Effect of mixer structure on liquid film formation and NOx conversion efficiency in Selective Catalytic Reduction system

Selective Catalytic Reduction as an efficient technology to reduce NO_x emission has been widely applied in exhaust aftertreatment system. However, the reductant injection causes inevitable impingement on the exhaust pipe and mixer surface and the liquid film will be generated. In this paper, the simulation of a double-blade mixer (Mixer 1) is verified by a bench test and other three kinds of mixer structures (baffle mixer: Mixer 2, blade-baffle mixer: Mixer 3, and blade mixer: Mixer 4) are simulated. The effect of the mixer structures on the liquid film formation, the pressure drop, the flow and temperature distribution after the mixers and ultimately the NO_x conversion efficiency are investigated. Results show that the liquid film is prone to form and accumulate at the connection region between the mixer and exhaust pipe, and liquid film tends to develop with the flow direction. Mixer 2 and Mixer 4 shows smaller liquid film area about 15 cm². The temperature distribution in four types of mixers shows negligible difference (within 4 °C). And NH₃ mass fraction distribution uniformity and pressure drop show a trade-off relationship. For the vertical injection system, it is found Mixer 3 gets the worst NO_x conversion efficiency, 57%, and Mixer 2 shows better NO_x conversion efficiency, 78%.