Identification of Film Break-Up for a Liquid Urea-Water-Solution and Application to CFD 2019-01-0983
The reduction of NOx-emissions from diesel engines is crucial for the fulfilment of environmental standards. Selective catalytic reduction (SCR) is a state-of-the-art way to achieve very low tailpipe NOx-emission levels. For an effective and efficient after treatment system, a homogeneous distribution of gaseous ammonia across the catalytic surface is essential. Therefore, a detailed understanding of the impingement of the injected urea water solution (UWS), its evaporation and transformation to gaseous ammonia is of vital importance. Due to the complex physics of the impingement process, the simulation of SCR systems with computational fluid dynamics (CFD) relies upon empirical models known as impingement maps. In the current study a droplet chain generator was used to investigate single droplet impingement of UWS. The impingement events were filmed with a high speed camera and then analysed using a post-processing routine scripted in MATLAB to analyse impact velocity and droplet diameter as well as to calculate the impingement Weber number. Together with the recorded surface temperature of the impingement target, an impingement map was drawn. The results revealed a wide temperature range relevant for SCR application were instead of pure film formation and evaporation an immediate boiling of the film and following thermal breakup was observed. Consequently, in comparison to the often used Bai-Gosman- or Bai-Onera-impingement-diagram, a significant amount of secondary droplets is created instead of all mass being transferred to liquid film. Although this film boiling has been described for water in literature before, to the best of our knowledge, it has never been implemented into CFD and applied to a complete SCR system. Implementing the observed behaviour by specifying a certain fraction of droplets to rebound from the surface in the specific regions of the impingement map, a significant improvement of the simulation results is demonstrated.
Max Quissek, Thomas Lauer, Oscar García-Afonso, Stewart Fowles