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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
111

On the effects of centrifugal forces in air-water two-phase flow regime transitions of an adiabatic helical geometry /

Young, Eric P. January 1900 (has links)
Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 116-124). Also available on the World Wide Web.
112

Microscale thermal management utilizing vapor extraction from a fractal-like branching heat sink /

Apreotesi, Mario A. January 1900 (has links)
Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 95-99). Also available on the World Wide Web.
113

An experimental and numerical study of flow distribution chambers

Wong, Voon Hon January 1999 (has links)
Flow distribution chambers are devices commonly used by the water industry to distribute flows in water and sewage treatment plants. These have simple designs, and are required to operate over a range of volumetric flowrates. Many chambers surveyed (Herbath and Wong, 1997b) were found to perform poorly. They suffered from flow mal-distribution, where the flow was not distributed according to design. The most common cause of flow mal-distribution was hypothesised to be due to the presence of a pipe bend below the chamber (Herbath and Wong, 1997a, 1997b). Therefore, an experimental and numerical study of the flow within a distribution chamber was conducted in this thesis to prove this hypothesis. A novel large-scale model (1: 13) of a typical distribution chamber was constructed. This allowed the collection of high quality and novel velocity and turbulence measurements near the free surface using hot film anemometry. The free surface location was measured using a vernier point gauge while the flow distribution between the outlets was metered by orifice plates. Records of the flow patterns were also kept. The experimental results showed that flow mal-distribution did not occur as expected since the model distribution chamber was designed with a long length of straight inlet pipe, to eliminate the suspected cause of flow mal-distribution. Novel velocity and water surface data were also collected in the experiments, which contributed towards the small body of knowledge in this area of research into flow distribution. CFD models of the physical model were created and solved using a commercial CFD code, CFX 4.1, developed by CFX International of AEA Technology. Steady state and transient two- and three-dimensional calculations of the symmetrical chamber were carried out in the course of the study. A novel adaptation of the existing code was made in obtaining solutions to the numerical models. A new solution strategy was made and refined in this stage of the research using the two-dimensional representation of the distribution chamber, for reasons of reduced computational time. Differencing schemes, surface sharpening, mass residuals, mesh refinement and different turbulence models were investigated during model refinement. The accuracies of the calculated results were determined by comparison with experimental results. It was found that the 3D model, incorporating the RNG k-c model, without surface sharpening, and using the Van Leer differencing scheme, gave good quantitative agreement with the experimental velocities, free surface location and flow distribution. The 2D results gave qualitatively good predictions. Quantitatively, the results were over-predicted which was due, to dimensional effects. The volume of the 2D model was reduced from the 3D model, while the inlet velocity was made the same. This replicated the momentum effects near the free surface that were the governing causes of flow mal-distribution. Nevertheless, this approach was much more practical in terms of computational effort. More importantly, the correct trends for flow mal-distribution could be predicted accurately. Therefore, the next stage of the research used the 2D model developed and validated here. This part of the research involved the novel adaptation of the existing symmetrical 2D results for investigating the asymmetric effects of pipe bends. Three different approaches for modelling the asymmetric effects of a pipe bend were investigated. The first, and the most simplistic, was to incline the incoming flow at an angle to the vertical. The second was to calculate the velocities and turbulence at the outlet of a simple 2D pipe bend, separate from the chamber. These calculated variables were then input into the chamber, to build up a picture of the asymmetric flow, iteratively. The third, and the most accurate method, was to couple the bend to the chamber. It was found that only the third method was capable of accurately representing the conditions within the chamber. Two different pipe bend. lengths were examined using the third approach. The distances chosen were typical of the bend distances found in some treatment plants. The results . from both simulations produced large flow mal-distribution and asymmetric flows within the chamber. A value of 10% difference between the flows from the two outlets was taken to be the maximum limit for mal-distribution. However, values of 44.5 % and 22.8 % were obtained for the larger pipe distance and short pipe distance respectively. Novel remediation strategies using numerical techniques were used to determine the most effective means of improving the flow distribution. The first, used a vertical flow splitter, placed directly above the chamber inlet. Although it altered the path of the jet, it was felt that it would be ineffective for all situations. Although the magnitude of the asymmetry was improved with the use of the splitter, the improvement was insufficient to warrant its recommendation. The other device tested was a horizontal plate located at a certain distance from the chamber inlet. For the longer bend case, a separation distance equivalent to two inlet hydraulic diameters was sufficient to deflect the jet, and reduced the magnitude of the flow asymmetry to around 2%. When the same plate location was used for the shorter bend case, the efficiency of the plate was reduced. Although there was an improvement in the distribution, the magnitude of the asymmetry was greater than 10%. The plate was subsequently lowered by half a hydraulic diameter. This gave a large improvement to the effectiveness of the plate, and the resulting asymmetry was reduced to 7.31 %. The horizontal plate was considered more promising since its function was to deflect and reduce the peak velocities of the jet. With the reduction in velocities, the magnitudes of the nonlinear terms in the Navier-Stokes equations are reduced. The solution to the equations would be more likely to be symmetrical.
114

