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Dissertation
Metamodeling for aerodynamic optimisation of a low-pressure compressor blade
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Year: 2019

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Dissertation
Master thesis : Metamodeling for aerodynamic optimisation of a low-pressure compressor blade
Authors: --- --- --- ---
Year: 2019 Publisher: Liège Université de Liège (ULiège)

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The company Safran Aero Boosters for which this master thesis was conducted is the world leader in conception of low-pressure compressors, also called boosters, for commercial turbofan engines. In a search of continued improvement, the engineers of the company are regularly confronted to the aerodynamic design of blades, a time consuming iterative procedure between the aerodynamics and mechanical teams. Within the conception process, the design space exploration consists in carrying out small modifications to a reference blade and then reassess its performances with long running CFD computations until finding the optimal one. In order to spare computational cost and time, metamodels could be used instead. Constructed on a limited database obtained with CFD computations, the metamodels give predictions on blade performances in an instant and can be intensively employed with optimisation algorithms. 

The main objective of this master thesis is to gain experience in the optimal construction of metamodels. In this context, highly predictive metamodels have been constructed on a large database and the relationships between the design variables modifications and blade performances analysed. In an attempt to reduce the computational cost associated to the creation of metamodels, a comparison of their predictability was made when they are constructed on alternative databases of reduced size. This study concluded that a database of 20 elements obtained with the Latin Hypercube sampling method appeared to be the most adequate choice in this application. 

After that, an enlargement of the design space was considered as modifications on the initial blade induced changes in other design variables. This enlarged design space was once again explored with the metamodels constructed this time with a reduced size database, result of the previous comparison, and with CFD computations on a smaller domain. In fact, with the reduced domain, the blades too different from the initial one encountered issues in convergence of their CFD computations. Nevertheless, the surrogate models were sufficiently predictive for this work and optimised blades were determined. Among those ones, the blade for which was predicted an improvement of both efficiency and operability satisfied all expectations. To finish this thesis, an analysis of the flow around these optimised blades was conducted in order to better understand the reasons behind the performances improvement with the blade geometry modifications.


Dissertation
Master thesis and internship[BR]- Master's thesis : Modelling of Gas Foil Bearing with a high order Discontinuous Galerkin Method[BR]- Integration Internship
Authors: --- --- --- ---
Year: 2022 Publisher: Liège Université de Liège (ULiège)

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In recent years, foil bearings, a special class of gas bearings, began to receive widespread converge in many scientific publications because of their advantages of environmental durability, higher reliability in operation (oil-free), and higher load capacity at high speed. Air Foil Bearing (AFB) supporting direct drive compressors or/and turbines have been gaining popularity in recent years. Mitis SA is developing a new generation of clean energy converters for decentralized Combined Heat and Power (CHP) based on flameless combustion chamber microturbines. Mitis SA is using AFB in their systems to guarantee an oil-free system and increase reliability. 
The overall objective of this work is to contribute to a better understanding of the AFB technology. The work focuses particularly on the behaviour of the lubrication gas film. Numerical simulations of the isothermal, steady state Reynolds equation using the Discontinuous Galerkin Finite Element Method (DG-FEM) are performed. Compared to classical FD and FV methods, such a method provides high accuracy in terms of interpolation and spectral properties on unstructured meshes without opting for large stencils. The work comprises the development and implementation of a numerical prediction code with the use of DG-FEM. The numerical aspect of the hyperbolic-elliptic problem has been addressed with the implementation of specific approaches. First, upwind flux has been proposed for the convective formulation, and then an incomplete internal penalty method (IIPM) has been considered to evaluate the diffusive interface flux. Newton-Raphson method has been used to solve the nonlinear equation. The residual and the inverse of the Jacobian have been computed using direct solver, which is based upon the LU factorization technique. All these strategies were implemented in ForDGe, an immersed boundary, and in turn, Adaptive Mech Refinement (AMR) on multiple order Cartesian grids, still under development at the University of Liège.
The modelling of the foil structure is built based upon existing models. The coupling between structural and fluid parts has been done at the level of the film thickness using the Simple Elastic Foundation Model (SEFM). The validity of the analysis and numerical code has been assessed by comparing predictions to experimental and/or numerical published data.
This work would be recognized as the reference, providing numerical results for GFBs used by Mitis SA and/or any kind of GFBs with variable geometries and different working gases and operating conditions.

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