Identification of Film Breakup for a Liquid Urea-Water-Solution and Application to CFD 2019-01-0983

The reduction of NO_x-emissions from diesel engines is crucial for the fulfilment of environmental standards. Selective catalytic reduction (SCR) is an effective way to achieve very low tailpipe NO_x-emission levels. For an 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 with respect to impingement velocity and droplet diameter as well as droplet Weber-number. Together with the recorded surface temperature of the impingement target, an impingement map was drawn. In contrast to previous investigations, these experiments were mainly carried out in the range of the critical temperature that separates impingement regimes that result in wall wetting and regimes with thermal rebound or break-up. The results revealed a temperature range that is relevant for SCR applications where instead of pure film formation and evaporation an immediate boiling of the film followed by 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. With the implementation of 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 could be demonstrated.