Session: 04-06 Combustion Dynamics - Modeling II

Submission Number: 176840

Injector Staging Effects on Combustion Instabilities in an Annular System and Comparison With Calculations Over a Full Operating Domain 

Thermoacoustic instabilities in annular combustors remain a major issue for the safe and reliable operation of aeronautical propulsion systems. These instabilities arise from the nonlinear coupling between acoustic modes and unsteady heat release and may lead to damaging limit-cycle oscillations. Reduced-order modeling (ROM) has recently emerged as a tool capable of predicting the saturation of such oscillations. Latour et al. (2025) demonstrated that this approach could reproduce the limit-cycle amplitude and mode structure at a single operating point in a lab-scale annular combustor. The present study extends this methodology to a much broader operating domain and shows that it is consistent for different injector staging strategies, which are highly relevant for thermoacoustic control.

The study is carried out on the MICCA annular combustor, comprising 16 spray-swirled injectors and fueled with liquid heptane. The operating domain consists of 35 points defined by five thermal powers (92 - 118 kW) and seven equivalence ratios (0.75 – 1.05). Injector staging is achieved by mixing two types of injectors, each characterized by a distinct flame response when used individually. Both injector types feature radial swirlers, characterized by different swirl numbers, 0.60 and 0.74, at a bulk velocity of 40 m/s. Different staging patterns are selected, each with different numbers and spatial location of the two injector types. These configurations provide a wide parametric basis to evaluate ROM predictions against experiments.

The ROM formulation consists of projecting the acoustic wave equation onto a set of acoustic modes, yielding coupled equations for the modal amplitudes and phases. These equations are then reformulated in terms of slow-flow variables (compared to the modal frequency of oscillation of the annular combustor), which capture the envelope dynamics of modal growth and saturation. Time averaging is applied to remove fast oscillations, leaving a compact system of ordinary differential equations that governs the nonlinear evolution of the acoustic field. This framework yields predictions of both effective growth rates and limit-cycle amplitudes. It also gives access to the spin ratio of azimuthal modes and anti-nodal line angle, which characterize the modal structure of the instability.

The nonlinear source term in the ROM is computed with Flame Describing Functions (FDFs). A key specificity of this study is that it relies only on velocity-based FDFs measured in the single-sector combustor SICCA, then converted into pressure-based FDFs and linearly extended to the annular conditions of MICCA. In SICCA, the velocity-based FDFs are obtained using OH* chemiluminescence to record variations of heat release rate, and laser Doppler velocimetry to measure velocity fluctuations at the injector exit. The facilities SICCA and MICCA share the exact same two spray-swirled injectors.

ROM predictions are directly compared to experimental measurements in MICCA for all 35 operating points and for the selected staging patterns. The results show that the ROM accurately retrieves the instability domain layout, distinguishing stable and unstable conditions across the operating grid. Predicted limit-cycle amplitudes agree well with experimental values at the selected operating points. The model suitably accounts for effects of the various injector staging arrangements.

V. Latour, D. Durox, A. Renaud and S. Candel (2025) Unified limit cycle amplitude prediction and symmetry breaking analysis of combustion instabilities. J. Fluid Mech.1007, A66.  https://doi.org/10.1017/jfm.2025.10

Presenting Author: Pierre-Alexandre Barré Laboratoire EM2C

Presenting Author Biography: 2nd year PhD student at laboratoire EM2C - Safran Aircraft Engines, working on thermo-acoustics in annular combustors.
Graduated from École Polytechnique and Imperial College London

Authors:

Pierre-Alexandre Barré Laboratoire EM2C
Sebastien Candel Laboratoire EM2C
Daniel Durox Laboratoire EM2C
Abel Faure-Beaulieu Safran Aircaft Engines
Antoine Renaud Laboratoire EM2C