Instantaneous heat transfers at the impact of a droplet onto a hot surface in the film boiling regime

Abstract

Heat and mass transfers at the impact of a droplet onto a hot solid surface are investigated experimentally. Millimetersized water droplets impinges onto a perfectly flat sapphire surface heated at 600°C. The temperature of the liquid inside the droplet is measured using the two-color laser-induced fluorescence (2cLIF) technique. Water is seeded with a temperature-sensitive fluorescent dye, while a nanosecond pulsed laser is used for the excitation of the fluorescence. The ratio of fluorescence signal detected in two appropriate spectral bands allows to determine the liquid temperature. One advantage of this non-intrusive optical technique is that it eliminates adverse effects associated with signal variations caused by droplet shape during its impact. In parallel, the temperature of the solid surface is characterized using infrared thermography. The latter measurements are made possible by the deposition of a nanosize coating of titanium aluminium nitride (TiAlN) on the upper surface of the sapphire window. Thanks to the high frame rate of the IR camera, the time evolution of the heat flux distribution at the solid surface can be reconstructed. A comparison of IR and 2cLIF techniques enable to correlate the heating of the liquid with the cooling of the wall. This reveals that most of the heat removed from the solid surface is devoted to the heating of the liquid, the energy used for liquid vaporization being significantly lower. IntroductionMany industrial applications require a rapid cooling of surfaces from high temperatures. Among the cooling technologies, spray cooling is certainly one of the most attractive for the thermal management of high heat flux systems. Compared to jet impingement, it has the capability of cooling a relatively wider surface area with a single nozzle. It also has an unrivalled cooling efficiency, meaning that significant quantities of coolant liquid can be saved to remove the same amount of heat. These features explain why it is widely employed in many industrial applications, especially in metal production and processing industry. However, while it is applied for decades, its integration remains a complex and cumbersome process because of still incomplete knowledge of the fluid flow and heat transfer characteristics. In particular, scientific investigations focused on individual droplets are still required to understand the underlying physics behind the interactions between droplets and a hot solid surface. When a drop impacts a hot wall, different behaviours are observed. The drop can spread over the solid surface and remain attached to it due to wettability forces. It can splash and creates several smaller secondary droplets or simply rebounds. Extensive experimental investigations were carried out in the past to characterize the parameters influencing the drop behavior at the impact. Among them, some can be related to the dynamic of the impacting droplets (velocity, diameter, etc.), the physical properties of the liquid (viscosity, surface tension, etc.), and the solid su...