Etude numérique et modélisation de la modulation de la turbulence dans un écoulement de nappe chargée de particules

Ce travail de thèse est consacré à l’étude numérique et théorique de la modulation de la turbulence par des particules. Cette étude s’appuie sur des résultats issus de simulations de type Euler/Lagrange qui résolvent directement les équations instantanées de la phase gazeuse et effectuent un suivi de trajectoires des particules. La configuration étudiée représente une nappe de particules injectées à haute vitesse dans une turbulence homogène isotrope décroissante. Le mouvement des particules est supposé uniquement gouverné par la force de traînée visqueuse. Le chargement en particules est suffisamment important pour que les particules influent sur la phase gazeuse (couplage inverse) mais suffisamment faible pour pouvoir négliger les collisions interparticulaires. Une analyse des équations de transport des principales grandeurs moyennes de l’écoulement est menée pour déterminer les effets directs et indirects des particules sur la turbulence fluide. L’étude des transferts d’énergie entre phases montre que la présence des particules tend à détruire la turbulence gazeuse au centre de la nappe et à l’augmenter à la périphérie. Ce dernier effet est causé par la forte corrélation entre la distribution de particules et la vitesse instantanée du gaz. Le modèle k - ε est ensuite étudié et la validité de ses hypothèses de fermeture en écoulement diphasique est éprouvée à l’aide de tests a priori. Une nouvelle formulation de type viscosité turbulente, fonction des paramètres diphasiques, est utilisée pour modéliser le tenseur de Reynolds du gaz. Une équation de Langevin diphasique est également testée pour modéliser les équations de vitesse de dérive et de covariance des fluctuations de vitesse fluide-particules. ABSTRACT : This work is devoted to the numerical and theoretical study of turbulence modulation by particles using direct numerical simulation for the continuous phase coupled with a Lagrangian prediction of trajectories of discrete particles. The configuration corresponds to a slab of particles injected at high velocity into an isotropic decaying turbulence. The motion of a particle is supposed to be governed only by the drag force. The particle mass loading is large so that momentum exchange between particles and fluid results in a significant modulation of the turbulence. Collisions are neglected. The momentum transfer between particles and gas causes a strong acceleration of the gas in the slab. In the periphery of the slab, the turbulence is enhanced due to the production by the mean gas velocity gradients. The analysis of the interphase transfer terms in the gas turbulent kinetic energy equation shows that the direct effect of the particles is to damp the turbulence in the core of the slab but to enhance it in the periphery. This last effect is due to a strong correlation between the particle distribution and the instantaneous gas velocity. Another issue concerns the k-ε model and the validity of its closure assumptions in twophase flows. A new eddy viscosity expression, function of particle parameters, is used to model the Reynolds stress tensor. The modelling of the gas turbulent dissipation rate is questioned. A two-phase Langevin equation is also tested to model drift velocity and fluid-particles velocity covariance equation

LES of knocking in engines using dual heat transfer and two-step reduced schemes

Large Eddy Simulation of knocking in piston engines requires high-fidelity physical models and numerical techniques. The need to capture temperature fields with high precision to predict autoignition is an additional critical constraint compared to existing LES in engines. The present work presents advances for LES of knocking in two fields: (1) a Conjugate Heat Transfer (CHT) technique is implemented to compute the flow within the engine over successive cycles with LES together with the temperature field within the cylinder head walls and the valves and (2) a reduced two-step scheme is used to predict both propagating premixed flames as well as autoignition times over a wide range of equivalence ratios, pressures and temperatures. The paper focuses on CHT which is critical for knocking because the gas temperature field is controlled by the wall temperature field and knocking is sensitive to small temperature changes. The CHT LES is compared to classical LES where the temperatures of the head and the valves are supposed to be homogeneous and imposed empirically. Results show that the skin temperature field (which is a result of the CHT LES while it is a user input for classical LES) is complex and controls knocking events. While the results of the CHT LES are obviously better because they suppress a large part of the empirical specification of the wall temperatures, this study also reveals a difficult and crucial element of the CHT approach: the description of exhaust valves cooling which are in contact with the engine head for part of the cycle and not in the rest of the cycle, leading to difficulties for heat transfer descriptions between valves and head. The CHT method is successfully applied to an engine studied at IFP Energies Nouvelles where knocking characteristics have been studied over a wide range of conditions

