PhD defense - B. Franzelli

Impact of the chemical description on direct numerical simulations  and large eddy simulations of turbulent combustion in industrial aero-engines

Delivered by INP Toulouse
Speciality: Energy and Transfers


September 19, 2011 - CERFACS

A growing need for numerical simulations based on reliable chemistries has been observed in the last years in order to develop new technologies which could guarantee the reduction of the enviromental impact on air transport. The description of combustion requires the use of detailed kinetic mechanisms for most hydrocarbons. Their use in turbulent combustion simulation is still prohibitive because of their high computational cost. Reduced chemistries and tabulation methods have been proposed to overcome this problem. Since all these reductions have been developed for laminar configurations, this thesis proposes to evaluate their performances in simulations of turbulent configurations such as a DNS of a premixed Bunsen methane/air flame and a LES of an experimental PRECCINSTA burner. The mechanisms are analysed in terms of flame structure, global burning parameters, flame length, prediction of major species concentrations and pollutant emissions.
An a priori methodology based on one-dimensional unstrained and strained laminar flames to evaluate the mechanism capability to predict three-dimensional turbulent flame features is therefore proposed. On the one hand when building a new reduced scheme, its requirements should be fixed compromising the computational cost, the robustness of the chemical description and the desired quality of results. On the other hand, the quality of DNS or LES results in three-dimensional configurations could be anticipated testing the reduced mechanism on  laminar one-dimensional premixed unstrained and strained flames.
Today, much of the current effort in combustion noise is the development of efficient numerical tools to calculate the noise radiated by flames. Although unsteady CFD methods such as LES or DNS can directly provide the acoustic field radiated by noise sources, this evaluation is limited to small domains due to high computational costs. Hybrid methods have been developed to overcome this limitation. In these schemes, the noise sources are decoupled from the radiated sound. The sources are still calculated by DNS or LES solvers whereas the radiated sound is evaluated by acoustic tools using an acoustic analogy.
In the present study, a numerical tool based on the Phillips analogy for low Mach number flows has been developed. This tool accounts for the role of the boundary conditions in the resulting acoustic field. Both LES and the acoustic solver developed here are used to assess the noise produced by a turbulent swirl-stabilized flame generated in a staged combustor. Good agreement is obtained between both techniques as long as the appropriate quantities are compared: the pressure signal obtained directly from LES contains a non negligible amount of hydrodynamic fluctuations that must be removed when a suitable comparison is sought with the acoustic solver.
The low Mach number assumption is completely realistic when considering the flow within a combustion chamber; it also allows for considerable simplifications when dealing with acoustic analogies. However, it cannot be used for the upstream (air-intake, compressors) and downstream (turbines, nozzle) sections of an aeronautical combustion chamber. A numerical tool is developed based on the quasi-1D Linearized Euler Equations in order to account for convective, non-isentropic and non-isenthalpic flows. By means of this tool, it is possible to estimate the acoustic boundary conditions that should be imposed at the inlet/outlet of a given combustion chamber when performing low-Mach number acoustic computations.

O. Gicquel	Professor - Ecole Centrale de Paris	Referee
H. Pitsch	Professor - RWTH Aachen, Germany	Referee
W. Jones	Professor - Imperial College, London	Member
J-F. Pauwels	Professor - Université Lille 1	Member
E.S. Richardson	Senior Researcher - University of Southampton	Member
A. Roux	Engineer - Turbomeca	Member
B. Cuenot	Senior researcher - CERFACS	Advisor

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