No description available.

Catalytic converters are widely used in the automotive industry to comply with increasingly stringent emissions regulations. The flow distribution across the catalyst substrate significantly aects its conversion eciency. Measuring the flow in a catalyst system is challenging; computational fluid dynamics (CFD) provides an alternative approach for the assessment of different design concepts and is therefore commonly employed to model flow behaviour. This thesis studies the application of CFD to modelling ow in a two-dimensional system consisting of a catalyst monolith downstream of a wide-angled planar diuser, with total included angle 60°. Computational models are developed using the commercial CFD software STAR-CCM+. Flow predictions are compared to experimental data collected by Mat Yamin, (2012) and also as part of this study. Measurements were obtained on a two-dimensional isothermal flow rig using particle image velocimetry (PIV) and hot-wire anemometry (HWA). Steady flow studies compare different methods of modelling the monolith. Models include the common approach of modelling the monolith as a porous medium and the computationally expensive individual channels model. A hybrid model is developed that combines the two approaches, benefiting from the respective merits of each method. Two monolith lengths are considered, with flow at varying Reynolds numbers. The porous model predicts the downstream velocity prole well for the shorter monolith but overpredicts flow maldistribution for the longer monolith. The inclusion of an entrance effect to account for the pressure losses associated with oblique entry into the monolith channels is studied. Best agreement in downstream velocity is observed when the pressure losses are limited using a critical angle approach. The individual channels model is found to be the most consistently accurate across monolith lengths, attributable to the accurate capture of flow behaviour upon entry into the monolith channels. A novel hybrid model is proposed, which combines the computational efficiency of the porous model with the geometrical accuracy of individual channels. The model is evaluated and is found to provide results similar to the individual channels model, with improved predictions of velocity maxima and minima. Pulsating flow studies present three transient flow regimes with similar inlet pulse shapes and varying Reynolds number and frequency. Predicted velocities in the diuser are in good agreement with PIV flow fields, however CFD predicts higher magnitudes at the shear layer. The model predicts large residual vortices present at the end of the cycle where experimental data shows none; it is concluded that CFD underpredicts turbulence diffusion. Evidence of cyclic variation in experimental data highlights the limitation of URANS turbulence models.

The study of magnetohydrodynamics unifies the fields of fluid mechanics and electrodynamics to describe the interactions between magnetic fields and electrically conducting fluids. Flows described by magnetohydrodynamics form a significant aspect in a wide range of engineering applications, from the liquid metal blankets designed to surround and remove heat from nuclear fusion reactors, to the delivery and guidance of nanoparticles in magnetic targeted drug delivery. The ability to optimize these, and other, processes is increasingly reliant on the accuracy and stability of the numerical models used to predict such flows. This thesis addresses this by providing a detailed assessment on the performance of two electromagnetically extended Reynolds-averaged Navier-Stokes models through computations of a number of electromagnetically influenced simple channel and Rayleigh-Bènard convective flows. The models tested were the low-Re k-ε linear eddy-viscosity model of Launder and Sharma (1974), with electromagnetic modifications as proposed by Kenjereš and Hanjalić (2000), and the low-Re stress-transport model of Hanjalić and Jakirlić (1993), with electromagnetic modifications as proposed by Kenjereš and Hanjalić (2004). First, a one-dimensional fully-developed turbulent channel flow was considered over a range of Reynolds and Hartmann numbers with a magnetic field applied in both wall-normal and streamwise directions. Results showed that contributions from the electromagnetic modifications were modest and, whilst both models inherently captured some of the reduction in mean strain that a wall-normal field imposed, results from the stress-transport model were consistently superior for both magnetic field directions. Then, three-dimensional time-dependent Rayleigh-Bènard convection was considered for two different Prandtl numbers, two different magnetic field directions and over a range of Hartmann numbers. Results revealed that, at sufficiently high magnetic field strengths, a dramatic reorganization of the flow structure is predicted to occur. The vertical magnetic field led to a larger number of thinner, more cylindrical plumes whilst the horizontal magnetic field caused a striking realignment of the roll cells' axes with the magnetic field lines. This was in agreement with both existing numerical simulations and physical intuition. The superior performance of the modified stress-transport model in both flows was attributed to both its ability to provide better representation of stress generation and other processes, and its ability to accommodate the electromagnetic modifications in a more natural, and exact, fashion. The results demonstrate the capabilities of the stress-transport approach in modelling MHD flows that are relevant to industry and offer potential for those wishing to control flow structure or levels of turbulence without recourse to mechanical means.