Theoretical analysis and simulation of methane/air flame inhibition by sodium bicarbonate particles

The capacity of sodium bicarbonate (NaHCO3)s powder to chemically reduce flame speeds and mitigate the effects of accidental explosions is well established. The inhibition of premixed hydrocarbon/air flames by monodisperse (NaHCO3)s solid particles is investigated, here, using theory and numerical simulations. First, an analytical solution for the temperature history of a solid (NaHCO3)s particle crossing a flame shows that the size of the largest (NaHCO3)s particle which can decompose inside the flame front, and act on chemical reactions efficiently, strongly depends on the flame speed. For various fuels and a wide range of equivalence ratios, particles with a strong potential for flame inhibition are identified: hence a criterion, on the maximum particle size, for efficient inhibition is proposed. Thereafter, a one-dimensional methane/air flame traveling in a premixed gas loaded with sodium bicarbonate is simulated using a chemical mechanism based on GRI-Mech, extended to include inhibition chemistry and reduced to 20 species with a DRGEP method (Pepiot-Desjardins and Pitsch, 2008). Inhibitor particle size and mass loading are varied to study the flame response to inhibition by (NaHCO3)s powders. Finally, two-dimensional simulations of a planar flame traveling in a flow with a non-uniform inhibitor mass loading distribution are analyzed. In the case of strong particle stratification, an acceleration of the flame is observed, instead of a mitigation. This fundamental mechanism may limit the actual potential of inhibition powders in real configurations

LES of explosions in venting chamber: A test case for premixed turbulent combustion models

This paper presents a new experimental and Large Eddy Simulation (LES) database to study upscaling effects in vented gas explosions. The propagation of premixed flames in three setups of increasing size is investigated experimentally and numerically. The baseline model is the well-known laboratory-scale combustion chamber from Sydney (Kent et al., 2005; Masri et al., 2012); two exact replicas at scales 6 and 24.4 were set up by GexCon (Bergen, Norway). The volume ratio of the three setups varies from 1 to more than 10,000, a variation unseen in previous experiments, allowing the exploration of a large range of Reynolds and Damköhler numbers. LES of gaseous fully premixed flames have been performed on the three configurations, under different operating conditions, varying the number of obstacles in the chamber, their position and the type of fuel (hydrogen, propane and methane). Particular attention is paid to the influence of the turbulent combustion model on the results (overpressure, flame front speed) comparing two different algebraic sub-grid scale models, the closures of Colin et al. (2000) and Charlette et al. (2002), used in conjunction with a thickened flame approach. Mesh dependency is checked by performing a highly resolved LES on the small-scale case. For a given scale and with a fixed model constant, LES results agree with experimental results, for all geometric arrangement of the obstacles and all fuels. However, when switching from small-scale cases to medium-scale or large-scale cases this conclusion does not hold, illustrating one of the main deficiencies of these algebraic models, namely the need for an a priori fitting of the model parameters. Although this database was initially designed for safety studies, it is also a difficult test for turbulent combustion models

Large-Eddy Simulation and experimental study of cycle-to-cycle variations of stable and unstable operating points in a spark ignition engine

This article presents a comparison between experiments and Large-Eddy Simulation (LES) of a spark ignition engine on two operating points: a stable one characterized by low cycle-to-cycle variations (CCV) and an unstable one with high CCV. In order to match the experimental cycle sample, 75 full cycles (with combustion) are computed by LES. LES results are compared with experiments by means of pressure signals in the intake and exhaust ducts, in-cylinder pressure, chemiluminescence and OH Planar Laser Induced Fluorescence (PLIF). Results show that LES is able to: (1) reproduce the flame behavior in both cases (low and high CCV) in terms of position, shape and timing; (2) distinguish a stable point from an unstable one; (3) predict quantitatively the CCV levels of the two fired operating points. For the unstable case, part of the observed CCV is due to incomplete combustion. The results are then used to analyze the incomplete combustion phenomenon which occurs for some cycles of the unstable point and propose modification of the spark location to control CCV

