Experimental and Numerical Investigations of Sprays in Crossflow in Aftertreatment Systems

Selective catalytic reduction (SCR) of oxides of nitrogen (NOx) with gaseous ammonia is the leading technology used to meet on- and off-highway NOx emission standards across the world. In typical SCR systems, a low-pressure injector introduces a solution of urea and water (UWS) into hot exhaust gases leading to atomization and subsequent spray processes that finally lead to production of gaseous ammonia. Through their synergetic effect, the UWS injector and mixing enhancement devices (such as static mixers or baffles) help deliver a uniform mixture of ammonia and NOx to the SCR catalyst with minimal urea-derived solid deposits. To develop an efficient and robust aftertreatment system, it is essential to have experimental and simulation capabilities to assess the behavior of sprays under flow conditions representative of engine exhaust. The experimental part of the present work uses an optically accessible, cold- or hot-flow capable test section (called Insitu test section) that allows optical observations and measurements of aftertreatment sprays in cross-flowing gas. Pitot tubes are used to measure gas velocity, and high-speed imaging is used in conjunction with laser diffraction drop size measurements to characterize the spray. CFD Simulations are conducted using the commercial finite volume code ANSYS FLUENT using the Lagrangian Drop - Eulerian Fluid framework. User defined functions are developed to prescribe accurate initial conditions for the spray, and custom MATLAB scripts are used for detailed post- processing. Predicted velocity fields, spray trajectories and drop sizes are compared against test data under both cold- and hot-flow conditions and effects of spray submodels and numerical model settings are investigated. Finally, implications of the findings from this study for diesel aftertreatment system CFD simulations are discussed.