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Title: Thermoelectric magnetohydrodynamics
Author: Dutta Gupta, Punya Brata
ISNI:       0000 0001 3436 3123
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1979
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The effect of the interaction of steady uniform transverse magnetic fields with the thermoelectic current in liquid metals that might exist at the nonisothermal, electically conducting interface of the metal liners, termed as TEMHD hereafter, is the subject of some theoretical and extensive experimental investigation reported in this thesis. The investigation has primarily been motivated by the possibility of significant consequences of such an effect in the lithium blankets of magnetically confined thermonuclear reactors (TNR) and the lack of any previous serious attempt to investigate this interaction experimentally. It is shown theoretically that at high Hartmann numbers (M) in various TEMHD configurations, the flow is equivalent to the ordinary MHD flow with an inherent pressure gradient due to the thermoelectric current and magnetic field, B. How well the TEMHD interaction is effective is measured in terms of a new nondimensional number D, the ratio of thermoelectric e.m.f. and the motion (determined by viscous and inherent force balance) induced e.m.f. Theoretical analysis for the secondary flow due to buoyancy in simple configurations shows that the secondary flow is at least an order of magnitude lower than the corresponding TEMHD flow. However, for other complex configurations there exists a B , a function of Rayleigh and Prandti numbers, in order to ignore the buoyancy effect. The experimental results obtained with Hg in Cu channels, as reported in this thesis, show that even in laboratory model experiments this TEMHD interaction in liquid metals in metal containers with non-isothermal interface does produce a significant body force. This body force is measurable in terms*of either the fluid flow velocity (stirring) or the static pressure gradient if the fluid flow is suitably constrained. It has been shown that these measurements provide a new method of finding experimentally the thermopower of liquid metals in magnetic fields as accurately as in conventional methods if the configuration is rightly chosen. These measurements are sensitive enough to the presence of traces of impurity to serve the purpose of determining the degree of refinement in metallurgical processes. Since lithium has an absolute thermopower, ignoring the sign, four times that of Hg, the scaling up of the TEMHD performance in Hg-Cu combination gives an adequate simulation of the blanket system TEMHD .With the complex geometry of TNR and the magnetic field system therein, the TEMHD configurations to be considered are very varied. Initially, to avoid buoyancy driven flow, simple thermally stable stratified configurations have been adopted in the form of (i) uniform horizontal copper pipes of circular cross-section (1.25 and 2.54 cm in diameter) with a sinusoidal (first harmonic) peripheral temperature distribution, and (ii) horizontal straight copper pipe with a rectangular cross-section (2i 2.5 cm square) with insulated sidewall interfaces and heated from the top and cooled at the bottom. Apart from the static pressure difference and velocity measurements, temperature oscillations are reported even in these basic thermally stable configurations which are suppressed at some field strength,B cr. For a fixed heat transfer,- the temperature in the TEMHD system has been found to drop with increase in B. These two unexpected experimental observations have been tentatively explained on the basis of finite amplitude instability due to magnetic-buoyancy force driven relaxation of isotherm curvature. In order to investigate TEMHD flow without pressure gradient, experimental investigations in (endless) annular channels with the top surface free, heated and cooled at the vertical sidewalls in uniform steady axial magnetic fields have been performed for mixed and totally conducting solid-liquid interfaces. Since in this configuration the buoyancy effect is inherently present, a few measurements of secondary flow and free surface profile are presented along with velocity and temperature measurements. The temperature at the hot wall increases with B as is normally expected, due to suppression of buoyancy induced vorticity by B. The critical magnetic field for transverse vorticity suppression and the build-up of magnetic drag by boundary layer formation has been found to occur at distinctly different values. By the choice of suitable nonlinear heating and mixed conducting interfaces, it has been shown that free shear layers could be generated. This experiment also confirms the possibility of higher TEMHD velocity as a function of B for aligned temperature gradient and B, as predicted by theoretical analysis. A discussion on the vorticity and stationary and drifting wave motion, as has been observed in these experiments, is provided. A simple novel structure of the lithium blanket of the TOKAMAK type of TNR in the form of double spiral modules is suggested to exploit TEMHD effect to pump lithium for heat transfer purpose. Limited experimental results that could be obtained on a laboratory model, with Hg as the fluid and the module fabricated with Cu are presented. The experimental measurements have been conducted with inclined tube manometers, flowmeters, mechanical veined vorticity-velocity probes, pitot tube • and thermoelectric potential probes. The thermoelectric potential probes, which have been iinverted as a modification of the ordinary potential probes to suit the TEMHD environment, provide simultaneous measurement of temperature and velocity at B > Bcr and the calibration procedure is much easier than for the alternative methods of measurement. A chapter on the review of literature relevant to TEMHD is provided to serve as a background of the investigation reported here, and a few areas of further investigation in TEMHD are suggested.
Supervisor: Not available Sponsor: Commonwealth Scholarship Commission in the United Kingdom ; Ministry of Education, India
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics