Quality Measures of Mixing in Turbulent Flow and Effects of Molecular Diffusivity

Abstract

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.

Unsteady Multiphase Simulation of Oleo-Pneumatic Shock Absorber Flow

“…The complexity of the interaction between the oil and gas observed in Figure 6c highlights the need for a quantitative approach to systematically assess the multiphase mixing as the stroke progresses from a straight line interface initially, shown in Figure 7a, advancing towards its full complexity as the gas bubble breaks into smaller ones. Slightly different variations in a mixing quality parameter were proposed in the literature to quantify mixing processes [39][40][41]. A similar idea is implemented here, where the mixing index (MI) in the present study, defined according to Equation (3), is used to isolate the interface between the two phases at a specific volume fraction value, taken to be α = 0.5 in all MI analyses presented.…”

Flow and Heat or Mass Transfer in the Chemical Process Industry

“…Turbulent flow effects on mixing were examined by Nguyen and Papavassiliou [12]. Qualitative measures of mixing effectiveness and mixing quality were suggested, and Lagrangian computations and direct numerical simulations of turbulent flow allowed the evaluation of these measures for mixing particles with different Schmidt numbers.…”