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Experiments and numerical simulations of interactions between transverse acoustic modes and cryogenic flames

Abstract : The general objective of this research is to contribute to the understanding of fundamental mechanisms leading to high frequency instabilities in liquid rocket engines. The process involves a tight coupling between combustion and transverse acoustic modes of the thrust chamber. This problem is investigated with a combination of experimental, numerical and modeling tools. Experiments are carried out on a model scale combustor comprising multiple coaxial injection units placed in a row and fed with liquid oxygen and gaseous methane. This experiment recreates some of the conditions prevailing in liquid rocket engines. The combustor was designed to allow a clear separation between the longitudinal and tranverse resonant modes. It is equipped with large windows providing optical access to the flames and with pressure transducers detecting fluctuations of this quantity in the chamber and in the propellant injection manifold. A toothed wheel modulator is used to periodically block an auxiliary nozzle and inject acoustic perturbations in the system. Digital imaging techniques are used to examine the flame dynamics. Systematic hot fire tests have been carried out at low (0.9 MPa), intermediate (3 MPa) and high pressure (6 MPa) to determine conditions where the flame is the most receptive to transverse acoustic modulations. A remarkable level of response was observed in the low pressure experiments. The level of oscillation was in that case around 8 \% of the mean pressure. The flame is strongly modified when the coupling takes place with the first transverse mode of the cavity, its spreading rate is augmented and its luminosity is increased. An intriguing reduction of the axial convection velocity is also observed with the high speed camera. Phase relations established between the pressure perturbations and the heat release in the chamber indicate that these two quantities feature similar spatial distributions. The intermediate pressure experiments carried out with a new injection head comprising 5 injectors at a higher rate of heat release indicate that the sharpness of resonance is reduced and that this can be attributed to a more intense level of temperature fluctuations in the system. Cold flow experiments were also carried out to examine the motion of injected streams of liquid oxygen and gaseous nitrogen when they are submitted to a resonant transverse acoustic excitation.
These experiments are complemented with numerical calculations carried out in the large eddy simulation (LES) framework. LES is used to examine the motion of multiple cold jets submitted to a transverse modulation. The oscillation induces a collective motion and mixing is intensified. A model is developed to represent the filtered rate of burning allowing a description of nonpremixed flames controlling cryogenic combustion. Initial calculations are carried out in a realistic multiple injector configuration fed with gaseous reactants. Two problems are envisaged on the modeling level. The first aims at describing how heat release fluctuations can be generated by tranverse velocity perturbations. An expression is devised which depends on the transverse velocity perturbation and on the sign of its gradient and its consequences are investigated. It is shown in particular that the model retrieves the pattern of heat release observed in some early experiments. The second model deals with the influence of temperature fluctuations on the resonance characteristics of a system. Direct simulation and analysis based on the method of averaging indicates that the response amplitude and the resonance sharpness are diminished in the presence of fluctuations, a phenomenon which seems to have been overlooked in the past but may have practical consequences. The knowledge gathered in these studies is intended to provide guidelines for further developments of computational tools aimed at the prediction of instabilities. It can also serve to develop design methods which would avoid the phenomenon.
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Franck Richecoeur. Experiments and numerical simulations of interactions between transverse acoustic modes and cryogenic flames. Electric power. Ecole Centrale Paris, 2006. English. ⟨tel-00345931⟩

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