Experimental analysis of the urea-water solution temperature effect on the spray characteristics in SCR systems

t One of the favored automotive exhaust aftertreatment solutions used for nitrogen oxides (NOx) emissions reductions is referred to as Selective Catalytic Reduction (SCR), which comprises a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx). It is customary with these systems to generate the NH3 by injecting a liquid aqueous urea solution (AUS-32) into the exhaust. The urea solution is injected into the exhaust and transformed to NH3 by various mechanisms for the SCR reactions. Understanding the spray performance of the AUS-32 injector is critical to proper optimization of the SCR injection system. Results were previously presented from imaging of an AUS-32 injector spray under hot exhaust conditions at the injector spray exit for an exhaust injection application. Those results showed substantial structural differences in the spray between room temperature fluid conditions, and conditions where the fluid temperature approached and exceeded 104° C and "flash boiling" of the fluid was initiated. The visualization techniques used in the testing permitted for the first time a quantification of the droplet size distribution changes under flash boiling conditions. However, the flash boiling could only be initiated at very high exhaust temperatures due to the construction of the injection unit. The spray investigations results presented in this paper follow up on the previous macroscopic imaging work by forcing flash boiling conditions even at reduced exhaust temperatures. The test facility concept and operation are described. The global spray structure changes observed in the previously published hot airflow measurements are confirmed, and conclusions drawn on the relative effects of injection air temperature and injected fluid temperature. Quantified comparisons of the spray atomization characteristics are presented.