Dominique Thévenin

Laminar premixed hydrogen/air counterflow flame simulations using Flame Prolongation of ILDM with differential diffusion

The cost of including full kinetics in realistic computations remains extremely high. This has led many researchers to develop reduction techniques for the chemistry. These methods are generally valid only in a very limited range of equivalence ratio, pressure, or temperature and require extensive human time to develop the reduced schemes. Recently, an automatic method based on intrinsic low-dimensional manifolds (ILDM) has been proposed. Because of ILDM, the reduction of detailed reaction schemes is much simuplified, leading to the fast generation of look-up tables containing the information corresponding to the reduced chemical schemes. Nevertheless, the ILDM method is not well suited to describe the low-temperature domain, since the dimension and therefore the complexity of the databases increase tremendously in this zone. In this work, we propose a new version of the ILDM method, called flame prolongation of ILDM (FPI), which enables us to solve the problem at low temperatures. We used laminar premixed free flames to extend the manifolds generated with ILDM, thus leading to smooth and accurate evolutions of the species along all the flame front. In order to demonstrate the interest of FPI, we computed the response of a premixed flame to straining using the double-premixed counterflow flame configuration. We show that using FPI, the correct evolution is obtained for all species from almost unstrained flames up to flames near extinction. The computational times are tremendously reduced with FPI in comparison with full chemistry.

Dynamics of flame/vortex interactions

Abstract Vortex interactions with flames play a key role in many practical combustion applications. Such interactions drive a large class of combustion instabilities, they control to a great extent the structure of turbulent flames and the corresponding rates of reaction, they occur under transient operations or when flames travel in ducts containing obstacles. Vortices of various types are often used to enhance mixing, organize the flame region, and improve the flame stabilization process. The analysis of flame/vortex interactions has value in the development of our understanding of basic mechanisms in turbulent combustion and combustion instability. The problem has been extensively investigated in recent years. Progress accomplished in theoretical, numerical and experimental investigations on flame/vortex interactions is reviewed in this article.

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.

Autoignition of turbulent non-premixed flames investigated using direct numerical simulations

Abstract The autoignition of a laminar non-premixed flame placed in a field of homogeneous isotropic turbulence has been studied previously using single-step chemistry and/or simplified models for diffusion processes. The existence of a specific value of the mixture fraction, called “most-reactive,” and the importance of the scalar dissipation rate to predict the ignition location were demonstrated. The effect of the turbulence intensity on the ignition time was found to be non-monotonic. In this work, we wish to assess the influence of more realistic chemistry and transport models on ignition location and time. To do so, direct simulations are carried out using a detailed reaction scheme, multicomponent diffusion velocities and accurate thermodynamic properties. We observe that the turbulent non-premixed flame ignites always faster than the laminar one, even for the highest Reynolds numbers investigated. The scalar dissipation rate can still be used to predict the ignition site, as was observed in simple chemistry simulations. But the most-reactive conditions must of course be determined using the detailed modeling, and cannot any more be analytically predicted. The interest of repeating the direct simulations to get rid of the influence of random initial conditions is also demonstrated.

Optimization and computational fluid dynamics

Generalities and methods.- A Few Illustrative Examples of CFD-based Optimization.- Mathematical Aspects of CFD-based Optimization.- Adjoint Methods for Shape Optimization.- Specific Applications of CFD-based Optimization to Engineering Problems.- Efficient Deterministic Approaches for Aerodynamic Shape Optimization.- Numerical Optimization for Advanced Turbomachinery Design.- CFD-based Optimization for Automotive Aerodynamics.- Multi-objective Optimization for Problems Involving Convective Heat Transfer.- CFD-based Optimization for a Complete Industrial Process: Papermaking.

Development of a parallel direct simulation code to investigate reactive flows

Abstract Solving the Navier-Stokes equations with detailed modeling of the transport and reaction terms remains at the present time a very difficult challenge. Direct simulations of two-dimensional reactive flows using accurate models for the chemical reactions generally require days of computing time on todays most powerful serial vector supercomputers. Up to now, realistic three-dimensional simulations remain practically impossible. Working with parallel computers seems to be at the present time the only possible solution to investigate more complicated problems at acceptable costs, however, lack of standards on parallel architectures constitutes a real obstacle. In this paper, we describe the structure of a parallel two-dimensional direct simulation code using detailed transport, thermodynamic and reaction models. Separating the modules controlling the parallel work from the flow solver, it is possible to get a high compatibility degree between parallel computers using distributed memory and message-passing communication. A dynamic load-balancing procedure is implemented in order to optimize the distribution of the load among the different nodes. Efficiencies obtained with this code on many different architectures are given. First examples of application conceding the interaction between vortices and a diffusion flame are shown in order to illustrate the possibilities of the solver.

Investigations of heat release, extinction, and time evolution of the flame surface, for a nonpremixed flame interacting with a vortex

Flame/vortex interactions to a great extent govern turbulent combustion. Flame roll-up due to vortices is also one of the most important phenomena driving combustion instabilities. An experimental investigation analyzes some fundamental features of a diffusion flame interacting with a vortex ring. A steady nonpremixed counterflow flame of air and hydrogen diluted with nitrogen is first established. A vortex ring is generated from a tube installed in the lower combustor nozzle and impinges on the flame. In the experiment described herein, the visualization of the flame front is achieved by OH planar laser-induced fluorescence (PLIF). The relevance of OH radicals as a marker of the reaction zone is discussed on the basis of direct numerical simulation (DNS) results. Scatter plots of correlations between OH concentration and heat release rate are also presented to derive a criterion of extinction. A detailed description of the interaction is given, showing a global enhancement of combustion due to the interaction with the vortex. Extinction processes occurring later are also described. The evolution of the flame surface during the interaction is extracted from the experimental visualizations. It is shown that extinctions are characterized by a reduction in flame surface area and that this decrease may be hidden by flame stretching and/or roll-up.

Impact of Stents and Flow Diverters on Hemodynamics in Idealized Aneurysm Models

Cerebral aneurysms constitute a major medical challenge as treatment options are limited and often associated with high risks. Statistically, up to 3% of patients with a brain aneurysm may suffer from bleeding for each year of life. Eight percent of all strokes are caused by ruptured aneurysms. In order to prevent this rupture, endovascular stenting using so called flow diverters is increasingly being regarded as an alternative to the established coil occlusion method in minimally invasive treatment. Covering the neck of an aneurysm with a flow diverter has the potential to alter the hemodynamics in such a way as to induce thrombosis within the aneurysm sac, stopping its further growth, preventing its rupture and possibly leading to complete resorption. In the present study the influence of different flow diverters is quantified considering idealized patient configurations, with a spherical sidewall aneurysm placed on either a straight or a curved parent vessel. All important hemodynamic parameters (exchange flow rate, velocity, and wall shear stress) are determined in a quantitative and accurate manner using computational fluid dynamics when varying the key geometrical properties of the aneurysm. All simulations are carried out using an incompressible, Newtonian fluid with steady conditions. As a whole, 72 different cases have been considered in this systematic study. In this manner, it becomes possible to compare the efficiency of different stents and flow diverters as a function of wire density and thickness. The results show that the intra-aneurysmal flow velocity, wall shear stress, mean velocity, and vortex topology can be considerably modified thanks to insertion of a suitable implant. Intra-aneurysmal residence time is found to increase rapidly with decreasing stent porosity. Of the three different implants considered in this study, the one with the highest wire density shows the highest increase of intra-aneurysmal residence time for both the straight and the curved parent vessels. The best hemodynamic modifications are always obtained for a small aneurysm diameter.

IGNITION DYNAMICS OF A DIFFUSION FLAME ROLLED UP IN A VORTEX

This article deals with the dynamics of ignition for a diffusion flame rolled up in a vortex. The flame is formed at the interface between a cold fuel and a hot oxidizer. The calculations are carried out in the thermodiffusive approximation, where all the volumetric expansion effects are neglected. Regimes of ignition are obtained for different vortex Reynolds numbers and oxidizer temperatures. By varying this last parameter, one observes three possible ignition modes: a diffusion flame ignition at moderate air temperatures, a premixed ignition in the core at high temperatures, and a mixed mode involving premixed and diffusion flames in the intermediate temperature range. Results of calculations are interpreted on the basis of one‐dimensional asymptotic analysis of unsteady strained laminar diffusion flames.

Progress in Numerical Combustion

Abstract This article begins with a synthetic presentation of key issues in the numerical description of combustion phenomena. Different levels of combustion modeling are identified and characterized. It is indicated how these modeling levels may be used to deal with fundamental questions or technological applications. Important advances have been made in detailed numerical modeling of complex flames and in direct simulation of flame/turbulence and flame/flow interactions. Results obtained in these areas have been employed to improve physical modeling methods which are currently used to calculate reactive flowfields in practical combustors operating in the turbulent regime. As physical modeling relies on average Navier-Stokes equations it requires closure rules for turbulent fluxes and for mean reaction rates. Considerable effort has been expanded to devise novel closure schemes or improve current models. Progress has been accomplished in the development of probabilistic methods in which the probability d...