The extent to which imposed oscillations were able to reduce emissions
of nitrogen oxides (NOx) has been quantified in ducted, bluff-body st
abilized flames and in a sector of a gas-turbine combustor. The amplit
ude of naturally occurring oscillations was controlled passively, by c
hanges in geometry and actively by the feedback of a signal from a pre
ssure sensor to different forms of actuator. These arrangements allowe
d direct comparison of flows with the same fuel, average flow rate, de
gree of premixedness, and overall equivalence ratio and different ampl
itudes of oscillation. With premixed methane and air flowing in a roun
d duct with stabilization by a disk, the amplitude of oscillations was
largest close to an equivalence ratio of unity and led to NOx emissio
ns some 20% less than those without oscillations. Similar results were
obtained when fuel was added through the center of the disk to create
premixedness, though with the expected larger concentrations. With tw
o stabilizers, to allow an annular flow of lean premixed methane and a
ir and a central core of richer mixture, as in some combustors for lan
d-based gas turbines, NOx concentrations increased with equivalence ra
tio of the central flow and were reduced by up to 40% by discrete-freq
uency oscillations. In these ducted flows, the emissions and their red
uction were independent of the oscillation frequency in the measured r
ange from 100 to 300 Hz. The gas-turbine combustor was operated at ove
rall equivalence ratios and preheat temperatures corresponding to crui
se and take-off, at atmospheric pressure and with kerosene and methane
as fuel. With kerosene, and oscillations such that the free-field noi
se increased by 6 dB, reductions of up to 40% were measured in the spa
tially averaged exit-plane concentrations and these became negligible
with a noise level of 2 dB. Similar but slightly smaller reductions we
re obtained with methane fuel and could be achieved without pressure w
aves and added noise by the operation of neighboring fueling devices w
ith different phases of the imposed oscillation. (C) 1998 by The Combu
stion Institute.