Numerical studies of gasoline direct injection engine processes

The GDI engine has a number of practical advantages over the more traditional port-fuel injection strategy, however a number of challenges remain the subject of continued research in an attempt to fully exploit the advantages of the GDI engine. These include complex in-cylinder flow fields and fuel-air mixing strategies, and significant temporal variation, both through an engine cycle and on a cycle-by-cycle basis. Despite advances in experimental techniques, the relative difficulty and cost of taking detailed measurements remains high, thus computational techniques are an integral part of research activities. The research work presented in this thesis has focused on the use of detailed 3D-CFD techniques for investigating physical phenomena of the in-cylinder flow field and fuel injection process in a single cylinder GDI engine with early injection event. A detailed validation of the numerical predictions of the in-cylinder flow field using both the RANS RNG k-ε turbulence model and the Smagorinsky LES SGS turbulence model was completed with both models showing good agreement against available experimental results. A detailed validation of the numerical predictions of the fuel injection process using a Lagrangian DDM and both RANS RNG k-ε turbulence model and Smagorinsky LES SGS turbulence model was completed with both models showing excellent agreement against experimental data. The model was then used to investigate the in-cylinder flow field and fuel injection process including research into: the three dimensional nature of the flow field; intake valve jet flapping, characterisation, causality and CCV, and whether it could account for CCV of the mixture field at spark timing; the anisotropic characteristics of the flow field using both the fluctuating velocity and turbulence intensity, including the increase in anisotropy due to the fuel injection event; the use of POD for quantitatively analysing the in-cylinder flow field; investigations into the intake valve, cylinder liner and piston crown spray plume impingement processes, including the use of a multi-component fuel surrogate and CCV of the formed liquid film; characterisation and CCV of the mixture field though the intake and compression strokes up to spark timing. Finally, the predicted turbulence characteristics were used to evaluate the resultant premixed turbulent combustion event using combustion regime diagrams

Approccio sinergico di metodi avanzati per la modellazione della combustione con codici CFD e solutori di cinetica chimica: dalle emissioni di particolato alla velocità laminare di fiamma dei combustibili carbonio-neutrali per modelli di combustione flamelet

