The importance of individual spray properties in performance improvement of a urea-SCR system employing flash-boiling injection

The appropriate mixing of a urea–water solution (UWS) with exhaust gases in a selective catalytic reduction system is crucial to efficiently reduce nitrogen oxides and diminish the deposition of liquid wall film. One of the methods to enhance the mixing of the UWS with the exhaust gases is a flash-boiling injection. Its positive effect has been linked with a reduced Sauter mean diameter (SMD) and improved evaporation, but the flash boiling influences all the spray parameters. For low-injection-pressure cases, the spray angle generally increases, and spray-tip penetration reduces. Moreover, the reduction of the droplet size is not manifested only by reduced average droplet diameter such as SMD, but also the shape of the droplet-size distribution is changed when the injected liquid is superheated. It has not been evaluated which of these factors is the most important. It is also unknown whether the synergy between these factors is an essential element. In this study, we aim to fill these gaps and provide the numerical analysis of the influence of individual parameters of urea–water solution sprays on the exhaust system’s performance, with particular reference to the changes caused by flash boiling. It was found that none of the spray parameters influenced by the flash boiling could alone explain the mixing enhancement. Nevertheless, the alteration of the droplet size distribution led to the highest simultaneous reduction of wall film and improvement of ammonia distribution uniformity. However, the most substantial drop in the wall film formation was achieved when the spray angle was increased. The results led to the conclusion that the most positive effect of the flash boiling results from the combination of all of the considered parameters, with the most significant contribution coming from the reduced droplet size. •The highest performance improvement was observed for the base superheated spray.•None of the evaluated spray parameters could alone explain the improvement.•All spray parameters altered by flash boiling contributed to the enhanced mixing.•The largest reduction of the wall film was achieved when the spray angle was raised.•The droplet size distribution had the highest combined effect.