Multi-dimensional simulations of cold-start transients in a catalytic converter under steady inflow conditions

A multi-channel model is used to study the impact of flow non-uniformity during cold-start transient operations of a catalytic converter. It is seen that inlet zone recirculation can lead to significant non-uniformity of the flow in the monolith, and this non-uniformity can lead to significant differences in ignition characteristics among the channels. These ignition differences are especially pronounced at lower exhaust temperatures, where the axial location of ignition can vary from one channel to another. It is suggested that this strong effect of temperature on ignition may explain some of the apparently contradictory conclusions about the impact of flow non-uniformities in the literature. The simulations here show that the index of non-uniformity, as defined in many past studies, is an inadequate measure of the full impact on ignition characteristics. For the same index of non-uniformity, the non-uniformity effects on ignition become less significant with increasing exhaust flow rate. This implies that more detailed simulations of flow and temperatures non-uniformities caused by the recirculation zones, heat losses at the boundaries and insufficient mixing upstream of the monolith can be relevant to practical applications.