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Effect of Injection Parameters on Spray Characteristics of Urea-SCR System
Abstract:
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 develops 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 angle, reversing injection angle helps the
spray atomize better with higher homogeneity. Injection parallel to
the radial direction acquires the shortest mixing distance.