Spray Characteristics of SCR Urea Swirl Nozzles: A Comparative Analysis Using Multiple Optical Particle Sizing Methods

Abstract Selective Catalytic Reduction (SCR) technology is extensively employed across power plants, diesel vehicles, marine engines, and industrial applications to mitigate NOx emissions, particularly in hydrogen and ammonia combustion systems. The urea injector nozzle's atomization performance critically impacts NOx conversion efficiency and urea crystallization. Pressure-swirl nozzles have gained prominence in SCR systems due to their structural simplicity, precise flow control, and superior atomization characteristics, producing hollow-cone spray patterns. This study introduces a novel Droplet Tracking Velocimetry (DTV) technique combining circular Hough transform with Voronoi tracking to analyze droplet size and velocity distributions in hollow-cone sprays. Comparative analysis with Phase Doppler Interferometry (PDI) and Malvern Laser Particle Size Analyzer revealed measurement variations due to differing accuracies and ranges. While PDI offers precise single-point diameter measurements, it fails to capture spatial spray heterogeneity. Similarly, for droplets smaller than 80 µm, the Malvern particle size analyzer faces several challenges, including light blocking, sample volume, and droplet uniformity. Additionally, the laser diffraction method tests the entire optical path, which passes through the entire hollow cone, leading to significant deviations. The DTV method demonstrated superior capability in simultaneously measuring droplet velocity and size distributions within defined ranges. Axial and radial measurements revealed distinct spray characteristics: Sauter Mean Diameter (SMD) showed progressive axial uniformity but significant radial growth, attributable to interactions between high-speed large droplets and low-speed small droplets. These findings provide crucial insights into swirl nozzle spray dynamics, supporting optimized SCR system design for enhanced NOx reduction performance.