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

Fluids ◽  
2018 ◽  
Vol 3 (3) ◽  
pp. 53 ◽  
Author(s):  
Quoc Nguyen ◽  
Dimitrios Papavassiliou
Keyword(s):  
Flow Field ◽  
Turbulent Flow ◽  
Quality Index ◽  
Schmidt Number ◽  
Wall Layer ◽  
Wall Region ◽  
Molecular Diffusivity ◽  
Mixing Quality ◽  
Schmidt Numbers ◽  
Logarithmic Layer

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.

Lagrangian Scalar Tracking for Laminar Micromixing at High Schmidt Numbers

2006 ◽  
Author(s):  
Christian Gobert ◽  
Florian Schwertfirm ◽  
Michael Manhart
Keyword(s):  
Scalar Field ◽  
Flow Field ◽  
Schmidt Number ◽  
Particle Density ◽  
Concentration Field ◽  
Flow Regimes ◽  
Effective Density ◽  
Wide Range ◽  
Schmidt Numbers ◽  
Very High

In many mixing processes the Schmidt number can easily reach very high values. In such cases the computation of the scalar field by Eulerian methods causes extremely high memory requirements. In the present study we circumvent this problem by adopting a Lagrangian particle tracking method to compute the scalar field in a laminar T-mixer configuration. The flow field is computed by direct numerical simulation (DNS). The movements of representative molecules are determined by the Langevin equation, describing convection by the flow-field and diffusion due to Brownian motion. The scalar field is computed by evaluation of the particle distribution. The particle density required for calculation of resolved concentration fields increases with increasing Schmidt number. For stationary flow regimes, the effective particle density could be increased via sampling in time. In the unsteady case, the effective density can be augmented by parallelization over the particles. No model is required in this approach, and concentration fields for very high Schmidt numbers can be computed. With this method is was possible to compute the fully resolved concentration field in a T-shaped micromixer at Schmidt number 3571 and Reynolds numbers 186 (steady) and 240 (unsteady). Schlu¨ter et. al. [CIT, 76(11), 2004] examined the stationary configuration experimentally. The agreement between numerical and experimental results is excellent. The developed method provides a possibility to compute the fully resolved scalar field in laminar flow regimes at a wide range of Schmidt numbers. It seems that in the cases under consideration Euler methods cannot provide such results. Therefore the proposed method represents a new basis for the prediction of mixing and the development of mixing models for high Schmidt number flows.

Spectrum of passive scalars of high molecular diffusivity in turbulent mixing

2013 ◽  
Vol 716 ◽  
Author(s):  
P. K. Yeung ◽  
K. R. Sreenivasan
Keyword(s):  
Reynolds Number ◽  
Turbulent Mixing ◽  
Kinematic Viscosity ◽  
Schmidt Number ◽  
Scalar Fields ◽  
Passive Scalar ◽  
Careful Examination ◽  
Passive Scalars ◽  
Molecular Diffusivity ◽  
Schmidt Numbers

AbstractWe consider the mixing of passive scalars transported in turbulent flow, with a molecular diffusivity that is large compared to the kinematic viscosity of the fluid. This particular case of mixing has not received much attention in experiment or simulation even though the first putative theory, due to Batchelor, Howells & Townsend (J. Fluid Mech., vol. 5, 1959, pp. 134–139), is now more than 50 years old. We study the problem using direct numerical simulation of decaying scalar fields in steadily sustained homogeneous turbulence as the Schmidt number (the ratio of the kinematic viscosity of the fluid to the molecular diffusivity of the scalar) is allowed to vary from $1/ 8$ to $1/ 2048$ for two values of the microscale Reynolds number, ${R}_{\lambda } \approx 140$ and $\approx $240. The simulations show that the passive scalar spectrum assumes a slope of $- 17/ 3$ in a range of scales, as predicted by the theory, when the Schmidt number is small and the Reynolds number is simultaneously large. The observed agreement between theory and simulation in the prefactor in the spectrum is not perfect. We assess the reasons for this discrepancy by a careful examination of the scalar evolution equation in the light of the assumptions of the theory, and conclude that the finite range of scales resolved in simulations is the main reason. Numerical issues specific to the regime of very low Schmidt numbers are also addressed briefly.

