ILASS–Europe 2019, 29th Conference on Liquid Atomization and Spray Systems, 2-4 September 2019, Paris, France This work is licensed under a Creative Commons 4.0 International License (CC BY-NC-ND 4.0). On the behaviour of urea on a heated wall - A revealed Leidenfrost-like temperature during urea thermolysis Louis-Marie Malbec* 1,2 , Chaouki Habchi 1,2 , Julien Bohbot 1,2 Scott Drennan 3 , Shaoping Quan 3 , Dan Maciejewski 3 1 IFP Energies nouvelles, 1 et 4 avenue de Bois-Préau, 92852 Rueil-Malmaison, France 2 Institut Carnot IFPEN Transports Energies, 1 et 4 avenue de Bois-Préau, 92852 Rueil- Malmaison, France 3 Convergent Science, Inc. - Middleton, WI 53562 - United States *Corresponding author: louis-marie.malbec@ifpen.fr Abstract The interaction of single droplets with hot solid surfaces is a fundamental process for a wide range of technical applications, such as urea-water solution (UWS) injection in selective catalytic reduction (SCR) systems. Specifically, the chemical evolution and film topology and state of UWS after its deposit on a hot surface are still not well known, although these phenomena are important for the SCR efficiency. In this work, we have experimentally and numerically investigated the behaviour of solid urea after being deposited on a heated surface. The experimental apparatus consists of a metal saucer whose surface can be heated up to 750K, on which a solid urea crystal is gently deposited. The evolution of the deposit is monitored with two cameras, showing top and side views, to record the dynamics and thermal behaviour of the urea. The images have shown four different urea thermolysis regimes. First, urea ball melts with a melting rate increasing with wall temperature (Tw). It forms a sessile droplet at the saucer bottom that maintains its shape for a long time indicating a very small thermolysis rate. Second, at a relatively low Tw, but higher than 406K, some bubbles appear in the molten sessile droplet due to urea thermolysis gaseous products (ammonia and isocyanic acid). These bubbles nucleate at the wall, especially near the meniscus of the liquid urea lens. This phenomenon is similar to nucleate boiling of hydrocarbons liquid film. In the third stage, the urea thermolysis rate reaches a maximum critical value for Tw between 600K and 650K. Then, the images show the formation of a deposit having a very small thermolysis rate. The evolution time to reach this final state has been measured from the images. In the fourth and final stage (Tw>650K), no deposit is formed on the wall and the molten urea droplet levitates and rebounds on the wall very similarly to a hydrocarbon droplet in the Leidenfrost regime. Therefore, Tw=650K may be considered as a Leidenfrost-like temperature for the urea. This temperature has been used to define the Leidenfrost regime for the urea in the map of the spray-wall interaction model. Finally, several CFD simulations have been carried in the third and fourth thermolysis stages. Thereby, the different measured evolution times have been used to check the thermolysis mechanism numerical results and to reveal the solid deposits composition that has proved to be composed of Cyanuric acid and Ammelide. Keywords Urea, SCR, liquid film, boiling, Leidenfrost temperature Introduction Urea/SCR system is currently an efficient method of NOx control for diesel engines. In a typical SCR system, urea is typically being used as an aqueous solution at its eutectic composition (32.5% wt. urea, marketed as Adblue™). This urea-water solution (UWS) is sprayed into the hot engine exhaust upstream of the SCR catalyst. It is commonly believed that water evaporates first, often after impingement of the spray on the wall as a liquid film. The remaining urea precipitates and completely melts at wall temperature higher than 406K, then decomposes into gas phase ammonia (NH3) and isocyanic acid (HNCO) and solid/liquid phase resulting from urea polymerization (such as biuret, cyanuric acid and ammelide). The amount and the state of these by-products depends mainly on the wall temperature. Previous studies has investigated these different processes [1–7]. However, these studies were considering UWS as the initial state. In this case, both the water evaporation and the decomposition of urea are occurring simultaneously. Especially, water evaporation potentially leads to the apparition of film boiling or Leidenfrost regime [8–11], when a vapor cushion prevents the droplet from wall wetting and therefore strongly affects thermal transfers and urea decomposition. In addition, once the water has completely evaporated, there is a need for an analysis of the decomposition of urea in the absence of water. Indeed, the thermolysis of a pure liquid urea film has not so far been studied at high wall temperature, especially during strong releases of NH3 and HNCO. Particularly, it would be interesting and useful for CFD models to know