Annular flow is an important flow regime in many industrial applications. The need for a better understanding of this flow regime is driven by the desire to improve the design of many terrestrial and space-based systems. Annular two-phase flow is frequently present in the drilling, production and transportation of oil and natural gas, boilers and condensers, and in heating and refrigeration systems. The flow regime is also important for the refueling of space vehicles, and heating and refrigeration systems for space use. Past studies on annular flow have dealt with varying the gas or liquid Reynolds numbers and studying the effect of such changes on the flow regimes and pressure drops. The effect of two other relevant dimensionless groups, namely the gas-to-liquid density ratio and the gas-to-liquid viscosity ratio, on the film characteristics are noticeably absent. As well, with the increased interest in the space environment, studies on the effect of the gravitational acceleration on two-phase flow would be beneficial. The effect of the gas density and the gravitational acceleration on the annular flow average film thickness and frictional pressure drop are examined. The film thickness was measured using two-wire conductance probes. Experimental data were collected in microgravity and hypergravity aboard the Novespace Zero-G Airbus microgravity simulator and normal gravity data were collected at the University of Saskatchewan. Data were collected for a range of annular flow set points by changing the liquid and gas mass flow rates. The liquid-to-gas density ratio was examined by collecting annular flow data using helium-water and air-water. The gravitational effect on the film thickness characteristics was examined by collecting the data during the microgravity and pull-up (hypergravity) portions of each parabolic flight. A direct comparison is possible between the normal gravity data and the microgravity data, due to the matching of the liquid and gas mass flow rates and the flow regime. The reduction in gravity causes the average film thickness to increase between two and four times from the normal gravity values. The microgravity average frictional pressure drop is within approximately 20% of the normal gravity pressure drop for the same flow conditions. For all gravity levels, the air-water and the helium-water flows give similar results, for both average film thickness and frictional pressure drop, when based on the specific energy of the gas. The hypergravity average film thickness results are larger than at normal gravity for the same flow conditions. However, no flow regime map exists for the hypergravity condition, so the similarity of the flow regime cannot be confirmed. The hypergravity flow appears more chaotic, and may be in the transition from a churn type flow. The average frictional pressure drop is increased by approximately 20% due to the increase in the gravitational acceleration. New non-dimensional equations, which include the effect of the gas density, are presented for each gravity level to predict the average film thickness and the average frictional pressure drop.

A study of the pressure drop characteristics of the flow of highly viscous fluids through small diameter orifices was conducted to obtain a better understanding of hydraulic fluid flow loops in vehicles. Pressure drops were measured for each of nine orifices, including orifices of nominal diameter 0.5, 1 and 3 mm, and three thicknesses (nominally 1, 2 and 3 mm), and over a wide range of flow rates (2.86x10sup-7/sup Q 3.33x10sup-4/sup msup3/sup/s). The fluid under consideration exhibits steep dependence of the properties (changes of several orders of magnitude) as a function of temperature and pressure, and is also non-Newtonian at the lower temperatures. The data were non-dimensionalized to obtain Euler numbers and Reynolds numbers using non-Newtonian treatment. It was found that at small values of Reynolds numbers, an increase in aspect ratio (length/diameter ratio of the orifice) causes an increase in Euler number. It was also found that at extremely low Reynolds numbers, the Euler number was very strongly influenced by the Reynolds number, while the dependence becomes weaker as the Reynolds number increases toward the turbulent regime, and the Euler number tends to assume a constant value determined by the aspect ratio and the diameter ratio. A two-region (based on Reynolds number) model was developed to predict Euler number as a function of diameter ratio, aspect ratio, viscosity ratio and generalized Reynolds number. This model also includes data at higher temperatures (20 and le; T and le; 50supo/supC) obtained by Mincks (2002). It was shown that for such highly viscous fluids with non-Newtonian behavior at some conditions, accounting for the shear rate through the generalized Reynolds number resulted in a considerable improvement in the predictive capabilities of the model. Over the laminar, transition and turbulent regions, the model predicts 86% of the data within and plusmn25% for 0.32 l/d (orifice thickness/diameter ratio) 5.72, 0.023 and beta; (orifice/pipe diameter ratio) 0.137, 0.09 Resubge/sub 9677, and 0.0194 and mu;subge/sub 9.589 (kg/m-s)

In this study, the flow plates of micro methanol fuel cells are designed and fabricated in-house through MEMS(Micro-Electro-Mechanical System) technology with deep UV lithography manufacturing processes (SU-8 photoresist) and micro electroforming manufacturing processes. The thesis investigates methanol and water crossover in a micro DMFC for serpentine flow field configuration. Experiments are conducted through various experiments with different operating conditions for the anode flow rate (2-10 sccm), cathode flow rate (100-500 sccm), methanol concentration (1, 2 and 3M), and temperature (25, 50 and 75¢J). Experimental results are presented in the form of polarization VI curves and PI curves under the above operating conditions. The experimental results show that the methanol and water crossover flux increases with increases in cell temperatures, methanol concentration and anode pressure drop. It is found that the fuel efficiency of the DMFC is closely related to the methanol crossover. Further examination of the relationship between the methanol crossover and cell performance reveals the possibility of reducing the methanol crossover by optimizing the anode flow rate.

