DROPSURF

The impact of a drop spray on a hot surface is one of the most efficient heat extraction methods. When the liquid meets the overheated wall, it heats up and starts boiling, which allows very high heat flux to be dissipated. It also allows to achieve a relatively uniform cooling thanks to the spatial distribution of drop impacts. Due to these many advantages, spray cooling is seen as a promising solution for the cooling of supercomputer processors, the servers in data centers, which requires during operation the dissipation of increasingly high heat flux densities. The DROPSURF project will explore the possibilities of texturing the exchange surface in order to intensify and optimize this cooling technique. While an increasing number of methods are available today to fabricate surfaces textured at spatial scales as small as one nanometer, the design of enhanced surfaces remains challenging due to the lack of knowledge about the mechanisms of heat and mass transfer involved when a drop impinges on these textures. In fact, many phenomena associated with heat and mass transfer are affected by surface texturing whatever its length scale: macro, micro or nanoscopic. Nanoscopic textures modify the wettability and thus the dynamics of the triple contact line, where most heat and mass transfers take place. At the micron scale, textures affect the density of nucleation sites. At larger scales, they can dramatically disrupt the flow field within the drop and in the vicinity of vapour bubbles. By tuning surface textures at different length scales, for example by means of hierarchical structures or heterogeneous surface properties (biphilic, thermally biconductive), a greater control is expected over the critical heat flux and the heat transfer coefficient. Accordingly, the main goal of the DROPSURF project is to establish design laws and models for heat and mass transfer that will guide the designing of enhanced surfaces. More generally, it aims to validate a rational and bottom-up design method that considers phenomena at the most relevant scales. To tackle this challenge, this project will rely on the interdisciplinary skills of three laboratories (LEMTA, IJL and IMFT) in the fields of thermal and optical diagnostics, interfacial phenomena and materials science, as well as the know-how of the company DEPHIS, which is specialized in the development and implementation of innovative coatings. The effects of texturing on wettability and interfacial transfers will be studied by means of simulations based on molecular dynamics. Coarse-grained approaches will extend the computational capabilities to fill the gap between the molecular scale and the microscopic scale, and thus gain an understanding of the nucleation and the transfer mechanisms in the microregion of the triple contact line. The effects of texturing will be also investigated using fundamental experiments to finely observe the phenomena associated with the growth of bubbles interacting with each other and the impact of individual droplets. Innovative optical diagnostics will be developed based on multiphoton fluorescence microscopy and ultra-rapid IR thermography to characterize heat and mass transfer very closely to the triple contact line. To complement this work, experimental results will be compared with direct numerical simulations that can take into account very accurately the phase change with the use of a tracking method for the interfaces. Finally, an upgrade of the fabrication processes will be undertaken in order to coat surfaces of a few hundred of cm2. This key step will allow to test spray cooling under the conditions of the industrial application. The analysis will focus on thermal performance, durability, resistance to wear, oxidation and thermal stress.