In this work

we revised the expression ofmixing intensity to describe the mixing output through a cross section in a flow system by considering heterogeneity of flow field

and carefully investigated the mixing process along a straight tube with expanding/contracting cross section by simulation method. The simulation results show that a sudden expansion of cross section has remarkable mixing intensification effect within a limited period (on the sub-second scale) or tube-length (on the millimeter scale)

corresponding to the generation of considerable local vortices determined by both the flow capacity and the ratio of cross section change; a sudden contraction of cross section has instantaneous but weak mixing intensification effect; through introducing a local expansion structure with proper length

as the combination of sudden expansion and sudden contraction

their mixing intensification effects could be superposed. Besides

the rationality and importance are experimentally verified to explore the time profile of mixing intensity and carry out the vortex analysis by simulation for enhancing the selectivity of a complicated reaction system. These progresses may lead to more meaningful quantitative description of mixing process in a flow microreactor for some specific chemical processes.