CFD modelling of solid propellant ignition

Lowe, C. January 1996 (has links)
Solid propellant is the highly energetic fuel burnt in the combustion chamber of ballistic weapons. It is manufactured, for this purpose, in either granular or stick form. Internal ballistics describes the behavior within the combustion chamber throughout the ballistic cycle upto projectile exit from the muzzle of the gun barrel. Over the last twenty years this has been achieved by modelling the process using two-phase flow equations. The solid granules or sticks constitute the first phase, which can be assumed to be incompressible over typical pressure ranges within the chamber. The gas-phase is composed of both the original ambient gas contained around the propellant and additional gas produced by the propellant gasifying on heating. Equations can be derived that describe the conservation of mass, momentum and energy in terms of average flow variables. The equations are a highly non-linear system of partial-differential- equations. High-speed flow features are observed in internal ballistics and ordinary fini te- difference methods are unsuitable numerical methods due to inaccurate prediction of discontinuous flow features. Modern shock-capturing methods are employed, which solve the system of equations in conservation form, with the ability to capture shocks and contact discontinuities. However, although the numerical solutions compare well with experiment over the bulk of the combustion chamber, the ignition models used in internal ballistics are unreliable. These are based on either gas or solid-surface temperature achieving some empirically measured 'ignition temperature' after which the propellant burns according to an empirical pressure dependent burning law. Observations indicate that this is not an adequate representation of ignition. Time differences between first solid gasification and ignition imply two distinct processes occurring. ]Further, ignition occurring in gas-only regions indicates that ignition is controlled by a gas-phase reaction. This thesis develops simple ideas to describe possible mechanisms for these physical observations. The aim is to provide an improved model of the ignition of solid propellant. A two stage reaction process is described involving endothermic gasification of the solid, to produce a source of reactant gas, followed by a very exothermic gas-phase ignition reaction. Firstly the gas-phase ignition is considered. A very simple reaction is suggested which is assumed to control the combustion of reactant gas, produced by solid gasification. Ignition is, by definition, the initiation of this exothermic reaction. Chemical kinetics are included in the gas-phase flow equations to explore the evolution of the reactant gas that is subject to changes in temperature and pressure. By assuming spatial uniformity, analytical solutions of the problem are deduced. The physical interpretation of the solution is discussed, in particular, the relationship between temperature, reactant concentration and ignition is explored. Numerical methods are required to solve the one-dimensional flow equations. Development of suitable CFD methods provides a method of solution. Finite-volume schemes, based on the original work by Godunov, are used to solve the conservation form of the equations. A simple test problem is considered whereby reactant gas is injected into a cylindrical combustion chamber. By examining the resulting flow histories, valuable information is gathered about the complicated coupling of chemistry and flow. Chemistry is included into a system of two-phase flow equations. By using standard averaging methods along with an equation for gas-phase species, equations are derived that describe the rate of change of average flo%v variables for both gas and particle phases. Numerical schemes are developed and some of the difficulties involved in two-phase flow systems, that are not an issue in single-phase flow, are presented. An internal ballistics application is considered as a test case and the solution discussed. The other important reaction involved in the combustion cycle, solid gasification, is explored. The model is based on detailed description of interphase mass and energy transfer at the solid-gas interface. This involves the solution of the heat conduction equation with a moving boundary that divides the solid and gas regions. Similar numerical schemes are constructed to solve the equations. Finally, this model is coupled with the equations of gas-phase reaction. This describes the complete cycle whereby increases in gas temperature cause the solid to increase in temperature and gasify. Subsequent gas-phase combustion of the reactant gases produces heat-transfer between the solid and gas and continues to accelerate gasification. Eventually this results in selfsustained combustion of the solid propellant.
115

