This article describes recent progress on premixed flame dynamics interacting with acoustic waves. Expressions are derived to determine the stability of combustors with respect to thermoacoustic oscillations. The validity of these expressions is general, but they are illustrated in laminar systems. Laminar burners are commonly used to elucidate the response of premixed flames to incoming flow perturbations, highlight the role of acoustic radiation in their stability, identify modes associated with thermoacoustic intrinsic instabilities and decipher the leading mechanisms in annular systems with multiple injectors. Many industrial devices also operate in a laminar premixed mode such as, for example, domestic gas boilers and heaters equipped with matrix burners for material processing in which unconfined flames are stabilized at one extremity of the system. This article proposes a systematic approach to determine the stability of all these systems with respect to thermoacoustic oscillations by highlighting the key role of the burner impedance and the flame transfer function (FTF). This transfer function links in frequency space incoming flow perturbations to heat release rate disturbances. This concept can be used in the turbulent flame case as well. Weakly nonlinear stability analysis can also easily be conducted by replacing the FTF by a flame describing function in the expressions derived in this work. The response of premixed flames to harmonic mixture compositions and flow-rate perturbations is then revisited and the main parameters controlling the FTF are described. A theoretical framework is finally developed to reduce the system thermoacoustic sensitivity by tailoring the FTF.

Numerical simulations and perturbation analysis of a radially imploding laminar premixed flame are used to study the mechanisms responsible for the generation of pressure fluctuations at flame fronts for various Lewis numbers. The relative importance of mechanisms based on unsteady heat release and on vorticity is investigated using an optimization methodology. Particular attention is paid to the influence of non-axisymmetric conditions and local flame curvature. It is shown that vorticity-based noise generation prevails for high-wavenumber, non-axisymmetric disturbances at all curvatures, while heat-release-driven noise generation dominates the axisymmetric and low-wavenumber regimes. These results indicate that short-wavelength vorticity waves actively participate in flame acoustic activity and can surpass acoustic output mechanisms based on heat-release fluctuations in the vicinity of the flame front.

Heat release rate fluctuations are the source of combustion instabilities that causerecurrent problems in many combustors. These quantities are difficult to measure andoptical techniques are generally implemented. The initial validation of an alternativemethod is presented in the case of pulsated laminar premixed flames. The technique relieson determining the travel time of ultrasonic waves propagating through the flow. The linkbetween heat release rate fluctuations and perturbations in the travel time of ultrasonicwaves is established. Measurements made with this method are compared with opticalmeasurements and an analytical model. A good agreement is obtained for the gain and phaseof the transfer function between perturbations in the sound travel time and heat releaserate disturbances when the flame is submitted to harmonic modulations of the flow. Thiswork validates the principle of the proposed technique and lays the foundations for futuredevelopments.

95Mo magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy is surprisingly sensitive to the local environment of tetrahedral molybdate species. A series of compounds related to expected crystallization products in nuclear waste glasses are probed to calibrate their spectral characteristics. Glasses formed with fast and slow quenching show a glassy peak corresponding to tetrahedral molybdate species. With slow quenching, a prominent sharp peak is observed, representing crystallinity. In sodium-borosilicate glasses with 2.5 mol% MoO3, the sharp peak corresponds to pure crystalline sodium molybdate. Cesium-sodium and lithium-sodium borosilicate glasses with Mo show crystalline peaks as well, and suggest that NMR may potentially be used to characterize mixed-cation molydates and more complex phase assemblages. While precise quantification of Mo in different phases is likely to be time-consuming, reasonable estimates can be obtained routinely, making 95Mo MAS NMR a useful tool for investigating phase separation and crystallization in model nuclear waste materials.