The main objective of this research is to analyze the performance of micro direct methanol fuel cell (£gDMFC) and observe the bubble behavior of carbon dioxide in the anode flow channel. The flow plate adopted in this study was manufactured through deep UV lithography manufacturing and micro-electroforming manufacturing process. The geometrical configuration of the flow field is in the serpentine form. Transparent acrylic (PMMA: Polymethylmethacrylate) was used to make the terminal plate placed on both sides of the cell in order to facilitate the observation of the bubble behavior of carbon dioxide in the anode flow channel. In this experiment, Micro Particle Image Velocimetry (£gPIV) is used in order to investigate the generation / removal process of carbon dioxide from the anode of micro direct methanol fuel cell (£gDMFC) through a visualized observation method. The behavior of carbon dioxide bubbles in liquidized methanol solution and micro flowfield is also explored. Major parameters of the experiment operation that consist of flow rate of anode and cathode, density of methanol and operational temperature are used to explore their influences on the fuel cell¡¦s polarization curve and power density. The results are presented by V-I curve and P-I curve. The relation between carbon dioxide bubble movement and behavior according to the anode pressure drop are also discussed.

Pressure drops over a packed bed of pebble bed reactor type are investigated. Measurement of porosity and pressure drop over the bed were carried out in a cylindrical packed bed facility. Air and water were used for working fluids. There are several parameters of the pressure drop in packed beds. One of the most important factors is wall effect. The inhomogeneous porosity distribution in the bed and the additional wetted surface introduced by the wall cause the variation of pressure drop. The importance of the wall effects and porosity can be explained by using different bed-to-particle diameter ratios. Four different bed-to-particle ratios were used in these experiments (D/dp = 19, 9.5, 6.33 and 3.65). A comparison is made between the predictions by a number of empirical correlations including the Ergun equation (1952) and KTA (by the Nuclear Safety Commission of Germany) (1981) in the literature. Analysis of the data indicated the importance of the bed-to-particle size ratios on the pressure drop. The comparison between the present and the existing correlations showed that the pressure drop of large bed-to-particle diameter ratios (D/dp = 19, 9.5and 6.33) matched very well with the original KTA correlation. However the published correlations cannot be expected to predict accurate pressure drop for certain conditions, especially for pebble bed with D/dp (bed-to-particle diameter ratio) </= 5. An improved correlation was obtained for a small bed-to-particle diameter ratio by fitting the coefficients of that equation to experimental database.

High Temperature Gas-cooled Reactor (HTGR) is a Generation IV reactor under consideration by Department of Energy and in the nuclear industry. There are two categories of HTGRs, namely, Pebble Bed Modular Reactor (PBMR) and Prismatic reactor. Pebble Bed Modular Reactor is a HTGR with enriched uranium dioxide fuel inside graphite shells (moderator). The uranium fuel in PBMR is enclosed in spherical shells that are approximately the size of a tennis ball, referred to as \fuel spheres". The reactor core consists of approximately 360,000 fuel pebbles distributed randomly. From a reactor design perspective it is important to be able to understand the fluid flow properties inside the reactor. However, for the case of PBMR the sphere packing inside the core is random. Unknown flow characteristics defined the objective of this study, to understand the flow properties in spherically packed geometries and the effect of turbulence models in the numerical solution. In attempt to do so, a steady state computational study was done to obtain the pressure drop estimation in different packed bed geometries, and describe the fluid flow characteristics for such complex structures. Two out of the three Bravais lattices were analyzed, namely, simple cubic (symmetric) and body centered cubic (staggered). STARCCM commercial CFD software from CD- ADAPCO was used to simulate the flow. To account for turbulence effects several turbulence models such as standard k-epsilon, realizable k-epsilon, and Reynolds stress transport model were used. Various cases were analyzed with Modified Reynolds number ranging from 10,000 to 50,000. For the simple cubic geometry the realizable k-epsilon model was used and it produced results that were in good agreement with existing experimental data. All the turbulence models were used for the body centered cubic geometry. Each model produced different results what were quite different from the existing data. All the turbulence models were analyzed, errors and drawbacks with each model were discussed. Finally, a resolution was suggested in regards to use of turbulence model for problems like the ones studied in this particular work.

