A CFD Study of Diesel Substrate Channels with Differing Wall Geometries

This paper describes efforts to use computational fluid dynamics (CFD) to provide some general insights on how wall-based protuberances affect the flow and thermal fields in substrates exposed to typical diesel engine exhaust conditions. The channel geometries examined included both square and round bumps as well as an extreme tortuous path design. Three different 2d CFD laminar-flow analyses were performed: (1) a transient fluid analysis to identify the existence of any vortex shedding in the vicinity of the bumps, (2) a steady-state fluid analysis to examine the velocity and pressure fields as well as momentum transport characteristics, and (3) a thermal analysis to examine the heat transport characteristics.

The model predicts no vortex shedding behind the bumps for the conditions and geometries examined, confirming the validity of a steady state approach and eliminating this possible transport mechanism. Recirculation zones are predicted near all the bumps with very weak levels of reversed flow. The flow does not appear to be significantly perturbed by the bumps, except for an extremely tortuous geometry. Because of inefficient perturbation, little or no improvement in the heat transport is observed with the inclusion of square and round bumps. However, heat transport was seen to improve with extreme flow perturbation, affording as much as a 40% decrease in the channel length to transfer equivalent energy. All channels have some pressure drop penalty compared to the straight channel case: up to 25% for half-square/round bumps and a factor of 6 for the tortuous path case. The pressure drop penalties would seem to make these designs unattractive for actual use, and will more than offset any transport improvements for all channels and flow conditions investigated. For some species, mass transport characteristics can be deduced by analogy. These observations are seen to be qualitatively similar to an experimental study of the emissions performance of similar substrate designs in the exhaust of HD diesel engines.