Multiphase flow is associated with concurrent flow of more than one phase (gas-liquid, liquid-solid, or gas-liquid-solid) in a conduit. The simultaneous flow of these phases in a flow line, may initiate a slug flow in the pipeline. Hydrodynamic slug flow is an alternate or irregular flow with surges of liquid slug and gas pocket. This occurs when the velocity difference between the gas flow rate and liquid flow rate is high enough resulting in an unstable hydrodynamic behaviour usually caused by the Kelvin-Helmholtz instability. Active feedback control technology, though found effective for the control of severe slugs, has not been studied for hydrodynamic slug mitigation in the literature. This work extends active feedback control application for mitigating hydrodynamic slug problem to enhance oil production and recovery. Active feedback Proportional-Integral (PI) control strategy based on measurement of pressure at the riser base as controlled variable with topside choking as manipulated variable was investigated through Olga simulation in this project. A control system that uses the topside choke valve to keep the pressure at the riser base at or below the average pressure in the riser slug cycle has been implemented. This has been found to prevent liquid accumulation or blockage of the flow line. OLGA (olga is a commercial software widely tested and used in oil and gas industries) has been used to assess the capability of active feedback control strategy for hydrodynamic slug control and has been found to give useful results and most interestingly the increase in oil production and recovery. The riser slugging was suppressed and the choke valve opening was improved from 5% to 12.65% using riser base pressure as controlled variable and topside choke valve as the manipulated variable for the manual choking when compared to the automatic choking in a stabilised operation, representing an improvement of 7.65% in the valve opening. Secondly, implementing active control at open-loop condition reduced the riser base pressure from 15.3881bara to 13.4016bara.

A study of two the sequential displacement of gas and liquid phases in microchannels for eventual application in temperature swing adsorption (TSA) methane purification systems was performed. A model for bulk fluid displacement in 200 m channels was developed and validated using data from an air-water flow visualization study performed on glass microchannel test sections with a hydraulic diameter of 203 m. High-speed video recording was used to observe displacement samples at two separate channel locations for both the displacement of gas by liquid and liquid by gas, and for driving pressure gradients ranging from 19 to 450 kPa m-1. Interface velocities, void fractions, and film thicknesses were determined using image analysis software for each of the 63 sample videos obtained. Coupled 2-D heat and mass transfer models were developed to simulate a TSA gas separation process in which impurities in the gas supply were removed through adsorption into adsorbent coated microchannel walls. These models were used to evaluate the impact of residual liquid films on system mass transfer during the adsorption process. It was determined that for a TSA methane purification system to be effective, it is necessary to purge liquid from the adsorbent channel. This intermediate purge phase will benefit the mass transfer performance of the adsorption system by removing significant amounts of residual liquid from the channel and by causing the onset of rivulet flow in the channel. The existence of the remaining dry wall area, which is characteristic of the rivulet flow regime, improves system mass transfer performance in the presence of residual liquid. The commercial viability of microchannel TSA gas separation systems depends strongly on the ability to mitigate the presence and effects of residual liquid in the adsorbent channels. While the use of liquid heat transfer fluids in the microchannel structure provides rapid heating and cooling of the adsorbent mass, the management of residual liquid remains a significant hurdle. In addition, such systems will require reliable prevention of interaction between the adsorbent and the liquid heat transfer fluid, whether through the development and fabrication of highly selective polymer matrix materials or the use of non-interacting large-molecule liquid heat transfer fluids. If these hurdles can be successfully addressed, microchannel TSA systems may have the potential to become a competitive technology in large-scale gas separation.

