Exhaust after-treatment devices for NOx reduction have become mandatory for achieving the strict diesel emission standards. The Selective Catalytic Reduction (SCR) method has proven to be efficient in this task. Nonetheless, in order to improve the efficiency of the system, the Urea-Water Solution (UWS) injection process needs to be properly characterized due to the limited geometry of the exhaust line and its flow conditions. In combination with the experimental analysis into the system in a dedicated test rig, Computational Fluid Dynamics (CFD) studies provide better insight of the physical phenomena. Therefore, the main objective of this investigation is to achieve validated droplet size and velocity distributions in the simulation similar when compared to experiments. Three different positions along the spray are evaluated for that. The methodology adopted includes an Eulerian-Lagrangian approach to study the UWS spray. The results obtained with it show a proper experimental validation as well as the Sauter Mean Diameter distribution for the conditions tested. The proposed model accurately reproduces the main spray characteristics for different injection pressures and ambient conditions. Thus, the main conclusions obtained sum up in a good methodology for predicting UWS sprays in SCR-like conditions.

The selective catalytic reduction (SCR) is a technology employed for NO x reduction purposes which is based on the injection of an Urea Water Solution (UWS) into the exhaust line. Conversion of this injected urea into ammonia is a key step to ensure high SCR efficiency. In order to study this phenomenon, a three-dimensional model of the urea–water injection process has been created to recreate realistic conditions. A Lagrangian–Eulerian approach has been followed to model liquid and gas phases, respectively. Droplet evaporation as well as relevant chemical processes have been included to recreate the thermolysis and hydrolysis phenomena, and the results have been validated against literature data. Then, the validated model has been applied to recreate an in-house experimental facility that measured spray macroscopic and microscopic characteristics by means of diffused back illumination (DBI) visualization. Probability density functions of the UWS droplet sizes as well as the velocity distributions have been obtained at three different regions of interest to be compared with the experimental data set. Contours of isocyanic acid and ammonia mass fractions have been included to show the chemical transformation from urea into its products. The model accurately replicates the experimental results, and it stands as a good methodology to predict the main spray characteristics as well as the chemical processes that take place in actual SCR systems.

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Selective Catalytic Reduction stands for an effective methodology for the reduction of NOx emissions from Diesel engines and meeting current and future EURO standards. For it, the injection of Urea Water Solution (UWS) plays a major role in the process of reducing the NOx emissions. A LES approach for turbulence modelling allows to have a description of the physics which is a very useful tool in situations where experiments cannot be performed. The main objective of this study is to predict characteristics of the flow of interest inside the injector as well as spray morphology in the near field of the spray. For it, the nozzle geometry has been reconstructed from X-Ray tomography data, and an Eulerian-Eulerian approach commonly known as Mixture Model has been applied to study the liquid phase of the UWS with a LES approach for turbulence modeling. The injector unit is subjected to typical low-pressure working conditions. The results extracted from it comprise parameters that characterize the hydraulic behavior as well as jet intact length. The conclusions drawn from the model depict differences in the flow behavior between the injector three orifices, with an under-prediction of nozzle and spray characteristics of LES formulation with respect to traditional RANS turbulence treatment.

The health awareness that has arisen from the COVID pandemic has been translated into interest in studying the contagion methods of airborne viruses. The cough mechanism is one cause of virion spread. To analyze this phenomenon, computational fluid dynamics (CFD) simulations of the human cough have been set up in closed room conditions. Fundamental droplet, air, and thermodynamic conditions for the problem have been extracted from the literature and applied to the simulations performed. Three typologies of cough have been computed corresponding to the maximum, minimum, and mean peak cough velocities of the human being, and their corresponding injection profiles. Coughs were simulated in transient conditions and the droplets were tracked following an Eulerian-Lagrangian approach and large eddy simulation (LES) formulation for the turbulence phenomenon. Results revealed droplet travel distances of almost 2 m for the strongest cough case. Also, the presence of droplets smaller than 5 µm of diameter, the main virus spreader, was considerably larger than particles of bigger size. Additionally, most of the droplets evaporate or fall to the ground within 2 s after their injection. The remaining ones that stayed above the waistline or remained suspended in the air followed trajectories coming from buoyancy effects rather than being driven by the initial velocity profile. Moreover, it was found how almost the totality of the droplets expelled followed a trajectory towards the floor. Droplets that did not evaporate during the fluid injection became airborne for at least 5 seconds after the coughing.