Large Eddy Simulation of Vented Deflagration

In order to understand gas explosion phenomena in industrial buildings, a reduced-scale vented combustion 6 chamber is investigated numerically. In this configuration, a flame is ignited in an initially quiescent flammable mixture and 7 propagates past solid obstacles, generating a strong pressure increase. The aim of this numerical study is twofold: The first 8 objective is to show how large eddy simulation manages to reproduce the parameters of critical relevance for this multiscale 9 problem, in particular the overpressure generated during the flame propagation. The second objective is to highlight that, even if 10 large- to small-scale turbulence effects play a crucial role in the flame development and the resulting overpressure, it is also 11 needed to correctly account for thermo-diffusive scale phenomena

Numerical Methods and Turbulence Modeling for LES of Piston Engines: Impact on Flow Motion and Combustion

In this article, Large Eddy Simulations (LES) of Spark Ignition (SI) engines are performed to evaluate the impact of the numerical set-up on the predicted flow motion and combustion process. Due to the high complexity and computational cost of such simulations, the classical set-up commonly includes "low" order numerical schemes (typically first or second-order accurate in time and space) as well as simple turbulence models (such as the well known constant coefficient Smagorinsky model (Smagorinsky J. (1963) Mon. Weather Rev. 91, 99-164). The scope of this paper is to evaluate the feasibility and the potential benefits of using high precision methods for engine simulations, relying on higher order numerical methods and state-of-the-art Sub-Grid-Scale (SGS) models. For this purpose, two high order convection schemes from the Two-step Taylor Galerkin (TTG) family (Colin and Rudgyard (2000) J. Comput. Phys. 162, 338-371) and several SGS turbulence models, namely Dynamic Smagorinsky (Germano et al. (1991) Phys. Fluids 3, 1760-1765) and sigma (Baya Toda et al. (2010) Proc. Summer Program 2010, Stanford, Center for Turbulence Research, NASA Ames/Stanford Univ., pp. 193-202) are considered to improve the accuracy of the classically used Lax-Wendroff (LW) (Lax and Wendroff (1964) Commun. Pure Appl. Math. 17, 381-398) - Smagorinsky set-up. This evaluation is performed considering two different engine configurations from IFP Energies nouvelles. The first one is the naturally aspirated four-valve spark-ignited F7P engine which benefits from an exhaustive experimental and numerical characterization. The second one, called Ecosural, is a highly supercharged spark-ignited engine. Unique realizations of engine cycles have been simulated for each set-up starting from the same initial conditions and the comparison is made with experimental and previous numerical results for the F7P configuration. For the Ecosural engine, experimental results are not available yet and only qualitative comparisons are performed to enforce the analysis and conclusions made on the F7P configuration. Regarding SGS models, only slight differences are found at the aerodynamic level even if sigma allows a better resolution of small structures of the velocity field. However, all results are in cycle-to-cycle variability envelopes from Granet (Granet et al. (2012) Combust. Flame 159, 1562-1575) and these single cycle computations don’t permit to distinguish clear improvements on macroscopic parameters such as resolved kinetic energy, heat release or mean in-cylinder pressure. Concerning numerical schemes, TTG schemes also allow a slighlty better resolution of small scale vortices but global quantities such as resolved kinetic energy and SGS viscosity are comparable. Nevertheless, clear differences appear between the different schemes in the combustion stroke. This is attributed to a better resolution of the flame-turbulence interaction process during the free flame propagation period, leading to an increase of the resolved part of heat release. It is also shown in this paper that an adjustment of the efficiency constant in the Thickened Flame (TF) model is compulsory to account for the over dissipation of the smallest resolved structures ifLWis used. In the light of these conclusions an hybrid setup, called ES O2 (Engine Stroke Optimal Order), which consists in using TTGC during combustion and LW elsewhere is proposed and applied to the two engines configurations. Results are in good agreement with the ones obtained in the case of a full TTGC simulation, while the CPU (Central Processing Unit) cost increase is only about 10% compared to LW. The accuracy of LW seems therefore to be sufficient for pure aerodynamic phases, while the use of TTGC only during combustion permits an improvement in the LES quality. The hybrid ES O2 method thus appears as an attractive approach to improve further calculations accuracy without being greatly penalized by additional CPU costs in multi-cycle simulations