In the post-operational phase of a Low/Intermediate-Low radioactive waste repository, gas will be generated in the caverns due to anaerobic corrosion of metals, and also chemical and microbial degradation of organic substances. Previous investigations on gas migration have indicated that discrete water conducting features (e.g. shear zones) are mainly responsible for gas transport from the caverns through the geosphere. Two phase flow processes occur in these water conducting features; the continuity and spatial distribution of pore spaces, the pore size distribution and the interfacial forces of the three phases gas-water-rock have a significant influence on gas transport.The main difficulties to be resolved when simulating two-phase flow processes in fractured rock are:- The description of the internal heterogeneity of the individual water conducting features. The influence of channelling along preferential flow paths is even more important than for single phase fluid flow, because gas transport takes place more or less exclusively along the most transmissive channels. - The determination of effective mass exchange coefficients of the relevant components of the system. Mass exchange may occur between three phases (gas-water-rock). It depends on the spatial distribution of water and gas along the water conducting features (i.e. specific surface of contact areas between phases), and on the solubility and diffusivity of the different components, but also on a couple of state variables of liquid phase (initial content of dissolve/free gas, initial pressure).The work presented in this thesis aims to improve the understanding of the physics of single and multiphase transport phenomena, to be able to develop a quantitative description of gas transport in shear zones to overcome in a satisfactory way the problems described above.

In oil and gas field operations, the dynamic interactions between reservoir and wellbore cannot be ignored, especially during transient flow in the near-wellbore region. As gas hydrocarbons are produced from underground reservoirs to the surface, liquids can come from condensate dropout, water break-through from the reservoir, or vapor condensation in the wellbore. In all three cases, the higher density liquid needs to be transported to the surface by the gas. If the gas phase does not provide sufficient energy to lift the liquid out of the well, the liquid will accumulate in the wellbore. The accumulation of liquid will impose an additional backpressure on the formation that can significantly affect the productivity of the well. The additional backpressure appears to result in a "U-shaped" pressure distribution along the radius in the near-wellbore region that explains the physics of the backflow scenario. However, current modeling approaches cannot capture this U-shaped pressure distribution, and the conventional pressure profile cannot explain the physics of the reinjection. In particular, current steady-state models to predict the arrival of liquid loading, diagnose its impact on production, and screen remedial options are inadequate, including Turner's criterion and Nodal Analysis. However, the dynamic interactions between the reservoir and the wellbore present a fully transient scenario, therefore none of the above solutions captures the complexity of flow transients associated with liquid loading in gas wells. The most satisfactory solution would be to couple a transient reservoir model to a transient well model, which will provide reliable predictive models to link the well dynamics with the intermittent response of a reservoir that is typical of liquid loading in gas wells. The modeling work presented here can be applied to investigate liquid loading mechanisms, and evaluate any other situation where the transient flow behavior of the near-wellbore region of the reservoir cannot be ignored, including system start-up and shut-down.

Cavitating venturies are one of the simplest devices to use on a flow line to control the flow rate without using complex valve and measuring systems. It has no moving parts and complex electronic systems. This simplicity increases the reliability of the venturi and makes it a superior element for the military and critical industrial applications. Although cavitating venturis have many advantages and many areas of use, due to the complexity of the physics behind venturi flows, the characteristics of the venturies are mostly investigated experimentally. In addition, due to their military applications, resources on venturi flows are quite limited in the literature. In this thesis, venturi flows are investigated numerically and experimentally. Two dimensional, two-dimensional axisymmetric and three dimensional cavitating venturi flows are computed using a commercial flow solver FLUENT. An experimental study is then performed to assess the numerical solutions. The effect of the inlet angle, outlet angle, ratio of throat length to inlet diameter and ratio of throat diameter to inlet diameter on the discharge coefficient, and the oscillation behavior of the cavitating bubble are investigated in details.

