On the onset of quench during spray cooling: The significance of oxide layers
On the onset of quench during spray cooling: The significance of oxide layers
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DOI：
10.1016/j.applthermaleng.2020.115682
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发表时间：
2020-07
影响因子：
6.4
通讯作者：
Kengo Tsukamoto;Yutaku Kita;S. Inoué;Takafumi Hamanosono;S. Hidaka;S. Ueoka;Hiroyuki Fukuda;M. Kohno;Y. Takata
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文献类型：
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作者：
Kengo Tsukamoto;Yutaku Kita;S. Inoué;Takafumi Hamanosono;S. Hidaka;S. Ueoka;Hiroyuki Fukuda;M. Kohno;Y. Takata
讨论了隔热层，特别是冶金应用中的氧化层对喷水冷却过程的影响。已经发现这种层提高了喷射液体开始接触热表面的淬火温度，大大提高了冷却速率。传统的，热阻为基础的模型可以预测淬火点定性的转变，虽然仍然存在显着的偏差，由于缺乏基本的了解液-固接触的开始。在本文中，我们进行了两组实验，试图阐明的淬火机制和氧化层的影响。首先，我们比较了不同的氧化层的不锈钢板的喷雾冷却过程中的温度历史。淬火温度根据氧化物层的组成和厚度而变化。此外，在高达350 °C的温度下观察到淬火，超过热力学润湿极限。然后，我们转移到单液滴碰撞实验，详细研究液滴行为的变化与表面温度。高速成像使我们能够识别液滴碰撞行为的转变，即沉积和反弹，这也发生在不同的壁温下，取决于氧化物层的组成。随后，我们计算了接触表面的温度假设的瞬态热传导的两个半有限体之间的接触。因此，液滴行为转变的起始点总是在接触表面温度约为100 ℃时。250 °C，与氧化层的成分和厚度无关。接触表面温度与壁面温度之差随氧化层热扩散系数的减小而增大，这是导致“表观”淬火温度不一致的直接原因。
The effect of thermally-insulating layer, particularly oxide layer as found in metallurgical applications, on the water spray-cooling process was discussed. Such layers have been found to increase the quenching temperature at which the sprayed liquid begins to contact the hot surface, greatly increasing the cooling rate. The conventional, thermal-resistance based model can predict the shift of the quenching point qualitatively, albeit significant deviations remain due to the lack of fundamental understanding of the onset of liquid–solid contact. In the present paper, we conducted two sets of experiments in an attempt to shed light on the quench mechanism and the effect of oxide layer. First, we compared temperature histories during spray cooling of a stainless-steel plate with various oxide layers. The quench temperatures varied depending both on the composition and the thickness of the oxide layer. Additionally, quench was observed at temperatures as high as 350 °C, exceeding the thermodynamic wetting limit. Then, we moved on to single droplet impingement experiments to investigate the change of droplet behavior with respect to the surface temperature in detail. High-speed imaging allowed us to identify the transition of droplet impact behavior i.e. deposition and bouncing, which also occurred at different wall temperatures depending on the composition of oxide layer. Subsequently, we calculated the contact surface temperature assuming the transient heat conduction for a contact between two semi-finite bodies. As a consequence, the onset of droplet behavior transition was always found at the contact surface temperature of ca. 250 °C regardless of the composition and thickness of the oxide layer. The difference between the contact surface temperature and the wall temperature increased as the thermal effusivity of the oxide layer decreased, which was a direct cause of the inconsistent “apparent” quenching temperature.