chapter  18
12 Pages

COUNTERCURRENT SHEAR LAYER CONTROL OF PREMIXED FLAMES

One of the problems in combustors that utilize premixed ames is the attainment

of stable performance over an extended range of operation (turndown ratio). The

condition, at which the combustion wave is driven back causing the ame to be

extinguished when the ow velocity exceeds the burning velocity everywhere in

the ow eld, is of particular interest to this study. The physical mechanisms

responsible for the blow-out limits and ame stabilization of jet ames is still

a topic of extensive research [1, 2]. The ame stabilization technique discussed

approach to this complex phenomenon, which can be summarized as follows.

The burning velocity depends upon a number of variables such as the mixture

strength, the ame temperature, and thermal and molecular diusivities of the

mixture [3]. In practical systems, the nozzle exit velocity prole has spatial vari-

ation that greatly in uences the ame stability. Typically, the ame is stabilized

within the shear layer region where the velocities are lower. When the ow rate

is very high, the velocity in the ame exceeds the burning velocity everywhere

and the ame blows out. It is generally accepted that the limiting speed that

can be achieved while keeping the ame anchored to the nozzle exit is deter-

mined by the maximum sustainable strain rate measured at the nozzle exit. By

the introduction of a small countercurrent ow, the strain eld in the nozzle

exit region is eectively altered to achieve conditions for ame anchoring and

stabilization.