Abstract
Ammonia preparation from urea-water solutions is a key feature to ensure an effective reduction of nitrogen oxides in selective catalytic reduction (SCR) systems. Thereby, air-assisted nozzles provide fine sprays, which enhance ammonia homogenization. In the present study, a methodology was developed to model the spray formation by means of computational fluid dynamics (CFD) for this type of atomizer. Experimental validation data was generated in an optically accessible hot gas test bench using a shadowgraphy setup providing droplet velocities and size distributions at designated positions inside the duct. An adaption of the turbulence model was performed in order to correct the dispersion of the turbulent gas jet. The spray modeling in the near nozzle region is based on an experimentally determined droplet spectrum in combination with the WAVE breakup model. This methodology was applied due to the fact that the emerging two-phase flow will immediately disintegrate into a fine spray downstream the nozzle exit, which is also known from cavitating diesel nozzles. The suitability of this approach was validated against the radial velocity and droplet size distributions at the first measurement position downstream the nozzle. In addition, the simulation results serve as a basis for the investigation of turbulent dispersion phenomena and evaporation inside the spray.
Highlights
Emission legislation is nowadays a crucial factor for the development of internal combustion engines in a wide range of applications
The working principle of an selective catalytic reduction (SCR) system is based on the injection of an aqueous urea solution into the hot exhaust gas
The simulations were carried out for three operating points of the test rig and are compared in detail with the experimental data derived from the optical investigations of Lieber et al [17]
Summary
Emission legislation is nowadays a crucial factor for the development of internal combustion engines in a wide range of applications. An adherence to the latest thresholds for pollutant emissions requires the use of aftertreatment systems with high conversion rates. Thereby, selective catalytic reduction (SCR) has proven to provide an effective method for cleaning the exhaust gas of diesel engines from nitrogen oxides, with efficiencies higher than 90% [1]. The working principle of an SCR system is based on the injection of an aqueous urea solution into the hot exhaust gas. Following the evaporation of the water content, urea undergoes thermo- and hydrolysis reactions and forms the reducing agent ammonia, which reacts with nitrogen oxides to pure nitrogen and water at the catalyst [4]
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