Structure of locally quenched highly turbulent lean premixed flames

Abstract

The local flame structure of a premixed swirl-stabilized gas turbine burner has been investigated, where “thickened flames” or flames in the “stirred reactor regime” are expected from the Borghi diagram. Simultaneous 2-D OH and temperature measurements show that the flame structure fluctuates between four typical flame modes: (1) flamelet-like burning, (2) modified preheat zone with sharp reaction zone, (3) locally quenched reaction zone, and (4) hot nonreacted holes. In mode (3), evidence for local quenching processes is found with no superequilibrium OH between fresh gas and recirculating burned gas. In about 10% of the obtained images, regions without detectable OH but with temperatures above 1300 K are seen, surrounded by sharp reaction zones with low thermal gradients (mode 4). Although the explanation is not clear yet, significant modifications of the reaction processes due to strain combined with transient effects seem to occur here. From 2-D and 3-D measurements, gradients and flame thickness distributions have been determined, showing strong fluctuation of the thermal gradients, but on average, no significantly broadened instantaneous flame fronts, contrary to the expectation of thick flames. Instead, mean gradients conditioned to the local reaction progress variable c=(T−T0)/(Tmax−T0)=0,50 result in a slightly thinned thermal thickness, and within the preheat zone at c=0.25 a thinning of about 30% is found. For an explanation. strained laminar calculations are helpful, indicating the same trend for lean CH4-air mixtures. Obviously, strain effects from medium- and large-scale eddies have much more influence than diffusive effects from entrained small eddies. While the thermal thickness distribution shows large fluctuations, the OH thickness distribution (deduced from the OH ascent) is narrow. Comparing the Oh ascent thickness (similar to the calculated width of the CH peak) with the inner layer Kolmogorov size, a ratio of order 1 is found, when local quenching occurs.