Characterization of strongly forced non-premixed methane jet flames
Abstract
Previous and current work show that flame length and soot luminosity
of laminar and turbulent nonpremixed jet flames can be significantly reduced
by high-frequency, high-amplitude forcing of the fuel flow rate. The current
work focuses on understanding the physical mechanisms responsible for these
changes to the flow/flame structure for the case where the forcing is sufficiently
strong that significant reverse flow exists at the exit of the fuel delivery tube.
The high-amplitude forcing is achieved by pulsing the fuel at the organ-pipe
resonance frequency of the fuel delivery tube. Quantitative mixture fraction
imaging in nonreacting jets indicates that the strongly pulsed jets exhibit
dramatically enhanced mixing as compared to unpulsed jets. In the pulsed
jets the mean mixture fraction falls below 0.08 in as little as 3 diameters
downstream of the nozzle exit and no pure jet fluid exists outside of the nozzle
due to in-tube mixing. Simultaneous OH and acetone planar laser induced
vi
fluorescence (PLIF) performed in methane jet flames shows that the OH zones
are much broader and more wrinkled in the resonantly pulsed jet as compared
to the unpulsed jet, and are greatly contorted by the vortical structures. A
unique feature of the pulsed flames is that the reaction zones appear to close
downstream of a vortical structure, just a few diameters downstream of the
nozzle exit, in a region where the mixture fraction imaging (for non-reacting
flows) shows reduced mixture fraction. Furthermore, the flame anchors on the
outer-upstream edge of the vortical structures where the fuel mixture fraction
is reduced due to enhanced entrainment. The significant in-tube premixing
and the enhanced entrainment appear to be the predominant mechanisms that
cause the reduction in length and luminosity of high-amplitude, high-frequency
pulsed nonpremixed jet flames.
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