An Experimental and Predictive Study of the Flow Field in Axisymmetric Automotive Exhaust Catalyst Systems

An experimental and theoretical investigation has been performed on the flow and pressure loss in axisymmetric catalytic converters and isolated monoliths under steady, isothermal flow conditions. Monolith resistance has been measured with a uniform, low turbulence, incident flow field. It has been found that the pressure loss expression for fully developed laminar flow is a good approximation to observations for x⁺ greater than 0.2. However, for x⁺ less than 0.2 the additional pressure loss due to developing flow is no longer negligible and a better approach is to use the correlation proposed by Shah (16). From experimental studies on the axisymmetric catalytic converters non-dimensional power law relationships have been derived relating maldistribution and pressure drop to expansion length, Re, and monolith length. These expressions are shown to generally fit the data well within ±5%. CFD predictions of the flow for a wide range of geometric configurations and flow conditions have shown that generally the system non-dimensional pressure loss can be predicted to within about 10% but that the maldistribution index is underpredicted within a range of 9-17.5%. It is believed that the pressure loss expression derived from 1-D flow studies is too simplistic. CFD predictions do, however, show the same pattern of change as observed experimentally and hence their performance in predicting trends is quite encouraging.