Thesis (Ph. D.)--State University of New York at Binghamton, Thomas J. Watson School of Engineeering and Applied Science, Department of Mechanical Engineering, 2009. / Includes bibliographical references.

The principal objectives of the work described in this thesis were: 1. To develop an improved multiphase CFD model for classifying cyclones and further improve understanding of the separation mechanism based on fluid flow and turbulence inside the cyclone. 2. To develop an improved Empirical model of classifying cyclones, covering a wide range of design and operating conditions. The multi-phase CFD model developed in this work is based on the approach reported by Brennan et al (2002) and Brennan (2003) using Fluent, and involves individual models for the air-core, turbulence, and particle classification. Two-phase VOF and mixture models for an air/water system were used to predict the air-core and the pressure and flow fields on 3D fitted fine grids. The turbulence was resolved using both DRSM (QPS) and LES turbulence models. The predicted mean and turbulent flow field from the LES and DRSM turbulence models were compared with the LDA measurements of Hsieh (1988). The LES model predicts the experimental data more accurately than the DRSM model. The standard mixture model (Manninnen et al, 1996) and the modified mixture model for a water/air/solids system were used to predict cyclone performance. The standard mixture model was able to predict classification efficiency reasonably at low solids concentrations, but under-predicts the recovery of coarse size fractions to underflow. To improve the predictions at moderate to high feed solids, the author modified the slip velocity with additional Bagnold dispersive forces, Saffman lift forces, and a hindered settling correction for particle drag in the mixture model superimposed on an LES turbulence model. Several cyclone geometries were used for validating the multiphase CFD model. The modified mixture model improves prediction of the separation of coarse size particles, and the predicted closely matches the experimental in various cyclones. The particle classification mechanism has been further elucidated using the simulated particle concentration distributions. At high solids concentrations, the modified CFD model predicts the efficiency curve reasonably well, especially the cut-size of the cyclone, but prediction of fine particle recovery to overflow is poor compared to the experimental data. It appears that the fines are significantly affected by turbulent dispersion and the flow resistance due to the high viscosity of the slurry at the apex is not sufficiently accounted for in the modified Mixture model. The improved multi-phase CFD model was validated against two sets of experimental data available in the literature: particle concentrations measured by gamma ray tomography data in a dense medium cyclone (Subramanian, 2002), and particle size distribution inside a hydrocyclone (Renner, 1976). Large eddy simulation (LES) with the modified Mixture model, including medium with a feed size distribution appears to be promising in predicting medium segregation inside a dense medium cyclone. The CFD predicted sample size distributions at different positions are reasonably comparable with Renner’s (1976) experimental data near the wall and in the bottom cone, but differ considerably near the forced vortex region, and also near the tip of the vortex finder wall. The CFD model shows no air-core formation at the low operating pressure used by Renner, which suggests his experiments involved an unusual/unstable forced vortex based cyclone separation. The effect of turbulence on fluid and solid particle motion was also studied in this thesis. The resolved turbulent fluctuations from LES of the hydrocyclone at steady flow were analysed using ensemble averaging. The ratio of the effective turbulent acceleration of each particle size to the centrifugal acceleration was calculated for various cyclones, which showed that turbulent mixing becomes less important with larger particles. The trends in this ratio correlate with the equilibrium positions of the particles from the multiphase LES. The analysis indicates that the short-circuiting might be aggravated by turbulent mixing across the locus of zero vertical velocity (LZVV) against the classification force, and along the vortex finder wall into the inner upflow region of the cyclone. An experimental study of the “fish-hook” effect was pursued in various industrial scale cyclones to evaluate the effect of various cyclone parameters. The observed diameter at which fine particle recovery starts to increase is mainly affected by feed solids content and spigot diameter, but less influenced by feed pressure. The observed particle recovery to the underflow at the fishhook dip size, the bypass, is always higher than the underflow water split. Any cyclone variable that affects the underflow water split, will also affect the bypass value. CFD studies showing high particle Reynolds numbers for coarse particles were used to provide a qualitative mechanism for fines reporting to the underflow in the wakes behind the larger particles (Tang et all. 1992). The Frachon and Cilliers (1999) model was used to fit and evaluate the fishhook parameters. The variations of these fishhook parameters were quantified for changes in cyclone design and operating conditions. The development of an improved empirical hydrocyclone model was attempted by collecting extensive historical data covering a wide range of cyclones. Additional experiments on 10 and 20 inch Krebs cyclones were performed to fill the gaps in the database, especially at low to moderate feed solids concentration and with different cone sections. Tangential velocity, turbulent diffusion, slurry viscosity and particle hindered settling correlations were identified from CFD as the key inputs to the particle classification mechanism for the empirical model. A new cyclone model structure based on a dimensionless approach has been developed. The model for , , Q gives a very good fit to the data, while the model for separation sharpness gave reasonable correlations with the cyclone design and operating conditions. 208 additional data sets were used to validate the new hydrocyclone model.