Estudo experimental da repartida assistida com água de uma linha de coreflow de óleo pesado-água / Experimental study of the water assisted restart of a heavy oil-water core flow line

Santos, Cynthia Amália Cardoso 16 August 2018 (has links)
Orientador: Antonio Carlos Bannwart / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica e Instituto de Geociências / Made available in DSpace on 2018-08-16T18:25:07Z (GMT). No. of bitstreams: 1 Santos_CynthiaAmaliaCardoso_M.pdf: 19031885 bytes, checksum: d7f4ccde723c3be25530db56830f68d3 (MD5) Previous issue date: 2010 / Resumo: Para que todo o potencial das reservas de óleo pesado existentes seja explorado são necessários o desenvolvimento de novas tecnologias e o aprimoramento de tecnologias já existentes. A produção e transporte deste tipo de óleo carregam consigo um conjunto de novos desafios devidos principalmente a sua alta viscosidade e densidade. A técnica de coreflow e uma das tecnologias ainda em desenvolvimento que busca uma resposta a estes desafios. Ela consiste na injeção de uma fina camada de água nas laterais do tubo, formando um filme ao redor do núcleo de óleo, evitando o contato direto entre o óleo e a parede do tubo. Nestas condições a perda de carga por atrito torna-se comparável a do escoamento monofásico de água. Algumas questões importantes sobre escoamento de óleos viscosos assistidos com água permanecem sem resposta, tal como o procedimento de repartida de uma linha apos uma parada súbita imprevista das bombas de óleo e de água. Este trabalho apresenta diversos experimentos de repartida de escoamento assistido com água. A repartida de uma linha parcialmente bloqueada com óleo viscoso, apos uma parada inesperada das bombas de água e óleo, foi realizada utilizando se somente água. Diversas configurações iniciais em termos de holdup de óleo, tempo em que o óleo permanece parado antes da repartida e vazão de água usada na repartida foram testadas / Abstract: In order to explore thoroughly the potential of existing heavy oil reserves it is necessary to develop new technologies and improve existing technologies. Heavy oil production and transport carry a set of new challenges due mainly its high viscosity and density. The core flow method is one of the new technologies in development that may provide an answer to those challenges. It consists in the lateral injection of a thin water layer that forms a film around de oil core, thereby avoiding direct oil-wall contact. In such conditions, frictional pressure drop becomes comparable to that observed in single phase water flow at mixture flow rate. Some important questions about the water-assisted flow of heavy oils remain unanswered, such as the possibility of re-starting the pipeline after an unexpected stop of the water and oil pumps. This work presents the results of several water assisted re-starting experiments. The re-start of a pipe partly blocked with viscous oil, after an unexpected stop of the water and oil pumps, was performed using a water flow only (i.e. a cleaning operation). Several initial configurations in terms of oil holdups, time in which the oil remain stopped before the re-start and water flow rate used in the re-start were tested / Mestrado / Explotação / Mestre em Ciências e Engenharia de Petróleo
116

