This work investigates the species separation in the rarefied flow of the argon-helium mixture through convergent-divergent micronozzles. Imposing a molecular mass ratio in the order of 10, the flow of this mixture can lead to the formation of serious nonhomogeneous phenomena such as the species separation. This study is performed in the ranges of 2.0–4.0 for the geometrical expansion ratio, 200–400 K for the wall temperature, and 0.003–1.454 for the inlet Knudsen number. The effects of these parameters are examined on the separative performances of micronozzle. The direct simulation Monte Carlo method is selected as the solution method because it can provide reliable solutions in the current rarefied flow regime study. The current study reveals two important separative effects in the mixture flow through micronozzles. The first effect is the lateral species separation, which results in the enrichment of heavier species near the centerline. The second effect is the streamwise separation, which leads to the enrichment of one species, mostly the lighter one, as the mixture passes through the micronozzle. The current results show that increasing the expansion ratio will enhance the lateral separation monotonically. However, there are specific wall temperature and Knudsen values, which can result in optimum lateral separative effects. In addition, it is observed that the expansion ratio has little effect on the streamwise separation. However, increasing either the wall temperature or the Knudsen number will enhance the streamwise separation, albeit with a limiting value at very high Knudsen numbers.

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In this paper, the effect of Magneto Hydro-Dynamics (MHD) on a polymer chain in the micro channel is studied by employing the Dissipative Particle Dynamics simulation (DPD) method. First, in a simple symmetric micro-channel, the results are evaluated and validated for different values of Hartmann (Ha) Number. The difference between the simulation and analytical solution is below 10%. Then, two types of polymer chain including short and long polymer chain are examined in the channel and the effective parameters such as Ha number, the harmony bond coefficient or spring constant (K), and the length of the polymer chain (N) are studied in the MHD flow. It is shown that by increasing harmony bond constant to 10 times with Ha = 20, the reduction of about 80% in radius of gyration squared, and half in polymer length compared to Ha = 1 would occur for both test cases. For short and long length of polymer, proper transfer of a polymer chain through MHD particles flow is observed with less perturbations (80%) and faster polymer transfer in the symmetric micro-channel.

A comparative study is conducted between the molecular and continuum approaches to treat the gas flow as well as the mixing problem in micro/nanoscale channels. The molecular-based simulations are performed using the direct simulation Monte Carlo DSMC method; however, the continuum-based simulations are accomplished using the finite-volume FV method incorporating suitable slip/jump boundary conditions for the gas mixture flows. Employing these two methods, we simulate the mixing process of two gases in mixers with different micro and nano scale sizes working in both slip and transitional flow regimes. The comparisons are provided for the corresponding results of these two methods including their flow velocities, mass flow rates, species mass fractions, and diffusion fluxes. Using the outcome of the performed comparisons, we eventually describe the effects of rarefaction on the achieved accuracy of the continuum-based method.

We present the rarefaction effects on diffusive mass transport in micro- and nanoscales using the results of direct simulation Monte Carlo DSMC method. Unlike the previous investigations, the momentum and heat contributions are eliminated from the computations via uniform velocity, pressure, and temperature field considerations. The effects of global Knudsen number on the diffusion phenomenon are studied for the same Peclet number and a unique mixer shape. The results indicate that there is considerable weakening in diffusion mechanism for high Knudsen number cases. As a result, the non-dimensional diffusive mass fluxes would decrease and the non-dimensional mixing length would increase as the Knudsen number increases. The effective diffusion coefficient is calculated throughout the mixer using the diffusive mass fluxes and the species mass fraction gradients. It is observed that the effective diffusion coefficient can vary considerably as a result of local rarefaction variations. It reaches to the lowest value at the point of confluence, where the maximum mass fraction gradient magnitude would occur for the species. Moving away from this point, the local rarefaction effects would weaken and the effective diffusion coefficient would reinforce subsequently. All the presented results indicate that there would be a convergent to a limiting behavior, which corresponds to the continuum mass diffusion case. Despite this, the local rarefaction level decreases continuously. Unfortunately, because of a considerable increase in the statistical fluctuations at very low rarefaction levels, the simulations do not provide reliable results in the limit of continuum regime.