Accurate estimation of the near-wall values of the wall-normal RMS turbulence velocity fluctuations is essential for obtaining accurate results for dispersion and deposition rate of micro and nano-particle. However, most RANS turbulence models overestimate the near-wall normal RMS values. Among the available turbulence models, the v2f model appears to predict the quadratic variation of the normal RMS velocity fluctuations in the near-wall regions with reasonable accuracy. Hence, in this study, the v2f model was used for evaluating the time-averaged fluid velocities and turbulence properties in a two-dimensional fully developed turbulent duct flow. The geometry and grid were generated by the OpenFOAM blockMesh utility, and the governing equations were solved by the OpenFOAM pisoFoam solver.

In this study, the accuracy of the discrete random walk (DRW) stochastic model in generating the instantaneous velocity fluctuations as seen by micro- and nano-particles in inhomogeneous turbulent flows was examined. Particular attention was given to the effects of the non-uniform normal RMS velocity fluctuations and turbulence time scale on the DRW model predictions. The trajectories of randomly injected point-particles with diameters ranging from 10 nm to 30 µm in a duct were evaluated using an in-house Matlab particle tracking code. The particle equation of motion included the drag and Brownian forces. The fully developed mean velocity and RMS fluctuation velocity profiles were exported from the v2f simulation and were used for the particle dispersion and transport analysis. It was assumed that the particle-laden flow is sufficiently dilute so that the particle-particle collisions and the two-way coupling effects of particles on the flow could be ignored. To incorporate the instantaneous turbulence velocity fluctuations effects on particle dispersion, the conventional DRW model (in the absence of drift corrections), which was originally developed for homogenous turbulent flows, was first used. It was shown that the conventional DRW model leads to superfluous migration of fluid-point particles toward the wall and erroneous particle deposition rate. A DRW model with an appropriate velocity gradient drift correction term was also tested. It was found that the predicted concentration profiles of tracer particles still are not uniform. It was hypothesized that the reason is because of the inhomogeneous turbulence time macroscale in the channel flow. A new drift correction term as a function of gradients of both RMS fluctuation velocity and the turbulence time macroscale was proposed. It was shown that the new improved DRW model with the velocity and time scale drift corrections leads to uniform distributions for fluid-point particles and reasonable concentration profiles for finite-size heavy particles. In addition, good agreement was found between the estimated deposition velocities of different size particles by the new improved-DRW model with the available experimental data as well as the predictions of the empirical models and earlier DNS numerical results.


Improved Drw Model for Prediction of Deposition and Dispersion of Nano- and Micro-Particles in Turbulent Flows

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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