Mechanism of Vapor Film Collapse During Rapid Quenching of Thin Wires in Liquid

快速淬火技术的发展对非晶、纳米晶等高性能材料的生产具有重要意义。采用超声振动的方法提高淬火过程中的冷却速度。一根加热到高温的细水平铂丝被迫以恒定的速度落入其中浸没有超声振动器的冷却剂浴中。将水和乙醇用作测试液体。研究了超声振动声压和频率对瞬态冷却特性的影响。随着声压的增加，膜态沸腾的最小热流点（MHF点）向较高的壁温方向移动，MHF点后的热流密度提高了200%以上。对水的影响更为显著。从理论上研究了冷却体表面汽膜破裂的机理。考虑了液体惯性、蒸汽粘性和可压缩性以及传热等因素的影响，对水平圆柱表面强制对流膜状沸腾汽膜稳定性进行了线性分析。与以前提出的理论不同，该理论可以解释MHF点处的壁厚随液体过冷度增加而增加的实验结果。并对水平铂丝在自然对流条件下淬火时的瞬态传热特性进行了实验研究。产生的蒸汽在金属丝上形成珠状气泡，这些气泡以几乎恒定的距离间隔开。对于这种情况，还发展了汽膜的线性稳定性理论。

Development of a rapid quenching technique is very important for the production of high performance materials such as amorphous and nano-crystalline alloys. Experiments were conducted to enhance the cooling speed during the quenching process by use of ultrasonic vibration. A thin horizontal platinum wire heated to a high temperature was forced to fall at a constant speed into a coolant bath in which an ultrasonic vibrator was immersed. Water and ethanol were used as test liquids. The effects of the sound pressure and the frequency of ultrasonic vibration on the transient cooling characteristics were studied. The minimum-heat-flux point (MHF point) of film boiling shifted toward a higher wall temperature with increasing sound pressure, and the heat flux after the MHF point was enhanced by more than 200 %. The effect was more significant for water. A theoretical study was conducted to clarify the mechanism of the collapse of vapor film on the cooled body. A linear stability analysis of vapor film in forced convection film boiling on a horizontal cylinder was presented that considered the effects of liquid inertia, vapor viscosily and compressibility, and heat transfer. Unlike the previously proposed theories, this theory could explain the experimental result that the wall superheat at the MHF point increased with increasing liquid subcooling. Experiments were also conducted to clarify the transientheat transfer characteristics during the quenching of a horizontal platinum wire under a natural convection condition. The generated vapor formed beads-like bubbles on the wire that were spaced at a nearly constant distance. A linear stability theory of the vapor film was also developed for this case.