Combustor equivalence ratio, effect

Pressure oscillations with RMS value up to 10 kPa in two models of lean-burn gas turbine combustors, with heat release around 100 kW, have been actively controlled by the oscillation of fuel flow. The flames were stabilized behind an annular ring and a step in one arrangement, and downstream of an expansion and aided by swirl in the other. Control was sensitive to the location of addition of oscillated fuel. Oscillations in the annular flow were attenuated by 12 dB for an overall equivalence ratio of 0.7 by the oscillation of fuel in the core flow and comprising 10% of the total fuel flow, but negligibly for equivalence ratios greater than 0.75. Oscillation of less than 4% of the total fuel in the annulus flow led to attenuation by 6 dB for all values of equivalence ratio considered. In the swirling flow, control was more effective with oscillations imposed on the flow of fuel in a central axial jet than in the main flow, and oscillations were ameliorated by 10 dB for equivalence ratio up to 0.75, above which the flame moved downstream so that the effectiveness of the actuator declined. The amelioration of pressure oscillations resulted in an increase in NOj, emissions by between 5% and 15%. 

Data from the present set of experiments suggest that the conversion of fuel nitrogen to TFN in jet-stirred combustors depends upon the equivalence ratio and average residence time of gases within the combustor, the fuel type and certain physical characteristics of the combustors. However, the effects of these primary variables on fuel nitrogen conversion appear to be related to their effects on the concentrations of unburned hydrocarbons and soot in the exhaust gases. These effects and their relationships to unburned hydrocarbon and soot concentrations are discussed below. 

This experimental investigation was motivated by the requirements of lean-premixed methane-air flames in modern gas-turbine combustors and the periodic extinction and relight observed close to the lean limit [1]. The first involves low equivalence ratios with possible dynamic effects, and the second involves a strain rate mechanism that may imply oscillations in bluff-body stabilized flames at all equivalence ratios. Opposed flames are used here to examine the nature of extinction, and to a lesser extent ignition to quantify extinction velocities and times and to determine limitations of this comparatively simple arrangement. The same arrangement was used in investigations of the corresponding isothermal flow [2]. 

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