Rarefied Gas Flow In Microchannel Research Articles

Second-order slip condition considering Langmuir isothermal adsorption for rarefied gas microflows

Effect of the slip boundary condition on rarefied gas flow simulations plays an important role to understand the behaviour of gas microflows in MEMS. Several second-order slip conditions were proposed by the models of the kinetic theory of gases to simulate the rarefied gas microflows, in which the so-called classical second-order slip condition was derived from the Karniadakis et al. model. In this paper, a new second-order slip condition is proposed to employ with the Navier-Stokes-Fourier equations for simulating the rarefied gas flows in microchannels. It is derived by combining the Langmuir isothermal adsorption and the Karniadakis et al. model, with the aim of achieving a more realistic physical model. The pressure-driven back-forward-step, the Couette and pressure-driven Poiseulle rarefied gas flows in microchannels are investigated to validate our new second-order slip condition. Slip velocities using our new second-order slip condition are better than those using the conventional Maxwell and the so-called classical second-order slip conditions, and are in very good agreement with the DSMC data for all cases considered.

Determination of tangential momentum accommodation coefficient and slip coefficients for rarefied gas flow in a microchannel

This paper presents an experimental study of rarefied gas flow in a trapezoidal microchannel with a constant depth of 103 µm, top width of 1143 µm, bottom width of 998 µm and length of 2 cm. The aim of the study is to verify the upper limit of the validity of the second-order slip boundary condition to model rarefied gas flows. The slip coefficients and the tangential momentum accommodation coefficient (TMAC) are determined for three different gases, viz. argon, nitrogen and oxygen, and it is observed that they compare well to the literature values. The range of mean Knudsen number (Knm) investigated is 0.007–1.2. The non-dimensional mass flow rate exhibits the well-known Knudsen minimum in the transition regime (Knm ~ 1). It is seen that the Navier–Stokes equation with a second-order boundary condition fits the data satisfactorily with a high value of correlation coefficient (r2 > 99.95%) in the entire range of Knm investigated. This work contributes by extending the range of Knudsen number studied in the context of validity of the second-order slip boundary condition.

Numerical study of rarefied gas flows in microchannels with oscillating elements

This paper present a numerical study of a free-molecular (Kn >10) gas flow in two devices with high-frequency oscillating boundary: (1) flow in plane channel with a series of oscillating barriers. (2) flow in a channel with wall performing forced harmonic motion. All calculations were performed using simplified collision-free version of Event-Driven Molecular Dynamics method (EDMD). It was shown that high-frequency oscillations have a significant influence on the gas flow, and this influence depends on molecules characteristic thermal speed. Thus it was shown, that this influence will be different for gases with different molecular masses, which leads to gas separation effect.

3D study of temperature drop behavior of subsonic rarefied gas flow in microchannel

3D Numerical study of temperature variation for subsonic rarefied gas flow in a microchannel is carried out using an in-house MPI-based parallelized DSMC code. The temperature drop in the microchannel decreases with an increase in the aspect ratio whereas it increases with an increase in the pressure ratio, the cross-aspect ratio (CAR), and the Knudsen number. 3D and 2D simulations results are compared and effect of the CAR and Knudsen number are brought out. Finally, a correlation that predicts the temperature drop is formulated along with a list of conditions that ensures a near isothermal flow.

On the numerical simulation of rarefied gas flows in micro-channels

Classical continuum methods break down for rarefied gas flows in micro-channels. Two non-continuum hydrodynamic models are investigated here: compressible Korteweg fluid-like and Bi-Velocity (Volume Diffusion) hydrodynamic models. The pressure driven rarefied gas flow in a rectangular micro-channel in the whole range of Knudsen number is numerically considered. The Korteweg model shows improved results in comparison with standard Navier–Stokes up to Knudsen number of unity. The Bi-Velocity method is found to allow a match between experiments and numerical results over the full range of Knudsen number.

