Caption:
Fig. 4. Mean temperature contours and streamlines of stable flame.
Context:
Figure 4 shows the mean temperature contours and pseudostreamlines
on the x-r plane based on the mean axialand radial velocity components
for a stable flame. A central torodial recirculation zone (CTRZ) is
established in the wake of the center body under the effects of the
swirling flow. The CTRZ, a form of vortex breakdown, serves as a flame
stabilization region, where hot products are mixed with the incoming mixture
of air and fuel. In addition, as a result of the sudden increase in
combustor area, a corner recirculation zone is formed downstream of the
backward-facing step.
Source:
Huang, Y., & Yang, V. (2004). Bifurcation of flame structure
in a lean-premixed swirl-stabilized combustor: transition from
stable to unstable flame. Combustion and Flame, 136(3),
383-389.
Abstract:
The present work addresses unsteady flame dynamics in a
lean-premixed swirl-stabilized combustor, with attention
focused on the transition of flame structure from a stable
to an unstable state. It was found that the inlet
temperature and equivalence ratio are the two most
important variables determining the stability
characteristics of the combustor. A slight increase in the
inlet mixture temperature across the stability boundary
leads to a sudden increase in acoustic flow oscillation.
One major factor contributing to this phenomenon is that
as the inlet mixture temperature increases, the flame,
which is originally anchored in the center recirculation
zone, penetrates into the corner recirculation zone and
flashes back, due to the increased flame speed. As a
consequence, the flame is stabilized by both the corner-
and the center-recirculating flows and exhibits a compact
enveloped configuration. The flame flaps dynamically and
drives flow oscillations through its influence on unsteady
heat release. This problem has not previously been studied
mechanistically. The results improve our understanding of
the mechanisms of initiation and sustenance of combustion
instabilities in gas-turbine engines with lean-premixed
combustion.
Notes:
The numerical units on the figure are not well described.
The color-coded temperatures are in degrees Kelvin (K).
The units at the bottom are presumably normalized units
for the grid used in the numerical simulation whose results
are shown in the various figures in the paper.
The diagram uses curved lines to indicate flow streamlines in this combustion chamber and overlays colored regions to indicate the temperature field. The full paper includes more elaborate variations of this figure showing sequences of snapshots in time demonstrating small scale oscillations (about 3KHz) as vortices are shed. The shedding of such vortices is similar to the shedding of vortices off the tip of an oar drawn through the water or the shedding of vortices from branches when the wind blows through trees, producing the familiar "whistling wind" sound. (RPF) Document contributed by Ngale Truong, Northeastern University, 2/2004.