Session: 16-01-01: NSF-funded Research (Grad & Undergrad)

Paper Number: 77071

Start Time: Wednesday, 02:25 PM

77071 - Magnetoconvection in a Horizontal Duct Flow at Very High Hartmann and Grashof Numbers 

Combined convection and magnetohydrodynamic effects dramatically change the nature of flows of electrically conducting fluids. A prominent example of technological applications is the liquid metal blankets of nuclear fusion reactors where an electrically conducting fluid serves as a coolant, radiation shield, and tritium breeder. A distinctive feature of this system is that the convection and magnetic field effects are both exceptionally strong.

This study addresses a recently discovered and still poorly understood phenomenon - the high amplitude fluctuations in flows in ducts and pipes, so-called magneto-convective fluctuations. They appear in the conditions of a very strong magnetic field effect, i.e. in the range of Hartmann numbers, where turbulence is fully suppressed by magnetic damping. In experiments, the magneto-convective fluctuations are manifested by oscillations of temperature with very high amplitude (up to 50K in some cases) and typical frequencies much lower than the frequencies of turbulence-induced fluctuations. The effect has potentially serious consequences for the design and operation of liquid metal blankets of future fusion reactors. Should the fluctuations appear in an actual blanket, they will lead to strong and unsteady thermal stresses in the walls. Due to their possibly very large amplitude, the stresses will threaten the structural integrity of a fusion reactor system. Significant effects on heat transfer, transport of tritium, and wall corrosion are also anticipated. It is yet impossible to say how realistic these expectations are since no experiments or computations at very high Hartmann and Grashof numbers typical for reactor conditions have been conducted so far.

Direct numerical simulations and linear stability analysis are carried out to study mixed convection in a horizontal duct with constant-rate heating applied at the bottom and imposed transverse horizontal magnetic field. A two-dimensional approximation corresponding to the asymptotic limit of very strong magnetic field effect is validated and applied, together with full three-dimensional analysis, to investigate the flow's behavior in the previously unexplored range of control parameters corresponding to typical conditions of a liquid metal blanket of a nuclear fusion reactor. 

It is found that the instability to quasi-two-dimensional rolls parallel to the magnetic field discovered at smaller Hartmann and Grashof numbers in earlier studies also occurs in this parameter range. Transport of the rolls by the mean flow leads to magnetoconvective temperature fluctuations of exceptionally high amplitudes. The fluctuations are not suppressed or even significantly reduced in amplitude by the very strong magnetic field. The amplitude remains high, reaching tens or hundreds (depending on the value of Grashof number) K in a typical duct geometry. This has significant far-reaching implications for mixing, heat and mass transfer, and structural integrity of reactor components. The most dangerous modes of instability have the form of rolls localized in the lower half of the duct and having the streamwise wavelength measured in horizontal half-widths of the duct

It is also demonstrated that the quasi-two-dimensional structure of flows at very high Hartmann numbers does not guarantee the accuracy of the classical two-dimensional approximation. The accuracy deteriorates at the highest Grashof numbers considered in the study.

Work is supported by the US NSF (Grant CBET 1803730 "Extreme magnetoconvection").

Presenting Author: Ruslan Akhmedagaev University of Michigan - Dearborn

Authors:

Ruslan Akhmedagaev University of Michigan - Dearborn
Oleg Zikanov University of Michigan - Dearborn
Yaroslav Listratov Moscow Power Engineering Institute