The response of slotted plate, Venturi meter and standard orifice to the presence of two phase, three phase and low differential flows was investigated. Two mixtures (air-water and air-oil) were used in the two-phase analysis while a mixture of air, water and oil was employed in the three-phase case. Due to the high gas void fraction (α>0.9), the mixture was considered wet gas. A slotted plate was utilized in the low differential pressure analysis and the discharge coefficient behavior was analyzed. Assuming homogeneous flow, an equation with two unknowns was obtained for the multi-phase flow analysis. An empirical relation and the differential response of the meters were used to estimate the variables involved in the equation. Good performance in the gas mass flow rate estimation was exhibited by the slotted and standard plates for the air-water flow, while poor results were obtained for the air-oil and air-water oil flows. The performance of all the flow meter tested in the analysis improved for differential pressures greater than 24.9 kPa (100 in_H2O). Due to the tendency to a zero value for the liquid flow, the error of the estimation reached values of more than 500% at high qualities and low differential pressures. Air-oil and air-water-oil flows show that liquid viscosity influences the response of the differential pressure meters. The best results for high liquid viscosity were obtained in the Venturi meter using the recovery pressure for the gas flow estimation at differential pressures greater than 24.9 kPa (100 in_H2O). A constant coefficient Cd was used for the low differential pressure analysis and results did show that for differential pressure less than 1.24 kPa (5 inH2O) density changes are less than 1% making possible the incompressible flow assumption. The average of the computed coefficients is the value of Cd.

Understanding the physics of multiphase plumes and their simulation through numerical modeling has been an important area of research in recent times in the area of environmental fluid mechanics. The two renowned numerical modeling types that are commonly used by researchers today to simulate multiphase plumes in nature are the mixed-fluid and the two-fluid integral models. In the present study, a detailed review was performed to study and analyze the two modeling approaches for the case of a double plume (upward moving inner plume with downward moving annular outer plume) with the objective of ascertaining which of these models represent the prototype physics in the integral plume model equations with a higher degree of completeness and accuracy. A graphical user interface was designed to facilitate running the models. By comparison to laboratory scale experimental data and through sensitivity analyses, a rigorous effort was made to determine the most appropriate choice of initial conditions needed at the start of the model computation and at the peeling locations and to obtain the most consistent values of the different model parameters that are necessary for calibration of the two models. Consequently, with these selected sets of initial conditions and model parameters, the models were run and their outputs compared against each other for three different case studies with ambient conditions typical of real environmental data. The dispersed phases considered were air bubbles in two cases and liquid CO2 droplets for the third case, with water as the continuous phase in all cases. The entrainment coefficient was found to be the most important parameter that affected the model results. In all the three case studies conducted, the mixed-fluid model was found to predict about 30% higher values for the peel heights and the DMPR (Depth of Maximum Plume Rise) than the two-fluid model.

Richtmyer-Meshkov instability (RMI) occurs when an interface of two fluids with different densities is impulsively accelerated. The main interest in RMI is to understand the growth of perturbations, and numerous theoretical models have been developed and validated against experimental/numerical studies. However, most of the studies assume very simple initial conditions. Recently, more complex RMI has been studied, and this study focuses on two cases: reshocked RMI and multiphase RMI. It is well known that reshock to the species interface causes rapid growth of interface perturbation amplitude. However, the growth rates after reshock are not well understood, and there are no practical theoretical models yet due to its complex interface conditions at reshock. A couple of empirical expressions have been derived from experimental and numerical studies, but these models are limited to certain interface conditions. This study performs parametric numerical studies on various interface conditions, and the empirical models on the reshocked RMI are derived for each case. It is shown that the empirical models can be applied to a wide range of initial conditions by choosing appropriate values of the coefficient. The second part of the study analyzes the flow physics of multiphase RMI. The linear growth model for multiphase RMI is derived, and it is shown that the growth rates depend on two nondimensional parameters: the mass loading of the particles and the Stokes number. The model is compared to the numerical predictions under two types of conditions: a shock wave hitting (1) a perturbed species interface surrounded by particles, and (2) a perturbed particle cloud. In the first type of the problem, the growth rates obtained by the numerical simulations are in agreement with the multiphase RMI growth model when Stokes number is small. However, when the Stokes number is very large, the RMI motion follows the single-phase RMI growth model since the particle do not rapidly respond while the RMI instability grows. The second type of study also shows that the multiphase RMI model is applicable if Stokes number is small. Since the particles themselves characterize the interface, the range of applicable Stokes number is smaller than the first study. If the Stokes number is in the order of one or larger, the interface experiences continuous acceleration and shows the growth profile similar to a Rayleigh-Taylor instability.

