CONTROL OF NOX EMISSIONS IN CONFINED FLAMES BY OSCILLATIONS

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.