Abstract
Rewetting is the establishment of water-surface contact that occurs during quenching of high temperature surfaces by water jet impingement. Rewetting is an unexpectedly complex phenomenon that has been reported to occur at surface temperatures significantly higher than the superheating limit of water. The presence of intermittently wet and dry episodes, and in particular the occurrence of so-called explosive boiling, is one of the theories to explain the contact of water with high temperature surfaces. However, there is a lack of experimental data in the literature to prove the presence of explosive boiling and intermittent wetting due to the small duration and scale of the rewetting phenomenon. In this study, recordings of the jet stagnation zone during rewetting are provided at a frame rate of 81 kfps. The high-speed recordings show a flashing regime consisting of intermittent (dry) bubble-rich and (wet) bubble-free periods at frequencies up to 40 kHz when the rewetted surface temperature exceeds the water superheat limit. As far as the authors know, these are the first direct observations of intermittent dry-wet periods occurring in the jet stagnation zone during quenching by water jet impingement. The dependency of the flashing frequency on initial surface temperature is quantified. A correlation between the size of the rewetting patch and the flashing frequency is found. Finally, a hypothesis to explain the role of water subcooling in maintaining the water-surface contact at surface temperatures well above the superheating limit of water is presented.
Highlights
Controlled quenching is used in steel industry in the socalled run out table (ROT), where hundreds of water jet arrays impinge on a moving
The events occurring in the jet stagnation zone during the first instants of quenching were recorded at high speed by use of a fixed borescope, installed in a tube that traverses the water tank and has a viewing window right above the jet stagnation zone
The possibility of rewetting happening at surface temperatures above the Thermodynamic limit of superheat (TLS) has been an important discussion in the field of quenching
Summary
Water jet impingement is widely used as a quenching technique in industry due to its high cooling potential. Experimental studies have reported rewetting to occur during subcooled water jet impingement at surface temperatures up to 900 ◦C (Ishigai, Nakanishi & Ochi 1978; Hatta, Kokado & Hanasaki 1983; Wang, Yu & Cai 2012). The phenomena occurring in the initial stages of quenching are recorded at 81 kfps by using the high-speed visualization technique developed by Leocadio et al (2017) Using this technique, we capture for the first time the presence of cyclic explosive boiling when rewetting occurs in smooth and sandblasted surfaces above 300 ◦C, leading to intermittent wet–dry periods.
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