EXPERIMENTAL AND MATHEMATICAL INVESTIGATION OF ENHANCING MULTIPHASE FLOW IN THE PIPELINE SYSTEMS

Al-saedi, Sajda S. 01 December 2020 (has links) (PDF)
The major challenge associated with saving energy in the pumping stations of the fluid transportation in the pipeline networks, especially the crude oil transportation for long-distance is drag forces. In other words, this grossly increases the drag form force and friction losses making fluids transport inside pipeline taken a long time to pass, that increases energy consumption and costs. Therefore, the effective solution to overcome these problems is added drag reduction materials (DRMs) with the main fluid using the drag reduction technique (DR). One of the most important drag reduction technique to enhance flow in the pipeline is an active drag reduction using DRMs. Where the DRMs can reduce drag forces in relatively small amounts part per million (ppm), as well as environment friendly. Thereby, the drag reduction enhancement is highly important in terms of fluid transportation in the many industrial applications. An experimental and mathematical study have been performed in the fully development flow to measure fluid characteristics and to evaluate %DR using various DRMs: polymers, surfactants, and nanoparticles in pipeline network. The active drag reduction experiments have been conducted in the rotational disk apparatus (RDA) and in the closed-loop recirculation system (CLRS) using different solutions of DRMs: individual, binary, and triple at different Reynolds numbers (Re) and at different concentrations. The morphological tests have been done employing XDR, TEM and SEM techniques. Mathematical model was presented to validate the experimental results using the statistic softwareV6.2. The results have been displayed with complete explanation, analysis, and conclusions. The results show that the %DR increases with increasing the velocity (Re) and concentration for the most of DRMs solutions. Also, the results confirm that the use of nanoparticle in complex solutions is more effective than using nanoparticle individually within the same work condition. further, the new complex solutions were formed in a manner that can contribute significantly to increase drag reduction performance and enhance shear resistance of the DRMs. Finally, all microscopy techniques confirm the fact that complex solutions were effectively formed and homogenized within the main fluid.
117

Downward two phase flow in vertical tubes

Chase, Sherwin January 1971 (has links)
No description available.
118

Development, Evaluation and Improvement of Correlations for Interphase Friction in Gas-Liquid Vertical Upflow

Clark, Randy R. Jr. 15 October 2015 (has links)
In this study, liquid-vapor vertical upflow has been research with the intent of finding an improved method of modelling the interphase friction in two-phase vertical flow in nuclear thermal-hydraulic codes. An improved method of modelling interphase friction should allow for better prediction of pressure gradient, void fraction and the phasic velocities. Data has been acquired from several available published resources and analyzed to determine the interphase friction using a force balance between the liquid and vapor phases. Using the Buckingham Pi Theorem, a dimensionless interphase friction force was tested and refined before being compared against seven other dimensionless parameters. Three correlations have been developed that establish a dimensionless interphase friction force as a function of the Weber number, the Froude number and the mixture Froude number. Statistical analysis of the three correlations shows that the mixture Froude number correlation should be the most accurate correlation. The correlations have a weakness that makes them ineffective mostly for bubbly flow and some slug flow scenarios, while they should perform significantly better for annular flow cases. Comparisons have been made against the interphase friction calculations published in the manuals of RELAP5/MOD2, RELAP5/MOD3.3, RELAP5-3D and TRACE. The findings have generally shown that the equations in the manuals provide very inaccurate approximations of the interphase friction compared to the interphase friction that was found via force balance. When analyzing the source code of RELAP5/MOD3.3, several differences were noticed between the source code and manual, which have been discussed. Calculations with the source code equations reveal that the source code provides a modestly improved prediction of the interphase friction force, but still has significant errors. Despite the fact that the manual and source code equations indicate that RELAP5/MOD3.3 should perform poorly in modelling interphase friction, actual RELAP5/MOD3.3 model runs perform very well in predicting pressure gradient, void fraction, the liquid and vapor velocities and the interphase friction force. This is largely due to RELAP5/MOD3.3 being able to adjust parameters to converge to a solution that fits within the boundary conditions established in the input file. Modifications to the RELAP5/MOD3.3 code were first made with the three correlations developed using dimensionless parameters, and were tested with data points that the RELAP5/MOD3.3 flow regime map had predicted would be annular flow. While the mixture Froude number correlation has been analyzed to be the most statistically accurate of the three correlations, it was found that the Weber number correlation performed best when implemented into RELAP5/MOD3.3. In a parametric study of the Weber number correlation, it performed optimally at 150% of the original correlation, improving upon the original RELAP model in almost every metric examined. Additional investigations were performed with individual annular flow correlations that model specific physical parameters. Results with the annular flow physical models were inconclusive as no particular model provided a significant improvement over the original RELAP5/MOD3.3 model, and there was no clear indication that combining the models would provide significant improvement. / Ph. D.
119

