Heat and mass transfer in automotive catalysts—The influence of turbulent velocity fluctuations

Abstract

Monolithic reactors employed in automotive exhaust gas aftertreatment typically operate under fluctuating flow conditions. These fluctuations in the volumetric flow rate through the catalyst are caused by engine pulsations and the upstream exhaust gas turbulence.In the current work, the effects of the turbulent-to-laminar transition on the conversion of pollutants under mass-transfer limited operation of a monolithic reactor are investigated using numerical simulations. The most notable effect of the turbulence is an introduction of temporal fluctuations to the observed conversion at the outlet. These fluctuations are attributed to variations in the axial velocity (and thus the retention time) in the channel.A simplistic model for a single monolith channel in a tanks-in-series framework is suggested that captures this effect of the upstream turbulence on the dimensionless heat and mass transfer numbers in automotive catalysts. The model can be used to resolve the transient variations of the heat and mass transfer when the upstream turbulent characteristics are known. The effects of such variations on the catalytic conversion are likely to be most pronounced for reactions with negative-order kinetics when operating close to ignition or extinction.