Browsing by Subject "buoyancy"
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Item Analysis of the Reactor Cavity Cooling System for Very High Temperature Gas-cooled Reactors Using Computational Fluid Dynamics Tools(2011-08-08) Frisani, AngeloThe design of passive heat removal systems is one of the main concerns for the modular Very High Temperature Gas-Cooled Reactors (VHTR) vessel cavity. The Reactor Cavity Cooling System (RCCS) is an important heat removal system in case of accidents. The design and validation of the RCCS is necessary to demonstrate that VHTRs can survive to the postulated accidents. The commercial Computational Fluid Dynamics (CFD) STAR-CCM+/ V3.06.006 code was used for three-dimensional system modeling and analysis of the RCCS. Two models were developed to analyze heat exchange in the RCCS. Both models incorporate a 180 degree section resembling the VHTR RCCS bench table test facility performed at Texas A&M University. All the key features of the experimental facility were taken into account during the numerical simulations. Two cooling fluids (i.e., water and air) were considered to test the capability of maintaining the RCCS concrete walls temperature below design limits. Mesh convergence was achieved with an intensive parametric study of the two different cooling configurations and selected boundary conditions. To test the effect of turbulence modeling on the RCCS heat exchange, predictions using several different turbulence models and near-wall treatments were evaluated and compared. The models considered included the first-moment closure one equation Spalart-Allmaras model, the first-moment closure two-equation k-e and k-w models and the second-moment closure Reynolds Stress Transport (RST) model. For the near wall treatments, the low y+ and the all y+ wall treatments were considered. The two-layer model was also used to investigate the effect of near-wall treatment. The comparison of the experimental data with the simulations showed a satisfactory agreement for the temperature distribution inside the RCCS cavity medium and at the standpipes walls. The tested turbulence models demonstrated that the Realizable k-e model with two-layer all y+ wall treatment performs better than the other k-e models for such a complicated geometry and flow conditions. Results are in satisfactory agreement with the RST simulations and experimental data available. A scaling analysis was developed to address the distortion introduced by the experimental facility and CFD model in simulating the physics inside the RCCS system with respect to the real plant configuration. The scaling analysis demonstrated that both the experimental facility and CFD model give a satisfactory reproduction of the main flow characteristics inside the RCCS cavity region, with convection and radiation heat exchange phenomena being properly scaled from the real plant to the model analyzed.Item Experimental investigation of a stratified buoyant wake(Texas A&M University, 2004-11-15) Kraft, Wayne NealAn existing water channel facility at Texas A&M University is used to experimentally study a stratified, buoyant wake. A cylindrical obstruction placed at the centerline of a developing Rayleigh-Taylor mixing layer serves to disturb the equilibrium of the Rayleigh-Taylor mixing layer. The development of the near wake in the presence of unstable stratification is examined, in addition to the recovery of the buoyancy driven mixing layer. Planar laser induced fluorescence (PLIF) is used to visualize the mixing layer / wake interactions, and qualitative observations of the behavior have been made. Also, quantitative measurements of velocity fluctuations and density fluctuations in the near wake have been obtained using particle image velocimetry (PIV) and a high resolution thermocouple system. These experimental measurements were used to investigate how the wake and buoyancy driven mixing layer interact. Finally, a mathematical model has been used to describe the decay of vertical velocity fluctuations in the near wake due to the effects of buoyancy.Item Experimental Investigation of Buoyancy Driven Mixing With and Without Shear at Different Atwood Numbers(2014-11-26) Akula, Bhanesh BThe first objective of the present work is to study Rayleigh-Taylor instability (RTI) mixing, and turbulent velocity statistics at a high Atwood number (At = (Ph -pl)=(Ph+pl)) of 0.75. Until now, no detailed experimental results were available at this Atwood number. The second objective is to study the mixing growth rate parameter variation, velocity statistics, and turbulence behavior of combined RTI and KHI (Kelvin - Helmholtz Instability) at different Atwood numbers. In the present study, a new multi layer gas tunnel facility was built at Texas A&M University to perform the experiments. This is a convective type system where fluids with different density (air and air-helium mixture), initially separated by partitions, start to mix in a transparent acrylic test section. A new density probe was developed using hot-wire anemometry techniques to make instantaneous density measurements inside the mixing layer. This probe along with a three wire probe is used to measure instantaneous velocity components and density simultaneously. Visualization experiments are performed to measure mixing heights and growth rates. For the first time, Particle Image Velocimetry (PIV) is also implemented for measuring RTI velocity statistics at these Atwood numbers. For the RTI experiments at Atwood number 0.75, the spike grew 1.8 times faster than the bubble, and also looked like fragmented dendrite like features rather than the classical mushroom structure. The velocity statistics measured at this Atwood number show self-similarity and scale well with the terminal bubble velocity. These scaling ratios and growth rate parameters are useful to validate existing RTI models. Combined instability experiments have shown that the initial mixing layer development is governed by KHI, and the late time mixing is governed by RTI. This transition between the two regimes is quantified through the Richardson number at four different Atwood numbers, and observed to be in between -0.8 to -2.3. In the combined instability experiments, almost two decades of inertial range scales confirming the Kolmogorov 5=3 scaling law are observed. The molecular mixing in these regimes, and the velocity PDF evolution during the transition are discussed.Item Wind- and Buoyancy-modulated Along-shore Circulation over the Texas-Louisiana Shelf(2013-07-22) Zhang, ZhaoruNumerical experiments are used to study the wind- and buoyancy-modulated along-shore circulation over the Texas-Louisiana continental shelf inshore of 50-m water depth. Most attention is given to circulation in the non-summer flow regime. A major focus of this study is on a unique along-shore flow phenomenon ? convergent along- shore flows, which is controlled jointly by wind forcing and buoyancy fluxes from the Mississippi-Atchafalaya river plume. The second problem addresses the forcing effect of buoyancy on the general along-shore circulation pattern over the shelf in non-summer. The convergent along-shore flows are characterized by down-coast flow from the northern shelf encountering up-coast flow from the southern shelf. This phenomenon is explored for both weather band and seasonal timescales. For the weather band, investigations are focused on non-summer convergent events. The formation of convergent flows is primarily caused by along-coast variation in the along-shore component of wind forcing, which in turn is due to the curvature of the Texas-Louisiana coastline. In general, along-shore currents are well correlated with along-shore winds. However, the points of convergence of currents and winds are not co-located; but rather, points of convergence of currents typically occur down-coast of points of convergence of wind. This offset is mainly caused by buoyancy forcing that forces down-coast currents and drives the point of convergence of currents further down-coast. No specific temporal shift pattern is found for the weather-band convergence, whereas monthly monthly mean convergence exhibits a prominent pattern of seasonal along-coast migration. Buoyancy forcing in the non-summer along-shore flow is investigated in detail in the second part of this study. During non-summer, under down-coast wind forcing, the Mississippi-Atchafalaya river plume exhibits a bottom-advected pattern, for which isopycnals strongly interact with the sea floor. The density front is fairly wide and spans nearly across the entire shelf. Within the front, vertical shear of the alongshore flow is in thermal wind balance with the cross-shore density gradient, and the shear causes a slight reversal of alongshore flow near the bottom. An alongshore flow estimated by the thermal wind relation, along with an assumption of zero bottom reference velocity, agrees well with the actual alongshore flow.