Benoit Fiorina

Modelling non-adiabatic partially premixed flames using flame-prolongation of ILDM

Many models are now available to describe chemistry at a low CPU cost, but only a few of them can be used to describe correctly premixed, partially premixed and diffusion combustion. One of them is the FPI model that uses two coordinates: the mixture fraction Z and the progress variable c. In this paper, we introduce a new evolution of the FPI method that can now handle heat losses. After a short review of kinetic models used in turbulent combustion, the main features of the new three-dimensional FPI method, in which we introduce a third coordinate for enthalpy h, are presented. First, a one-dimensional radiative premixed flame validation case is presented for a large range of radiative heat losses. Second, we present the results of simulations of two laminar burners. Both the fully and the partially premixed burner simulations give a good estimation of all the flame features such as the flame stabilization (driven by heat losses), the flame structure and the profile of major and minor species.

Three-dimensional boundary conditions for numerical simulations of reactive compressible flows with complex thermochemistry

The Navier-Stokes characteristic boundary conditions (NSCBC) is a very efficient numerical strategy to treat boundary conditions in fully compressible solvers. The present work is an extension of the 3D-NSCBC method proposed by Yoo et al. and Lodato et al. in order to account for multi-component reactive flows with detailed chemistry and complex transport. A new approach is proposed for the outflow boundary conditions which enables clean exit of non-normal flows, and the specific treatment of all kinds of edges and corners is carefully addressed. The proposed methodology is successfully validated on various challenging multi-component reactive flow configurations.

Using self-similar properties of turbulent premixed flames to downsize chemical tables in high-performance numerical simulations

Detailed chemical mechanisms have to be incorporated in turbulent combustion modelling to predict flame propagation, ignition, extinction or pollutant formation. Unfortunately, hundreds of species and thousands of elementary reactions are involved in hydrocarbon chemical schemes and cannot be handled in practical simulations, because of the related computational costs and the need to model the complexity of their interaction with turbulent motions. Detailed chemistry may be handled using look-up tables, where chemical parameters such as reaction rates and/or species mass fractions are determined from a reduced set of coordinates, progress variables or mixture fractions, as proposed in ILDM, FPI or FGM methods. Nevertheless, these tables may require large computer memory spaces and non-negligible access times. This issue becomes of crucial importance when running on massively parallel computers: to implement these databases in shared memories would induce a large number of data exchanges, reducing the overall code performance; on the other hand duplicating databases in every local processor memory may become impossible either for large databases or small local memories. This work proposes to take advantage of the self-similar behaviour of turbulent premixed flames to reduce the size of these chemical databases, specifically when running on massively parallel machines, under the FPI (Flame Prolongation of ILDM) framework. Several approaches to reduce the database are investigated and discussed both in terms of memory requirements and access times. A very good compromise is obtained for methane–air turbulent premixed flames, where the size of the database is decreased by a factor of 1000, while the access time is reduced by about 60%.

Multicomponent real gas 3-D-NSCBC for direct numerical simulation of reactive compressible viscous flows

Abstract The topic of this paper is to propose an extension of the classical one-dimensional Navier–Stokes boundary conditions (1-D-NSCBC) for real gases initially developed by Okong’o and Bellan [1] to a 3-D-NSCBC formulation based on the work of Lodato et al. [2] and Coussement et al. [3] . All the differences due to the real gas formulation compared to the perfect gas formulation proposed in [3] are emphasized. A new way of determining the pressure relaxation coefficient is introduced for handling transcritical flows crossing the boundary. The real gas 3-D-NSCBC are then challenged on several test cases: a two-dimensional subsonic vortex convection, a subsonic supercritical bubble convection and a flame vortex interaction. All these test cases are performed by direct numerical simulation of multicomponent flows. It shows the stability of the boundary conditions without creating any numerical artifact.

