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The 4 December 2019 at 10h00

PhD defense – Gauthier WISSOCQ: “Investigation of lattice Boltzmann methods for turbomachinery secondary air system simulations”

Jean-François BOUSSUGE |  JCA CONFERENCE ROOM, CERFACS, Toulouse, France |  

Abstract

Over the past decades, the optimization of turbomachinery efficiency has led to a constant increase in the air temperature of the primary vein. Nonetheless, large temperatures can considerably reduce the engine’s lifetime due to excessive thermal loads or uncontrolled clearances affected by thermal dilatation. An efficient and well-designed cooling system is therefore necessary. This is the role of the \textit{bore cooling} circuit, composed of successive rotating cavities in which a competition takes place between inertia, temperature gradients and forced convection induced by an axial throughflow. These phenomena give birth to complex, unsteady, non-axisymmetric flows of a priori unknown periodicity. Simulating such flows is a major challenge for numerical modeling since it requires solvers adapted to long and three-dimensional unsteady computations. The present thesis is devoted to the investigation of a particular method for the simulation of such flows: the lattice Boltzmann method (LBM). It combines the advantages of being inherently unsteady, relatively fast and perfectly adapted to complex three-dimensional geometries.
First, a study of instabilities occurring in rotating cavities subject to radial temperature gradients is proposed. To that end, linear stability analyses based on a local approach are applied to cases of annular geometries, representative of the axial planes of the rotating cavities. They are used to determine the flow structure in a linear regime as well as the critical Rayleigh and Reynolds numbers for instability occurrence. However, these analyses do not account for the non-linear effects of the limit cycle, which requires another method.
The thesis then focuses on the potential of the LBM for such simulations. A detailed investigation of the numerical instabilities that may occur under the conditions of application of the method is proposed. A particular methodology developed during this thesis, based on von Neumann’s approach, enables us to clearly identify the waves propagated by the scheme and underlines the numerical phenomena at the origin of instabilities. This study highlights the effect of many parameters on numerical stability such as the choice of the lattice and the collision model. A further analysis of regularized models highlights two fundamental properties of these schemes that have a strong influence on numerical stability for subsonic flows.
Applications of LBM to rotating cavity flows are finally performed. The commercial software PowerFLOW is used, being the only LBM solver mature enough to model perfect gases. The code is evaluated on academic cases of increasing complexity (two-dimensional cavity, closed cavity and rotating cavity with cooling throughflow). The results are compared with linear stability analyses, computations from the literature and experimental data. Finally, a multi-stage configuration is simulated, for which a conjugate heat transfer coupling (CHT) is carried out in order to take radiative transfers into account and make best use of the experimental data. The results highlight very good estimates of temperature distributions, hinting towards a good modelling of the complex phenomena contributing to heat transfer.

Jury

Pierre Sagaut                     Aix-Marseille Université (France)                                  Advisor

Jean-François Boussuge   CERFACS Toulouse (France)                                       Co advisor

Florian De Vuyst                UTC, Compiègne (France)                                             Referee

Tony Arts                           Von Karman Institute, Rhode St Genèse (Belgique)      Referee

Françoise Bataille              Université de Perpignan (France)                                  Member

Nicolas Gourdain              ISAE-Supaero, Toulouse (France)                                  Member

Farid Benyoucef                Safran AE Villaroche (France)                                       Invited member

 

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