DSMC investigation of rarefied gas flow through diverging micro- and nanochannels

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$^{\circ}$ to 17$^{\circ}$ are described at different pressure ratios (1.5${\le}{\prod}{\le}$2.5) and Knudsen numbers (0.03${\le}$Kn${\le}$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.Comment: Accepted manuscript; 25 Pages and 11 Figures; "Microfluidics and Nanofluidics

A DSMC investigation of gas flows in micro-channels with bends

Pressure-driven, implicit boundary conditions are implemented in an open source direct simulation Monte Carlo (DSMC) solver, and benchmarked against simple micro-channel flow cases found in the literature. DSMC simulations are then carried out of gas flows for varying degrees of rarefaction along micro-channels with both one and two ninety-degree bends. The results are compared to those from the equivalent straight micro-channel geometry. Away from the immediate bend regions, the pressure and Mach number profiles do not differ greatly from those in straight channels, indicating that there are no significant losses introduced when a bend is added to a micro-channel geometry. It is found that the inclusion of a bend in a micro-channel can increase the amount of mass that a channel can carry, and that adding a second bend produces a greater mass flux enhancement. This increase happens within a small range of Knudsen number (0.02 Knin 0.08). Velocity slip and shear stress profiles at the channel walls are presented for the Knudsen showing the largest mass flux enhancement

The GASMEMS network: Rationale, programme and initial results

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.GASMEMS is an Initial Training Network supported by the European Commission, which aims at training young researchers in the field of rarefied gas flows in MEMS, and at structuring research in Europe in the field of gas microflows in order to improve global fundamental knowledge and enable technological applications to an industrial and commercial level. The partners and the global objectives of this 4 year programme are detailed, and some initial results are presented. First experimental data about the flow of binary gas mixtures through rectangular microchannels are successfully compared with continuum and kinetic models, in the slip flow and early transition regimes. The behaviour of these mixtures has also been simulated in triangular microchannels, for the whole range of the Knudsen number, using a kinetic approach and the McCormack model. Heat transfer in plane microchannels has been numerically investigated, pointing out compressibility and rarefaction effects. The effect of thermal creep has been studied comparing BGK, Smodel and ellipsoidal model with the solution from the full Boltzmann equation. A semi-analytical model of the Knudsen layer has been developed and used to simulate the problem of thermal transpiration in a microchannel. Gaseous flows through rough microchannels have been simulated using kinetic theory and DSMC method, the wall roughness being simulated as a highly porous medium of variable thickness.This study is funded by the European Community's Seventh Framework Programme FP7/2007-2013 under grant agreement ITN GASMEMS n° 215504

Simulating fluid flows in micro and nano devices : the challenge of non-equilibrium behaviour

We review some recent developments in the modelling of non-equilibrium (rarefied) gas flows at the micro- and nano-scale, concentrating on two different but promising approaches: extended hydrodynamic models, and lattice Boltzmann methods. Following a brief exposition of the challenges that non-equilibrium poses in micro- and nano-scale gas flows, we turn first to extended hydrodynamics, outlining the effective abandonment of Burnett-type models in favour of high-order regularised moment equations. We show that the latter models, with properly-constituted boundary conditions, can capture critical non-equilibrium flow phenomena quite well. We then review the boundary conditions required if the conventional Navier-Stokes-Fourier (NSF) fluid dynamic model is applied at the micro scale, describing how 2nd-order Maxwell-type conditions can be used to compensate for some of the non-equilibrium flow behaviour near solid surfaces. While extended hydrodynamics is not yet widely-used for real flow problems because of its inherent complexity, we finish this section with an outline of recent 'phenomenological extended hydrodynamics' (PEH) techniques-essentially the NSF equations scaled to incorporate non-equilibrium behaviour close to solid surfaces-which offer promise as engineering models. Understanding non-equilibrium within lattice Boltzmann (LB) framework is not as advanced as in the hydrodynamic framework, although LB can borrow some of the techniques which are being developed in the latter-in particular, the near-wall scaling of certain fluid properties that has proven effective in PEH. We describe how, with this modification, the standard 2nd-order LB method is showing promise in predicting some rarefaction phenomena, indicating that instead of developing higher-order off-lattice LB methods with a large number of discrete velocities, a simplified high-order LB method with near-wall scaling may prove to be just as effective as a simulation tool