A Method To Accelerate LES Explicit Solvers Using Local Time-Stepping

a large disparities of geometrical length scales are often encountered. Inside a combustor, for example, the ratio between the diameter of the injection holes and the size of the entire combustion chamber may present several orders of magnitude. When considering an explicit solver for fully compressible Navier-Stokes equations, the global time step is constrained through a CFL-like condition by the size of the smallest cells in the overall computational domain. Local renement of the injector leads to an inhomogeneous mesh and the former restriction drastically alters the overall solver eciency. A new local time-stepping (LTS) method is proposed to address this issue. The domain is divided into subgrids composed of cells that have similar sizes. Flow equations are simultaneously advanced on each subgrid which have a local time step adapted to satisfy the local CFL condition. The accuracy of the method has been veried on a simple convection case using a test code. The method has also been implemented in a large eddy simulation (LES) explicit solver and successfully tested for an acoustic wave propagation. It has been nally used in the two-dimensional large eddy simulation of a turbulent jet.

Modeling Challenges in Computing Aeronautical Combustion Chambers

This article reviews the modeling challenges for performing Large Eddy Simulations of aero-nautical combustion chambers. Since the kerosene is injected in a liquid phase into the combustion chamber, the description of the atomization is of primary importance. The article first discusses the numerous numerical challenges encountered during this process, which leads to the formation of small droplets that constitute a spray. The existing numerical and modeling methods to describe a spray of kerosene droplets are then presented. The article then focuses on the description of the complex combustion kinetics. Hundreds of species and thousands of reactions have to be considered to predict ignition, flame stabilization and pollutant emissions. Due to lengthy computational times, detailed chemical schemes are too large to be directly used in CFD. This article then presents the major existing chemical reduction strategies. Significant interactions of the reactions layers with the flow vortices occur at the subgrid scale. The question of turbulent combustion modeling is therefore discussed in an LES context. Finally, the prediction of soot and NOx formation is presented. The review is illustrated by several examples representative of practical situations encountered in aeronautical combustors.

Large Eddy Simulation of Swirling Kerosene/Air Spray Flame Using Tabulated Chemistry

Accurate characterization of swirled flames is a key point in the development of more efficient and safer aeronautical engines. The task is even more challenging for spray injection systems. On the one side, spray interacts with both turbulence and flame, eventually affecting the flame dynamics. On the other side, spray flame structure is highly complex due to equivalence ratio inhomogeneities caused by the evaporation process. Introducing detailed chemistry in numerical simulations, necessary for the prediction of flame stabilization, ignition and pollutant concentration, is then essential but extremely expensive in terms of CPU time. In this context, tabulated chemistry methods, expressly developed to account for detailed chemistry at a reduced computational cost in Large Eddy Simulation of turbulent gaseous flames, are attractive. The objective of this work is to propose a first computation of a swirled spray flame stabilized in an actual turbojet injection system using tabulated chemistry. A Large Eddy Simulation of an experimental benchmark, representative of an industrial swirl two-phase air/kerosene injection system, is performed using a standard tabulated chemistry method. The numerical results are compared to the experimental database in terms of mean and fluctuating axial velocity. The reactive two-phase flow is deeper investigated focusing on the flame structure and dynamics.Copyright

A Filtered Tabulated Chemistry Model for Large Eddy Simulation of Reactive Flows

A new model called F-TACLES (Filtered Tabulated Chemistry for LES) is developed to introduce tabulated chemistry methods in LES of turbulent premixed combustion. The main objective is to recover the correct propagation speed of the flltered flame front when the sub-grid scale wrinkling vanishes. The filtered flame structure is mapped by 1-D filtered laminar premixed flames. The methodology is first applied to 1-D filtered laminar flames. Computations show the capability of the model to recover the laminar flame speed and the correct chemical structure when flame wrinkling is fullyresolved on the LES lter scale. The model is then extended to turbulent regimes by introducing sub-grid scale wrinkling effects on the flame propagation. LESs of a 3-D turbulent premixed flame are performed on dierent grids, with different flame filter sizes. Objectives are to analyze the influence of the grid size, the flame filter size and the sub-grid flame wrinkling on the model performances. All these computations are compared to experimental data.