This study entails an investigation of a novel moment closure, originally constructed for rarefied-gas prediction, to the modelling of inert, dilute, disperse, particle flows. Such flows are important in many engineering situations. As one example, in internal-combustion engines, fuel is often injected as a spray of tiny droplets and, during combustion, a cloud of tiny soot particles can be formed. These particle phases are often difficult to model, especially when particles display a range of velocities at each location in space. Lagrangian methods are often too costly and many Eulerian field-based methods suffer from model deficiencies and mathematical artifacts. Often, Eulerian formulations assume that all particles at a location and time have the same velocity. This assumption leads to nonphysical results, including an inability to predict particle paths crossing and a limited number of boundary conditions that can be applied. The typical multi-phase situation of many particles is, in many ways, similar to that of a gas compressed of a huge number of atoms or molecules. It is therefore expected that powerful techniques from the kinetic theory of gases could be applied. This work explores the advantages of using a modern fourteen-moment model, originally derived for rarefied gases, to predict multi-phase flows. Details regarding the derivation, the mathematical structure, and physical behaviour of the resulting model are explained. Finally, a numerical implementation is presented and results for several flow problems that are designed to demonstrate the fundamental behaviour of the models are presented. Comparisons are made with other classical models.

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The transport of multi-phase flow in pipelines can be met in a wide range of industrial applications, including the oil and gas industry, showing great savings in developments. In addition, as the exploration of new fields in oil and gas expands to harsh environments, such as ocean or polar, the multi-phase flow transport sometimes becomes the only feasible option. The important features of such multi-phase flow applications include flow regimes, pressure drop and liquid holdup. The precise estimation of these parameters has significant technical and economical impacts on the design and operation of an oil and gas pipelines. Many prediction correlations and methods have been developed; computational fluid dynamics (CFD) being one of them. This type of modelling approach has many advantages over the conventional approaches such as its ability to solve 3D transient problems; offering access to a wealth of information which with conventional techniques is extremely difficult to obtain. Therefore, interest in applying CFD for multi-phase flow transport in pipelines has been on the rise. This thesis is aimed at presenting CFD simulations based on the use of the Volume of Fluid model (VOF) approach for various conditions of gas-liquid turbulent flow in a horizontal circular pipe. In the current VOF formulation in addition to the secondary phase transport equation, a geometric reconstruction technique based on a piecewise-linear interface construction approach is used for reconstructing the interface. A number of multi-phase studies using different turbulence models to the current one have recently appeared in the open literature for simple flow geometries such as rectangular channels. However, most of them assume specific boundary conditions (such as fully-separated phases for stratified flows, the use of square wave at the inlet to represent slug flow or imposing an interfacial disturbance to initiate slugging). These require case-by-case empirical information such as, interfacial roughness for stratified- or slug frequency for intermittent-flow. However, most of them have not presented any detailed validation of their results. The former two points are very crucial for the design of transport pipelines as a pre-knowledge of the operative flow regime and empirical information are not available at the design stage. The predictive accuracy of the present simulations is tested against most common mechanistic approaches and detailed measurements of stratified two-phase flow in a horizontal pipe of Strand (1993) and have been found to be in reasonable quantitative agreement. For the intermittent flow type cases, the numerical results are qualitatively compared against experiments in a horizontal pipe of Al-alweet (2008). The computed flow data of intermittent flow type are further tested against some empirical and mechanistic correlations; the numerical results are qualitatively in a reasonable agreement. Gas compressibility effects on the simulations of slug flow are also explored and are found to bring about some positive benefit. Overall, the predictive accuracy of the present approach is reasonable and promising, demonstrating the ability of the model to predict different types of flow regimes found in two-phase pipe flows. Furthermore, the proposed model shows potential for general applicability to the design of two-phase pipeline systems as it does not require pre-knowledge of the flow regime or any case-by-case empirical information.

