Thesis (MScEng (Process Engineering))--Stellenbosch University, 2008. / A full scale, three dimensional, transient, mathematical model was developed to simulate fluid flow and mixing in gas stirred ladles. The volume of fluid (VOF) and discrete phase (DPM) models were used in combination to account for multiphase aspects, and a slightly modified version of the standard - model was employed for turbulence modelling. The model was validated to compare well against published physical modelling results. Model results were interpreted from the fundamental grounds of kinetic energy transport within the ladle. This approach led to the specification of three key measures of mixing efficiency: The rate and efficiency of kinetic energy transfer from the buoyant gas to the bulk steel as well as the total kinetic energy holding capacity of the ladle. These measures describe the quantity of mixing in any specific ladle setup, whereas the traditional measure of mixing time reflects mixing quality, i.e. the degree of kinetic energy distribution through the entire ladle. The model was implemented in designed experiments to assess various operating and design variables pertaining to mixing quantity and quality. Considerable time was invested in finding the correct balance between numerical accuracy and computational time so that the model could be used to generate the required data within the given time frame. Experiments on the operating variables drew an important distinction between factors influencing the shape and the strength of gas induced flow patterns. Flow pattern strengthening variables, such as gas purge rate, significantly affected the quantity of mixing, but had a limited effect on mixing quality. Variables such as mass loading that influence the shape of the flow patterns had much larger potential to influence both the quantity and quality of mixing. Minimization of turbulence losses in the region of the plume eye was identified as the primary outcome of ladle design. It was shown that a taller vessel allowed more distance over which the plume could disperse, thereby reducing velocity gradients and subsequent turbulence generation at the free surface. Multiple tuyere systems yielded similar improvements by dividing the gas flow into several weakened plumes. Surface wave formation was investigated as an added mixing mechanism and demonstrated to be impractical for application in full scale gas stirred ladles. The conditions for resonance between the surface wave and the bubble plume were met only in vessels with a very low aspect ratio. Performance improvements offered by swirl in these ladles could easily be replicated in more practical ways. This study demonstrated the potential of mathematical modelling as a tool for in-depth investigation into fluid flow and mixing in the hostile environment of a full scale gas stirred ladle. Scaled-down cold models are the only alternative and can offer no more than a reasonably reliable predictive framework. The ease of flow data extraction from the numerical model also proved invaluable in facilitating a fundamental understanding of the effects of various important independent variables on ladle hydrodynamics. At this stage of development, however, the model is recommended for use on a comparative basis only. Two important developments are required for complete quantitative agreement: The inclusion of turbulence modulation by the bubbles and the increased turbulence kinetic energy dissipation rate in the vicinity of the free surface. A general strategy was developed to account for these effects and it compared favourably with published cold model results. Further research is required to generalize this approach for application in full scale gas stirred ladles.

<p>The development process for new aircraft configurations needs to be more efficient in terms of performance, cost and time to market. The potential to influence these factors is highest in early design phases. Thus, high confidence must be established in the product earlier than today. To accomplish this, the concept of virtual product development needs to be established. This implies having a mathematical representation of the product and its associated properties and functions, often obtained through numerical simulations. Building confidence in the product early in the development process through simulations postpones expensive testing and verification to later development stages when the design is more mature.</p><p>To use this in aerodynamic design will mean introducing more advanced physical modelling of the flow as well as significantly reducing the turn around time for flow solutions.</p><p>This work describes the benefit of using parallel computers for flow simulations in the aircraft design process. Reduced turn around time for flow simulations is a prerequisite for non-linear flow modelling in early design stages and a condition for introducing high-end turbulence models and unsteady simulations in later stages of the aircraft design process. The outcome also demonstrates the importance of bridging the gap between the research community and industrial applications.</p><p>The computer platforms are very important to reduce the turn around time for flow simulations. With the recent popularity of Linux–clusters it is now possible to design cost efficient systems for a specific application. Two flow solvers are investigated for parallel</p><p>performance on various clusters. Hardware and software factors influencing the efficiency are analyzed and recommendations are made for cost efficiency and peak performance.</p> / Report code: LiU-TEK-LIC-2006:27.

The development process for new aircraft configurations needs to be more efficient in terms of performance, cost and time to market. The potential to influence these factors is highest in early design phases. Thus, high confidence must be established in the product earlier than today. To accomplish this, the concept of virtual product development needs to be established. This implies having a mathematical representation of the product and its associated properties and functions, often obtained through numerical simulations. Building confidence in the product early in the development process through simulations postpones expensive testing and verification to later development stages when the design is more mature. To use this in aerodynamic design will mean introducing more advanced physical modelling of the flow as well as significantly reducing the turn around time for flow solutions. This work describes the benefit of using parallel computers for flow simulations in the aircraft design process. Reduced turn around time for flow simulations is a prerequisite for non-linear flow modelling in early design stages and a condition for introducing high-end turbulence models and unsteady simulations in later stages of the aircraft design process. The outcome also demonstrates the importance of bridging the gap between the research community and industrial applications. The computer platforms are very important to reduce the turn around time for flow simulations. With the recent popularity of Linux–clusters it is now possible to design cost efficient systems for a specific application. Two flow solvers are investigated for parallel performance on various clusters. Hardware and software factors influencing the efficiency are analyzed and recommendations are made for cost efficiency and peak performance. / Report code: LiU-TEK-LIC-2006:27.

