Effect of Sample Start Temperature during Transient Boiling Water Heat Transfer
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TRODUCTION
WATER is commonly used in metallurgical and materials manufacturing operations to cool products at a prescribed rate or down to a given temperature. Example processes include casting, hot deformation processes, and heat-treatment processes. In these processes, water can be imparted onto the surface of the product in a variety of ways including via jet, spray, and mist systems, and via agitated and stagnant pools. In most industrial metallurgical operations, the temperature of the component being cooled is significantly greater than the boiling point of water, and hence transient boiling water conditions are experienced. Quantitative knowledge of the heat transfer between a product and chill water and its variation as a function of the surface temperature is critical. Process models often require techniques to model boiling water heat transfer effectively and to illustrate how these boiling curves are affected by many factors including the starting surface temperature of the component being cooled. An industrial example of this is the start-up phase of the direct chill (DC) casting process, where the temperature of the ingot in contact with the water evolves temporally, or the runout table (ROT) cooling of steel, where the surface temperature of the steel changes as it comes in contact with impinging jets. This article outlines a technique to incorporate the effect of the starting temperature of the D. LI, formerly Postdoctoral Student, Department of Materials Engineering, University of British Columbia, is with Belvac Production Machinery, Lynchburg, VA 24502-4203. M.A. WELLS, Associate Professor, S.L. COCKCROFT, Professor, and E. CARON, Postdoctoral Candidate, are with the Department of Materials Engineering, University of British Colombia, Vancouver, BC, Canada V6T 1Z4. Contact e-mail: [email protected] Manuscript submitted November 1, 2006. Article published online November 6, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
component being cooled on the boiling heat transfer, so that it makes it relatively easy to generate the relevant boiling curves that should be applied. During transient boiling water heat transfer, large variations in the rate of heat extraction at the surface of the component are experienced during cooling. Since microstructure and product property evolution are related to the thermal history experienced by the material in many processes, a quantitative understanding of the cooling rates experienced by the component during quenching is necessary to find the optimum way to control and improve quality.
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BOILING WATER HEAT TRANSFER
A typical pool boiling curve is shown in Figure 1,[1] which shows the relationship of heat flux to surface superheat (the difference between the surface temperature and the water saturation temperature). In general, the boiling curve can be divided into four regimes of behavior as follows: (1) free convection cooling (DT < DTA); (2) nucleate boiling (DTA < DT < DTC); (3) transition boiling (DTC < DT < DTD); and (4) stable film boiling (DT > DTD). As the sur
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