Flow distribution and pressure drop in diffuser-monolith flows: Implications to the automotive catalytic converter design

Most current automotive catalytic converters use diffusers to distribute the flow field inside the monolithic bricks where catalysis takes place. While the characteristics and performance of a simple diffuser flow are well documented, the influence of downstream brick resistance is not clear. In this study the trade-off between flow-uniformity and pressure drop of automotive catalytic converters is investigated numerically and experimentally for selected cases. A rudimentary reaction kinetic model is developed mathematically to examine its impact on the predicted flow field. The monolithic brick resistance is formulated from the pressure gradient of fully developed laminar duct-flow and corrected for the entrance effect. A distribution index is formulated to quantify the degree of non-uniformity in selected test cases. The test matrix covers a range of different inlet pipe bending angles, diffuser angles, inlet Reynolds numbers and flow resistances (brick types). Point-velocity measurements using Laser Doppler Velocimetry and pressure drop measurements at selected flow sections are conducted to verify the simulation results. The experimental results show fairly good agreements with the numerical predictions. As expected, flow distributions within the monolith are found to depend strongly on inlet pipe bending angles and diffuser performance, which is modified by brick resistance. Pressure drop due to the headers and brick resistance, and their relative roles are also identified. The implications of these data for converter design are discussed in terms of the trade-off between flow-uniformity and pressure drop. The conversion efficiencies of catalytic converters are also discussed in terms of the radial and the axial variations regarding the velocity fields.