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<p>Three-dimensional turbulent flow in a cylindrical hydrocyclone is considered and studied by means of computational fluid dynamics using software packages CFX and Fluent. The aim has been to identify the methods that can be used for accurate simulation of the flow in three-dimensional configurations in hydrocyclones at high swirl numbers.</p><p>As a starting point, swirling pipe flows created by tangential inlets, where detailed experimental data were available in literature, were considered. It was found that the velocity profiles for the flow with a swirl number of 2.67 could be predicted accurately using a Reynolds stress model and an accurate numerical discretization on a fine-enough mesh. At a higher swirl number, 7.84, under-prediction in the tangential velocity profiles was observed; however the prediction of the axial velocity profiles was satisfactory. The validated methods were then used to simulate the flow in a cylindrical hydrocyclone at a swirl number as large as 21. The calculated tangential velocity profiles were compared against experimental data measured with a pitometer. Acceptable agreements were recorded except near the geometric axis of the cyclone. Due to the lack of the aircore in the numerical model, disagreements near the axis of the cyclone could be expected to some extent.</p><p>Numerical experiments performed in the present work indicated that the RNG k-ε model is not likely to be capable to predict highly swirling flows accurately and a Reynolds stress model is required. For three-dimensional models, where the computing capacity and the available memory set strong restrictions on the computational mesh, optimizing the maximum mesh resolution available play an important role on the accuracy and stability of the solution procedure. The most stable results in the present study were found using the Reynolds stress model proposed by Launder et al. on an as regular and structured mesh as possible using a higher order discretization scheme in Fluent. Therefore, the meshing capabilities of the pre-processor, the available turbulence models and the accuracy of the numerical methods must be considered in parallel. Acceptable results were also generated using the Baseline Reynolds stress model implemented in CFX, however, only with a transient procedure which was likely to be more time-consuming.</p><p>Present simulations present a complex flow structure in the cylindrical cyclone with a double axial flow reversal. The effect of such a flow pattern on the fractionation of the fibres with small differences in density needs to be investigated in future studies.</p>

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A well-functioning, energy-efficient ventilation system is of vital importance to offices, not only to provide the kind of comfortable, healthy indoor environment necessary for the well-being and productive work performance of occupants, but also to reduce energy use in buildings and the associated impact of CO2 emissions on the environment. To achieve these goals impinging jet ventilation has been developed as an innovative ventilation concept. In an impinging jet ventilation system, a high momentum of air jet is discharged downwards, strikes the floor and spreads over it, thus distributing the fresh air along the floor in the form of a very thin shear layer. This system retains advantages of mixing and stratification from conventional air distribution methods, while capable of overcoming their shortcomings. The aim of this thesis is to reach a thorough understanding of impinging jet ventilation for providing a good thermal environment for an office, by using Computational Fluid Dynamics (CFD) supported by detailed measurements. The full-field measurements were carried out in two test rooms located in a large enclosure giving relatively stable climate conditions. This study has been divided into three parts where the first focuses on validation of numerical investigations against measurements, the second addresses impacts of a number of design parameters on the impinging jet flow field and thermal comfort level, and the third compares ventilation performance of the impinging jet supply device with other air supply devices intended for mixing, wall confluent jets and displacement ventilation, under specific room conditions. In the first part, velocity and temperature distributions of the impinging jet flow field predicted by different turbulence models are compared with detailed measurements. Results from the non-isothermal validation studies show that the accuracy of the simulation results is to a great extent dependent on the complexity of the turbulence models, due to complicated flow phenomena related to jet impingement, such as recirculation, curvature and instability. The v2-f turbulence model shows the best performance with measurements, which is slightly better than the SST k-ω model but much better than the RNG k-ε model. The difference is assumed to be essentially related to the magnitude of turbulent kinetic energy predicted in the vicinity of the stagnation region. Results from the isothermal study show that both the SST k-ω and RNG k-ε models predict similar wall jet behaviours of the impinging jet flow. In the second part, three sets of parametric studies were carried out by using validated CFD models. The first parametric study shows that the geometry of the air supply system has the most significant impact on the flow field. The rectangular air supply device, especially the one with larger aspect ratio, provides a longer penetration distance to the room, which is suitable for industrial ventilation. The second study reveals that the interaction effect of cooling ceiling, heat sources and impinging jet ventilation results in complex flow phenomena but with a notable feature of air circulation, which consequently decreases thermal stratification in the room and increases draught discomfort at the foot level. The third study demonstrates the advantage of using response surface methodology to study simultaneous effects on changes in four parameters, i.e. shape of air supply device, jet discharge height, supply airflow rate and supply air temperature. Analysis of the flow field reveals that at a low discharge height, the shape of air supply device has a major impact on the flow pattern in the vicinity of the supply device. Correlations between the studied parameters and local thermal discomfort indices were derived. Supply airflow rates and temperatures are shown to be the most important parameter for draught and stratification discomfort, respectively. In the third part, the impinging jet supply device was shown to provide a better overall performance than other air supply devices used for mixing, wall confluent jets and displacement ventilation, with respect to thermal comfort, heat removal effectiveness, air exchange efficiency and energy-saving potential related to fan power.

