Abstract
Results from numerical simulations of the mixing of two puffs of scalars released in a turbulent flow channel are used to introduce a measure of mixing quality, and to investigate the effectiveness of turbulent mixing as a function of the location of the puff release and the molecular diffusivity of the puffs. The puffs are released from instantaneous line sources in the flow field with Schmidt numbers that range from 0.7 to 2400. The line sources are located at different distances from the channel wall, starting from the wall itself, the viscous wall layer, the logarithmic layer, and the channel center. The mixing effectiveness is quantified by following the trajectories of individual particles with a Lagrangian approach and carefully counting the number of particles from both puffs that arrive at different locations in the flow field as a function of time. A new measure, the mixing quality index Ø, is defined as the product of the normalized fraction of particles from the two puffs at a flow location. The mixing quality index can take values from 0, corresponding to no mixing, to 0.25, corresponding to full mixing. The mixing quality in the flow is found to depend on the Schmidt number of the puffs when the two puffs are released in the viscous wall region, while the Schmidt number is not important for the mixing of puffs released outside the logarithmic region.
Highlights
Turbulent flow is known to promote mixing in chemical reactors and other industrial processes.turbulent diffusion, which is one of the defining characteristics of turbulent flow [1], leads to enhancement in mixing [2]
A number of industrial applications depend on turbulent mixing [3,4,5,6,7], including combustion processes, and the design and operation of continuous stirred tank reactors (CSTRs) and plug flow reactors (PFRs), in addition to environmental processes that are important for weather changes and pollution dispersion [8,9]
Such an approach has been implemented to investigate the changes of the shape of a puff or scalar markers released in the middle of a turbulent flow channel [44] and to investigate mixing [45]
Summary
Turbulent flow is known to promote mixing in chemical reactors and other industrial processes. It is feasible to simulate the dispersion of particles that mark the trajectory of scalar quantities (like heat or mass concentration) in turbulent flow These computations are based on the Lagrangian framework, and they consist of tracking individual scalar markers in a flow field obtained through a computational fluid dynamics model. A DNS/LST approach is employed to investigate mixing from instantaneous line sources of scalars released at different locations of a turbulent channel flow. Such an approach has been implemented to investigate the changes of the shape of a puff or scalar markers released in the middle of a turbulent flow channel [44] and to investigate mixing [45].
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