Experimental investigation on the injection of an urea–water solution in hot air stream for the SCR application: Evaporation and spray/wall interaction

In automotive application, Selective Catalytic Reduction (SCR) system is used to control the NOx emissions from Diesel engines. The reducing agent, ammonia NH3, is produced by urea decomposition in spraying Urea–Water Solution (UWS) in hot exhaust gas. UWS evaporation is a crucial step in SCR system process. The effects of turbulence and spray/wall interactions are of paramount importance for the SCR process, not taken into account in the quiescent environment conditions usually considered in experiments related in the literature. In this work, UWS spray is investigated in a hot air stream. Test bench was designed to reach conditions similar to Diesel exhaust conditions in term of dimensions, temperature, air mass flow rate and injection strategies. UWS was sprayed in hot air stream canalized in transparent duct to enable optical access. Promising experimental techniques were performed to study the spray/wall interaction and spray evaporation in hot stream. An adapted backlight imaging technique was used to visualize the formation and development of liquid film caused by the impaction of spray on the duct wall. The variation of the area of the formed liquid film is quantified for several working conditions. UWS spray in the hot stream is examined with the Laser light Sheet Imaging (LSI) technique. The principle of this technique is to illuminate liquid drops with a laser sheet and to record the scattered-light at 90° with a CCD camera. Such measurements allow determining a 2D distribution of liquid that indicates the distribution of the liquid phase in a stream cross-section. We show that the evolution of 2D liquid distribution contains global information on the UWS drops evaporation and an evaporation rate can be estimated considering the Mie-scattered-light theory and D-square evaporation law. The experimental work is completed with a theoretical work that leads to the calculation of evaporation time of UWS drop in different aerothermodynamic conditions. Finally, this study promises interesting perspectives for the investigation of UWS spray evaporation in hot gas stream.