International audienceCombustion dynamics of a V-flame in an afterburner-type configuration are investigated using high-order compressible large eddy simulations (LES) and compared to experimental results. Both self-excited longitudinal (100 Hz) and transverse (1400 Hz) modes observed in the experiments are captured by LES and instability mechanisms are discussed. LES results for all modes are compared to a Helmholtz solver output, showing that the transverse mode appearing in the LES is the 1Lx-2Ty-0Tz eigenmode of the chamber, affecting the velocity field symmetrically. The 1Lx fluctuation causes a symmetric flame roll-up which increases heat release rate fluctuations, closing the feedback loop. The 2Ty component of the mode is active along the flame holder axis and causes not only transverse fluctuations but also a reorganization of the mean flame along two main zones located on both sides of the zero acoustic velocity plane, a feature that has not been reported before. Dynamic mode decomposition (DMD) is used to extract the structure of the transverse mode from LES snapshots which is found to match the Helmholtz solver prediction. This study confirms the capacity of high-order LES to capture not only low-frequency oscillations but also high-order frequency transverse modes in combustion chambers
LES is used to study self-excited transverse modes in an atmospheric, square combustor (p = 1 bar). Simulations over a range of different mass flow rates show that transverse modes are present for all cases and exhibit varying RMS pressure amplitudes up to 0.4 bar. Analysis of LES results shows that transverse modes are due to a lock-in mechanism between an hydrodynamic unstable mode typical of swirling flows (the PVC mode or Processing Vortex Core) and the cavity modes. A control method using damping compliant walls (named DCWs) is applied to control the acoustic mode in the LES and to characterize the PVC in the absence of acoustic forcing. This method shows that the highest pressure oscillations appear when the PVC frequency is close to the frequency of the first transverse acoustic mode. A 3D Helmholtz solver is then used to predict the stability limits obtained by 3D LES. To capture transverse modes, a new flame transfer function (FTF) formulation is derived where local heat release perturbations are controlled by the orthoradial acoustic velocity fluctuations. The FTF is measured in the LES and when it is included in the Helmholtz solver, it allows to recover the stability zones observed in the LES.
Steady methane/air laminar premixed flames stabilised on a cylindrical bluff body subjected to a continuous rotation are analysed using joint direct numerical simulations (DNS) and experiments. DNS are carried out using a 19 species scheme for methane/air combustion and a lumped model to predict the cylinder temperature. Rotation of the cylinder induces a symmetry breaking of the flow, and leads to two distinct flame branches in the wake of the cylinder. DNS are validated against experiments in terms of flame topologies and velocity fields. DNS are then used to analyse flame structures and thermal effects. The location and structure of the two flames are differently modified by rotation and heat transfer: a superadiabatic flame branch stabilises close to the hot cylinder and burns preheated fresh gases while a subadiabatic branch is quenched over a large zone and anchors far downstream of the cylinder. Local flame structures are shown to be controlled to first order by the local enthalpy defect or excess due to heat transfer between the cylinder and the flow. An analysis of the local wall heat flux around the cylinder shows that, for low rotation speeds, the superadiabatic flame branch contributes to wall heat fluxes that considerably exceed typical values found for classical flame/wall interactions. However, for high rotation speeds, fluxes decrease because the cylinder is surrounded by a layer of burned gases that dilute incoming reactants and shield it from the flame.
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