Flow Distribution and Pressure Drop in Diffuser-Monolith Flows

Abstract

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 paper, the trade-off between flow-uniformity and pressure drop of an axisymmetric automotive catalytic converters is studied numerically and experimentally for selected cases. The monolithic brick resistance is formulated from the pressure gradient of fully developed laminar duct-flow and corrected for the entrance effect. The monolithic brick downstream stabilizes the diffuser flows both physically and computationally. A distribution index was formulated to quantify the degree of nonuniformity in selected test cases. The test matrix covers a range of different diffuser angles and flow resistance (brick types). For simplicity, an axisymmetric geometry is chosen. Flow distribution within the monolith was found to depend strongly on diffuser performance, which is modified strongly by brick resistance. Pressure drop due to the headers and brick resistance and their relative roles is also identified. The implications of these data for converter design are discussed in terms of the trade-off between flow-uniformity and pressure drop.