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<p>Steel refining via ladle treatment is critical to final product quality in the steel manufacturing process. The process of ladle refining serves to assist in the removal of non-metallic inclusions, which can impact steel product fatigue strength, impact toughness, and corrosion resistance. While the steelmaking industry has in place best practices for the process, it remains costly to performing trial and error testing on the ladle. In addition, an understanding of the flow phenomena within the ladle during operation can provide industry with key knowledge necessary to improve the efficiency and throughput of the process.</p> <p> </p> <p>The method by which this research aims to address this is through the development of a comprehensive computational fluid dynamics (CFD) model of the steelmaking ladle. Such a model, capable of predicting the inclusion removal process and flow patterns within the ladle, would serve to provide the necessary information to advance steelmaking efficiency and improve product quality. A full scale unsteady state three dimensional CFD model has been developed to predict removal of inclusion during gas-stirring in a ladle. The Eulerian-Eulerian model was used to simulate the multiphase flow, the Population Balanced Model (PBM) has been used to describe the inclusion distribution. The phenomena of bottom-blow argon bubble coalescence and breakup were considered. </p> <p> </p> <p>Additionally, a model has been developed to predict inclusion removal during operation. For the inclusion removal model, the CFD-PBM coupled method has been proposed to investigate the inclusion behavior. This includes representing phenomena such as inclusion-bubble collision, inclusion removal by attachment to the ladle refractory, and inclusion capture by slag floating on the surface of the melt. The unified computational model for simulation of fluid flow and inclusion removal was validated against industry measurements provided by Nucor Steel. </p> <p> </p> <p>Using this CFD model and a ladle geometry and set of baseline conditions provided by Nucor Steel, studies were carried out to examine flow development, gas bubble distribution, and inclusion removal. Examining the impacts of inclusion size on removal rate indicated that larger inclusions are removed faster. This agreed with both industry expectations and data found in published literature. In addition, the model predicts that bubble-inclusion collision are primarily responsible for 99% inclusion removal in a gas-stirred ladle.</p>

The main aim of this research was to investigate the mechanism of cavitation in conforming and partially conforming tribological conjunctions. The effect of cavitation on load carrying capacity and frictional performance of is also investigated. This is important with regards to fuel efficiency in internal combustion (IC) engines. Friction accounts for 15–20% of IC engine losses. The piston–cylinder system contributes to 40–50% of these, with the compression ring(s) being responsible for most of this. This is because the primary function of the ring is to seal the combustion chamber, thus small emerging gaps lead to increased friction. In fact, compression ring(s) expend 3–5% of engine input fuel energy. The share of frictional losses of engine bearings is approximately 20–25%. Traditionally, prediction of performance of tribological conjunctions has been studied using Reynolds equation. When the effect of cavitation is considered, various cavitation algorithms with associated boundary conditions for lubricant rupture and reformation are proposed. These include Elrod, and Elrod and Coyne algorithms, as well as boundary conditions such as Swift-Stieber, JFO and Prandtl-Hopkins. There are a number of assumptions embodied in these approaches, as well as the use of Reynolds equation itself. These approaches do not uphold the continuity of mass and momentum in multi-phase flow, in cavitation beyond the lubricant film rupture. A detailed methodology for multi-phase flow, comprising simultaneous solution of Navier-Stokes, energy and lubricant rheological state equations is developed.

Hydraulic fracturing has recently been the completion of choice for most tight gas bearing formations. It has proven successful to produce these formations in a commercial manner. However, some considerations have to be taken into account to design an optimum stimulation treatment that leads to the maximum possible productivity. These considerations include, but not limited to, non-Darcy flow and multiphase flow effects inside the fracture. These effects reduce the fracture conductivity significantly. Failing to account for that results in overestimating the deliverability of the well and, consequently, to designing a fracture treatment that is not optimum. In this work a thorough investigation of non-Darcy flow and multi-phase flow effects on the productivity of hydraulically fractured wells is conducted and an optimum fracture design is proposed for a tight gas formation in south Texas using the Unified Fracture Design (UFD) Technique to compensate for the mentioned effects by calculating the effective fracture permeability in an iterative way. Incorporating non-Darcy effects results in an optimum fracture that is shorter and wider than the fracture when only Darcy calculations are considered. That leads to a loss of production of 5, 18 percent due to dry and multiphase non-Darcy flow effects respectively. A comparison between the UFD and 3D simulators is also done to point out the differences in terms of methodology and results. Since UFD incorporated the maximum dimensionless productivity index in the fracture dimensions design, unlike 3D simulators, it can be concluded that using UFD to design the fracture treatment and then use the most important fracture parameters outputs (half length and CfDopt) as inputs in the simulators is a recommended approach.

