the base (edge) of the flame detached from
the burner rim, oscillated at half the flickering
frequency, and blew off eventually. Numerical
simulations with full chemistry predicted the internal
flame structure and unsteady flame behavior: flame
flickering, tip separation, base detachment,
oscillation, and blowoff, in good agreement with the
experiment. The mechanism of the edge diffusion flame
oscillation was due to a cyclic series of events: (1)
flame-base detachment and drifting downstream as a
result of weakening due to dilution and a momentary
increase in the entrainment-flow velocity associated
with the vortex evolution, (2) fuel-air mixing in
widened, lower-speed, wake space between the flame base
and the burner rim, and (3) flame-base propagation
through the flammable mixture layer back to the burner
rim. A peak reactivity spot (reaction kernel) at the
edge diffusion flame controlled the unsteady behavior
through its dramatic changes in characteristics from the
passively drifting to (premixed-type) propagating phase
during a cycle. Because a mixing time of approximately
100 ms was required before propagation was enabled, a
subsequent vortex evolved and passed. Thus, the
flame-base oscillation was strongly coupled with the
buoyancy-driven vortex evolution and the oscillation
frequency was locked-in to half the flame-flickering
frequency. The results have implications in turbulent
flame structure; more specifically, the local
extinction-mixing-reignition processes, in that the slow
molecular mixing can become rate-limiting and the edge
diffusion flame structure can be significantly
different, depending on the phase in the process.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899