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Stability of MHD Buoyancy Driven Flows Presented by Naveen Vetcha (UCLA) With contribution from: Sergey Smolentsev (UCLA) Rene Moreau (Prof., Lab. EPM, ENSHM de Grenoble, France) Mohamed A. Abdou (Prof., UCLA) FNST MEETING August 18-20, 2009 UCLA

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Outline Importance of the study Problem formulation Linear stability analysis Results and discussions Conclusions Future work

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Calculations by Smolentsev. Buoyancy flows are dominant in the blanket Nusselt number increase by 20-50% (Authie et. al., European Journ. of Mech. B/Fluids, 22, 2003, 203-220) can lead to higher heat loss to He which is undesirable. Recirculation flows may lead to “hot spots” at the interface. Temperature fluctuations caused due to the instability may lead to fatigue of the wall material. Recirculation flows may lead to local Tritium accumulation making its extraction more complicated. Will affect temperature distribution in the FCI. ############################# Linear stability analysis is performed in order to predict conditions under which flow becomes turbulent, but quantitative knowledge of the turbulent flow requires full numerical modeling or experiment. Due to buoyancy the characteristic velocity is higher (~25-30 cm/s) compared to that in the forced flow (10 cm/s). What type of flow regime prevails in the poloidal ducts? Is it laminar or turbulent ?

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Two particular cases: upward and downward flows Sketch illustrating the forced flow direction with respect to the gravity vector and the magnetic field (Smolentsev et.al.). ParameterITERDEMO OB Ha650012,000 Re30,00060,000 Gr7.0x10 9 3.5x10 12 Basic dimension less flow parameters. Poloidal flow. ITER, DEMO OB (outboard). In the downward flows, recirculation flows can be predicted for both ITER and DEMO. This suggests that instability mechanisms in upward and downward flows are different. In DEMO, Gr is sufficiently higher ~ 10 12. This makes the problem really complicated.

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Problem formulation Hartmann effect Buoyancy term Volumetric heat flux n = 1: buoyancy assisted (upward) flow; n = 2: buoyancy opposed (downward) flow. The flow is quasi-two-dimensional (Q2D) The FCI acts as ideal thermal/electric insulator Inflectional instability is dominating over “thermal-buoyant” instability Inlet/outlet effects are not considered at

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Linear Stability analysis All the flow variables are split up into mean and fluctuating parts. We look for solution Modified Orr-Sommerfeld equation Stable, Unstable;Neutral. at We look at the sign of by solving an Eigenvalue problem. -wave number, where is amplification factor.

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Results. Upward flow: flow patterns Upward flow Re = 10,000; Gr = 4e+07 and Ha = 100. Base Velocity Vorticity Boundary layer Internal shear layer Two types of instabilities have been observed. Primary instability. Inflectional instability associated with the internal shear layer. Secondary instability. The vortices generated at the inflection points make the side layer unstable.

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Different types of Instabilities Re = 5e+04 Re = 1e+05 Re = 2e+06 Unstable Ha = 10

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At Re~10 3 -10 4, Gr~10 6 -10 9 the flow is stable if Ha>250-800 In the range of flow parameters relevant to DCLL, the mixed convection upward flow seems to be stable Results. Upward flow: neutral stability curves

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Downward flow Re = 30,000; Gr = 1e+08 and Ha = 1500. Base Velocity Results. Downward flow: flow patterns As the velocity profile shows a shear layer a shear layer instability is expected and two rows of vortices appeared. Flow is observed to be unstable at very high Hartmann numbers also. This instability at high Hartmann number shows that downward flow can be unstable under the blanket conditions. need to extend the analysis to solve the complete set of governing equations. Internal shear layer Boundary layer Vorticity

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Results. Downward flow: neutral stability curves The critical Hartmann number is comparable to the Ha relevant to the blanket conditions. In the range of flow parameters relevant to DCLL, the mixed convection downward flow can be unstable.

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Conclusions Upward flow: The primary instability is Inflectional instability due to the internal shear layer Primary vortices destabilized the side layer. The critical Ha is very small compared to the fusion relevant values, so the upward flow is stable (considering the effects of only buoyancy). Downward flow: Instability mechanism appears to be same as that of upward flow but inflectional instability is stronger due to the recirculation. The critical Ha is comparable to the fusion relevant values so this flow can be unstable. Need for the full study of the downward flow.

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Future work The same analysis should be repeated by including the effect of thermal-buoyant instability. Small conductivity of walls leads to M shaped profiles in the channel which will increase the chances of flow becoming unstable, this has to be studied. It was observed that slip flow increase the chances of instability (Smolentsev. S, MHD duct flows under hydrodynamic slip condition, Theor. Comp. Fluid. Dyn., 2009). Extend the analysis to three dimensional disturbances. Full numerical modeling of the flow.

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