Mechanism of Solid-Liquid Contact During Rapid Quenching of Thin Wires in Liquids

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Development of a rapid immersion cooling technique is very important as a basis of the production of fibers of a new class of high performance materials, such as amorphous alloys, nanocrystalline alloys etc., by melt spinning process. Also, understanding of the mechanism of the incipience of solid-liquid contact during the immersion cooling process is very important because it is closely connected with the vapor explosion phenomena. In this study, experiments were conducted to investigate the fluid flow and heat transfer characteristics during the rapid quenching of thin horizontal platinum wires (0.3 and 0.5 mm in diameter) in a pool of subcooled liquids (water, CaCl_2/water solution and ethanol). The falling speed of the test wire ranged from 0.1 to 1.5m/s, the subcooling of quenching liquid from 20 to 80K,and the concentration of CaCl_2 from 0 to 40%. For water and CaCl_2/water solution, the solid-liquid contact during the quenching process was measured using a kind of conductance p … More robe. The results are summarized as follows : (1) The transient boiling curve obtained from the cooling curve has two minimum-heat-flux points. (2) At a wall temperature higher than the upper minimum-heat flux point(named the M1 point), the wire is covered with vapor film and the occurrence of solid-liquid contact is very rare. When the wall superheat decreases below the M1 point, both the solid-liquid signal and the heat flux increase rapidly. (3) In the region of wall superheat exceeding the lower minimun-heat-flux point (named the M2 point) , the wall superheat at the M1 point and the heat flux increases with increasing liquid subcooling, falling velocity and CaCl_2 concentration. (4) The wall superheat at the M2 point is constant irrespective of the liquid subcooling and falling velocity. It increases with increasing CaCl_2 concentration. (5) In the region of wall superheat important for the production of amorphous alloys, the average heat flux increases as much as 140% by use of the CaCl_2 solution. The mechanism of solid-liquid contact was studied theoretically from the viewpoint of hydrodynamic stability of thin vapor film formed on the wire surface. The theory considered the linear stability of disturbance superimposed on the vapor film. The motion was assumed to be governed by the wave equation on the liquid side and by the laminar flow equation on the vapor side. The theoretical prediction of the heat transfer coefficient at the neutral stability point compared well with that at the M1 point for water and ethanol. Less