Active control of combustion oscillations in a lean premixed combustor by secondary fuel injection coupling with chemiluminescence imaging technique

Abstract

An active control of combustion driven oscillations occurring in a lean premixed model combustor is demonstrated by means of a secondary fuel injection. An unstable condition, which leads to self-excited pressure oscillations beyond 170 dB, is chosen as control target. Keeping the inlet air temperature, the air mass flow rate and the overall equivalence ratio, respectively at 700 K, 78 g/s and 0.50, different secondary fuel injection strategies are examined. First, a series of experiments of steady secondary fuel injection is performed using five types of injectors, with secondary fuel percentage (SFP) varying from 2% to 10%. Effects of the angle, location and amount of injection on pressure levels are investigated. A favorable injector is selected from a point of view of control effectiveness, which is evaluated by the reduction in the peak pressure. The degree of phase coupling between pressure and heat-release rate is visualized by a phase-locked chemiluminescence imaging technique. This information is used to detect local sources of instability. To further improve control effectiveness, a closed-loop control, which was designed by the mixed H 2/H ∞ control algorithm, is applied with the selected injector. The closed-loop control with 2.5% SFP performs peak pressure reduction of 27.7 dB, which is almost 10 dB larger than that of the steady injection method. An explanation for the difference in the reduction levels is given by showing pressure-chemiluminescence correlation maps. On the other hand, little difference exists between the NO x emissions of the steady injection and the closed-loop control. An obvious effectiveness of the closed-loop control is confirmed without losing the low NO x capability.