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Artykuły w czasopismach na temat "Buoyancy-driven MHD"
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DAVOUST, L., M. D. COWLEY, R. MOREAU i R. BOLCATO. "Buoyancy-driven convection with a uniform magnetic field. Part 2. Experimental investigation". Journal of Fluid Mechanics 400 (10.12.1999): 59–90. http://dx.doi.org/10.1017/s002211209900645x.
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In this paper, an experimental study of laminar magnetohydrodynamic (MHD) buoyancy-driven flow in a cylindrical cell with axis horizontal is described. A steady uniform magnetic field is applied vertically to the mercury-filled cell, which is also subjected to a horizontal temperature gradient. The main features of this internal MHD thermogravitational flow are made experimentally evident from temperature and electric potential measurements. Whatever the level of convection, raising the Hartmann number Ha to a value of the order of 10 is sufficient to stabilize an initially turbulent flow. At much higher values of the Hartmann number (Ha∼100) the MHD effects cause a change of regime from boundary-layer driven to core driven. In this latter regime an inviscid inertialess MHD core flow is bounded by a Hartmann layer on the horizontal cylindrical wall and viscous layers on the endwalls. Since the Hartmann layer is found to stay electrically inactive along the cell, the relevant asymptotic (Ha[Gt ]1) laws for velocity and heat transfer are found from the balance between the curl of buoyancy and Lorentz forces in the core, together with the condition that the flow of electric current between core and Hartmann layer is negligible. A modified Rayleigh number RaG/Ha2, which is a measure of the ratio of thermal convection to diffusion when there is a balance between buoyancy and Lorentz forces, is the determining parameter for the flow.
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Hanasz, Michał, K. Otmianowska-Mazur, H. Lesch, G. Kowal, M. Soida, D. Wóltański, K. Kowalik, R. K. Pawłaszek i B. Kulesza-Żydzik. "Cosmic-ray driven dynamo in galactic disks". Proceedings of the International Astronomical Union 4, S259 (listopad 2008): 479–84. http://dx.doi.org/10.1017/s1743921309031147.
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AbstractWe present new developments on the Cosmic–Ray driven, galactic dynamo, modeled by means of direct, resistive CR–MHD simulations, performed with ZEUS and PIERNIK codes. The dynamo action, leading to the amplification of large–scale galactic magnetic fields on galactic rotation timescales, appears as a result of galactic differential rotation, buoyancy of the cosmic ray component and resistive dissipation of small–scale turbulent magnetic fields. Our new results include demonstration of the global–galactic dynamo action driven by Cosmic Rays supplied in supernova remnants. An essential outcome of the new series of global galactic dynamo models is the equipartition of the gas turbulent energy with magnetic field energy and cosmic ray energy, in saturated states of the dynamo on large galactic scales.
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Davoust, L., R. Moreau, M. D. Cowley, P. A. Tanguy i F. Bertrand. "Numerical and analytical modelling of the MHD buoyancy-driven flow in a Bridgman crystal growth configuration". Journal of Crystal Growth 180, nr 3-4 (październik 1997): 422–32. http://dx.doi.org/10.1016/s0022-0248(97)00238-8.
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Prakash, J., S. Gouse Mohiddin i S. Vijaya Kumar Varma. "Free Convective MHD Flow Past a Vertical Cone with Variable Heat and Mass Flux". Journal of Fluids 2013 (18.11.2013): 1–8. http://dx.doi.org/10.1155/2013/405985.
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A numerical study of buoyancy-driven unsteady natural convection boundary layer flow past a vertical cone embedded in a non-Darcian isotropic porous regime with transverse magnetic field applied normal to the surface is considered. The heat and mass flux at the surface of the cone is modeled as a power law according to qwx=xm and qw*(x)=xm, respectively, where x denotes the coordinate along the slant face of the cone. Both Darcian drag and Forchheimer quadratic porous impedance are incorporated into the two-dimensional viscous flow model. The transient boundary layer equations are then nondimensionalized and solved by the Crank-Nicolson implicit difference method. The velocity, temperature, and concentration fields have been studied for the effect of Grashof number, Darcy number, Forchheimer number, Prandtl number, surface heat flux power-law exponent (m), surface mass flux power-law exponent (n), Schmidt number, buoyancy ratio parameter, and semivertical angle of the cone. Present results for selected variables for the purely fluid regime are compared with the published results and are found to be in excellent agreement. The local skin friction, Nusselt number, and Sherwood number are also analyzed graphically. The study finds important applications in geophysical heat transfer, industrial manufacturing processes, and hybrid solar energy systems.