Modeling Two-Phase Flow in the Downcomer of a Once-Through Steam Generator using RELAP5/MOD2

Clark, Randy Raymond 31 January 2012 (has links)
The purpose of this study is to develop an accurate model of the downcomer of the once-through steam generator (OTSG) developed by Babcock & Wilcox, using RELAP5/MOD2. While the physical model can be easily developed, several parameters are left to be adjusted to optimally model the downcomer and match data that was retrieved in a first-of-a-kind (FOAK) study conducted at Oconee Unit I in Oconee, South Carolina. Once the best-fit set of parameters has been determined, then the model must be tested for power levels exceeding that for which the steam generator was originally designed, so as to determine the power level at which a phenomenon known as flood-back becomes a concern. All known previous studies that have been conducted using RELAP5/MOD2 have shown that RELAP over-predicts interphase friction. However, all of those studies focused on heated two-phase upflow, whereas the downcomer is modeled as adiabatic two-phase downflow. In this study, it is found that the original slug drag model for RELAP5/MOD2 developed by Idaho National Engineering Laboratory (INEL) under-predicts the interphase friction between the liquid and vapor phase within the downcomer. Using a modified version of the original slug drag model created by Babcock & Wilcox (B&W), an optimum multiplier is found for each power level. An increase of 1181% in interphase friction over the INEL slug drag model, which equals an increase of 4347% for the default B&W model provides the most accurate results for all power levels studied. Emphasis is also placed on modeling the orifice plate of the OTSG downcomer which has been added to stabilize pressure fluctuations between the downcomer and tube bundle of the OTSG. While several different schemes are explored for modeling the orifice plate, a branch connection with an inlet area 14.22% of that of the downcomer is used to model the orifice plate along with the volume that transitions the two-phase downflow to horizontal flow into the tube nest of the OTSG. Power levels exceeding that for which the steam generator was designed are tested in RELAP using the slug drag multiplier to determine at which power level a liquid level would occur and would flood-back become a concern. In this study, it is determined that a liquid level would form at 135% power and that at any higher power level, flood-back would be of concern for any user of the steam generator. / Master of Science
120

MECHANISTIC MODELLING OF CRITICAL HEAT FLUX ON LARGE DIAMETER TUBES

BEHDADI, AZIN 11 1900 (has links)
Heavy water moderator surrounding each fuel channel is one of the important safety features in CANDU reactors since it provides an in-situ passive heat sink for the fuel in situations where other engineered means of heat removal from fuel channels have failed. In a critical break LOCA scenario, fuel cooling becomes severely degraded due to rapid flow reduction in the affected flow pass of the heat transport system. This can result in pressure tubes experiencing significant heat-up during early stages of the accident when coolant pressure is still high, thereby causing uniform thermal creep strain (ballooning) of the pressure tube (PT) into contact with its calandria tube (CT). The contact of the hot PT with the CT causes rapid redistribution of stored heat from the PT to CT and a large heat flux spike from the CT to the moderator fluid. For conditions where subcooling of the moderator fluid is low, this heat flux spike can cause dryout of the CT. This can detrimentally affect channel integrity if the CT post-dryout temperature becomes sufficiently high to result in continued thermal creep strain deformation of both the PT and the CT. A comprehensive mechanistic model is developed to predict the critical heat flux (CHF) variations along the downward facing outer surface of calandria tube. The model is based on the hydrodynamic model of \cite{Cheung/Haddad1997} which considers a liquid macrolayer beneath an elongated vapor slug on the heated surface. Local dryout is postulated to occur whenever the fresh liquid supply to the macrolayer is not sufficient to compensate for the liquid depletion within the macrolayer due to boiling on the heating surface. A boundary layer analysis is performed, treating the two phase motion as an external buoyancy driven flow, to determine the liquid supply rate and the local CHF. The model takes into account different types of flow regime or slip ratio. It is applicable for a calandria vessel as well, under a sever accident condition where a thermal creep failure is postulated to occur if sustained CHF is instigated in the surrounding shield tank water. Model shows good agreement with the available experimental CHF data. The model has been modified to take into account the effect of subcooling and has been validated against the empirical correction factors. / Dissertation / Doctor of Philosophy (PhD)

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