Fuzzy Comprehensive Analysis of Static Mixers Used for Selective Catalytic Reduction in Diesel Engines

The proper selection of a relevant mixer generally requires an effective assessment of several models against the application requirements. This is a complex task, as traditional evaluation methods generally focus only on a single aspect of performance, such as pressure loss, mixing characteristics, or heat transfer. This study assesses a urea-based selective catalytic reduction (SCR) system installed on a ship, where the installation space is limited and the distance between the urea aqueous solution injection position and the reactor is low; therefore, the static mixer installed in this pipeline has special performance requirements. In particular, four evaluation indices are used in this study: The B value, C value, pressure loss correction factor (Z′), and the ratio of the required distance to the equivalent diameter of the pipe (L_V/D) when the velocity field after the mixer attains uniformity. Six types of static mixers were simulated with varying concentrations, flow speeds, and positions. A fuzzy comprehensive evaluation method was introduced to evaluate and compare the related advantages and disadvantages. The results showed that 1) mixing performance was related to the shape of the mixer and had no direct relationship with flow velocity. 2) For the same mixer position, the lower the urea concentration, the greater the difficulty of evenly mixing the solution. 3) At a constant urea concentration, the mixing performance improved when the mixer was closer to the injection inlet. 4) The installation of a GK mixer in the SCR system of a 9L20C diesel engine was best.