Numerical and Experimental Investigation of Mixing Quality for Pulsed Mixing Flows with Application

Amin  Reihani, University of Michigan

For many diesel aftertreatment methods, such as urea SCR and DPF regenerations, a secondary flow is injected into lean exhaust. In these methods, mixing of the injected fluid with the exhaust flow is challenging. An even greater mixing challenge occurs with a process referred to as Rapidly Pulsed Reductants or RPR, where HC is injected in rapid pulses ahead of a LNT in order to expand the operating window of the LNT to higher temperatures and flow rates. Here good radial mixing is desired with minimal axial mixing. The objective of this study is the numerical and empirical investigation of the mixing of pulsed gaseous hydrocarbons into the main exhaust flow of a bench flow catalyst test reactor. Hence, the injection of propane, a representative hydrocarbon, as a high frequency pulse into the lean exhaust gas has been studied.
The first challenge in the current study is to find an efficient approach to uniformly mix the injected HC and the main flow in the radial direction of the test tube, while simultaneously maintaining axially unmixed flow, in order to obtain a rectangular plug flow HC pulse. Using analytic fluid mechanics approaches, CFD simulations were performed to find the most effective design among a number of general mixing designs. Furthermore, a new method has been employed to empirically evaluate the mixing uniformity and the effects of arrangement and flow parameters on the mixing process, using a Diesel Oxidation Catalyst (DOC) and a Fast Flame Ionization Detector (FFID). The results were analyzed to find an optimal and robust mixing condition.
In order to find a precise numerical approach with reasonable computational cost for simulation of pulsed mixing flows, we replicated our experimental data implementing different numerical models. It was observed that the turbulence model and turbulent transport parameters have a significant effect on the spatial and temporal distribution of species in the flow field. Finally, an efficient numerical and empirical approach for study of mixing quality in pulsed mixing flows is suggested to extend the replication of our results to a wider range of flow conditions.

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