For power plants that make use of high energy boiler feed pumps, there is a risk that the boiler feed pump may experience cavitation and overheating at low load and start-up conditions. These plants make use of a leak off or recirculation system that diverts some of the flow back to the feed water tank, ensuring that a minimum flow through the pump is maintained at low load and start-up operating conditions. The recirculation valve, also known as a leak off valve, experiences a very high pressure difference and cavitation pitting is common due to the water being close to saturation. There are various ways in which the recirculation flow is controlled in the industry such as open orifice, on/off binary type control valves, automatic recirculation valves (ARC) or modern modulating leak off systems. The valves themselves can also be simple plug type or make use of pressure staging to reduce the risk of cavitation. This project involves modelling the flow system around the boiler feed pump and its control for the various architectures employed in Eskom. This is to assist in understanding the reasons for cavitation damage that is found in some recirculation valves as well as the low load capability of the system. Single stage components with extremely high pressure drops are singled out as components with the highest risk of cavitation in the systems. Although extremely high pressure drops are found across the leak off valves themselves, the majority of the valves are multistage valves which are specifically designed to accommodate cavitation development and are therefore not of major concern. Some of the findings of the study are: The rule of thumb used within Eskom to determine the amount of pressure reducing stages on leak off valves could be more conservative. The specification of new valves and components for the leak off systems requires accurate specification based on detailed process models, such as the ones developed for this study. The full range of all possible operational cases must also be considered during the design.

Gravity driven trickle flow of a liquid over a fixed bed in the presence of a gaseous phase is widely encountered throughout the process industry. It is one of the most common ways of contacting multi-phase fluids for reaction or mass transfer purposes. The presence of three phases greatly complicates the mathematical modelling of trickle-bed reactors and makes a description from first principles difficult. Trickle flow performance is usually characterized in terms of hydrodynamic parameters. One such parameter is the liquid holdup. The value and morphology (shape or texture) of the holdup influences the catalyst contacting, wetting, mass transfer characteristics and ultimately the performance of the trickle flow unit. This study is limited to the air-water-glass spheres system with no gas flow. It is partitioned into three sections. An investigation into the nature of the residual liquid holdup in beds of spherical particles revealed that the general assumption that all residual liquid is held in the form of pendular rings at particle contact points proves to be untrue. Instead, indication is that 48 % of the residual holdup is present in the form of agglomerated liquid globules in interstices of low local porosity. Theoretical residual liquid holdup models and residual liquid holdup-based mass transfer models should include this phenomenon. In a subsequent section, the influence of the prewetting procedure on the operating holdup is investigated. Three distinct limiting cases are identified: Kan-wetted, Levec-wetted and non-wetted. A volumetric utilization coefficient that describes the extent to which the bed is irrigated is developed. It indicates that large fractions of the bed remain non-irrigated in the Levec- and non-wetted modes. A momentum balance-based model is adopted to predict the Kan-wetted mode holdup. This model was successfully extended to predicting the holdup in the Levec- and non-wetted modes by simple incorporation of the volumetric utilization coefficient. The predictive capability of this model is highly satisfactory, especially in light of it using only the classical Ergun constants and no fitted parameters (AARE = 9.6 %). The differences in the hysteresis behaviour of holdup and pressure drop in the different modes are attributed to differences in the morphology of the operating holdup. The existence of the three limiting prewetted modes is confirmed by residence time distribution (RTD) analysis of the stimulus-response behaviour of the system. This behaviour was quantified using a NaCl tracer and conductivity measurements at both the inlet and outlet of a bench scale bed. The analyses show that: · There are large fractions of the holdup that is inaccessible to the tracer in the Levec-wetted and non-wetted modes. · The mixedness in the three prewetted modes differ appreciably, with the Kan-wetted mode clearly less mixed than the Levec-wetted mode. The RTD analyses also confirm the existence of the three prewetting modes in a porous system (spherical a-alumina), with a large fraction of the holdup being inaccessible to the tracer in the Levec-wetted mode. This study emphasizes the role of the morphology of the various types of liquid holdup on the hydrodynamic performance of a trickle flow unit. It is apparent that aspects of the morphology depend strongly on phenomena like globule formation, hysteresis and flow and prewetting history that have not been adequately recognized to date. The visualization of the various modes of trickle flow is an intellectual platform from which future studies may be directed. / Dissertation (MEng)--University of Pretoria, 2004. / Chemical Engineering / Unrestricted

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Numerical modeling and prediction techniques are used to determine pressure drop, minimum fluidization velocity and segregation for bubbling fluidized beds. The computational fluid dynamics (CFD) code Multiphase Flow with Interphase eXchange (MFIX) is used to study a two-stage reactor geometry with a binary mixture. MFIX is demonstrated to accurately predict pressure drop versus inlet gas velocity for binary mixtures. A new method is developed to predict the pressure drop versus inlet gas velocity and minimum fluidization velocity for multi-component fluidized beds. The mass accounting in the stationary system (MASS) method accounts for the changing bed composition during the fluidization process by using a novel definition for the mass fractions of the bed not yet fluidized. Published experimental data for pressure drop from single-, binary- and ternary-component fluidized bed systems are compared to MFIX simulations and the MASS method, with good agreement between all three approaches. Minimum fluidization velocities predicted using correlations in the literature were compared with the experimental data, MFIX, and the MASS method. The predicted minimum fluidization velocity from the MASS method provided very good results with an average relative error of ±4%. The MASS method is shown to accurately predict when complex multi-component systems of granular material will fluidize. The MASS method and MFIX are also used to explore the occurrence and extent of segregation in multi-component systems. The MASS method and MFIX are both shown to accurately predict the occurrence and extent of segregation in multi-component systems. / Master of Science