Intra-Channel Mass and Heat-Transfer Modeling in Diesel Oxidation Catalysts

We consider the effect of intra-channel mass and heat transfer in modeling the performance of diesel oxidation catalysts. Many modeling studies have assumed that the intra-channel flow is laminar and, thus, heat and mass transfer between the bulk gas and wall are appropriately described using correlations for fully-developed laminar flow. However, recent experimental measurements of CO and hydrocarbon oxidation in diesel exhaust reveal that actual mass-transfer rates can deviate significantly from those predicted by such correlations. In particular, it is apparent that there is a significant dependence of the limiting mass-transfer rate on the channel Reynolds number. Other studies in the literature have revealed similar behavior for heat transfer. We speculate that this Reynolds number dependence results from boundary-layer disturbances associated with washcoat surface roughness and/or porosity. When we apply experimental mass and heat- transfer correlations to multi-channel simulations of a diesel oxidation catalyst, the steady-state conversions differ significantly from those obtained assuming fully-developed laminar flow. These results suggest that assuming fully-developed laminar flow may not always be appropriate.