The present research focuses on the development and computer implementation of a novel threedimensional, anisotropic turbulence model not only capable of handling complex geometries but also the turbulence driven secondary currents. The model equations comprise advanced algebraic Reynolds stress models in conjunction with Reynolds Averaged Navier-Stokes equations. In order to tackle the complex geometry of compound meandering channels, the body-fitted orthogonal coordinate system is used. The finite volume method with collocated arrangement of variables is used for discretization of the governing equations. Pressurevelocity coupling is achieved by the standard iterative SIMPLE algorithm. A central differencing scheme and upwind differencing scheme are implemented for approximation of diffusive and convective fluxes on the control volume faces respectively. A set of algebraic equations, derived after discretization, are solved with help of Stones implicit matrix solver. The model is validated against standard benchmarks on simple and compound straight channels. For the case of compound meandering channels with varying sinuosity and floodplain height, the model results are compared with the published experimental data. It is found that the present method is able to predict the mean velocity distribution, pressure and secondary flow circulations with reasonably good accuracy. In terms of engineering applications, the model is also tested to understand the importance of turbulence driven secondary currents in slightly curved channel. The development of this unique model has opened many avenues of future research such as flood risk management, the effects of trees near the bank on the flow mechanisms and prediction of pollutant transport.

In this thesis, a new turbulence model developed previously for channel and flat plate flows is evaluated for flat plate flows with pressure gradient. For this purpose a flow solver, which uses boundary layer equations as the governing equations and Von Karman momentum integral equation for the calculation of skin friction, is developed. It is shown that the error of the new turbulence model, in predicting the velocity profile, is less than 5 % for the flat plate flows without pressure gradient and less than 10 % for the flat plate flows with favorable pressure gradient. It is also shown that results with an error in the order of 20 % can be achieved for the flat plate flows with adverse pressure gradient.

This thesis focuses on the study of flow and heat transfer in porous media in both laminar and turbulent flow regimes, by using Volume Averaged Reynolds Navier Stokes (VARNS) approach. The main concern is to investigate the possibility of using porous media for the gas turbine blade cooling. Very recently, using this technique in blade cooling, particularly with internal cooling, has motivated many researchers due to an effective enhancement in the blade cooling. In this study turbulence is represented by using the Launder-Sharma low-Reynolds-number k-Îμ turbulence model, which is modified via proposals by Nakayama and Kuwahara (2008) and Pedras and de Lemos (2001) for extra source terms in the turbulent transport equations to account for the porous structure, which is treated as rigid and isotropic. Due to the changing of the effective porosity as the clear fluid region is approached, the porosity and additional source term in the macroscopic Reynolds averaged Navier-Stokes equations are relaxed across a thin transitional layer at the edges of the porous media. This is achieved by utilizing exponential damping relations to consider these changes. The Local Thermal Equilibrium (LTE) (one-energy equation) model is used for the thermal analysis in porous media. In order to investigate the validity of the extended model, laminar and turbulent flow in different cases, fully developed and developing flows, have been considered. For laminar flows, fully developed plane channel flows with one and two porous layers, a channel with a single porous block and partially filled porous channel flows have been examined for the purpose of validating the extra drag terms in the momentum equations. For the validation purpose for turbulent flows in porous media, the extended model has been tested in homogeneous porous media, turbulent porous channel flows, turbulent solid/porous rib channel flows, and repeated turbulent porous baffled channel flows. Results of all laminar cases show excellent qualitative agreements with the available numerical calculations and experimental data. Results of all turbulent cases show that the extended model returns generally satisfactory accuracy through the comparisons with the available data, except for some predictive weaknesses in regions of either impingement or adverse pressure gradients, both of which are largely due the underlying eddy-viscosity model formulation employed. Thus, from all results, it can be confirmed that the extended model is promising for engineering applications.