Search

In the start-up process of practical gas turbine applications, the modulation of operating condition, especially the variation of global equivalence ratios (ϕ g l o), could cause the variations of stability regimes. The above process is accompanied by the increase of total air flow rates ( m ̇ a i r) when approaching the stable operation condition, and thus the effect of this parameter should also be considered in the operating condition modulation process. This work presents an experimental investigation into the transitions of thermoacoustic modes and flame dynamics in a centrally-staged swirl combustor over a range of ϕ g l o and m ̇ a i r. The present thermoacoustic system exhibits two transitions of thermoacoustic modes with ϕ g l o variation, showing distinct characteristics of pressure oscillation (p ′) and flame structures. To further explore the detailed variation of p ′ characteristics and its link to flame dynamics during two transitions, a nonlinear dynamic analysis of p ′ including its phase and recurrence plots as well as a proper orthogonal decomposition (POD) study have been conducted. The thermoacoustic network analysis and local Rayleigh index analysis have clarified the mode transition mechanism with varying ϕ g l o and m ̇ a i r. The present results can provide a guideline for quantifying the stability regimes of fuel-flexible combustors during the operating condition modulation processes. • Transitions of thermoacoustic modes and flame dynamics are characterized. • Effects of equivalence ratios and air flow rates on the transitions are examined. • Transient processes during different transitions are observed. • Transient mechanisms of thermoacoustic modes and couplings are elucidated. [ABSTRACT FROM AUTHOR]

The present study investigates the effect of the exit geometry of the main stage on thermoacoustic instabilities in a model combustor with a dual-swirl layout. In practical combustors, the main stage has a much larger airflow than the pilot stage, therefore dominating the aerodynamics of the global flame. In this paper, six different geometries of the main-stage exit are designed to cover the common styles in engineering applications. Their effects on thermoacoustic instability and flame shape are examined experimentally. Sudden transitions from the attached flame to the blurred flame are found in most of the exit geometries, excluding the outer-rough case. This transition of flame shape triggers the onset of strong thermoacoustic oscillation, leading to a jump in the pressure fluctuation amplitude. Frequency drop and the acoustic mode shift are also found at the transition point. Flame dynamics are further analyzed using the dynamic mode decomposition method. It is concluded that the outer-rough case has the best thermoacoustic stability within the tested range. The mechanisms of the observed phenomena are analyzed by measuring the reacting flow fields. It is found that the increased roughness of the outer wall can prevent the flame shape transition and the onset of thermoacoustic instability, possibly due to the weakened vortex structures and strain rate in the outer shear layer. [ABSTRACT FROM AUTHOR]