Large Eddy Simulation of self excited azimuthal modes in annular combustors

doi:10.1016/j.proci.2008.05.033

Proceedings of the Combustion Institute

Volume 32, Issue 2, 2009, Pages 2909-2916

https://doi.org/10.1016/j.proci.2008.05.033 Get rights and content

Abstract

While most academic set ups used to study combustion instabilities are limited to single burners and are submitted mainly to longitudinal acoustic modes, real gas turbines exhibit mostly azimuthal modes due to the annular shape of their chambers. This study presents a massively parallel Large Eddy Simulation (LES) of a full helicopter combustion chamber in which a self-excited azimuthal mode develops naturally. The whole chamber is computed from the diffuser outlet to the high pressure stator nozzle. LES captures this self-excited instability and results (unsteady pressure RMS and phase fields) show that it is characterized by two superimposed rotating modes with different amplitudes. These turning modes modulate the flow rate through the 15 burners and the flames oscillate back and forth in front of each burner, leading to local heat release fluctuations. LES demonstrates that the first effect of the turning modes is to induce longitudinal pulsations of the flow rates through individual burners. The transfer functions of all burners are the same and no mechanism of flame interactions between burners within the chamber is identified.

Introduction

In the highly competitive field of power generation, gas turbines have gained an increasing role over the years. New emission regulations and growing energy demand increase the weight on the research and development of gas turbine. Substantial advances have been made and ever more complex designs have been developed to meet the increasingly stringent regulations. Unfortunately, new designs sometimes are subject to combustion instabilities [1], [2], [3], [4]. In the case of annular combustion chambers containing multiple burners, these instabilities often take the form of azimuthal modes. These instabilities need to be predicted to evaluate their effects on the turbine operation. They also raise fundamental issues in terms of mechanisms and modeling:

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Modes appearing in annular combustion chambers are often controlled by the first (and sometimes second) azimuthal acoustic mode [5], [6], [7]. These modes can appear as standing wave modes or rotating modes. Both cases are observed in gas turbines [4]. Paschereit et al. [7], [8] propose a non-linear theoretical approach showing that standing wave modes can be found at low oscillation amplitudes but that only one rotating mode is found for large amplitude limit cycles.
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The models used to predict stability in annular chambers are usually based on a one-dimensional network view of the chamber [8], [9], [10] in which each burner is only influenced by the flow rate fluctuation it is submitted to by the azimuthal acoustic mode. All burners are supposed to have the same transfer function (i.e. the same relation between inlet burner velocity and reaction rate fluctuations). This may not be the case in practice: in liquid-fueled rocket engines or more generally in burners containing multiple jets [11], the interaction between neighbouring flames can lead to instability. This may happen in gas turbines too and require other modeling approaches than the existing ones.

Using experiments to study these issues is difficult because azimuthal modes cover the complete span of the combustion chamber and such test rigs are expensive and rare. A new approach is now possible using massively parallel computations and Large Eddy Simulation (LES). A compressible LES solver has the capacity to predict instabilities in a reacting flow configuration [3], [12]. By running such a code on a massively parallel machine (typically around 1000 processors), it is now possible to compute a full combustion chamber and study the mechanisms leading to the growth of azimuthal instabilities. In this paper a full combustion chamber LES of an helicopter turbine is presented. The LES tool and the models are first described before presenting the configuration which is a helicopter chamber demonstrator. The LES results are described next before measuring the transfer functions of each individual burner in the chamber and verifying if burners respond similarly to perturbations or if interactions between burners lead to a more complex instability mechanism.

Section snippets

LES and numerical models

Recent studies using LES have shown the potential of this approach for reacting flows (see reviews in [3] or [13]). LES is able to predict mixing [14], [15], [16], [17], [18], stable flame behaviour [19], [20], [21], [22] and flame acoustic interaction [17], [23]. It is also used for flame transfer function evaluation [24], [25]. Here a fully compressible, unstructured, explicit code is used to solve the multi-species Navier–Stokes equations with realistic thermochemistry on unstructured grids

Target configuration

This study focuses on an approximatively 20 cm radius annular helicopter combustion chamber equipped with 15 burners (Fig. 1). The combustion chamber is approximatively 5 cm high. Each burner contains two co-annular counter-rotating swirlers. The fuel injectors are placed in the axis of the swirlers. To avoid uncertainties on boundary conditions the chamber’s casing is also computed. The computational domain starts after the inlet diffuser and ends at the throat of the high pressure stator. In

LES of a self-excited azimuthal mode

The full chamber (15 sectors and burners) LES is initialised using a single sector LES with periodic boundary conditions which is copied over the other 15 sectors. The final mesh contains 9,009,065 nodes and 42,287,640 tetrahedra. Figure 2 shows a detailed view of the mesh resolution for $z = 0$ and $y > 0$ . Of course, the choice of an adequate mesh for such a large domain is a critical question. The present grid was chosen using tests performed on single sector computations. The same reacting flow was

Individual burner transfer function

Since each burner is axially forced by the azimuthal acoustic mode, it is possible to evaluate its response to the flow rate oscillations by computing the transfer function between inlet velocity fluctuations and global sector unsteady heat release [38], [39]. The correlated variables are $Ω$ , the unsteady heat release averaged in each sector and u, the bulk velocity through each burner. Figure 12 shows the modulus n and the phase $τ$ of the ratio $Ω / u$ for each of the 15 burners.

Amplitudes and

Conclusion

A full combustion chamber LES of an helicopter chamber was performed using massively parallel computing. Including the chamber’s casing, the swirlers and a choked high pressure stator reduces uncertainties on boundary conditions. The LES reveals that a self-excited mode at 740 Hz grows and reaches a limit cycle where two counter-rotating modes with different amplitudes modulate the flow rate through the 15 burners. This leads to strong modulations of the flame position and sometimes to

Acknowledgments

The authors wish to acknowledge the help of Turbomeca (Dr. C. Bérat and Dr. V. Moureau). The support of the Barcelona Computing Center and to Cray Inc. for providing the computer power necessary for this simulation is gratefully acknowledged.

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Large Eddy Simulation of self excited azimuthal modes in annular combustors

Abstract

Introduction

Section snippets

LES and numerical models

Target configuration

LES of a self-excited azimuthal mode

Individual burner transfer function

Conclusion

Acknowledgments

Combust. Sci. Tech.

J. Sound Vib.

J. Fluid Mech.

Combust. Flame

Ann. Rev. Fluid Mech.

J. Turb.

Phys. Fluids

Phys. Fluids

J. Fluid Mech.

Int. J. Num. Meth. Fluids

Int. J. Num. Meth. Fluids

Combust. Sci. Technol.

Flow Turb. Combust.

Combust. Flame