This paper examines the streamwise dispersion of passive contaminant molecules released in a time-dependent laminar flow through a tube in the presence of boundary absorption or a catalytic wall reaction, which causes a depletion of contaminant in the flow. A finite-difference implicit scheme has been used to solve the unsteady convective–diffusion equation for all time. Here it is shown how the mixing of the cross-sectionally integrated concentration of contaminant molecules is influenced by the frequency of pressure pulsation and the heterogeneous reaction at the boundary. The behaviour of the dispersion coefficient due to the shear effects of steady, oscillatory, and the combined action of steady and periodic currents have been examined separately. The comparison reveals that for all cases the dispersion coefficient asymptotically reaches a stationary state after a certain time and it decreases with the absorption parameter. The increased wall absorption causes negatively skewed deviations from Gaussianity.

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