Computational Study of Urea–Water Solution Sprays for the Analysis of the Injection Process in SCR-like Conditions

Abstract

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.

Numerical Analysis of Urea to Ammonia Conversion in Automotive Selective Catalytic Reduction Realistic Conditions

“…In computational terms, the geometry employed in which the transport equations are to be solved is the one used by Payri et al, 25 which consists of a simplification of the experimental injection chamber, reducing it into a cube of 70 mm × 70 mm × 70 mm ( Figure 5 ) without affecting the accuracy of the presented model. The computational geometry consists of two surfaces that act as the velocity inlet and pressure outlet boundary conditions, respectively.…”

“…The distribution is controlled by the scale and shape parameters. In order to analyze the differences between the inert and the chemical simulations, the very same parameters have been chosen by Payri et al., 25 who found that the scale parameter was highly influenced by the inner geometry of the injector. Therefore, k = 3 stands for the selected shape parameter, while the scale parameter has been set to d 0 = 0.3 d n , where d n is the nozzle diameter.…”

“… 33 The mesh sensitivity study has also been carried out to determine the optimal mesh to resolve the presented problem. The base element size was set to 1.5 mm, which was increased and reduced for mesh independence purposes, concluding into a total cell count of 800,000 cells with a base size of 0.75 mm, which will possibly be increased by the AMR tool up to a minimum cell size of 0.19 mm, as it happened in the work of Payri et al 25 The results of this study are presented in Figure 6 , and an example of the mesh is included in Figure 7 , where an example of the AMR effects can be seen in the refined cells. The resolution in the near-wall region is not sufficient to properly resolve the boundary layer, as the y + value of the first cell has a value of 30.…”

Numerical analysis of the flow rig for UWS spray examination in exhaust system-relevant conditions

“…The flash-boiling injection is particularly advantageous as it provides a high level of urea decomposition and uniform ammonia distribution [4,5]. However, those sprays are characterised by significantly smaller droplets, lower velocity and increased angles, and therefore are highly exposed to the drift [3,7,10,11,13,14,[17][18][19].…”

“…A rise in the flow velocity and temperature enhanced the spray penetration in the longitudinal and transverse directions. Payri et al [14] examined the UWS spray atomisation at a flow temperature of 350°C in the test rig by numerical methods and experimental validation. The spray formation was highly influenced by the droplet's momentum.…”