Comparative study between computational and experimental results for binary rarefied gas flows through long microchannels (original) (raw)
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Flow rate measurement of rarefied binary gases in long rectangular microchannels
2011
The flow rate of binary gas mixtures through rectangular long microchannels is measured and compared to the numerical solution of the McCormack kinetic model. The microchannels are etched in silicon, and each individual channel has width=21, height=1.15, length=5000 μm. The measurement refers to He/Ar and He/Kr gas mixtures and are based on the constant volume method. The microchannel is placed between an upstream and a downstream reservoir having different pressures. The flow through the microsystem is maintained by the pressure drop between the containers and the flow rate is determined from the pressure variations in the reservoirs. In the case of He/Ar, measurements have been performed for several values of its concentration varying between zero and one, while in the case of He/Kr only a concentration equal to 0.5 is considered. The pressure ratio between the two containers is in the range of 3-7 and the corresponding average Knudsen numbers are in the range of 0.12-0.98. The results of the flow rate measurement are compared to the discrete velocity solution of the McCormack kinetic model and very good agreement between experiment and simulation has been obtained for all flow configurations. The relative discrepancy between the experimental and numerical results is in the range of the experimental uncertainty.
Gas separation in rarefied binary gas mixture flows through long tapered microchannels
The rarefied gas flow of binary gas mixtures through diverging and converging microchannels is investigated based on linear kinetic theory. The analysis is valid in the whole range of the Knudsen number. Indicative results for the molar flow rates and the axial pressure and molar fraction distributions are provided for the gas mixtures of He-Xe and NeAr flowing through tapered channels of specific geometry due to a given pressure difference. The dependency of the computed fluxes and of the induced separation effect on the species of the gas mixture, its molar fraction as well as on the gas-surface interaction is examined and discussed in detail.
Flow rate measurements of binary gas mixtures through long trapezoidal microchannels
Journal of Physics: Conference Series, 2012
The flow rate of two noble gas mixtures, namely He/Ar and He/Kr, is measured through a microsystem containing 400 long trapezoidal microchannels placed in parallel configuration. Each microchannel has a trapezoidal cross section with long base 5.38 micrometers and height 1.90 micrometers, while its length is 5000 micrometers. The experiment is based on the constant volume method. The flow is driven by pressure gradient. The flow rate measurements refer to downstream pressures of 15.1 kPa and 8.05 kPa. The pressure ratio is in the range of 3-7 and 4-7 for the larger and smaller downstream pressures, respectively. The investigated rarefaction range is in the slip and early transition regions. The concentration of He varies from zero to one. The measured flow rates are compared to the corresponding computational ones obtained by the numerical solution of the McCormack kinetic model. Very good agreement between the experimental and computational results is reached. The difference between the corresponding results is less than the experimental uncertainty. Typical pressure and concentration profiles along the axis and the velocity profiles in the center of the channel obtained from the numerical solution are also presented.
Kinetic calculation of rarefied gaseous flows in long tapered rectangular microchannels
2014
Gaseous flows in microsystems have attracted considerable attention in fluid dynamic communities over the last few years. When the size of the device is in the range of microns, the molecular mean free path becomes comparable with the device size, and the details of the molecular interactions need to be taken into account. The proper description of such microflows requires the consideration of the velocity distribution function of the molecules and kinetic equations. The scope of the present paper is to discuss the determination of the behavior of pressure driven rarefied gas flows in microchannels at the kinetic level. As a new application of the methodology, preliminary results are presented for pressure driven flows of single gases through long rectangular tapered microchannels, which have constant widths but varying depths along the axis of the channel. The kinetic calculation is based on the solution of the linearized Bhatnagar-GrossKrook (BGK) equation and refers to the determ...
DSMC investigation of rarefied gas flow through diverging micro-and nanochannels
2017
Direct simulation Monte Carlo (DSMC) method with simplified Bernoulli-trials (SBT) collision scheme has been used to study the rarefied pressure-driven nitrogen flow through diverging microchannels. The fluid behaviours flowing between two plates with different divergence angles ranging between 0 o to 17 o are described at different pressure ratios (1.5≤Π≤2.5) and Knudsen numbers (0.03≤Kn≤12.7). The primary flow field properties, including pressure, velocity, and temperature, are presented for divergent microchannels and are compared with those of a microchannel with a uniform cross-section. The variations of the flow field properties in divergent microchannels, which are influenced by the area change, the channel pressure ratio and the rarefication are discussed. The results show no flow separation in divergent microchannels for all the range of simulation parameters studied in the present work. It has been found that a divergent channel can carry higher amounts of mass in comparison with an equivalent straight channel geometry. A correlation between the mass flow rate through microchannels, the divergence angle, the pressure ratio, and the Knudsen number has been suggested. The present numerical findings prove the occurrence of Knudsen minimum phenomenon in micro-and Nano-channels with non-uniform cross-sections.
