Citation: Picus, C.M.; Mihai, I.; Suciu, C. Pseudo-Desublimation of AdBlue Microdroplets through Selective Catalytic Reduction System Microchannels and Surfaces. Micromachines 2023, 14, 1807. https://doi.org/10.3390/ mi14091807 Academic Editors: Stéphane Colin and Xuan Zhang Received: 30 June 2023 Revised: 13 September 2023 Accepted: 19 September 2023 Published: 21 September 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). micromachines Article Pseudo-Desublimation of AdBlue Microdroplets through Selective Catalytic Reduction System Microchannels and Surfaces Claudiu Marian Picus , Ioan Mihai * and Cornel Suciu * Faculty of Mechanical Engineering, Automotive and Robotics, Stefan cel Mare University, 720229 Suceava, Romania; claudiu.picus@usm.ro * Correspondence: mihai.i@usm.ro (I.M.); suciu@usm.ro (C.S.); Tel.: +40-735173288 (I.M.); +40-742007820 (C.S.) Abstract: In the present paper, the occurrence and development of the pseudo-desublimation process of AdBlue microdroplets in the microchannels and surfaces of catalytic reduction systems (SCR) are reported. In order to understand how the pseudo-desublimation process develops, the influence of heat flux values on the heat transfer of AdBlue injection was analysed, taking into account the structure of the microchannels inside the SCR and the overall configuration of the installation. The evolution of the AdBlue vapour flow in the SCR system was simulated, as well as the temperature variation along an SCR microchannel through which the mixture flows. An experimental set-up was designed in order to visualise and interpret the processes at the onset of pseudo-desublimation. The results described in this paper confirm the existence of a pseudo-desublimation process that occurs only under certain temperature conditions when AdBlue is injected into SCR systems. The characteristics of the crystals formed and their growth rate depend on the working temperature, which could be controlled by efficient preheating methods immediately after engine start. A better understanding of the process will allow the development of methods of avoiding solid depositions on SCR system components, which has a direct impact on SCR catalyst performance and durability. Keywords: microchannels; pseudo-desublimation; SCR; AdBlue; NOx 1. Introduction The Euro 7 standards for cars, proposed to come into effect in 2025, include a number of stricter emission regulations. Although the new values of NOx (nitrogen oxides) emission limits are not much lower than in the Euro 6d standard, the cold emissions test will mean big increases in costs, mainly for diesel engines. At present, coding to Euro 6d regulations, petrol engines have a limit of 60 mg/km for NOx emissions, while diesel engines have a limit of 80 mg/km. With the Euro 7 rules, the NOx emission limit will be 60 mg/km for all types of internal combustion engines. There is a one-off derogation from 60 mg/km to 75 mg/km NOx for small commercial vehicles up to 3.5 tonnes with a specific power output of up to 35 kW (48 hp). Emissions will be measured using the same method as in Euro 6d, called RDE (Real Driving Emissions), which has been found to be viable and conclusive. The European Commission’s communication does not explicitly mention the introduction of a short-distance cold emissions test for small commercial vehicles with a whole-trip specific power of 10 km. Basically, the kilometre limit will be identical to the normal long-distance test, but will be very difficult to achieve [1,2]. Other changes brought about by the Euro 7 rules include extending the period of compliance of vehicles with emission standards from 100,000 km and 5 years, as was the case under Euro 6d rules, to 200,000 km and 10 years, as the average age of cars in Europe approaches 12 years [1,2]. The amount of pollutants according to European requirements for engines up to 35 Kw diesel and petrol are shown in Table 1, comparatively for the Euro 6d and Euro 7 standards, where PM represents the accepted particulates or atmospheric particulate matter and PN is the particle number (symbol N), which is the number of constituent particles in the analysed Micromachines 2023, 14, 1807. https://doi.org/10.3390/mi14091807 https://www.mdpi.com/journal/micromachines