26 research outputs found
Detailed Numerical Simulation of Multi-Dimensional Plasma Assisted Combustion
Interaction between flames and plasmas are the guiding thread of this work. Nanosecond Repetitively Pulsed (NRP) discharges are non-thermal plasmas which have shown interesting features for combustion control. They can interact with flames not only through heat, but also chemically by producing active species. In this work, fully-coupled plasma assited combustion simulations are targeted. To achieve this goal, plasma discharge capabilities are built in the low temperature plasma code, AVIP. The corresponding numerical methods, as well as validation cases regarding each set of equations, are first presented. To simulate plasma discharges, the coupled drift-diffusion equations and the Poisson equation are considered. AVIP is coupled to the AVBP code which solves the reactive Navier-Stokes equations to describe combustion phenomena. In a second part, we start by constructing and validating a fully-detailed chemistry for methane-air mixtures in zero-dimensional reactors before reducing it for multi dimensional simulations. The multi-dimensional streamer simulation capabilities of the code are then assessed using simple chemistries. All the validated parts of the code come together in a fully detailed simulation of ignition using NRP discharges. We finish by discussing phenomenological models built upon the knowledge that we gained from fully-detailed simulations. In a last part, finally, attempt to solve the Poisson and generalized Poisson equations using neural networks, which have a potential for speedup compared to classical linear solvers, is carried out
High-order methods for diffuse-interface models in compressible multi-medium flows: a review
The diffuse interface models, part of the family of the front capturing methods, provide an efficient and robust framework for the simulation of multi-species flows. They allow the integration of additional physical phenomena of increasing complexity while ensuring discrete conservation of mass, momentum, and energy. The main drawback brought by the adoption of these models consists of the interface smearing, increasing with the simulation time, therefore, requiring a counteraction through the introduction of sharpening terms and a careful selection of the discretization level. In recent years, the diffuse interface models have been solved using several numerical frameworks including finite volume, discontinuous Galerkin, and hybrid lattice Boltzmann method, in conjunction with shock and contact wave capturing schemes. The present review aims to present the recent advancements of high-order accuracy schemes with the capability of solving discontinuities without the introduction of numerical instabilities and to put them in perspective for the solution of multi-species flows with the diffuse interface method.Engineering and Physical Sciences Research Council (EPSRC): 2497012.
Innovate UK: 263261.
Airbus U
Modeling and Simulation of Metallurgical Processes in Ironmaking and Steelmaking
In recent years, improving the sustainability of the steel industry and reducing its CO2 emissions has become a global focus. To achieve this goal, further process optimization in terms of energy and resource efficiency and the development of new processes and process routes are necessary. Modeling and simulation have established themselves as invaluable sources of information for otherwise unknown process parameters and as an alternative to plant trials that involves lower costs, risks, and time. Models also open up new possibilities for model-based control of metallurgical processes. This Special Issue focuses on recent advances in the modeling and simulation of unit processes in iron and steelmaking. It includes reviews on the fundamentals of modeling and simulation of metallurgical processes, as well as contributions from the areas of iron reduction/ironmaking, steelmaking via the primary and secondary route, and continuous casting
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Simultaneous phase-stability/-split computation for multiphase oil-displacement simulation
Solvent injection is a widely used method for enhanced oil recovery. Phase behavior of reservoir-oil/injection-gas mixtures should be effectively used for successful implementation of solvent injection. Complex phase behavior involving three hydrocarbon phases has been observed for many solvent injection processes at temperatures typically below 120°F. Well-known examples are CO₂ injection for West Texas oils and enriched gas injection for Alaskan viscous oils, for which the multiphase behavior consisted of the oleic, solvent-rich liquid, and gaseous phases.
Such multiphase behavior makes it challenging to study details of solvent injection. Firstly, it is computationally difficult to robustly solve for multiphase behavior using an equation of state. Secondly, how the interplay between multiphase flow and multiphase behavior affects oil displacement is much more involved than the traditional gas injection problem with only two hydrocarbon phases. This research is concerned with two main technical challenges in multiphase behavior modeling for solvent injection: robust multiphase flash calculation, and quantification of the miscibility development through three-hydrocarbon-phase flow.
In the initial part of this dissertation, a novel algorithm is presented for multiphase isobaric isothermal flash. The formulation is derived from global minimization of the Gibbs free energy using the tangent plane defined at an equilibrium phase composition at a specified temperature and pressure. The new algorithm solves for two groups of stationary points of the tangent-plane-distance (TPD) function: tangent and non-tangent stationary points of the TPD function. Equilibrium phases, at which the Gibbs free energy is tangent to the TPD function, are found as a subset of the solution.
