Experimental investigation of urea-water sprays in selective catalytic reduction (SCR) systems

Urea-SCR (selective catalytic reduction) is currently the state-of-the-art NOx reduction technique for Diesel engines. The main challenges for the implementation of mobile urea-SCR systems include rapid decomposition and homogeneous distribution of urea and the mitigation of deposit formation. A key affecting these performance factors is the injection of the urea-water solution (UWS). However, urea-water sprays under exhaust flow conditions have not been studied extensively so far. The present study is a comprehensive analysis of the behavior of the UWS spray. The investigation was conducted with a 3-hole commercial SCR injector. The spray properties under exhaust flow conditions are described by non-intrusive optical techniques: shadow imaging and Mie scattering. The spray cooling effect on the exhaust channel wall is analyzed based on the temporal and spatial evolution of the wall temperature using infrared thermography. This work is also complemented with the investigation of the specific behavior of UWS droplet during the wall impact over a wide range of temperatures and urea concentrations. This fundamental study provides information on the characteristics of UWS droplets evaporation time, the boiling regimes and the wetting conditions, which enhances the understanding of the wall impingement of urea-water sprays.