A numerical study of the interaction between flow, reactions and thermal effects in a planar two dimensional model of a catalytic convertor is presented. A typical catalytic convertor consists of an inlet manifold, a catalytic monolith and an outlet manifold. The catalytic monolith is modeled as a multi-channel structure. Two different planar 2D geometries are compared: a full-scale geometry with 85 channels and a reduced-scale model with 21 channels. The diverging diffuser section of the inlet manifold, which lies immediately upstream the catalytic monolith, induces flow non-uniformity along the transverse direction. The flow immediately upstream of the multi-channel monolith is highly non-uniform. However, the frictional pressure drop in the individual channels tends to reduce this non-uniformity within individual channels. A "flow distribution index" - ratio of actual flow-rate in a channel to the ideal expected flow-rate if the flow was uniform - is defined to numerically characterize the flow mal-distribution across the various channels in the monolith. The velocity and scale effects are analyzed. Higher inlet velocities induce more flow non-uniformity. Catalytic oxidation of CO represented by power-law kinetics is employed to study the role of flow distribution. The flow distribution affects the conversion in the catalytic convertor, with different conversions in central and peripheral channels of the monolith. The net conversion from the device is marginally lower than what is predicted by single channel simulations with an assumption of uniform flow distribution. The thermal effects due to heat losses and reactions also in turn affect flow distribution. The flow tends to be more uniform when heat losses are included in the non-adiabatic simulations for the geometric parameters and kinetics considered in this work. (C) 2011 Elsevier Ltd. All rights reserved.