Bi-Stable Flame Behaviour of Heavy Duty Gas Turbine Burner: RANS, LES and Experiment Comparison

Abstract

VeLoNOx™ series (Very Low NOx) burners have been developed by Ansaldo Energia to satisfy the most restrictive emissions limits required by various countries. For these combustion systems the characterization of the field of stable operating conditions is a key design element. In particular, flame transition is of paramount importance for the stability of these combustion systems. Usually, this phenomenon occurs during load ramps and is characterized by a change of the flame shape, occurring in a well-defined load range. Proper flame shape control is essential to achieve more stable operating conditions up to base load. At transition, experiments in a full scale pressure test-rig clearly show both a modification of the flame shape, a change of the measured air side burner effective area, and a reduction of the combustion chamber wall temperatures. LES simulations were conducted using the same boundary conditions as RANS simulations discussed in a previous work [1]. In this paper, we show that both RANS and LES were able to correctly predict flame transition and a RANS-LES-Experiment comparison is performed.

Flame Double Input Describing Function analysis

“…The FDF can predict the existence of oscillations of the two modes independently, but will fail in predicting their stability, as the latter is connected to the nonlinear coupling between the two modes. This phenomenon is usually called mode-switching, and was observed also by Moeck and Paschereit [21] and in gas turbines experiments by Cazalens et al [22] and Anisimov et al [23] . In [21] it was shown that mode-switching can be attributed to the existence of an unstable quasiperiodic attractor in the phase-space of thermoacoustic trajectories, which the FDF framework cannot calculate.…”

“…For a fixed point, both amplitudes vanish and J simply contains the growth rates σ 1 and σ 2 on the main diagonal, retrieving the classic linear stability result. For a limit cycle solution (say of mode 1), the Jacobian takes the form (23) and has eigenvalues ∂ σ 1 /∂ A 1 A 1 and σ 2 . Because A 1 is positive, the stability is determined by the sign of ∂ σ 1 / ∂ A 1 (the FDF condition) and σ 2 .…”

Study on the Flame Transition Characteristics of a Gas Turbine Combustor

“…With the continuous advancement of advanced, efficient, and clean gas turbine combustion technologies [8], coupled with increasingly stringent emission regulations [9,10], the development of the gas turbine encounters new challenges and demands, including the imperative to simultaneously achieve for higher efficiency, lower NO x and CO emissions, greater fuel flexibility, and a larger operation window [11,12]. The wider operation window is characterized by lower park loads [13], indicating the operational state of the gas turbine at lower loads, and higher ramp gradients, denoting a greater rate of change when adjusting the load, which showcases the greater flexibility and adaptability of gas turbines various operational conditions [14][15][16]. Specifically, increasing the equivalence ratio to enhance combustion is very effective in extending the operation window.…”

“…Additionally, the trails of the pilot flame in the left image are open, whereas they are closed and slightly rolled around the chamber centerline in the right image. Rofi et al [16,38] employed both experiments and Reynolds Averaged Navier-Stokes (RANS) simulations to explore the flame shape before and after flame transition in a single burner combustion chamber developed by Ansaldo Energia. Their findings revealed distinctive flame shape transitions during load ramps, providing valuable insights into the dynamics of combustion stability and emission characteristics of their gas turbine combustor.…”

“…Their findings revealed distinctive flame shape transitions during load ramps, providing valuable insights into the dynamics of combustion stability and emission characteristics of their gas turbine combustor. During the development of Ansaldo Energia's coaxial multi-stage lean premix burner, an obvious sudden change in flame shape occurs during loading, and an inverse sudden change takes place during deloading, which is recognized as "flame transition" or "flame switch" [16,38]. As depicted in Figure 2, the left side shows the flame image before the flame transition at the end of the flame chamber during the test, while the right side illustrates the flame image after the flame transition from the same perspective.…”