Yutaku Kita - February 23, 2021

𝗤𝘂𝗲𝗻𝗰𝗵 𝗠𝗲𝗰𝗵𝗮𝗻𝗶𝘀𝗺𝘀 𝗼𝗳 𝗦𝗽𝗿𝗮𝘆 𝗖𝗼𝗼𝗹𝗶𝗻𝗴 - 𝗳𝗿𝗼𝗺 𝗦𝗶𝗻𝗴𝗹𝗲 𝗗𝗿𝗼𝗽𝗹𝗲𝘁 𝘁𝗼 𝗦𝗽𝗿𝗮𝘆 𝗟𝗲𝘃𝗲𝗹 Due to its excellent heat transfer performance, spray cooling has been widely used in the fields including electronics, fire and nuclear safety, and metallurgy, to name a few. Despite, the complex, multiphysics, multiscale and transient nature of spray cooling remains unclear. One of the long-standing open questions is the onset of “quenching”—the rapid cooling of a hot object induced the transition from film to nucleate boiling. Accurately predicting when quenching occurs is paramount to the optimization of steel manufacturing as it determines the thermal history of the workpiece, and thus, the final properties. However, quenching temperatures ever reported range between 200 °C and over 500 °C. While spray conditions, surface roughness and wettability have been known to influence the quenching temperature, little is known about the effect of oxide layers present on the surface. Fig. 1 shows cooling curves obtained by our spray experiments for stainless steel plates with various oxide layers. Clearly, the quenching temperatures are inconsistent and even reported to be over 350 °C which exceeds the thermodynamic limit of superheat (so water should not touch the surface!). To address this controversy, we then investigate the fundamental process of droplet impact onto a surface heated at different temperature levels. High-speed imaging allows to identify the transition of droplet impact behavior, which corresponds to the onset of quenching. Instead of conventionally-defined quenching temperature as in the literature, we defined so-called “contact surface temperature” calculated via transient heat conduction analysis. Consequently, we found a unique contact surface temperature (ca. 250 °C) for the droplet behavior transition regardless of the composition and thickness of the oxide layer (Fig. 2). Although the meaning of this 250 °C is still under investigation (and happy to discuss here), our findings will provide one perspective of quenching mechanism and inform the optimization of spray cooling. In this talk, I would also like to discuss how such local and microscopic phenomena of single droplets can be linked to the macroscopic results of spray cooling.