Catalytic converters are widely used in the automotive industry to comply with increasingly stringent emissions regulations. The flow distribution across the catalyst substrate significantly affects its conversion efficiency. Measuring the flow in a catalyst system is challenging; computational fluid dynamics (CFD) provides an alternative approach for the assessment of different design concepts and is therefore commonly employed to model flow behaviour. This thesis studies the application of CFD to modelling ow in a two-dimensional system consisting of a catalyst monolith downstream of a wide-angled planar diffuser, with total included angle 60°. Computational models are developed using the commercial CFD software STAR-CCM+. Flow predictions are compared to experimental data collected by Mat Yamin, (2012) and also as part of this study. Measurements were obtained on a two-dimensional isothermal flow rig using particle image velocimetry (PIV) and hot-wire anemometry (HWA). Steady flow studies compare different methods of modelling the monolith. Models include the common approach of modelling the monolith as a porous medium and the computationally expensive individual channels model. A hybrid model is developed that combines the two approaches, benefiting from the respective merits of each method. Two monolith lengths are considered, with flow at varying Reynolds numbers. The porous model predicts the downstream velocity prole well for the shorter monolith but overpredicts flow maldistribution for the longer monolith. The inclusion of an entrance effect to account for the pressure losses associated with oblique entry into the monolith channels is studied. Best agreement in downstream velocity is observed when the pressure losses are limited using a critical angle approach. The individual channels model is found to be the most consistently accurate across monolith lengths, attributable to the accurate capture of flow behaviour upon entry into the monolith channels. A novel hybrid model is proposed, which combines the computational efficiency of the porous model with the geometrical accuracy of individual channels. The model is evaluated and is found to provide results similar to the individual channels model, with improved predictions of velocity maxima and minima. Pulsating flow studies present three transient flow regimes with similar inlet pulse shapes and varying Reynolds number and frequency. Predicted velocities in the diffuser are in good agreement with PIV flow fields, however CFD predicts higher magnitudes at the shear layer. The model predicts large residual vortices present at the end of the cycle where experimental data shows none; it is concluded that CFD underpredicts turbulence diffusion. Evidence of cyclic variation in experimental data highlights the limitation of URANS turbulence models.
|Date of Award||2016|
|Supervisor||Stephen Benjamin (Supervisor), Humberto Medina (Supervisor) & Carol Roberts (Supervisor)|
- Computational fluid dynamics
- Turbulence modelling