Turbulent particle-laden flows occur extensively in various environmental, biological, and industrial processes. Therefore, improving the knowledge base of the details of particle-turbulence interactions in turbulent two-phase fluid-solid flows is vital for handling the challenges facing in these processes. While different approaches can be used to explore the characteristics of the turbulent flows, the direct numerical simulation (DNS) has been recognized as the most accurate method. The DNS approach directly solves the Navier-Stokes equations and evaluates the instantaneous fluid velocities resolving all fluctuations of turbulence down to the Kolmogorov scale.

The DNS, in conjunction with the Lagrangian particle tracking, has been successfully used for simulation of dilute two-phase flows. Although the DNS approach is an accurate method for evaluating turbulent flows, the predicted deposition velocities of small size particles appear to underestimate the experimental data for the Stokes number, St, in the range of 0.1\<St\<5. (Here St is the non-dimensional particle relaxation time.) Therefore, this study is devoted to providing an understanding of the causes of the discrepancies between the predictions of DNS for particle deposition velocities with the experimental data. Particular attention is given to the effects of the gravity direction, the lift force, and the wall roughness which can affect the deposition velocity predicted by the DNS-particle tracking approach. The goal is to improve the DNS predictions of particle deposition.

In this investigation, a dilute gas-solid turbulent flow in a channel was studied. A pseudo-spectral DNS code was used to solve the Navier-Stokes equations, and to generate the instantaneous turbulent velocity fluctuation field for the gas flow. Under the one-way coupling assumption, the gas flow carries the micro- and nano-particles, but the influence of particles on the flow can be neglected. To provide an understanding of the transport behavior of particles of different sizes, 200,000 monodisperse point particles with Stokes numbers of 0.1, 1, 5, 25, and 125 were introduced randomly in the channel. The corresponding Lagrangian particle equation of motion, including Stokes drag, gravity, and the lift forces are solved, and the trajectories of the particles for the duration of 10,000 wall units were evaluated for dilute suspension. The trap boundary condition on the lower and upper walls of the channel was assumed, and the deposition rates of particles with different sizes were evaluated and recorded as a function of time. Ensemble and time averaging of the simulation results were performed, and the corresponding concentration profiles and the deposition velocities of particles were evaluated for various conditions. A series of simulations were performed, and the effects of wall roughness, lift force, and the gravity direction on the deposition rate were carefully examined. It was found that the surface roughness and the direction of gravity in conjunction with the lift force significantly affect the fine particle deposition rate and could improve the agreement of the DNS simulation with the available experimental data.


A Dns Study of the Dispersion and Deposition of Nano- and Micro-Particles in a Turbulent Channel Flow

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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