Ad oggi la combustione è una delle tecnologie più ampiamente utilizzate nell’industria dei trasporti e dell’energia, entrambi settori cardine dell’economia. La ricerca e il progresso scientifico sono stimolati delle normative sempre più stringenti ad andare verso lo sviluppo di processi a basse emissioni, in particolar modo di anidride carbonica. Di vitale importanza è la continua ottimizzazione di questi dispositivi, la quale può essere perseguita tramite via sperimentale o mediante simulazioni. La fluidodinamica computazionale (CFD) e le simulazioni di cinetica chimica offrono strumenti avanzati per lo studio del processo di combustione. Lo scopo di questa tesi è l’impiego di queste tecniche computazionali applicate al caso dei motori a combustione interna per indagare alcune problematiche di interesse, quali le emissioni di particolato e il possibile uso di combustibili e/o vettori energetici privi di carbonio. • In primis è proposto un excursus sull’uso di combustibili surrogati necessari per rappresentare i più complessi combustibili reali, come la benzina. Questi sono formulati con una composizione ad hoc per replicare le proprietà di interesse del combustibile di riferimento. A seguito della formulazione, si discute dell’uso dei surrogati nelle simulazioni 3D CFD motore, presentando l’impatto che l’approccio della modellazione della fase lagrangiana ha sulla distribuzione spaziale della tendenza a produrre particolato per motori ad iniezione diretta. In fine, è proposta una metodologia per la modellazione della propagazione del fronte di fiamma per miscele di toluene, n-eptano ed isottano nel caso dei modelli di combustione appartenenti alla famiglia dei modelli “flamelet”. La strategia presentata è flessibile in quanto basata su regole di miscela per il calcolo della velocità laminare di fiamma del surrogato finale, a partire dalle tre correlazioni ricavate da simulazioni di cinetica chimica per ciascun componente. • Nella seconda parte si discute sulle potenzialità della sinergia tra cinetica chimica e simulazione CFD motore per la formazione di particolato, inquinante dannoso per l’ambiente e la salute, ma estremamente complesso da modellare. Nell’ambito del metodo delle sezioni, la cinetica chimica è impiegata per produrre tabulazioni delle costanti di reazione ricavate ad-hoc sulla composizione di surrogati di benzine, con e senza contenuto di ossigenati, e nel caso estremo di etanolo puro. L'efficacia qualitativa è stata testata in simulazioni CFD 3D di un motore premiscelato. Per quanto sofisticate, queste tabulazioni si basano su due fattori decisi a priori: la composizione del combustibile e il tipo di reattore impiegato nelle simulazioni di cinetica chimica. Il solutore 3D CFD usato in questo studio offre due possibili tipi di reattori per la tabulazione completamente: uno diffusivo e uno premiscelato. Nel caso di una combustione parzialmente premiscelata, la scelta per la tabulazione non è scontata. È proposta una modifica al codice sorgente per passare da una libreria all'altra in base ai risultati di una verifica eseguita in ciascuna cella dove è in atto l’ossidazione del combustibile. Tra i diversi criteri di verifica esaminati, ne viene proposto uno nuovo basato sul modello di combustione utilizzato. Una verifica preliminare è presentata per casi test. • In fine, nella terza parte, la cinetica chimica è adoperata per la derivazione di correlazioni di input per la velocità laminare di fiamma per miscele di ammoniaca e idrogeno. Le simulazioni di cinetica chimica per fiamme monodimensionali a propagazione libera sono utilizzate per le procedure di fitting da cui si ottengono correlazioni ad-hoc per ciascuna miscela e infine è proposta un’unica correlazione parametrizzata rispetto anche al contenuto di idrogeno.Combustion is still widely employed in both energy and transportation sectors, which are the backbone of the global economy. Given the current policies and regulations, the scientific community is making endeavors to achieve carbon-neutral processes. Within this framework, further optimization of combustion device operation can be achieved either via experiments and/or by simulations. Computational Fluid Dynamics (CFD) and chemical kinetics simulations offer an advanced tools to the investigation of the combustion process. The object of this thesis is the use of these tools for advanced application for internal combustion engines focusing on two essential facets: emissions and the use of carbon-neutral fuels. • In the first part, the necessity for chemical kinetics simulations and the use of surrogate fuels in 3D CFD engine simulations is discussed in terms of composition definition, the lagrangian phase modelling approaches for the fuel spray (lumped or multicomponent) and its influence over the sooting tendency of the stratified charge, and finally the flame propagation. For this last, a flexible approach based on mixing rules and flame propagation modelling based on correlations derived from chemical kinetics is presented for Toluene Reference Fuels. • In the second part of the thesis, the synergy between chemical kinetics and engine simulations is discussed focusing on the particulate matter modelling. On one hand, this pollutant is damaging the environment and the health of living being since nano particles can enter the respiratory system, on the other hand it is one of the most complex types of emission to model. As a further improvement to the current sectional method, a customization of source terms reaction rate constants via look-up tables is presented and achieved by chemical kinetics simulations. The qualitatively efficacy in catching the sooting threshold is tested in 3D CFD simulations of a premixed engine fed with gasoline surrogates with and without oxygenated content, and the extreme case of pure ethanol. Then, the case of partially premixed combustion is investigated since it may potentially occur in case of sudden evaporation of liquid film deposits under specific operational conditions. The tabulation of chemical reaction rates is based on two factors that are decided a priori: the fuel composition, thus the importance of surrogate fuels previously discussed, and the type of rectors. The simulation framework employed in this study offers two possibilities for the table generation: a pure diffusive flamelet or a premixed constant pressure reactor. In case of partially premixed combustion the choice is not so straightforward. A modification on the fortran-based source code is operated to switch between the two libraries accordingly to the results of a cell-wise test. For this purpose, different criteria are tested for assessing the predominant type of combustion for the cell-wise test, and a new one is proposed based on the employed combustion model. Premixed combustion is initially assumed, and if required, the input table is switched from the default premixed one to the diffusive one. Finally, the efficacy is checked for two test cases. • In third part, the chemical kinetics is employed for developing input correlations for modelling carbon-neutral fuels combustion in flamelet CFD codes. Chemical kinetic simulations are employed to derive input correlations for flame propagation in case of carbon-neutral fuels. One-dimensional freely propagating flames are employed to build an extensive database for several ammonia-hydrogen blend, and a single correlation for laminar flame speed is derived with hydrogen content as an additional parameter

Coconut Phytophthora. Workshop proceedings, 26-30 October 1992, Manado, Indonesia

Ce séminaire sur le Phythophthora du cocotier avait pour but de réunir tous les acteurs d'un projet démarré en 1990 et de faire le point sur les études réalisées pour la connaissance et la lutte contre ce champignon. Ce projet a donné une grande priorité aux échanges d'informations et aux complémentarités entre les équipes de recherche européennes et tropicale

Performance of surface structures subjected to subsurface soil erosion

Several geotechnical engineering structures (e.g. pavements, slabs-on-grade, footings) transfer pressure to the ground through a contact area with the supporting soil. Design of these structures usually assumes that full contact is established throughout the service life of the structure. Erosion of the subgrade soil is a common mechanism that can lead to the development of subsurface voids and consequently a contact loss between the structure and the supporting soil characterized by a void space under the structure. The performance of surface structures such as concrete pavements, slabs-on-grade, and sidewalks subjected to subsurface soil erosion is investigated experimentally and numerically in this thesis. The experiments were performed in a rigid tank holding a prism of sand with artificially created soil erosion; supporting a rigid steel plate. The surface deformation and contact pressure at the plate-soil interface were measured to quantify the effect of subsurface soil erosion on the stresses developing in the surface structure. Numerical modeling was conducted using 2D elasto-plastic finite element analysis. The model was first validated using the experimental results. Several scenarios were then considered where parameters such as the volume of soil erosion, its location, and the magnitude of the load the concrete slab is subjected to, were varied. Results of this investigation indicated that when the void space is centered under the slab-on-grade, tensile stresses developing in the structure increased as the size of the void increased causing ultimately the failure of the slab. It has also been concluded that when the void is offcentered, the supporting soil around the void space is likely to experience shear failure before excessive tensile stresses develop in the outermost fibers of the structure