Pulsating Flow in a Planar Diffuser Upstream of Automotive Catalyst Monoliths: A CFD Study

Catalytic converters are used in the automotive industry to reduce pollutant emissions, however maldistribution of flow in the catalyst strongly affects its conversion efficiency. Computational fluid dynamics (CFD) is commonly employed to model flow behaviour. This study investigates the application of CFD to a two-dimensional system consisting of a catalyst monolith downstream of a wide-angled planar diffuser presented with pulsating flow. Flow predictions are compared to particle image velocimetry (PIV) flow fields in the diffuser. A porous medium approach is used for modelling the flow inside the catalyst monolith, with an entrance effect accounting for extra pressure losses due to oblique entry. Predicted velocities show good qualitative agreement with experimental data, with CFD predicting less mixing in the shear layer between the central jet and the recirculation regions. This can be explained by the inability of the turbulence model (unsteady Reynolds Stress Transport) to accurately predict turbulent diffusion. Vorticity in the diffuser is in good qualitative agreement, however CFD predicts higher magnitudes than PIV and the model shows considerably higher residual vorticity at the end of the cycle. As well as the low turbulence diffusion in the model, dissimilarity of vorticity fields is also potentially attributed to cyclic variability in the measured flow field.