<p>This thesis deals with the heat evolution duringsolidification and its relation to the formation of porosity.It intends to improve the current understanding of theformation of porosity in cast materials with special interestin nodular cast iron and the spray forming process. Twodifferent systems, a Fe-based alloy, Cast iron, and a Ni-basedalloy, Inconel 625, are examined. The effect on the heatevolution of the morphology and the processing parameters inspray forming are treated.</p><p>An evaluation of the microstructural features, segregationbehavior and physical properties such as latent heat of fusionis performed byusing thermal analysis under cooling ratesranging from 0.1 to 104 K/s. In order to achieve this amodified differential thermal analysis (DTA) equipment, amirror furnace and levitation casting are used. Results arepresented in terms of the fraction of solidified, the coolingrate and the microstructure observed. The measured latent heatof fusion is not constant throughout the solidificationprocess. Variations in morphology and cooling rate affect therelease of the latent heat.</p><p>A thermodynamic model is used to describe the experimentalobservations and to explain the formation of pores in nodularcast iron by taking into consideration the formation of latticedefects during the liquid/solid transformation. In this casethe formation of porosity is regarded as a consequence ofchanges in the volume fraction ratio graphite/ during thesolidification process.</p><p>A numerical model of the spray forming process is developedby means of CFD modelling and compared with experimentalmeasurements performed in an industrial facility. Stagnationpressure measurements provided information about the gas flowvelocity and an analysis of the overspray powder providedinformation about the particle thermal history. Evaluation ofthe deposit was also performed. It is observed that the processconditions in spray forming promote non-equilibriumsolidification even though solidification at the deposit occursat a lower rate. In this case the porosity formed near theinterface substrate/deposit depends largely on the substratetemperature. The presence of certain reactive elements, such astitanium, affects the porosity levels in the rest of thedeposit.</p><p><b>Keywords:</b>Thermal Analysis, Nodular Cast Iron, Inconel625, CFD, Flow Assesment, Multiphase Flow, Spray Deposition,Microporosity, Superalloys</p>

This thesis reports an experimental investigation of low Reynolds number particle-laden turbulent flows in a horizontal plane channel. Experiments were conducted over a smooth wall and over two rough surfaces made from sand grain and gravel of relative roughness k/h ≈ 0.08 and 0.25, respectively, where k is the roughness height and h is the channel half-height. The flow was loaded with small solid particles with diameters less than 1/10 of the length scale of the energy-containing eddies, and whose concentrations decreased with time due to sedimentation. A novel particle image velocimetry (PIV) method that employed colour filtering for phase discrimination was used to measure the velocities of the fluid and solid particles. Over the smooth wall, the particles mean velocity, turbulence intensities and Reynolds shear stress matched those of the unladen flow very well. There were substantial differences between particle and fluid profiles over the rough wall, which include more rapid reduction in the particle mean velocity and significantly larger turbulence intensities and Reynolds shear stress compared to the unladen flow values. Stratification of the particle concentration led to attenuation of the fluid wall-normal turbulence intensity. This effect was nullified by the roughness perturbation leading to collapse of the wall-normal turbulence intensities over the rough wall. The streamwise turbulence intensity also collapsed over the rough wall but it was found that particles augmented the fluid Reynolds shear stress due to enhanced correlation between the rough wall streamwise and wall-normal velocity fluctuations. A quadrant decomposition of the fluid Reynolds shear stress also revealed corresponding enhancements in ejections and sweeps, the dominant contributors to the Reynolds shear stress, over the rough wall. Based on two-point correlations between the velocity fluctuations and between the velocity fluctuations and swirling strength, it was concluded that both wall roughness and particles modified the turbulence structure by increasing the size of the larger-scale structures. The idea of eddies growing from the wall, thereby enhancing communication between the inner layer and outer parts of the flow, has implications for wall-layer models that assume that the outer layer is detached from the turbulence in the inner region.