Pool boiling cooling for melt spinning quench wheels
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I.
INTRODUCTION
M E L T spinning rapid solidification involves applying a molten metal alloy to the outside surface of a rotating wheel. A thin ribbon of alloy is drawn from the melt and quenched by the wheel surface. The problem of maintaining a low average casting surface temperature for continuous ribbon casting can be solved in a variety of ways: single phase forced convection, forced convection boiling, film vaporization, spray cooling, and pool boiling. In laboratory devices, where wheel size and cost are relatively unimportant, any of the cooling methods might be implemented to suit particular needs. The key advantage offered by pool boiling for production devices is simple geometry: a capped, plain hollow cylinder with a liquid feed and vapor exit port. Excellent uniform heat transfer in the boiling, centrifugally accelerated, annular liquid layer does not require finned heat exchange surfaces, flow constrictions to increase convection, or thick cylinder walls for beat spreading; as a result, cylinder wall thickness is dictated by mechanical design, wheel compactness is maximized, and axial scalability is straightforward. The complexity of auxiliary equipment will depend on the choice of boiling fluid and operating pressure, but in the simple case of atmospheric boiling water, inexpensive, low pressure, rotating seals may be used to vent steam directly without heat exchangers. The casting surface of the wheel sees extreme variations in applied heat flux, but in many cases this variation will damp out to present the interior with a circumferentially average flux. Although boiling may be suitable in cases with circumferential variations in heat flux, the analysis is more complicated and must include the effects of ribbon to wheel heat transfer.
II.
POOL BOILING
The convection heat transfer of forced flow systems is replaced by bubble induced convection in pool boiling
R. S. MILLER is Mechanical Engineer with General Electric Company, Corporate Research and Development, Schenectady, NY 12301. Manuscript submitted May 25, 1982. METALLURGICAL TRANSACTIONS B
cooled designs. The increasing bubble generation with heat flux results in rapidly increasing boiling heat transfer coefficients. While a number of correlations are available for predicting heat transfer coefficients in pool boiling, problems exist in their use for melt spinning applications. The key problems are that acceleration is either ignored or predicted to have a significant effect in various correlations, and melt spinning produces heat fluxes which are generally much higher than those of the data used to construct the correlations. An example of a pool boiling correlation based on bubble microconvection phenomenology and including an acceleration dependence, is the widely cited correlation of Rohsenow 1 (Eq. [1]). The accuracy of the correlation in accounting for flux and fluid property effects in boiling is adequate and well documented at one gravity acceleration; however, the predicted heat transfer coefficient dependence on acceleration to
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