Fig. 4. Mean temperature contours and streamlines of stable flame.
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.
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.
Full paper (PDF)
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.
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.
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