Gas dynamics at the micro-scale: A review of progress in hydrodynamic modelling

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.We review some recent developments in the modelling of non-equilibrium (rarefied) gas flows at the micro- and nano-scale using extended hydrodynamic models. Following a brief exposition of the challenges that non-equilibrium poses in micro- and nano-scale gas flows, we outline the field of extended hydrodynamics, describing the effective abandonment of Burnett-type models in favour of high-order regularised moment equations. We then review the boundary conditions required if the conventional Navier-Stokes-Fourier (NSF) fluid dynamic model is applied at the micro scale, describing how 2nd-order Maxwelltype conditions can be used to compensate for some of the non-equilibrium flow behaviour near solid surfaces. While extended hydrodynamics is not yet widely-used for real flow problems because of its inherent complexity, we finish with an outline of recent ‘phenomenological extended hydrodynamics’ (PEH) techniques — essentially the NSF equations scaled to incorporate non-equilibrium behaviour close to solid surfaces — which offer promise as engineering models.This work is funded in the UK by the Engineering and Physical Sciences Research Council through grants EP/F002467/1, EP/D07455X/1, EP/D007488/1 and EP/F028865/1

Simulation of copper-water nanofluid in a microchannel in slip flow regime using the lattice Boltzmann method with heat flux boundary condition

Laminar forced convection heat transfer of water–Cu nanofluids in a microchannel is studied using the double population Thermal Lattice Boltzmann method (TLBM). The entering flow is at a lower temperature compared to the microchannel walls. The middle section of the microchannel is heated with a constant and uniform heat flux, simulated by means of the counter slip thermal energy boundary condition. Simulations are performed for nanoparticle volume fractions equal to 0.00%, 0.02% and 0.04% and slip coefficient equal to 0.001, 0.01 and 0.1. Reynolds number is equal to 1, 10 and 50.The model predictions are found to be in good agreement with earlier studies. Streamlines, isotherms, longitudinal variations of Nusselt number and slip velocity as well as velocity and temperature profiles for different cross sections are presented. The results indicate that LBM can be used to simulate forced convection for the nanofluid micro flows. They show that the microchannel performs better heat transfers at higher values of the Reynolds number. For all values of the Reynolds considered in this study, the average Nusselt number increases slightly as the solid volume fraction increases and the slip coefficient increases. The rate of this increase is more significant at higher values of the Reynolds number

The Rationality of 2-D Simplification of Low-Speed Short Microchannel Flows

[[abstract]]The Direct-simulation Monte Carlo (DSMC) method has been employed to analyze the rationalityof the 2-D simplification for a 3-D straight rectangular cross-sectional channel, a 3-D straight rectangular cross-sectional channel with microstructures and a 3-D straight rectangular cross-sectional channel with a backward-facing step. An implicit treatment for low-speed inflow and outflow boundaries for the DSMC of the microchannel flow is employed. The 3-D microchannel flows are simulated with the various cross-aspect rations ranging between 1 and 5. The calculated heat and flow properties in the 3-D cases are compared with the results of the 2-D case. It shows that when the aspect ratio is less than 3, the two extra side walls in the 3-D case have significant effects on the heat transfer and flow properties. When the aspect ratio increases, the flow properties and heat transfer characteristics of the 3-D simulations tend to approach that of the 2-D results. This paper also provides the imformaion of the approaching level for different geometry of the microchannels. It is found that the approaching level becomes slower when the geometry of the channel becomes more complicated. In the present study , when the cross-section aspect ratio is 5 , the approaching level of the above mentioned three different geometry of microchannels are found to be about 99%, 97.8% and 97% respectively.[[journaltype]]國內[[incitationindex]]EI[[booktype]]紙本[[countrycodes]]TW

Gas Flows in Microsystems

International audienc