The goal of this work is focused on improving fuel and exhaust mixing using virtual methods in order to optimize exhaust aftertreatment hardware. The aftertreatment application of interest is converting diesel fuel that is injected into an exhaust tailpipe over of a fuel reformer catalyst. The objective is to fully vaporize the atomized fuel droplets and mix the fuel vapor to create a uniform fuel vapor distribution at the inlet of the fuel reformer catalyst. A method is developed to model the fuel sprays from low-pressure injectors. This model is integrated into a simulation tool of fuel-exhaust vaporization and mixing. Standard spray measurement techniques are used to measure the key characteristics of the spray. The spray model defines the atomized drop size distribution and volume flux distribution using probability density functions that are independent of the fuel injection rate, resulting in a simple input file for the simulation. A variety of fuel injection profiles that capture the transient nature of the fuel reformer operation are specified in a user input file. The simulation of fuel-vapor distribution is qualitatively validated with measurements of exhaust temperatures exiting the fuel reformer catalyst. The simulation tool is used to optimize the mixing hardware, resulting in a uniform fuel vapor distribution. The hardware configuration is tested in the engine laboratory and shows a reduced exhaust temperature spread.