직접모사법을 이용한 형상 변화에 따른 미소채널 유동 해석

In the present study, to investigate the characteristics of subsonic rarefied gas flows in micro-channels with shape change, flow simulations of micro-channels with different geometries were conducted using a direct simulation Monte-Carlo (DSMC) solver based on unstructured meshes. For simulating micro-channel flows like pressure driven flows, the pressure boundary conditions at the inlet and the exit were additionally applied. Verification of the boundary condition was conducted by comparing with other researchers’ DSMC simulations with the pressure boundary condition. After verification of the boundary condition, the straight micro-channel of 15μm × 1μm was considered for the baseline channel. The pressure conditions at the inlet and the exit were 5×105 Pa and 1.66×105 Pa, respectively. The inlet gas temperature and wall temperature were both 300K, and a fully diffuse reflecting wall was considered. Three types of change of shapes were considered. The change of the length, height and bend angles were treated for this study. Firstly, to investigate the effects of the change of length, half and twice of the baseline channel length were considered. It was shown that the pressure gradient was dominantly affected by the change of the lengths. As a result, the velocity was more accelerated for short length channel than for the baseline channel. On the other hand, when the heights were become higher, the changes of flow properties were greater than the baseline channel due to the area of boundary layers or the forces by the pressure gradient. Finally, the effects of the bend angles in micro-channels were investigated with several bend angles. The angles were considered from 0 to 135 degrees. When the angle became greater, the flow separation was generated at the concave type corner. For larger angles, the variation of flow properties at the corner became greater. Flow properties at the exit were different trend from those at the corner. Pressure and density at the exit were not dependent on the bend angles, whereas the velocity at the exit was increased according to the bend angles.

Simulation of Rarefied Gas Flows in Microchannels

In this work the rarefied gas flows in microchannels is investigated. With low pressure owing to low gas density the gas slippage affetcting the channel walls is observed. The task of sliding flow on initial segments of flat (two-dimensional) channel of uniform cross section is solved. Intercomparison of two calculation methods is carried out. The analytical method is based on approximate approach developed by S. M. Targ. The small channel extension does not always allow application of the formulas of calculation of hydraulic resistance, received for long (theoretically infinitely long) channels. The physics of process are also answered, more by development of flow on the so-called initial segment, when the flow characteristics change not only on a cross section of the channel, but also along its length. As a result of the reasonable simplification of the initial tasks it was possible to receive the simple formulas for practical calculations of the main hydrodynamic characteristics and the value of velocity and pressure of flow on channel length. The numerical solution of the task is carried out with the control-volume method in an ANSYS CFX system with use of structured computational mesh, constructed in ANSYS ICEM CFD. The task is solved in a flat statement: cells are located in one layer and on the sidewalls of layer the symmetry conditions are set. The closed system of the Navier-Stokes equations is solved. The laminar flow mode taking into account a slipping condition on wall of “Finite Slip Wall” is considered. Calculations are carried out for Knudsen numbers from 0 to 0,1 with step in 0,02. The intercomparison of the modeling results obtained by analytical and numerical methods showed good agreement.

Lattice Boltzmann simulation of rarefied gas flows in microchannels

For gas flows in microchannels, slip motion at the solid surface can occur even if the Mach number is negligibly small. Since the Knudsen number of the gas flow in a long microchannel can vary widely and the Navier-Stokes equations are not valid for Knudsen numbers beyond 0.1, an alternative method that can be applicable to continuum, slip and transition flow regimes is highly desirable. The lattice Boltzmann equation (LBE) approach has recently been expected to have such potential. However, some hurdles need to be overcome before it can be applied to simulate rarefied gas flows. The first major hurdle is to accurately model the gas molecule and wall surface interactions. In addition, the Knudsen number needs to be clearly defined in terms of LBE properties to ensure that the LBE simulation results can be checked against experimental measurements and other simulation results. In this paper, the Maxwellian scattering kernel is adopted to address the gas molecule and surface interactions with an accommodation coefficient (in addition to the Knudsen number) controlling the amount of slip motion. The Knudsen number is derived consistently with the macroscopic property based definition. The simulation results of the present LBE model are in quantitative agreement with the established theory in the slip flow regime. In the transition flow regime, the model captures the Knudsen minimum phenomenon qualitatively. Therefore, the LBE can be a competitive method for simulation of rarefied gas flows in microdevices.

Statistical simulation of rarefied gas flows in micro-channels

Rarefied gas flows through micro-channels are simulated using particle approaches, named as the information preservation (IP) method and the direct simulation Monte Carlo (DSMC) method. In simulating the low speed flows in long micro-channels the DSMC method encounters the problem of large sample size demand and the difficulty of regulating boundary conditions at the inlet and outlet. Some important computational issues in the calculation of long micro-channel flows by using the IP method, such as the use the conservative form of the mass conservation equation to guarantee the adjustment of the inlet and outlet boundary conditions and the super-relaxation scheme to accelerate the convergence process, are addressed. Stream-wise pressure distributions and mass fluxes through micro-channels given by the IP method agree well with experimental data measured in long micro-channels by Pong et al. (with a height to length ratio of 1.2:3000), Shih et al. (1.2:4800), Arkilic et al. and Arkilic (1.3:7500), respectively. The famous Knudsen minimum of normalized mass flux is observed in IP and DSMC calculations of a short micro-channel over the entire flow regime from continuum to free molecular, whereas the slip Navier–Stokes solution fails to predict it.