In this thesis a set of boundary conditions for multiphase flow is suggested. Characteristic-based boundary conditions are reviewed for single-phase flow. The problem of open-boundary conditions is investigated, and to avoid drifting values, the use of control functions is proposed. The use of control functions is also verified with a new test which assesses the quality of the boundary conditions. Particularly, P- and PI-control functions are examined. PI-controllers have the ability to specify a given variable exactly at the outlet as well as at the inlet, without causing spurious reflections which are amplified. Averaged multiphase flow equations are reviewed, and a simplified model is established. This model is used for the boundary analysis and the computations. Due to the averaging procedure, signal speeds are reduced to the order of the flow speed. This leads to numerical challenges. For a horizontal channel flow, a splitting of the interface pressure model is suggested. This bypasses the numerical problems associated with separation by gravity, and a physical realistic model is used. In this case, the inviscid model is shown to possess complex eigenvalues, and still the characteristic boundary conditions give reasonable results. The governing equations are solved with a Runge-Kutta scheme for the time integration. For the spatial discretisation, a finite-volume and a finite-difference method are used. Both implementations give equivalent results. In single-phase flow, the results improve significantly when a numerical filter is applied. For two-dimensional two-phase flow, the computations are unstable without a numerical filter.

This work presents a number of the practical aspects of developing reduced- order models (ROMs) based on proper orthogonal decomposition (POD). ROMS are derived and implemented for multiphase flow, quasi-2D nozzle flow and 2D inviscid channel flow. Results are presented verifying the ROMs against existing full-order models (FOM). POD is a method for separating snapshots of a flow field that varies in both time and space into spatial basis functions and time coefficients. The partial differential equations that govern fluid flow can then be pro jected onto these basis functions, generating a system of ordinary differential equations where the unknowns are the time coefficients. This results in the reduction of the number of equations to be solved from hundreds of thousands or more to hundreds or less. A ROM is implemented for three-dimensional and non-isothermal multiphase flows. The derivation of the ROM is presented. Results are compared against the FOM and show that the ROM agrees with the FOM. While implementing the ROM for multiphase flow, moving discontinuities were found to be a ma jor challenge when they appeared in the void fraction around gas bubbles. A point-mode POD approach is proposed and shown to have promise. A simple test case for moving discontinuities, the first order wave equation, is used to test an augmentation method for capturing the discontinuity exactly. This approach is shown to remove the unphysical oscillations that appear around the discontinuityin traditional approaches. A ROM for quasi-2D inviscid nozzle flow is constructed and the results are com- pared to a FOM. This ROM is used to test two approaches, POD-Analytical and POD-Discretized. The stability of each approach is assessed and the results are used in the implementation of a ROM for the Navier-Stokes equations. A ROM for a Navier-Stokes solver is derived and implemented using the results of the nozzle flow case. Results are compared to the FOM for channel flow with a bump. The computational speed-up of the ROM is discussed. Two studies are presented with practical aspects of the implementation of POD- based ROMs. The first shows the effect of the snapshot sampling on the accuracy of the POD basis functions. The second shows that for multiphase flow, the cross- coupling between field variables should not be included when computing the POD basis functions.

Flow mechanisms affect transport processes during condensation. Most studies on two-phase flow regimes are qualitative in nature, typically providing only information to guide the identification of the respective regimes and the transitions between them. These studies have, however, not yielded quantitative information to assist the development of pressure drop and heat transfer models. Such qualitative studies have also yielded results with considerable variability between investigators. A comprehensive investigation of flow mechanisms, void fraction, pressure drop and heat transfer during condensation of R404A in microchannels was conducted. In contrast to all prior investigations, high-speed video recordings and image analyses were used to directly measure void fraction, slug frequencies, vapor bubble velocity, vapor bubble dimensions and liquid film thicknesses in tube diameters ranging from 0.508 to 3.00 mm. Experiments were conducted at reduced pressures and mass fluxes ranging from 0.38 to 0.77 and 200 to 800 kg m-2 s-1, respectively, to document their influences on the condensation process at local vapor qualities ranging from 0 to 1. This information was used to develop a model for the void fraction in condensing flows. A complementing set of heat transfer and pressure drop measurements were conducted on the same geometries at similar conditions, and the void fraction model was used in conjunction with these measurements to develop improved heat transfer and pressure drop models. This comprehensive set of experiments and analyses yields a self-consistent and accurate treatment of high-pressure condensation in small hydraulic diameter geometries. Furthermore, the heat transfer model was found to agree well with condensing ammonia and carbon dioxide data that were obtained at significantly different conditions in different tube diameters. The added physical understanding of the condensation process and the models developed will serve as important building blocks for the design of microscale condensers and thermal systems.

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 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.

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>

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