Unlike the traditional flash algorithms, the new algorithm does not require finding false solutions for robust multiphase flash. The advantage of the new algorithm in terms of robustness is shown to be more pronounced for more complex phase behavior, for which multiple local minima of the TPD function are present. It can be robustly initialized even when no K value correlation is available for the fluid of interest; e.g., multiphase behavior involving a solvent-rich liquid phase.
The final part of this dissertation presents a straightforward application of a mass conservation equation to explain and quantify the local oil displacement efficiency in three-hydrocarbon-phase flow. Mass conservation dictates how components must partition into phases upon a multiphase transition (e.g., between two and three phases) in multiphase convective flow. Detailed analysis of multiphase compositional flow equations leads to the distance parameter that quantifies the level of the miscibility developed between a displaced phase and a displacing phase in the presence of other immiscible phases. This distance parameter becomes zero when multicontact miscibility is developed, for example, between the oleic and solvent-rich liquid phases in the presence of the gaseous phase in low-temperature COâ‚‚ flooding.
However, the application of the distance parameter is complicated when a composition path is calculated by using the equation-of-state compositional formulation that takes into account volume change on mixing. In such an application, the mapping of the distance parameter from volume space to composition space was performed, which made the calculated distance parameter less accurate near a displacement front where the solvent concentration rapidly changes.
In this research, the distance parameter is applied directly in volume space for a given composition path. This is a more direct and accurate way to validate the utility of the distance parameter to quantify the local displacement efficiency in three-phase flow. A composition path in three-phase oil displacement is obtained by numerically solving 1-D convective compositional flow equations with no volume change on mixing in this research. The new flash algorithm mentioned above is implemented in this in-house slim-tube simulator. In case studies based on experimental data, the distance parameter is shown to successfully quantify the local oil displacement efficiency in three-phase flow. It properly captures the effects of numerical dispersion and relative permeability on the development of multicontact miscibility. This is because the distance parameter is derived by a simple rearrangement of the weak form of a compositional flow equation.Petroleum and Geosystems Engineerin
Solid sponges as support for heterogeneous catalysts in gas-phase reactions
Solid sponges combine large specific surface areas and low pressure losses with excellent heat transport properties. Thus, they are promising catalyst supports for endo- and exothermic processes. Nevertheless, design tradeoffs regarding the porosity and window diameter of solid sponges with respect to high catalyst densities, low pressure losses, and high effective thermal conductivities remain unsolved. Therefore, a 2-d pseudo-homogeneous multi-scale reactor model for solid sponges is developed in this work. The model is validated against polytropic lab-scale experiments for the methanation of carbon dioxide in a fixed-bed reactor. In order to quantify and analyze the design tradeoffs, the model is used to solve the outlined multi-objective optimization problem. Moreover, tailored graded solid sponges with an optimal porosity distribution in the radial direction are introduced to successfully resolve the existing design tradeoffs
Modeling dispersed gas-particle turbulence in volcanic ash plumes
This PhD thesis focuses on numerical and analytical methods for simulating the dynamics of volcanic ash plumes. The study starts from the fundamental balance laws for a multiphase gas\u2013 particle mixture, reviewing the existing models and developing a new set of Partial Di\ufb00erential Equations (PDEs), well suited for modeling multiphase dispersed turbulence. In particular, a new model generalizing the equilibrium\u2013Eulerian model to two-way coupled compressible \ufb02ows is developed. The PDEs associated to the four-way Eulerian-Eulerian model is studied, investigating the existence of weak solutions ful\ufb01lling the energy inequalities of the PDEs. In particular, the convergence of sequences of smooth solutions to such a set of weak solutions is showed. Having explored the well-posedness of multiphase systems, the three-dimensional compressible equilibrium\u2013Eulerian model is discretized and numerically solved by using the OpenFOAM\uae numerical infrastructure. The new solver is called ASHEE, and it is veri\ufb01ed and validated against a number of well understood benchmarks and experiments. It demonstrates to be capable to capture the key phenomena involved in the dynamics of volcanic ash plumes. Those are: turbulence, mixing, heat transfer, compressibility, preferential concentration of particles, plume entrainment. The numerical solver is tested by taking advantage of the newest High Performance Computing infrastructure currently available. Thus, ASHEE is used to simulate two volcanic plumes in realistic volcanological conditions. The in\ufb02uence of model con\ufb01guration on the numerical solution is analyzed. In particular, a