Microfluidics and Nanofluidics, 2010
The flow of binary gas mixtures through long micro-channels with triangular and trapezoidal cross sections is investigated in the whole range of the Knudsen number. The flow is driven by pressure and concentration gradients. The McCormack kinetic model is utilized to simulate the rarefied flow of the gas mixture, and the kinetic equations are solved by an upgraded discrete velocity algorithm. The kinetic dimensionless flow rates are tabulated for selected noble gas mixtures flowing through micro-channels etched by KOH in silicon (triangular and trapezoidal channels with acute angle of 54.74°). Furthermore, the complete procedure to obtain the mass flow rate for a gas mixture flowing through a channel, based on the dimensionless kinetic results, which are valid in each cross section of the channel, is presented. The study includes the effect of the separation phenomenon. It is shown that gas separation may change significantly the estimated mass flow rate. The presented methodology can be used for engineering purposes and for the accurate comparison with experimental results.
Icnmm 2007-30048 Transport Phenomena Through Gaseous Mixtures in Microchannels
2007
m ABSTRACT In practice, one deals with gaseous mixtures more frequen y than with a single gas. However, very few papers about t transport phenomena through a mixture of rarefied gases w published. The aim of this work is to present a general a proach to calculations of mass, heat and momentum trans through gaseous mixtures over the whole range of the gas r efaction. Results on some classical problems such as slip c efficient, Poiseuille flow, Couette flow and heat transfer are gi ven for a gaseous mixture. A comparison with results correspon ing to a single gas is carried out. Such a comparison shows peculiarities of the transport phenomena in mixtures.
Statistical Modeling of Rarefied Gas Flows in Microchannels
2007
Lattice Boltzmann method (LBM) and direct simulation Monte Carlo (DSMC) method are used for analysis of moderate Knudsen number phenomena. Simulation results are presented for pressure driven isothermal microchannel flow at various pressure ratios. Analytical equations for non-linear pressure distribution and velocity profiles along the channel axis are used to verify the present LBM and DSMC results. We conclude that the LBM method can be used as an alternative model to DSMC simulations.
The contribution of diffusion to gas microflow: An experimental study
Moderately rarefied gas flows are clearly distinguished from viscous flow in the continuum regime and from molecular diffusion at high rarefaction. They are an intermediate of the two border cases referred to as slip flow and transition regime flow. Here we present a new pencil-and-paper approach for modeling flows in these regimes by a superposition of convection and Fickian diffusion. It allows us to predict mass flows for helium, argon, nitrogen and carbon dioxide in microducts with parallel walls and with slightly varying cross-section. The model was validated by measurement series taken from literature and by own permeation experiments on tapered microchannels. Analytical investigation of the approach showed that the diffusive flow is proportional to the cross-sectional area at the channel entrance. Hence the mass flow in a tapered channel is unequal in both directions when diffusion dominates due to increased rarefaction. In contrary to the common Maxwellian slip approach the superposition model describes the data reliably. From this we conclude that deviations from continuum behavior in the intermediate cannot be explained by slip flow at the walls and tangential momentum accommodation, but by Fickian diffusion. Now predictions are possible without any usage of fitted parameters such as the Tangential Momentum Accommodation Coefficient (TMAC).
Physics of Fluids, 2005
The flow of binary gaseous mixtures through rectangular microchannels due to small pressure, temperature, and molar concentration gradients over the whole range of the Knudsen number is studied. The solution is based on a mesoscale approach, formally described by two coupled kinetic equations, subject to diffuse scattering boundary conditions. The model proposed by McCormack substitutes the complicated collision term and the resulting kinetic equations are solved by an accelerated version of the discrete velocity method. Typical results are presented for the flow rates and the heat fluxes of two different binary mixtures ͑Ne-Ar and He-Xe͒ with various molar concentrations, in two-dimensional microchannels of different aspect ͑height to width͒ ratios. The formulation is very efficient and can be used instead of the classical method of solving the Navier-Stokes equations with slip boundary conditions, which is restricted by the hydrodynamic regime. Moreover, the present formulation is a good alternative to the direct simulation Monte Carlo method, which often becomes computationally inefficient.