The Effect of Transient Fuel Staging on Self-Excited Instabilities in a Multi-Nozzle Model Gas Turbine Combustor

Abstract

Combustion instability in gas turbines can be mitigated using active techniques or passive techniques, but passive techniques are almost exclusively used in industrial settings. While fuel staging, a common passive technique, is effective in reducing the amplitude of self-excited instabilities in gas turbine combustors at steady-state conditions, the effect of transients in fuel staging on self-excited instabilities is not well understood. This paper examines the effect of fuel staging transients on a laboratory-scale five-nozzle can combustor undergoing self-excited instabilities. The five nozzles are arranged in a four-around-one configuration and fuel staging is accomplished by increasing the center nozzle equivalence ratio. When the global equivalence ratio is φ = 0.70 and all nozzles are fueled equally, the combustor undergoes self-excited oscillations. These oscillations are suppressed when the center nozzle equivalence ratio is increased to φ = 0.80 or φ = 0.85. Two transient staging schedules are used, resulting in transitions from unstable to stable operation, and vice-versa. It is found that the characteristic instability decay times are dependent on the amount of fuel staging in the center nozzle. It is also found that the decay time constants differ from the growth time constants, indicating hysteresis in stability transition points. High speed CH* chemiluminescence images in combination with dynamic pressure measurements are used to determine the instantaneous phase difference between the heat release rate fluctuation and the combustor pressure fluctuation throughout the combustor. This analysis shows that the instability onset process is different from the instability decay process.

Comparison of Equivalence Ratio Transients on Combustion Instability in Single-Nozzle and Multi-Nozzle Combustors

“…The transient is executed by varying the equivalence ratio with a proportional control valve upstream of the fuel-injection point in the nozzles. The transient equivalence ratio change for both single-nozzle combustor (SNC) and multi-nozzle combustor (MNC) configurations is conducted the same way as described by Culler et al [19]. The main fuel and air are mixed ahead of a choke plate, far upstream of the combustor to ensure perfect mixing.…”

“…The timescale of a transition can be determined by fitting a logistic curve to the envelope of the dynamic pressure signal, calculated using the analytic signal, and evaluating the width of the curve, as described in Culler et al [19]. The general logistic equation that describes the transition between two system limits is shown in Eq.…”

“…The general logistic equation that describes the transition between two system limits is shown in Eq. 1 [19]. The Plimit-cycle and Pstable are the pressure fluctuation magnitudes of the pressure fluctuations in selfexcited limit-cycle unstable and stable conditions, respectively.…”

“…They found that when staging fuel is introduced into the middle nozzle, not only does the distribution of heat release rate change throughout the flame, but also, the regions of highest heat release rate fluctuation oscillate out-of-phase with neighboring flames to suppress the instability. Building from the steady-state work on fuel staging, Culler et al [19] studied the transient behavior of fuel staging in the same multi-nozzle combustor. They found that the characteristic instability decay timescale depended on the amount of staging fuel added, where larger amounts of staging fuel resulted in shorter decay timescales.…”

Comparison of Center Nozzle Staging to Outer Nozzle Staging in a Multi-Flame Combustor

“…Depending on the timescale and direction of this unsteady change, nonlinear behaviors such as triggering [9] or hysteresis [10] may occur. Recent work by Culler et al [11] considered the effect of timevarying axisymmetric staging on self-excited instabilities using the same multiple nozzle combustor in the present work. Their study specifically examined the effect of impulse (very fast) transients in staging equivalence ratio on the onset and decay of combustion instability.…”