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Multiphysical modelling and simulation of the ignition transient of complete solid rocket motors

Abstract : Solid rocket motors (SRMs) use the combustion of a solid material, the propellant, as an energy source.A crucial step in the operation of such an engine is its ignition, during which the surface of the propellant must be heated by about 400 degrees to initiate combustion.This is usually done by letting a hot jet impact the surface.The ignition of an SRM involves a wide variety of phenomena, including: combined heat transfer between the igniter gases and the propellant, pyrolysis of the propellant below the surface, release of gaseous species that burn in an intense flame attached to the surface, heating of the propellant by radiation emitted from the gas phase, compressible multiphase flow in the combustion chamber, supersonic flow in the nozzle.The multiphysical nature and the disparities in space-time scales make it impossible to simulate ignition using a single tool that would include a complete modelling of all the phenomena. Typically, the propellant flame is so thin that it cannot be reasonably resolved in a CFD mesh for a complete motor. In addition, it involves stiff and potentially complex chemical kinetics.This is why the classical approach is to use a 1D model of the propellant combustion, at each boundary face of the CFD mesh belonging to the propellant surface. Thus, all the physico-chemical and numerical complexity of solving this combustion is encapsulated in a dynamic boundary condition. However, the existing 1D models are very simplified, impacting the fidelity of the reproduction of ignition in some motors.In this thesis, we choose to use a more advanced 1D approach, which includes a numerical resolution of the flame, able to use complex or global kinetics. Specific attention is paid to the mathematical analysis of the 1D model in steady state, through the study of a travelling combustion wave, clarifying the notion of eigenvalue for the regression speed. To simulate unsteady combustion, a semi-discretisation in space is obtained by the method of lines. The differential-algebraic nature of the resulting system of equations is clearly exposed, allowing for the choice of efficient integration methods to solve the propellant dynamics with high order in time and adaptive time step.In order to ensure an accurate reproduction of the ignition of different propellants, an optimisation process is developed to automatically parameterise the model, allowing for a good agreement between experimental and simulated ignition times.The 1D model is then coupled with the semi-industrial 3D CFD code CEDRE from ONERA, in order to allow for the simulation of ignition in complete motors. The coupling is initially operated at order 1, but techniques are suggested to allow for a higher-order and adaptive coupling.In order to verify the effect of the 1D representation of the flame, a more detailed but more expensive coupling is also developed, where the flame is solved in the CFD code itself.The comparison of the two approaches on an academic 2D configuration allows to verify the consistency and accuracy of the new approach.The coupling between the 1D model and the CFD code developed during this thesis and the interdisciplinary approaches used offer new perspectives both for the development of mathematical tools for adaptive coupling strategies for a wide range of applications, allowing to optimise the accuracy and the cost of the computations, as well as for a better reproduction of ignition in various motors.
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Submitted on : Tuesday, May 17, 2022 - 4:05:12 PM
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Laurent Francois. Multiphysical modelling and simulation of the ignition transient of complete solid rocket motors. Numerical Analysis [cs.NA]. Institut Polytechnique de Paris, 2022. English. ⟨NNT : 2022IPPAX004⟩. ⟨tel-03670668⟩

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