Dynamics of Leidenfrost drops studied by X-ray imaging

Abstract

When a liquid drop impacts on a sufficiently heated surface above the liquid’s boiling point, a vapor layer is formed beneath the liquid drop, inhibiting the liquid’s contact to the surface, known as Leidenfrost effect. Here one of the critical issues is reduction of the heat flux between solid and liquid for many industrial applications such as fuel injection and spray cooling. For example, Fukushima nuclear disaster in Japan in 2011 happened by the cooling failure of the reactor, mostly because spray cooling didn`t work well. Thus, proper understanding of the Leidenfrost effect is very important to exact and effective heat transfer analysis. To study the dynamics of a Leidenfrost drop, it is necessary to visualize in real time the interface profiles between the liquid drop and the vapor layer. It is a challenge to clearly visualize the interface profiles with conventional optical imaging due to strong light reflection and scattering of visible light. Interferometry or total-internal-reflection microscopy, although recently being tried, is also not appropriate to track the steep and complex profiles of the liquid-vapor(L-V) interface. In this thesis, important dynamics related with Leidenfrost drops are studied including the transient dynamics, jetting dynamics, contact/bouncing dynamics of Leidenfrost drop. The key idea is employing ultrafast X-ray imaging which enables direct visualizing of liquid-vapor interface in high spatial (~2 μm) and temporal resolutions (~3.67 μs) thanks to its high penetration ability and little refraction for hard X-ray. First, with ultrafast phase-contrast X-ray imaging, transient dynamics of the Leidenfrost phenomenon in drop impact is studied, with direct visualization of the liquid-vapor interface profiles in real time. A vapor disk with a homogeneous thickness, developed during drop impact, grows in thickness following the Fourier law. At a certain thickness of the vapor disk, ripples are generated at its periphery due to capillary waves, and their amplitude rapidly increases while they propagate to the center. Interestingly, rippling enhances drop vaporization rate significantly. Second, jetting dynamics of the Leidenfrost drop is studied, with direct visualization of the liquid-vapor interface profiles in real time. Here, the formation of a vapor cavity beneath the Leidenfrost drop and then downward ejection of a jet into the cavity is first observed in this study. The cavity is induced mostly by the retraction of the drop and the jetting is caused by the convergence of capillary waves along the liquid-cavity interface. Finally, a jetting criterion is established based on viscous dissipation of capillary waves: [Oh·We1.5] ≤ 7.6 ± 0.7. Last, contact and bouncing dynamics of Leidenfrost drop is also studied by X-ray imaging. For the first time, it is studied that from contact and levitation to bouncing of the liquid drop with time and increasing substrate temperature. It is revealed that there are 4 regimes based on contact and bouncing characteristics. Between classical contact and Leidenfrost regimes, it is found that there is bouncing regime without the formation of the complete vapor layer, with large range of temperature. Here, I develop a reliable method of contact time determination measuring the intensity for X-ray refraction beam, hard to measure by conventional X-ray projection images. Last, it is understood that the amount of contact depending on substrate temperature strongly affects to the bouncing time of the liquid drop. In this thesis, many physical phenomena related to the dynamic Leidenfrost drop are studied. As shown in this thesis, X-ray imaging has a great potential and performances in study of Leidenfrost drop It will be great help to substantial insight for further analytical, numerical, and experimental work on the Leidenfrost phenomenon, which can be greatly helpful for many industrial applications such as fuel combustion or spray cooling.