Modeling the depleting mechanism of urea‐water‐solution droplet for automotive selective catalytic reduction systems

The current work aims to develop a reliable theoretical model capable of simulating the depletion process of urea‐water‐solution (UWS) droplets injected in a hot exhaust stream as experienced in an automotive urea‐based selective catalytic reduction system. A modified multicomponent vaporization model is presented and implemented in the current study to simulate the behavior of UWS droplet in heated environment. Although water depletion is modeled as a vaporization process, urea depletion is modeled using two different approaches: (i) vaporization and (ii) direct thermal decomposition. The suitability of both depletion approaches is assessed in the current study by comparison with experimental data of the decay of a single UWS droplet in a quiescent heated environment. The decay rate of UWS droplet is accurately predicted with the multicomponent vaporization model. The possibility of internal gasification is demonstrated. Based on the complex decomposition behavior of urea, the current study proposes a decomposition mechanism for UWS droplet. The suitability of implementing the rapid mixing approach is assessed through comparison with the diffusion limit approach at various operating conditions and initial UWS droplet sizes. © 2011 American Institute of Chemical Engineers AIChE J, 2011