No description available.

This thesis presents the results of steady, Reynolds-averaged Navier-Stokes (RANS) computations for jet flow emanating from a generic turbofan engine exhaust. All computations were performed with commercial solver FLUENT v6.2.16. Different turbulence models were evaluated. In addition to turbulence modeling issues, a parametric study was considered. Different modeling approaches for turbulent jet flows were explained in brief, with specific attention given to the Reynolds-averaged Navier-Stokes (RANS) method used for the calculations. First, a 2D ejector problem was solved to find out the most appropriate turbulence model and solver settings for the jet flow problem under consideration. Results of one equation Spalart-Allmaras, two-equation standart k-&amp / #949 / , realizable k-&amp / #949 / , k-&amp / #969 / and SST k-&amp / #969 / turbulence models were compared with the experimental data provided and also with the results of Yoder [21]. The results of SST k-&amp / #969 / and Spalart-Allmaras turbulence models show the best agreement with the experimental data. Discrepancy with the experimental data was observed at the initial growth region of the jet, but further downstream calculated results were closer to the measurements. Comparing the flow fields for these different turbulence models, it is seen that close to the onset of mixing section, turbulence dissipation was high for models other than SST k-&amp / #969 / and Spalart-Allmaras turbulence models. Higher levels of turbulent kinetic energy were present in the SST k-&amp / #969 / and Spalart-Allmaras turbulence models which yield better results compared to other turbulence models. The results of 2D ejector problem showed that turbulence model plays an important role to define the real physics of the problem. In the second study, analyses for a generic, subsonic, axisymmetric turbofan engine exhaust were performed. A grid sensitivity study with three different grid levels was done to determine grid dimensions of which solution does not change for the parametric study. Another turbulence model sensitivity study was performed for turbofan engine exhaust analysis to have a better understanding. In order to evaluate the results of different turbulence models, both turbulent and mean flow variables were compared. Even though turbulence models produced much different results for turbulent quantities, their effects on the mean flow field were not that much significant. For the parametric study, SST k-&amp / #969 / turbulence model was used. It is seen that boundary layer thickness effect becomes important in the jet flow close to the lips of the nozzles. At far downstream regions, it does not affect the flow field. For different turbulent intensities, no significant change occurred in both mean and turbulent flow fields.