Impact dynamics and morphology of urea-water-solution droplets impinging on a hot plate under urea-SCR relevant conditions: Influence of surface tension

Abstract

NOx conversion efficiency of urea-selective-catalytic reduction (SCR) systems are governed by dispersion of urea-water-solution (UWS) injected in exhaust manifold. Impingement of larger size urea droplets on mixture distribution fans as well as to the internal surfaces of SCR systems will lead to formation of deposits, which has potential to deteriorate the effectiveness of urea-SCR system. In this work, detailed analyses on droplet-wall interactions of UWS droplets impinging on a hot plate under urea-SCR-relevant conditions have been presented. The effect of lowering surface tension of UWS on droplet morphology and impact dynamics were also explored. The surface tension of UWS was lowered from 73.7 to 30.2 mN/m by adding a surfactant (DDA75).Distinct modes of droplet impact viz., deposition, thermal atomization, rebound and breakup were identified. The DDA75 droplets showed a significant increase in maximum spreading factor (βmax) by 37% due to 59% reduction in surface tension. New empirical correlation was developed to predict βmax for UWS and DDA75 droplets, which considers the effect of wall temperature on spreading process; the predicted βmax values for different liquids including water, hydrocarbon and alternative fuels viz., n-heptane, n-decane, Jatropha biodiesel and camelina-based alternative jet-fuel had a mean error less than 8.2% across all wall temperature conditions. The drop-size distributions of secondary DDA75 droplets revealed considerably narrower drop-size distribution (up to 36%) compared to UWS droplets. The surfactant-added-UWS droplets have the potential to enhance NOx reduction in SCR systems through better evaporation and mixing and also through reduction in the formation of urea deposits.