parametric analysis is performed, based on: 1) the kinematic decoupling model; 2) the subgrid scale model for turbulence; 3) the discretization resolution. In a one-dimensional and steady-state approximation, the multiphase \ufb02ow model is used to derive a model for volcanic plumes in a calm, strati\ufb01ed atmosphere. The corresponding Ordinary Di\ufb00erential Equations (ODEs) are written in a compact, dimensionless formulation. The six non-dimensional parameters characterizing a multiphase plume are then written. The ODEs is studied both numerically and analytically. Di\ufb00erent regimes are analyzed, extracting the \ufb01rst integral of motion and asymptotic solutions. An asymptotic analytical solution approximating the model in the general regime is derived and compared with numerical results. Such a solution is coupled with an electromagnetic model providing the infrared intensity emitted by a volcanic ash plume. Key vent parameters are then retrieved by means of inversion techniques applied to infrared images measured during a real volcanic eruption
Supercritical reactive flow modeling in LRE thrust chambers
This CFD study is devoted to the characterization and the analysis of the flow field and heat loads evaluation in oxygen/methane liquid rocket engines. Thanks to CFD we obtain a compromise between details and experimental cost and it is possible a full scale engine analysis as support to the engine design phase. Our work is focused on the heat flux evaluation, hot gas and on flame structure in a thrust chamber. The importance of flame position leads to a study of mesh refinement of post tip. As consequence, a small recirculation zone near the post tip is identified and is studied to guarantee a stable flame also in term of position in the chamber. Also three large recirculation zones of hot gases are located in the combustion chamber and their presence assures, near the walls and the plate, the presence of high heat fluxes. We considered the usage of EOSs and a preliminary analysis was realised before the CFD simulations. Simple test cases are simulated and used to identify the best numerical strategy employ. Finally, we reproduced a simulation of the DEMO a regenerative cooled chamber LOX/methane, obtaining by combustion process simulation the heat flux at the chamber and nozzle walls
Modeling of combustion and propulsion processes of a new concept gun using a gaseous propellant
The combustion light gas gun (CLGG) uses a low molecular weight gas as the propellant to burn, expand and propel the projectile out of the barrel with higher muzzle velocities.In order to better understand the interior ballistic process of CLGG, an multidimensional combustion and flow model for CLGG is established. It contains unsteady Reynolds-averaged Navier-Stokes (RANS) equations, the RNG k
Modélisation numérique d’écoulement diphasique compressible et transport réactif en milieux poreux - Applications à l'étude de stockage de CO2 et de réservoir de gaz naturel.
Human activity in the subsurface has rapidly been expanding and diversifying (waste disposal, new mining technologies, high-frequency storage of energy), while the public and regulatory expectations keep growing. The assessment of each step of underground operations requires careful safety and environmental impact evaluations. They rely on elaborate simulators and multiphysics modeling. With its process-based approach, reactive transport simulation provides an effective way to understand and predict the behavior of such complex systems at different time and spatial scale.This work aims at incorporating a compressible multiphase flow into conventional reactive transport framework by an operator splitting approach. A multiphase flow module is developed in the HYTEC reactive transport software. A new approach is then developed to fully couple multiphase multicomponent compressible flow, the complex thermodynamic description of the fluid properties, with existing reactive transport codes. The method is implemented in HYTEC. Some validation is provided, before application to the simulation of underground storage of CO2 and associated impurities.Les activités humaines dans la subsurface se développent rapidement (stockage de déchets,nouvelles techniques minières, stockage à haute fréquence de l’énergie), alors que dans le même temps les attentes du public et des autorités s’intensifient. L’évaluation de chaque étape de ces opérations souterraines repose sur des études détaillées de la sûreté et des impacts environnementaux.Elles reposent sur des simulateurs élaborés et sur de la modélisation multiphysique. Avec leur approche orientée processus, les simulations en transport réactifs proposent une méthode efficace pour comprendre et prévoir le comportement de ces systèmes complexes, à différentes échelles de temps et d’espace.Le but de ce travail est d’intégrer la résolution de l’écoulement diphasique compressible dans le cadre de codes de transport réactifs à l’aide d’une méthode de séparation d’opérateurs. Un module multiphasique a été créé dans le code de transport réactif HYTEC. Une nouvelle approche a ensuite été développée pour coupler écoulement multicomposant multiphasique compressible, description de propriétés thermo-dynamiques complexes pour les fluides, avec des codes de transport réactif. La méthode a été intégrée dans HYTEC. Des cas de validation sont proposés, puis des exemples d’application pour la simulation du stockage souterrain de CO2 et des impuretés associées