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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.
