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Turbulent multiphase Large Eddy Simulation of reheat combustion

MEGEP (Mécanique, Energétique, Génie civil & Procédés) [Subject to defense authorization]

Lien Youtube (soon)

Reheat combustors – also called afterburners or sequential combustors – are used in certain aircraft turbojet engines and industrial gas turbines. The focus of this work, aircraft reheat chambers are placed downstream of the low pressure turbine stages for supersonic aircraft and they are used to produce additional thrust for specific parts of the flight envelope by burning additional fuel using the residual oxygen in the core flow. As the requirements of jet engines for global efficiency, thrust and emissions are ever-evolving, afterburner designs have to be adapted accordingly. The complex multi-scale flow physics inside reheat chambers poses several challenges for the development of new chamber designs and architectures. To overcome these limitations, Large Eddy Simulations is a very promising tool but it needs additional developments to increase its fidelity for the flow conditions found in afterburners. In particular, the accurate prediction of the liquid kerosene phase poses problems as the direct resolution of the liquid phase is not affordable computationally, so an Euler-Lagrange approach is adapted in this work. However, the complex interaction of the high speed gas with the Lagrangian liquid phase needs additional consideration for the particular operating conditions found in afterburner, as most of these models were developed for conventional combustion chambers. In the first part this thesis, the modeling of the injection of kerosene using jet-in-crossflow (JICF) injectors in a high speed, high temperature, vitiated environment, typical of reheat chambers is treated. It is shown that the interaction of the JICF and also the individual droplets with the gaseous flow is stronger than what is typically seen in conventional chambers, and Lagrangian models have to be modified to correctly represent the liquid phase in reheat conditions. Second, diffusion flames are treated as they play a major role in afterburners due to the poor mixing of the fuel vapor with the vitiated air stream. As a first step, the chemical description of diffusion flames is investigated. A 2-step kerosene-air scheme, originally developed for premixed reheat flames, is adapted for diffusion flames by optimizing its transport properties using asymptotic theory of counterflow-diffusion flames. Then, the treatment of turbulent diffusion flames is studied in the context of “no-model” LES of diffusion flames. It is shown that the mesh and numerical stabilizing operators have a very strong influence on no-model diffusion flames, to the point where physics becomes secondary. An approach based on thickening, called Diffusion Flame Thickness Control (DFTC) is proposed to remedy the so-called under-resolution issues and resolve the diffusion flame fronts on sufficient number of grid points. For each topic, the developed models are validated by simulating the 3D postcombustion-relevant experimental test bench of Georgia-Tech, showing the applicability of the developments in a complex, multi-phase and multi-regime reheat combustor.

Jury

Pr. Ronan Vicquelin	Université Paris-Saclay	Reviewer, Chair of the Jury
Pr. Antonio Andreini	Universitá degli studi di Firenze	Reviewer
Pr. Simone Hochgreb	University of Cambridge	Examiner
Dr. Guillaume Ribert	INSA Rouen Normandie	Examiner
Dr. Julien Tillou	Safran Aircraft Engines	Invited
Dr. Thierry Poinsot	CERFACS	Thesis supervisor
Dr. Laurent Gicquel	CERFACS	Thesis co-supervisor