Malcolm, David George (1968) Thermo-anemometry in magnetohydrodynamics. PhD thesis, University of Warwick.

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Abstract

The problem of developing a reliable technique for measuring instantaneous velocities at an arbitrary point in a magnetohydrodynamic (MHD) flow of mercury is the primary concern of the thesis. The solution to this problem is urgently needed in order that measurements may be taken in practical MHD devices and in order that the interesting theoretical work which has emerged in the past decade may be substantiated by experimental evidence. Much of the information on measurement techniques contained herein will be applicable to other liquid metal flows. The recent advances in the area of constant temperature hot-film anemometry have made this technique the best one for instantaneous measurements in low-temperature liquid metal systems.

The thesis is divided into two parts. The study of thermoanemometry is described in chapter 2 and its application to some MHD experiments is described in chapter 3. Chapter 2 is divided into two main sections, §2.2 and §2.3, which
discuss the theory of thermo-anemometry in mercury and demonstrate its application in calibration exercises. The hot-film sensors tested are the tiny (0.030mm diameter and 0.5mm length) quartz-insulated platinum film sensors which are now made available for work in electrically conducting fluids by Thermo-Systems Inc., St,Paul, Minnesota, U.S.A. An account of our attempts to construct somewhat similar sensors in the laboratory is given in Appendix B.

In addition to discussing various calibration problems, such as MHD effects on free convective heat transfer from the hot-film and its dynamic response, chapter 2 develops, and demonstrates by experiment, new methods for the direct measurement of low turbulence intensi.ties in low Prandtl number fluids. The simple formulae presented are likely to be more accurate in the majority of cases than more sophisticated methods involving linearization of the anemometer voltage with respect to velocity because of their lack of sensitivity to signal drift. Chapter 3 discusses theoretical aspects in S3.2 and their experimental substantiation in §3.3 for two different types of MHD flow. The first of these is electrically driven flow in mercury between two circular, highly conducting electrodes Which are placed opposite one another in insulating planes with a magnetic field normal to their surfaces. The steady flow is examined theoretically by an asymptotic analysis and experimentally using Pitot tubes and electric potential probes. The experiments give some insight into the behaviour of these probes in MHD situations. This same flow is studied using a hot-film sensor with good results. This experiment points to the need for an extensive study of the MHD effects on hotfilm heat transfer and the associated MHD errors in measurements for various orientations of the sensor relative to the magnetic field.

Hot-film techniques are then used to examine the instability and subsequent wave-like secondary flow (when the Hartmann number, M, is very high) in this same electrically driven flow. These phenomena are interesting but very difficult to understand. It is shown that the transverse magnetic field, if strong enough, can control the stability of, and secondary flow in, the electrically driven shear layprs through direct action on the most unstable three-dimensional disturbances even though most of the vorticity of these disturbances is parallel to the magnetic field. An attempt is made to discuss the physical mechanism involved. The second tyPe of MHD flow investigated is turbulent vorticity suppression behind a square-array wire grid in a transverse magnetic field. The exper-Iment,is of a preliminary nature but demonstrates some interesting phenomena which can be explained in order of magnitude terms.

Both of the MHD flows studied demonstrete various aspects of the behaviour of the hot-film sensors as well as being interesting in themselves.
The paper listed in the Table of Contents under Appendix A as "Malcolm (1968b)'may be read as an extended summary of the thesis.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QB Astronomy T Technology > TA Engineering (General). Civil engineering (General)
Library of Congress Subject Headings (LCSH):	Magnetohydrodynamics, Anemometer, Fluid dynamic measurements
Official Date:	August 1968
Dates:	Date Event August 1968 Submitted
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Alty, C. J. N.
Sponsors:	National Research Council of Canada; University of Saskatchewan
Extent:	1 volume (various pagings)
Language:	eng

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