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Wood, Charles E., i Chris J. Lawn. "Two-phase MHD energy conversion from buoyancy-driven flows of liquid metal coolant in a fusion reactor". Fusion Engineering and Design 151 (luty 2020): 111288. http://dx.doi.org/10.1016/j.fusengdes.2019.111288.
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Liu, Zhipeng, Chaowei Jiang, Xueshang Feng, Pingbing Zuo i Yi Wang. "Numerical Simulation of Solar Magnetic Flux Emergence Using the AMR–CESE–MHD Code". Astrophysical Journal Supplement Series 264, nr 1 (22.12.2022): 13. http://dx.doi.org/10.3847/1538-4365/ac9d2b.
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Abstract Magnetic flux emergence from the solar interior to the atmosphere is believed to be a key process in the formation of solar active regions and driving solar eruptions. Due to the limited capabilities of observations, the flux emergence process is commonly studied using numerical simulations. In this paper, we develop a numerical model to simulate the emergence of a twisted magnetic flux tube from the convection zone to the corona, using the AMR–CESE–MHD code, which is based on the conservation-element solution-element method, with adaptive mesh refinement. The results of our simulation agree with those of many previous studies with similar initial conditions, but by using different numerical codes. In the early stage, the flux tube rises from the convection zone, being driven by magnetic buoyancy, until it reaches close to the photosphere. The emergence is decelerated there, and with the piling up of the magnetic flux, the magnetic buoyancy instability is triggered, which allows the magnetic field to partially enter into the atmosphere. Meanwhile, two gradually separated polarity concentration zones appear in the photospheric layer, transporting the magnetic field and energy into the atmosphere through their vortical and shearing motions. Correspondingly, the coronal magnetic field is also reshaped into a sigmoid configuration, containing a thin current layer, which resembles the typical pre-eruptive magnetic configuration of an active region. Such a numerical framework of magnetic flux emergence as established will be applied to future investigations of how solar eruptions are initiated in flux emergence active regions.
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Subhrajit Kanungo i Tumbanath Samantara. "Flow And Heat Transfer of Unsteady Two-Phase Boundary Layer Flow Past an Inclined Permeable Stretching Sheet with Electrification of Particles". CFD Letters 15, nr 5 (16.03.2023): 134–44. http://dx.doi.org/10.37934/cfdl.15.5.134144.
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In the present study, an analysis has been carried out for a particle laden boundary layer flow with existence of electrification of particles has been studied over an inclined permeable stretching sheet. In most of the MHD fluid flow problems, either the plate is externally supplied by the magnetic/electric field or the fluid is electrically conducting. In the present problem, neither the plate is electrified nor the fluid is electrically conducted, but due to the random motion of the particles, collision of particle-particle and particle–wall, the particles are electrified. This electric field affects the fluid flow and heat transfer of the flow problem. Again, in the previous literatures, Buoyancy force is considered in momentum equations of fluid phase only. But in reality, both the phases are affected by the buoyancy force. For this reason, a reasonable mathematical model for two-phase buoyancy driven flow has been formulated with the consideration of electrification of particles in both fluid and particle phase. The governing system of PDEs are transferred to system of ODEs by applying similarity transformations and then computed by implementing Runga-Kutta method. The impact of electrification and other fluid parameters on flow and heat transfer has been studied. The results are represented through graphs and tables
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Gouse, Mohiddin, Anwar Bég i Vijaya Varma. "Numerical study of free convective MHD flow past a vertical cone in non-Darcian porous media". Theoretical and Applied Mechanics 41, nr 2 (2014): 119–40. http://dx.doi.org/10.2298/tam1402119g.
