Quantitative Evaluation of Passive Scalar Flow Mixing – A Review of Recent Developments

Flow mixing has always been of particular interest for chemical engineering, combustion, and energy industries as well as other related engineering applications. Mixing always plays a crucial role because of its ability to uniformly distribute the passive scalar in the flow field without dynamic influences to the flow field itself. This paper reviews studies which quantitatively evaluated passive scalar mixing in various engineering applications. On the one hand, passive scalar mixing in turbulence has been reviewed from the aspect of energy spectrum cascades and small temporal-spatial scale analysis, mixing time, unmixedness, mixing time-scale ratio, and relative mixing intensity ratio using DNS. Secondly, several different concepts (percentage mixing, mixing intensity and degree of mixing) to quantify the flow mixing in microfluidic systems were introduced and compared thoroughly, and it is believed that these methods can be extended to other macro fluid engineering systems. Furthermore, a CFD-based approach using the statistics of temporal and spatial distribution of fluid particles was reviewed, but cannot be extended to engineering applications with special requirements of mixing time.