Twin-screw pumps are economical alternatives to the conventional multiphase system and are increasingly used in the oil and gas industry due to their versatility in transferring the multiphase mixture with varying Gas Void Fraction (GVF). Present work focuses on the experimental and numerical analysis of twin-screw pumps for different operating conditions. Experimental evaluation aims to understand steady state and transient behavior of twin-screw pumps. Detailed steady state evaluation helped form better understanding of twin-screw pumps under different operating conditions. A comparative study of twin-screw pumps and compressors contradicted the common belief that compressor efficiency is better than the efficiency of twin-screw pumps. Transient analysis at high GVF helped incorporate necessary changes in the design of sealflush recirculation loop to improve the efficiency of the pump. The effect of viscosity of the sealflush fluid at high GVF on pump performance was studied. Volumetric efficiency was found to be decreased with increase in viscosity. Flow visualization was aimed to characterize phase distribution along cavities and clearances at low to high GVF. Dynamic pressure variation was studied along the axis of the screw which helped correlate the GVF, velocity and pressure distribution. Complicated fluid flow behavior due to enclosed fluid pockets and interconnecting clearances makes it difficult to numerically simulate the pump. Hence design optimization and performance prediction incorporates only analytical approach and experimental evaluation. Current work represents an attempt to numerically simulate a multiphase twin-screw pump as a whole. Single phase 3D CFD simulation was performed for different pressure rise. The pressure and velocity profile agreed well with previous studies. Results are validated using an analytical approach as well as experimental data. A two-phase CFD simulation was performed for 50% GVF. An Eulerian approach was employed to evaluate multiphase flow behavior. Pressure, velocity, temperature and GVF distributions were successfully predicted using CFD simulation. Bubble size was found to be most dominant parameter, significantly affecting phase separation and leakage flow rate. Better phase separation was realized with increased bubble size, which resulted in decrease in leakage flow rate. CFD results agreed well with experimental data for the bubble size higher than 0.08 mm.

This thesis presents the study of laminar to turbulent transition of particle laden flows. When a flow becomes turbulent, the drag increases one order of magnitude compared to a laminar flow, therefore, much research is devoted to understand and influence the transition. Previous research at the Linne Flow Centre at KTH has concentrated on the understanding of the bypass transition process of single-phase fluids. Though there are still questions, the principles of this process are now, more or less, known. However, little is known of the influence of particles on transition. While experiments in the 1960s already showed that particles can reduce the friction in turbulent channel flows significantly. The question explored in this thesis is whether this can be attributed to their influence on transition. The initial onset of transition has been investigated with both modal and non-modal linear stability analysis in a Poiseuille flow between two parallel plates. Particles are introduced as a second fluid and they are considered to be solid, spherical and homogeneously distributed. When the fluid density is much smaller than the particle density, ξ (≡ ρf/ρp) &lt;&lt; 1, an increase of the critical Reynolds number is observed. However, transient growth of streamwise vortices resulting in streaks is not affected by inclusion of particles. Particles with ξ ∼ 1 hardly seem to have an effect on stability. Although linear analysis shows that particles hardly influence the transient growth of disturbances, they might affect other (non-linear) stages of transition. To investigate such effects, the full Navier-Stokes equations for 3D Poiseuille flow between two parallel plates are numerically solved and particles are introduced as points with two-way coupling. For particles in a channel flow with ξ&lt;&lt;1, results show that the transition to turbulence is delayed for mass fractions ƒ (=mp N / ρf) larger than 0.1. For a mass fraction of ƒ=0.4 the initial disturbance energy needed to get a turbulent flow increases with a factor of four. Even if lower particle mass fractions ƒ are used, locally there could be large particle mass fractions. Therefore, the next step is to investigate the generation of local large particle mass fractions ƒ. Such particle clusters can be as large as the typical flow structures in the flow, like streak width and vortex size. Then they might change the flow field and (in)stability mechanisms. Numerical simulations of bypass transition in a boundary layer flow are used to determine whether particles cluster and where they tend to cluster. It is found that point particles with ξ&lt;&lt;1 and a large particle relaxation time tend to move in the low speed regions of the flow. In case of streaks, the low speed streaks are most favourable. For smaller particle relaxation times, particles act as tracers and do not have a preferential position and are homogeneously distributed. For particles with ξ∼1 the linear stability analysis showed no transition effect at any ƒ. However, one effect neglected until now is that of particle size. For particles with dimensions of the same order of magnitude of the flow disturbance, particles might influence the flow field. To investigate whether such particles migrate towards positions where they can affect transition some exploratory numerical simulations and experiments are performed. Numerically, the lateral migration of large particles (H/d=5) with ξ=1 in a 3D Poiseuille flow between two parallel plates is investigated. In laminar channel flow, large particles tend to move laterally due to shear to an equilibrium position. For a single large particle some key parameters for migration are identified: the size of the particle and the velocity of the fluid. When multiple particles are present, they tend to form particle trains. If particles are close, they influence each other and the equilibrium position shifts towards the wall, where the final position is dependent on the inter particle spacing. Also, not one steady equilibrium position is present, but particles move around an equilibrium position. Experimentally, migration of particles in bypass transition with ξ=1 is investigated to find out whether neutrally buoyant particles have a preferential position within streaks. The first results with tracer particles (d∼50μm) and few large particles (d∼200μm) do not show detectable preferential positioning. / <p>QC 20131030</p>