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A numerical study of buoyancy-driven unsteady natural convection boundary layer flow past a vertical cone embedded in a non-Darcian isotropic porous regime with transverse magnetic field applied normal to the surface is considered. The heat and mass flux at the surface of the cone is modeled as a power-law according to qw(x) = xm and q*w (x) = xn respectively, where x denotes the coordinate along the slant face of the cone. Both Darcian drag and Forchheimer quadratic porous impedance are incorporated into the two-dimensional viscous flow model. The transient boundary layer equations are then non-dimensionalized and solved by the Crank-Nicolson implicit difference method. The velocity, temperature and concentration fields have been studied for the effect of Grashof number, Darcy number, Forchheimer number, Prandtl number, surface heat flux power-law exponent (m), surface mass flux power-law exponent (n), Schmidt number, buoyancy ratio parameter and semi-vertical angle of the cone. Present results for selected variables for the purely fluid regime are compared with the non-porous study by Hossain and Paul [9] and are found to be in excellent agreement. The local skin friction, Nusselt number and Sherwood number are also analyzed graphically. The study finds important applications in geophysical heat transfer, industrial manufacturing processes and hybrid solar energy systems.
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Suzuki, Takeru K., Yasuo Fukui, Kazufumi Torii, Mami Machida, Ryoji Matsumoto i Kensuke Kakiuchi. "Investigating Magnetic Activity in the Galactic Centre by Global MHD Simulation". Proceedings of the International Astronomical Union 11, S322 (lipiec 2016): 137–40. http://dx.doi.org/10.1017/s1743921316012461.
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AbstractBy performing a global magnetohydrodynamical (MHD) simulation for the Milky Way with an axisymmetric gravitational potential, we propose that spatially dependent amplification of magnetic fields possibly explains the observed noncircular motion of the gas in the Galactic centre (GC) region. The radial distribution of the rotation frequency in the bulge region is not monotonic in general. The amplification of the magnetic field is enhanced in regions with stronger differential rotation, because magnetorotational instability and field-line stretching are more effective. The strength of the amplified magnetic field reaches ≳ 0.5 mG, and radial flows of the gas are excited by the inhomogeneous transport of angular momentum through turbulent magnetic field that is amplified in a spatially dependent manner. As a result, the simulated position-velocity diagram exhibits a time-dependent asymmetric parallelogram-shape owing to the intermittency of the magnetic turbulence; the present model provides a viable alternative to the bar-potential-driven model for the parallelogram shape of the central molecular zone. In addition, Parker instability (magnetic buoyancy) creates vertical magnetic structure, which would correspond to observed molecular loops, and frequently excited vertical flows. Furthermore, the time-averaged net gas flow is directed outward, whereas the flows are highly time dependent, which would contribute to the outflow from the bulge.
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Hanasz, M., D. Woltanski i K. Kowalik. "Interstellar and intergalactic dynamos". Proceedings of the International Astronomical Union 8, S294 (sierpień 2012): 225–36. http://dx.doi.org/10.1017/s1743921313002573.
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AbstractWe review recent developments of amplification models of galactic and intergalactic magnetic field. The most popular scenarios involve variety of physical mechanisms, including turbulence generation on a wide range of physical scales, effects of supernovae, buoyancy as well as the magnetorotational instability. Other models rely on galaxy interaction, which generate galactic and intergalactic magnetic fields during galaxy mergers. We present also global galactic-scale numerical models of the Cosmic Ray (CR) driven dynamo, which was originally proposed by Parker (1992). We conduct a series of direct CR+MHD numerical simulations of the dynamics of the interstellar medium (ISM), composed of gas, magnetic fields and CR components. We take into account CRs accelerated in randomly distributed supernova (SN) remnants, and assume that SNe deposit small-scale, randomly oriented, dipolar magnetic fields into the ISM. The amplification timescale of the large-scale magnetic field resulting from the CR-driven dynamo is comparable to the galactic rotation period. The process efficiently converts small-scale magnetic fields of SN-remnants into galactic-scale magnetic fields. The resulting magnetic field structure resembles the X-shaped magnetic fields observed in edge-on galaxies.
Rozprawy doktorskie na temat "Buoyancy-driven MHD"
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Gupta, Adhip. "A Monolithic Finite Element Formulation for Magnetohydrodynamics Involving a Compressible Fluid". Thesis, 2023. https://etd.iisc.ac.in/handle/2005/6188.
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This work develops a new monolithic finite-element-based strategy for Magnetohydrodynamics (MHD)
involving a compressible fluid based on a continuous velocity-pressure formulation. The entire formula-
tion is within a nodal finite element framework, and is directly in terms of physical variables. The exact
linearization of the variational formulation ensures a quadratic rate of convergence in the vicinity of
the solution. Both steady-state and transient formulations are presented for two- and three-dimensional
flows. Several benchmark problems are presented, and comparisons are carried out against analytical
solutions, experimental data, or against other numerical schemes for MHD. We show a good coarse-mesh
accuracy and robustness of the proposed strategy, even at high Hartmann numbers.
Części książek na temat "Buoyancy-driven MHD"
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Müller, Ulrich, i Leo Bühler. "Buoyancy driven MHD flows". W Magnetofluiddynamics in Channels and Containers, 163–78. Berlin, Heidelberg: Springer Berlin Heidelberg, 2001. http://dx.doi.org/10.1007/978-3-662-04405-6_12.
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Davoust, L., R. Moreau i R. Bolcato. "Roads to Turbulence for an Internal MHD Buoyancy-Driven Flow Due to a Horizontal Temperature Gradient". W Transfer Phenomena in Magnetohydrodynamic and Electroconducting Flows, 123–33. Dordrecht: Springer Netherlands, 1999. http://dx.doi.org/10.1007/978-94-011-4764-4_9.
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Phethean, Jordan J. J., Martha Papadopoulou i Alexander L. Peace. "Dense melt residues drive mid-ocean-ridge “hotspots”". W In the Footsteps of Warren B. Hamilton: New Ideas in Earth Science. Geological Society of America, 2022. http://dx.doi.org/10.1130/2021.2553(30).
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ABSTRACT The geodynamic origin of melting anomalies found at the surface, often referred to as “hotspots,” is classically attributed to a mantle plume process. The distribution of hotspots along mid-ocean-ridge spreading systems around the globe, however, questions the universal validity of this concept. Here, the preferential association of hotspots with slow- to intermediate-spreading centers and not fast-spreading centers, an observation contrary to the expected effect of ridge suction forces on upwelling mantle plumes, is explained by a new mechanism for producing melting anomalies at shallow (<2.3 GPa) depths. By combining the effects of both chemical and thermal density changes during partial melting of the mantle (using appropriate latent heat and depth-dependent thermal expansivity parameters), we find that mantle residues experience an overall instantaneous increase in density when melting occurs at <2.3 GPa. This controversial finding is due to thermal contraction of material during melting, which outweighs the chemical buoyancy due to melting at shallow pressures (where thermal expansivities are highest). These dense mantle residues are likely to locally sink beneath spreading centers if ridge suction forces are modest, thus driving an increase in the flow of fertile mantle through the melting window and increasing magmatic production. This leads us to question our understanding of sub–spreading center dynamics, where we now suggest a portion of locally inverted mantle flow results in hotspots. Such inverted flow presents an alternative mechanism to upwelling hot mantle plumes for the generation of excess melt at near-ridge hotspots, i.e., dense downwelling of mantle residue locally increasing the flow of fertile mantle through the melting window. Near-ridge hotspots, therefore, may not require the elevated temperatures commonly invoked to account for excess melting. The proposed mechanism also satisfies counterintuitive observations of ridge-bound hotspots at slow- to intermediate-spreading centers, yet not at fast-spreading centers, where large dynamic ridge suction forces likely overwhelm density-driven downwelling. The lack of observations of such downwelling in numerical modeling studies to date reflects the generally high chemical depletion buoyancy and/or low thermal expansivity parameter values employed in simulations, which we find to be unrepresentative for melting at <2.3 GPa. We therefore invite future studies to review the values used for parameters affecting density changes during melting (e.g., depletion buoyancy, latent heat of melting, specific heat capacity, thermal expansivity), which quite literally have the potential to turn our understanding of mantle dynamics upside down.
Streszczenia konferencji na temat "Buoyancy-driven MHD"
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Agarwal, Ramesh. "Numerical study of MHD suppression of secondary motion in buoyancy-driven flows". W Fluids 2000 Conference and Exhibit. Reston, Virigina: American Institute of Aeronautics and Astronautics, 2000. http://dx.doi.org/10.2514/6.2000-2457.
