In the current paper, three dimensional fluid flow and pressure loss characteristics are investigated numerically for the geometry of a catalytic converter with two substrates inside. Various relative positions of two substrates are considered: air gap widths in the axial direction and different alignments of substrate's channels in the lateral direction. Studies are focused on the effects of those relative positions on flow uniformity inside both substrates and additional pressure loss through converter system.
Under the same flow conditions and a constant air gap width, the stagger alignment in the lateral direction, shown in Fig. 1 (a), gives the highest additional pressure loss (pressure drop through air gap). Additional pressure loss drops with the increase of air gap width and reaches the lowest value at certain air gap width, then increases with the increases of gap width. The highest extra pressure loss, which is 6 mbar under a moderate exhaust mass flow rate, is found when no air gap is presented between two substrates. Pressure loss is reduced by 33 % when bias stagger alignment is presented which is shown in Fig. 1 (b). The percentage of pressure loss reduction from stagger to bias staggers alignments decreases with the increase of air gap width. At an air gap width of 12 mm, both alignments give the same extra pressure loss of 3 mbar. Results show significant dependence of extra pressure loss on the thickness of substrate wash coat. As thickness increases from 1.0 mil to 2.0 mil, extra pressure loss increases from 6 mbar to almost 26 mbar.
Flow uniformity index inside front substrate decreases as air gap distance increases. The opposed trend is found inside the rear substrate. The converter efficiency is defined to quantify overall flow distribution inside a converter with multiple substrates. It is interested to notice that efficiency drops slightly with increase of air gap between two substrates.
The study on the effect of substrate length ratio reveals that, contrary to traditional thought, a much shorter rear substrate will leads to higher converter efficiency.