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BACG symposium on natural convection and crystal growth
Journal of Crystal Growth 11(1971) 104
North-Ho/land Publishing Co.
NEWS, REPORTS AND ANNOUNCEMENTS
BACG SYMPOSIUM ON NATURAL CONVECTION AND CRYSTAL GROWTH University of Birmingham, U.K., 6 April 1971
The British Association for Crystal Growth held a symposium on this topic at the University of Birmingham on 6 April 1971. The main aim was to introduce crystal growers, and those interested in castings, to the basic phenomena of natural convection. Members who arrived the previous evening were entertained with a fascinating lecture by ProfessorJ. W. Elder(Manchester University) on “The role of thermal turbulence in the earth’s mantle”. This included a spectacular film of a volcanic erruption. The morning session was devoted to four papers reviewing natural convection. Dr. E. Jakeman (RRE) in his introductory lecture explained the concept of hydrodynamic stability and went on to illustrate the methods by which stability problems are treated theoretically. Mr. P. Mason (Meteorological Office) considered the effects of rotation on natural convection and iilustrated part of his talk with a film showing flow in a rotating annulus with an impressed temperature gradient. In his talk on thermosolutal convection Dr. A. Gill (Cambridge University) described mixing phenomena which occur when two diffusing entities (e.g. solute and heat) produce contributions to the overall density gradient which oppose each other. A film of the mixing layers formed between salt and sugar solutions was shown. The behaviour of fluids at high Rayleigh numbers and the transition to turbulence was described by Professor J. W. Elder in the final morning’s talk. Many of the fluids of most interest to crystal growers (e.g. liquid metals, molten oxides and salts) have low Prandtt numbers and unfortunately it is just this class of fluids for which our understanding of convective behaviour is poorest. Some of the complexities of the 104
behaviour of such fluids were evident in the first two papers in the afternoon which described experimental studies of non-steady convection in liquid metals. Mr. M. G. Skafel (Manchester University) reported measurements of the power spectrum of temperature fluctuations in a vertical annulus of liquid mercury with radial heat flow whilst Dr. D. T. J. Hurle (RRE) described sinusoidal temperature oscillations in liquid gallium contained in a horizontal boat and heated from the side. The final two papers, by Dr. B. Cockayne (RRE) and Dr. J. D. Hunt (Oxford University) reviewed the state of our knowledge of convection in crystal growth melts and in castings respectively. Dr. Cockayne discussed the results of model flow experiments and their limitations together with visual observations and temperature measurements on oxide melts. Dr. Hunt reviewed his recent work on the role of convection in the “mushy” zone on the formation of “A” and “V” segregates. He also described the very recent U.S. work on the role of thermosolutal convection in producing “freckles” in uni-directionally solidified castings. The meeting ended with a lively discussion period. The success of thermosolutal models in explaining some segregation phenomena in castings is impressive and the discussion showed that a number of people felt that an investigation of thermosolutal effects in melt and flux growth would be worthwhile. The potential complexity of convective motion in crystal growth melts is formidable but, hopefully, those who attended will have learnt of something which they can relate to their own techniques of crystal growth or metal casting. D. T. J. HURLE RRE, United Kingdom
North-Ho/land Publishing Co.
NEWS, REPORTS AND ANNOUNCEMENTS
BACG SYMPOSIUM ON NATURAL CONVECTION AND CRYSTAL GROWTH University of Birmingham, U.K., 6 April 1971
The British Association for Crystal Growth held a symposium on this topic at the University of Birmingham on 6 April 1971. The main aim was to introduce crystal growers, and those interested in castings, to the basic phenomena of natural convection. Members who arrived the previous evening were entertained with a fascinating lecture by ProfessorJ. W. Elder(Manchester University) on “The role of thermal turbulence in the earth’s mantle”. This included a spectacular film of a volcanic erruption. The morning session was devoted to four papers reviewing natural convection. Dr. E. Jakeman (RRE) in his introductory lecture explained the concept of hydrodynamic stability and went on to illustrate the methods by which stability problems are treated theoretically. Mr. P. Mason (Meteorological Office) considered the effects of rotation on natural convection and iilustrated part of his talk with a film showing flow in a rotating annulus with an impressed temperature gradient. In his talk on thermosolutal convection Dr. A. Gill (Cambridge University) described mixing phenomena which occur when two diffusing entities (e.g. solute and heat) produce contributions to the overall density gradient which oppose each other. A film of the mixing layers formed between salt and sugar solutions was shown. The behaviour of fluids at high Rayleigh numbers and the transition to turbulence was described by Professor J. W. Elder in the final morning’s talk. Many of the fluids of most interest to crystal growers (e.g. liquid metals, molten oxides and salts) have low Prandtt numbers and unfortunately it is just this class of fluids for which our understanding of convective behaviour is poorest. Some of the complexities of the 104
behaviour of such fluids were evident in the first two papers in the afternoon which described experimental studies of non-steady convection in liquid metals. Mr. M. G. Skafel (Manchester University) reported measurements of the power spectrum of temperature fluctuations in a vertical annulus of liquid mercury with radial heat flow whilst Dr. D. T. J. Hurle (RRE) described sinusoidal temperature oscillations in liquid gallium contained in a horizontal boat and heated from the side. The final two papers, by Dr. B. Cockayne (RRE) and Dr. J. D. Hunt (Oxford University) reviewed the state of our knowledge of convection in crystal growth melts and in castings respectively. Dr. Cockayne discussed the results of model flow experiments and their limitations together with visual observations and temperature measurements on oxide melts. Dr. Hunt reviewed his recent work on the role of convection in the “mushy” zone on the formation of “A” and “V” segregates. He also described the very recent U.S. work on the role of thermosolutal convection in producing “freckles” in uni-directionally solidified castings. The meeting ended with a lively discussion period. The success of thermosolutal models in explaining some segregation phenomena in castings is impressive and the discussion showed that a number of people felt that an investigation of thermosolutal effects in melt and flux growth would be worthwhile. The potential complexity of convective motion in crystal growth melts is formidable but, hopefully, those who attended will have learnt of something which they can relate to their own techniques of crystal growth or metal casting. D. T. J. HURLE RRE, United Kingdom