Direct numerical simulations and models for hot burnt gases jet ignition

This work uses multiple three-dimensional Direct Numerical Simulations (DNSs) to i) investigate the ig- nition process of a cold lean premixed mixture at atmospheric conditions by a jet of hot burnt gases that may be cooled before injection ii) evaluate models able to predict the outcome of such a scenario in terms of ignition. Understanding and being able to model ignition of cold premixed mixtures by hot burnt gases is essential to design systems like engines (to ensure ignition) and flameproof enclosures (to prevent ignition). Limited work has focused on the combined effects of the jet injection speed and temperature on ignition. This is difficult to do by using experiments only and DNS is a natural approach to gain knowledge on that point. By varying the hot jet injection speed and temperature, the three- dimensional, kinetically detailed, DNSs allow a parametric study of the impact of these parameters on the ignition process and provide data to build and test models. Simulations prove that jet injection speed and temperature (usually less than the adiabatic flame temperature because of cooling effects through the injection hole) directly govern ignition. Chemical Explosive Mode Analysis (CEMA) is used to char- acterize the reacting flow structure which is strongly impacted by the jet injection speed. Based on the DNSs conclusions, a zero-dimensional Lagrangian model where a small element of the jet burnt gases mixes at a certain rate with the fresh gases while it potentially ignites is found to be a good candidate to predict the outcome of an ignition sequence (success or failure)

Influence of kinetics on DDT simulations

Deflagration to Detonation Transition (DDT) is an intricate problem that has been tackled numerically, until recently, using single-step chemical schemes. These studies (summarized in Oran and Gamezo, 2007) [1] showed that DDT is triggered when a gradient of reactivity forms inside a pocket of unreacted material. However, recent numerical simulations of hydrogen/air explosions using detailed reaction mechanisms (Liberman et al., 2010; Ivanov et al., 2011) [2], [3] showed that detonation waves can emerge from the flame brush, unlike what was usually seen in the single-step simulations. The present work focuses on chemistry modeling and its impact on DDT. Using the idealized Hot Spot (HS) problem with constant temperature gradient, this study shows that, in the case of hydrogen/air mixtures, the multi-step chemical description is far more restrictive than the single-step model when it comes to the necessary conditions for a hot spot to lead to detonation. A gas explosion scenario in a confined and obstructed channel filled with an hydrogen/air mixture is then considered. In accordance with the HS analysis, the Zeldovich’s (1970) mechanism [4] is responsible for the detonation initiation in the single-step case, whereas another process, directly involving the deflagration front, initiated DDT in the complex chemistry case. In the latter, a shock focusing event leads to DDT in the flame brush through Pressure Pulse (PP) amplificatio

Time scale analysis of the homogeneous flame inhibition by alkali metals

A time scale analysis of the homogeneous flame inhibition problem is carried out to identify the main param- eters controlling the gas phase chemical interaction of the alkali metal inhibitors with the flame chemistry. First, kinetic sub-models for the interaction of alkali metals with the flame are analyzed to show that a simplified 2-step inhibition cycle can capture the essential features of this interaction. Second, it is shown that this cycle is auto-catalytic, which explains the high efficiency of alkali metals in inhibiting flames even at low concentrations. Third, the time scales associated to this inhibition cycle are linked to the free flame termina- tion time scale via a non-dimensional parameter characterizing the efficiency of an inhibitor at promoting radical scavenging. It is shown that this parameter accounts for the main trends observed in the literature and can also be used to provide estimates for the chemical flame suppression limit