Experimental study of droplets impact onto heated walls using combined optical techniques: single droplets, multiple droplets and sprays

Abstract

Droplets and sprays impact onto heated walls have drawn much attention from the Thermal Engineering community because of its many applications that allow uniform and efficient heat exchange, especially for the cooling of high-temperature parts. However, most of the experimental studies in the literature only visualize the droplets impact or use intrusive measurement methods. This lead to arbitrary analyses and characterizations of the droplets impact fluid dynamics and limited investigation of the involved heat transfer processes. Furthermore, little is known about multiple droplets impact, which may be an important leap to associate single-droplet phenomena to spray cooling applications. The present research project looks to bring light to this droplet-spray correlation by proposing the use of combined optical techniques to characterize the impact and cooling of a single droplet, multiple droplets and dilute sprays. The simultaneous measurement techniques are: shadowgraph for the droplet's size and velocity, planar laser-induced fluorescence for the droplet's temperature, infrared thermography for the surface temperature, and total internal reflection for the droplet-wall contact. These techniques were carefully chosen to be applicable for all the test conditions defined in this project. The test sections (sapphire dove prism with coating and metallic sheet) were designed to allow to most combined measurement techniques possible. This effort to measure all these parameters will allow making a complete characterization of the thermal-hydraulics during droplets' impact. With direct observations through imaging and with detailed mass and energy balance modeling, conclusive information of droplets-impact cooling will be obtained, like heat transfer coefficients, Leidenfrost temperatures, and droplets interactions' effects. Hence, better and physics-based models for droplets and spray cooling can be developed, as well as more a comprehensive mapping of droplets impact regimes. Part of the equipment is already existent in the host institution, which helps to decrease substantially the cost of the experimental apparatus. Two direct doctoral students will work on this project. The first will compare single droplets' behaviors to spray cooling in the Leidenfrost regime, and the other will focus on multiple droplets impact in the Leidenfrost regime. Both of them will have the opportunity for a co-mentored doctoral diploma working at the LEMTA at the Université de Lorraine, in Nancy, France, where they will perform experiments with a different apparatus to compare with the results obtained in the new apparatus proposed in this project, as well as test other experimental conditions. (AU)