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Wang, Hongyan, i Xidong Zhang. "Effect of Nature Convection on Heat Transfer in the Liquid LiPb Blanket". W 16th International Conference on Nuclear Engineering. ASMEDC, 2008. http://dx.doi.org/10.1115/icone16-48698.
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In some liquid blankets (or liquid blanket module) of fusion reactor, the liquid metal, i.e. LiPb flow, as only tritium breeder is characterized by lower outlet temperature and slower flow velocity that allows the utilization of relatively mature material technology [1–2]. The magnetohydrodynamic (MHD) flow and heat transfer become very complex resulting from the differential heating of walls of the channels, especially adjacent to the First Wall (FW), and internal heat sources inside of the liquid LiPb. The nature convection of the liquid LiPb, due to thermal diffusion, in the poloidal channel adjacent to the FW in the presence of the strong magnetic field of the blanket has been considered and studied. The temperature distribution is changed and there is a strong thermal coupling, modifying importantly the magnitude of the flow. The effect of the buoyancy on pressure driven duct flows has been investigated. The buoyant convection was found to be sufficiently strong to impose its flow pattern on the cross flow in the region of intense volumetric heating.
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Steen, Benjamin, i Kamran Siddiqui. "Turbulent Flow Behaviour in a Pipe Fully Submerged in a Hot Fluid". W ASME 2017 Fluids Engineering Division Summer Meeting. American Society of Mechanical Engineers, 2017. http://dx.doi.org/10.1115/fedsm2017-69541.
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We report on an experimental study conducted to investigate the flow behaviour in a heat exchanger pipe submerged in a hot stagnant fluid. Particle Image Velocimetry (PIV) was used to measure the two-dimensional velocity field in the mid-vertical plane of the tube. Fluid temperatures in the cross-sectional plane were also measured using thermocouples. The mode of heat transfer into the pipe was mixed convection where both inertia and buoyancy contributed to the convection. The results show that when the contribution of buoyancy-driven flow (natural convection) was smaller than that of the inertia-driven flow (forced convection), in an originally turbulent flow, the shear-induced turbulence dominated the flow and the turbulent velocity profile was not influenced by the heat input. In an originally laminar flow, the role of buoyancy was primarily limited to the initiation of instabilities in the laminar flow to trigger the turbulence transition. The temperature profiles indicate the presence of stably stratified layer inside the pipe in originally laminar flow regime that suppressed the heat transfer rate. In originally turbulent regime, the fluid temperature field was nearly uniform indicating efficient flow mixing.
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Haque, Mohammad Rejaul, i Amy Rachel Betz. "Numerical Study of Natural Convection Heat Transfer Over a Cooling Stage". W ASME 2017 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2017. http://dx.doi.org/10.1115/imece2017-71868.
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Numerical study of natural convection phenomenon close to a cooling stage has been investigated in a two dimensionally controlled environment. The study was conducted for ambient air which was kept at constant temperature (22°C) and Prandtl number of this Newtonian fluid was taken as 0.71. The cooling stage was kept at 5°C and height of the stage was considered 0.02 m. The stage was located at the bottom on the computational domain. The center of cooling stage was placed at X = 0.35 m and X = 0.5 m respectively from the left boundary of the domain. Later, computational analysis was performed to solve coupled momentum and energy equations for appropriate boundary conditions. The study was performed for a range of Rayleigh number from 102 to 107. Thermal and hydrodynamic behavior was reported in terms of isotherms, streamlines and average Nusselt number calculation. The position of the stage significantly effects heat transfer and flow fields. Nusselt number was evaluated close to cooling stage. Streamlines resulted huge recirculation region which was symmetric about the vertical mid-centerline of the domain for cooling stage located at X = 0.5 m. The center of core vortices shifted near to the cooling stage as Rayleigh number increases ensuring enhancement of heat transfer. Additionally, increasing Rayleigh number induces significant buoyancy driven flow. The velocity of this driven flow increases towards left and right wall as Rayleigh number increases. Velocity profile was also evaluated due to flow inside the enclosure. A parabolic variation was observed for horizontal velocity component near the isothermal walls and it was found less significant compared to vertical component due to buoyancy driven flow. Moreover, the asymmetric distribution of isotherms created by eccentric position of the stage resulted better enhancement than centric position of the cooling stage. Finally, this results would help the researchers find the optimal position of any sample on a cooling stage subjected to convection phenomenon. This could be significant in the collection of experimental data for condensation and frost formation.