Numerical simulations and analysis of the turbulent flow field in a practical gas turbine engine combustor

2021 ◽  
pp. 095765092110632
Author(s):  
Veeraraghava R Hasti ◽  
Prithwish Kundu ◽  
Sibendu Som ◽  
Jay P Gore
Keyword(s):  
Flow Field ◽  
Turbulent Flow ◽  
Numerical Simulations ◽  
Gas Turbine ◽  
Adaptive Mesh Refinement ◽  
Three Dimensional ◽  
Adaptive Mesh ◽  
Turbulent Structures ◽  
Cut Cell ◽  
Turbulent Flow Field

The turbulent flow field in a practical gas turbine combustor is very complex because of the interactions between various flows resulting from components like multiple types of swirlers, dilution holes, and liner effusion cooling holes. Numerical simulations of flows in such complex combustor configurations are challenging. The challenges result from (a) the complexities of the interfaces between multiple three-dimensional shear layers, (b) the need for proper treatment of a large number of tiny effusion holes with multiple angles, and (c) the requirements for fast turnaround times in support of engineering design optimization. Both the Reynolds averaged Navier–Stokes simulation (RANS) and the large eddy simulation (LES) for the practical combustor geometry are considered. An autonomous meshing using the cut-cell Cartesian method and adaptive mesh refinement (AMR) is demonstrated for the first time to simulate the flow in a practical combustor geometry. The numerical studies include a set of computations of flows under a prescribed pressure drop across the passage of interest and another set of computations with all passages open with a specified total flow rate at the plenum inlet and the pressure at the exit. For both sets, the results of the RANS and the LES flow computations agree with each other and with the corresponding measurements. The results from the high-resolution LES simulations are utilized to gain fundamental insights into the complex turbulent flow field by examining the profiles of the velocity, the vorticity, and the turbulent kinetic energy. The dynamics of the turbulent structures are well captured in the results of the LES simulations.

Turbulent Mass Transfer Simulations of Binary Electrolyte in Parallel-Plate Electrode Channel

2012 ◽  
Vol 550-553 ◽  
pp. 2014-2018
Author(s):  
Xiao Lan Zhou ◽  
Cai Xi Liu ◽  
Yu Hong Dong
Keyword(s):  
Mass Transfer ◽  
Turbulent Flows ◽  
Primary Objective ◽  
Wall Turbulence ◽  
Limiting Current ◽  
Solid Boundary ◽  
Wall Region ◽  
Schmidt Numbers ◽  
Near Wall ◽  
Turbulent Mass Transfer

Electrochemical mass transfer in turbulent flows and binary electrolytes is investigated. The primary objective is to provide information about mass transfer in the near-wall region between a solid boundary and a turbulent fluid flow at different Schmidt numbers. Based on the computational fluid dynamics and electrochemistry theories, a model for turbulent electrodes channel flow is established. The turbulent mass transfer in electrolytic processes has been predicted by the direct numerical simulation method under limiting current and galvanostatic conditions, we investigate mean concentration and the structure of the concentration fluctuating filed for different Schmidt numbers from 0.1 to 100 .The effect of different concentration boundary conditions at the electrodes on the near-wall turbulence statistics is also discussed.

Turbulent flow-field effects in a hybrid CFD-CRN model for the prediction of NO and CO emissions in aero-engine combustors

Fuel ◽  
2018 ◽  
Vol 215 ◽  
pp. 853-864 ◽  
Author(s):  
A. Innocenti ◽  
A. Andreini ◽  
D. Bertini ◽  
B. Facchini ◽  
M. Motta
Keyword(s):  
Flow Field ◽  
Turbulent Flow ◽  
Field Effects ◽  
Aero Engine ◽  
Turbulent Flow Field
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