Effect of Injection Parameters on Spray Characteristics of Urea-SCR System

Urea-SCR system is one of the after-treatment methods for diesel engines, which could effectively reduce the NOX emissions and enable diesel engines to meet increasingly stringent emission legislations. Within the urea-SCR system, characteristics of urea-solution spray, especially the distribution uniformity of spray droplets as well as gaseous NH3 within the exhaust pipe, play an important role in the efficiency of catalytic reduction. In this paper, an SCR spray visualization test bench was set up. Urea-solution from a non-air-assist injector is injected into the steady stream of simulated exhaust gas flow. The transient characteristics of spray are recorded by high-speed photography. Specific spray characteristics in the original photographs, i.e. mixing distance and degree of uniformity are extracted. The influence of injection pressure and injection angle on spray characteristics are tested in different sets of experiments. Wall impingement on the inner surface of the pipe is also captured in several experiments and discussed. The experimental results reveal that: within a reasonable injection pressure range, the spray developes without impinging on the pipe wall. When the injection pressure becomes lower, the urea-solution droplet distribution shows better axial concentration uniformity. The mixing distance decreases as injection pressure decreases. Meanwhile, compared to the forward, i.e. downstream, direction of injection angel, reversing injection angle helps the spray atomize better with higher homogeneity. Injection parallel to the radial direction acquires the shortest mixing distance.