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Merzari, Elia, Paul Fischer i Hisashi Ninokata. "Numerical Simulation of the Flow in a Toroidal Thermosiphon". W ASME-JSME-KSME 2011 Joint Fluids Engineering Conference. ASMEDC, 2011. http://dx.doi.org/10.1115/ajk2011-03084.
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Buoyancy-driven flows are widespread in diverse engineering applications. Such flows have been studied in great detail theoretically, experimentally, and numerically. The prototype of passive, residual heat removal systems is the toroidal thermosiphon. The stability properties of such systems were first examined in detail by Creveling et al. in the mid-1970s, who reported flow reversals and instability in this geometry. Traditionally, however, the stability analysis of natural convection loops has been confined to one-dimensional calculations, on the argument that the flow would be monodimensional when the ratio between the radius of the loop and the radius of the pipe is much larger than 1. Nevertheless, accurate velocity measurements of the flow in toroidal loops have shown that the flow presents three-dimensional effects. In the present work we analyze the stability problem in a toroidal loop and then use computational fluid dynamics to evaluate the relative importance of these three-dimensional effects with regard to stability. We performed a series of high-fidelity numerical simulations using the spectral element code Nek5000. We compared the results to the available data and calculations performed with the code STAR-CCM+ 5.06. The results show a much richer dynamics than expected from either previous calculations or stability theory. The results also point to some outstanding issues in the RANS modeling of such flows.
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Oh, Chang H., i Eung S. Kim. "Study on Air Ingress Mitigation Methods in the Very High Temperature Gas Cooled Reactor (VHTR)". W ASME/JSME 2011 8th Thermal Engineering Joint Conference. ASMEDC, 2011. http://dx.doi.org/10.1115/ajtec2011-44417.
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An air-ingress accident followed by a pipe break is considered as a critical event for a very high temperature gas-cooled reactor (VHTR) safety. Following helium depressurization, it is anticipated that unless countermeasures are taken, air will enter the core through the break leading to oxidation of the in-core graphite structure. Thus, without mitigation features, this accident might lead to severe exothermic chemical reactions of graphite and oxygen depending on the accident scenario and the design. Under extreme circumstances, a loss of core structural integrity may occur along with excessive release of radiological inventory. Idaho National Laboratory under the auspices of the U.S. Department of Energy is performing research and development (R&D) that focuses on key phenomena important during challenging scenarios that may occur in the VHTR. Phenomena Identification and Ranking Table (PIRT) studies to date have identified the air ingress event, following on the heels of a VHTR depressurization, as very important (Oh et al. 2006, Schultz et al. 2006). Consequently, the development of advanced air ingress-related models and verification and validation (V&V) requirements are part of the experimental validation plan. This paper discusses about various air-ingress mitigation concepts applicable for the VHTRs. The study begins with identifying important factors (or phenomena) associated with the air-ingress accident using a root-cause analysis. By preventing main causes of the important events identified in the root-cause diagram, the basic air-ingress mitigation ideas can be conceptually derived. The main concepts include (1) preventing structural degradation of graphite supporters; (2) preventing local stress concentration in the supporter; (3) preventing graphite oxidation; (4) preventing air ingress; (5) preventing density gradient driven flow; (6) preventing fluid density gradient; (7) preventing fluid temperature gradient; (7) preventing high temperature. Based on the basic concepts listed above, various air-ingress mitigation methods are proposed in this study. Among them, the following one mitigation idea was extensively investigated using computational fluid dynamic codes (CFD) in terms of helium injection in the lower plenum. The main idea of the helium injection method is to replace air in the core and the lower plenum upper part by buoyancy force. This method reduces graphite oxidation damage in the severe locations of the reactor inside. To validate this method, CFD simulations are addressed here. A simple 2-D CFD model was developed based on the GT-MHR 600MWt as a reference design. The simulation results showed that the helium replaces the air flow into the core and significantly reduces the air concentration in the core and bottom reflector potentially protecting oxidation damage. According to the simulation results, even small helium flow was sufficient to remove air in the core, mitigating the air-ingress successfully.