Turbulent solid-liquid internal flows are encountered in many engineering applications, such as mining, chemical, and petroleum. The technical and economical burden of experimental tests and the lack of generalization of the many simplified physically based models made Computational Fluid Dynamics (CFD) a commonly-used approach in recent years. Among the CFD models, the two-fluid ones are actually the only possible way to simulate dense flows. However, even for pipe flows, the existing two-fluid models appear numerically unstable and computationally expensive; these features could complicate, and even prevent, their application to more complex flows of engineering interest. In this thesis a new two-fluid model for the simulation of turbulent solid-liquid flows in pipeline systems is presented. The model solves a double-averaged formulation of the mass and momentum conservation equations for both phases, coupled by means of interfacial momentum transfer terms. The model is robust and numerically stable and requires rather low computer time to procure converged solution due to the peculiar way in which the key physical mechanisms governing solid-liquid flows are modeled: phase diffusion fluxes are introduced in all conservation equations to reproduce the effect of the turbulent dispersion of particles; the presence of other particles on the interfacial momentum transfer is taken into account by considering their effect on a mixture viscosity: a wall function is employed in order to model the mechanical contribution to the wall shear stress. Three test cases are investigated. At first, the turbulent flow of solid-liquid mixtures in horizontal pipes is considered, comparing the predictions of the proposed model to both experimental data from open literature and the estimations of previous two-fluid models. Afterwards, the flow of solid-liquid slurries in a sudden expansion in a rectangular duct is simulated, evaluating the phenomenological consistency of the solution on the grounds of the theoretical background inferred from literature. At last, the model is applied to geometry of engineering interest, which is a wellhead choke valve for oil & gas applications, in order to preliminary investigate the effect of the presence of solid particles in the flow on the dissipation characteristics of the device.
Flussi turbolenti solido-liquido in pressione sono frequentemente incontrati in molti ambiti dell’ingegneria, tra cui quelli minerario, chimico e petrolifero. Le difficoltà tecniche e l’onerosità economica delle prove sperimentali, oltre alla mancanza di generalità dei modelli concettuali, hanno reso la Fluidodinamica Computazionale un approccio comunemente seguito negli ultimi anni. Tra i modelli numerici, quelli del tipo Euler-Euler sono di fatto la scelta obbligata qualora si vogliano simulare flussi ad alta concentrazione di particelle. Tuttavia, anche per il caso di condotta indefinita, i modelli Euler-Euler esistenti appaiono numericamente instabili e computazionalmente onerosi; queste caratteristiche possono complicare, e al limite impedire, la loro applicazioni a flussi complessi di interesse ingegneristico. In questa tesi viene proposto un nuovo modello Euler-Euler per la simulazione di flussi turbulenti solido-liquido in sistemi di condotte. Il modello risolve una formulazione due volte mediata delle equazioni di conservazione della massa e del momento della quantità di moto di entrambe le fasi, e queste equazioni sono accoppiate mediante termini che rappresentano il trasferimento di momento all’interfaccia. Il modello è robusto, numericamente stabile e richiede un tempo di calcolo relativamente breve per giungere a convergenza. Queste caratteristiche dipendono dal modo in cui vengono modellati i principali meccanismi fisici alla base del fenomeno: flussi diffusivi compaiono in tutte le equazioni di conservazione per riprodurre l’effetto della dispersione turbolenta delle particelle; l’effetto delle particelle sul trasferimento di momento all’interfaccia viene considerato attraverso il suo effetto sulla viscosità della miscela; una legge opportuna viene applicata per rappresentare il contributo meccanico dello sforzo di parete. Sono analizzati tre casi test. Dapprima, è simulato il flusso turbolento di miscele solido-liquido in condotte orizzontali indefinite, confrontando la soluzione del modello proposto ai dati sperimentali di letteratura e alle previsioni di altri modelli Euler-Euler. Successivamente, è studiato il flusso di miscele solido-liquido attraverso un brusco allargamento in una condotta rettangolare, analizzando la consistenza fenomenologica della soluzione alla luce delle basi teoriche riportate in letteratura. Infine, è stato applicato il modello ad una geometria di interesse ingegneristico, cioè una valvola di tipo “choke” per applicazioni oil & gas, al fine di analizzare preliminarmente l’effetto della presenza di particelle solide nel flusso sulle caratteristiche dissipative del manufatto.
Two fluid model for solid liquid flows in pipeline systems
MESSA, GIANANDREA VITTORIO
Abstract
Turbulent solid-liquid internal flows are encountered in many engineering applications, such as mining, chemical, and petroleum. The technical and economical burden of experimental tests and the lack of generalization of the many simplified physically based models made Computational Fluid Dynamics (CFD) a commonly-used approach in recent years. Among the CFD models, the two-fluid ones are actually the only possible way to simulate dense flows. However, even for pipe flows, the existing two-fluid models appear numerically unstable and computationally expensive; these features could complicate, and even prevent, their application to more complex flows of engineering interest. In this thesis a new two-fluid model for the simulation of turbulent solid-liquid flows in pipeline systems is presented. The model solves a double-averaged formulation of the mass and momentum conservation equations for both phases, coupled by means of interfacial momentum transfer terms. The model is robust and numerically stable and requires rather low computer time to procure converged solution due to the peculiar way in which the key physical mechanisms governing solid-liquid flows are modeled: phase diffusion fluxes are introduced in all conservation equations to reproduce the effect of the turbulent dispersion of particles; the presence of other particles on the interfacial momentum transfer is taken into account by considering their effect on a mixture viscosity: a wall function is employed in order to model the mechanical contribution to the wall shear stress. Three test cases are investigated. At first, the turbulent flow of solid-liquid mixtures in horizontal pipes is considered, comparing the predictions of the proposed model to both experimental data from open literature and the estimations of previous two-fluid models. Afterwards, the flow of solid-liquid slurries in a sudden expansion in a rectangular duct is simulated, evaluating the phenomenological consistency of the solution on the grounds of the theoretical background inferred from literature. At last, the model is applied to geometry of engineering interest, which is a wellhead choke valve for oil & gas applications, in order to preliminary investigate the effect of the presence of solid particles in the flow on the dissipation characteristics of the device.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/74528