Computing Separated Flows in MEMS Devices

2002 ◽  
Author(s):  
Oktay Baysal ◽  
Alim Rustem Aslan
Keyword(s):  
Temperature Jump ◽  
Slip Flow ◽  
Fluid Flows ◽  
Synthetic Jets ◽  
Separated Flows ◽  
Navier Stokes ◽  
Mems Devices ◽  
Micro Channels ◽  
Analytical Formulae ◽  
Compressibility Effects

Fluid flows in micro devices span the entire Knudsen (Kn) number regime. Depending on the Kn range, a full continuum or a full free-molecular analysis may be applicable. In the present study, flows in the Kn range of 10−3 to 10−1 are considered and they are modeled using a conventional Navier-Stokes solver. Its boundary conditions, however, have been modified to account for the slip velocity and the temperature jump conditions encountered in these micro-sized geometries. The computations have been performed for straight micro channels, a micro backward facing step, and a micro filter. The present results are then compared with analytical formulae and other computations available in the literature. The results indicate that the rarefaction and compressibility effects present in these micro devices have been accurately predicted. In the case of slip flow, the separation is found to occur at a higher Reynolds number compared to the corresponding no-slip flow case. As the next step of the study, micro synthetic jets will be computed and the optimal cavity actuator geometries will be sought for desired flow deflections.

Conjugate Heat Transfer Performances for Gaseous Flows in Short Micro Channels

2014 ◽  
Author(s):  
Giulio Croce ◽  
Michele A. Coppola ◽  
Olga Rovenskaya
Keyword(s):  
Heat Transfer ◽  
Conjugate Heat Transfer ◽  
Temperature Jump ◽  
Slip Flow ◽  
Heat Transfer Analysis ◽  
Navier Stokes ◽  
Performance Parameters ◽  
Micro Channels ◽  
Gaseous Flows ◽  
Slip Flow Regime

A fully conjugate heat transfer analysis of gaseous flow, within slip flow regime, in short microchannels is presented. A Navier Stokes code, coupled with Maxwell slip and Smoluchowski temperature jump models, is adopted. The main focus is on the interaction between compressibility and heat transfer; in particular, due to the link between temperature and velocity field in highly compressible flow, it is important to recast the channel performance parameters in order to take into account the flow cooling due to the conversion between internal and kinetic energy. Results are presented for Nusselt number and a corrected heat sink thermal resistance, as well as resulting wall temperature.

Three Dimensional Numerical Analysis of Laminar Slip-Flow Heat Transfer in Rectangular Microchannels

2008 ◽  
Author(s):  
H. D. Madhawa Hettiarachchi ◽  
Mihajlo Golubovic ◽  
William M. Worek
Keyword(s):  
Heat Transfer ◽  
Boundary Condition ◽  
Temperature Jump ◽  
Slip Flow ◽  
Three Dimensional ◽  
Velocity Slip ◽  
Thermal Boundary Condition ◽  
Navier Stokes ◽  
Constant Wall Temperature ◽  
Rectangular Microchannels

Slip-flow and heat transfer in rectangular microchannels are studied numerically for constant wall temperature (T) and constant wall heat flux (H2) boundary conditions under thermally developing flow. Navier-Stokes and energy equations with velocity slip and temperature jump at the boundary are solved using finite volume method in a three dimensional cartesian coordinate system. A modified convection-diffusion coefficient at the wall-fluid interface is defined to incorporate the temperature-jump boundary condition. Validity of the numerical simulation procedure is stabilized. The effect of rarefaction on heat transfer in the entrance region is analyzed in detail. The velocity slip has an increasing effect on the Nusselt (Nu) number whereas temperature jump has a decreasing effect, and the combined effect could result increase or decrease in the Nu number. For the range of parameters considered, there could be high as 15% increase or low as 50% decrease in fully developed Nu is plausible for T thermal boundary condition while it could be high as 20% or low as 35% for H2 thermal boundary condition.

Computational Analysis of Conjugate Heat Transfer in Gaseous Micro Channels

2013 ◽  
Author(s):  
Giulio Croce ◽  
Olga Rovenskaya ◽  
Paola D’Agaro
Keyword(s):  
Heat Transfer ◽  
Compressible Flows ◽  
Conjugate Heat Transfer ◽  
Computational Analysis ◽  
Slip Flow ◽  
Heat Transfer Analysis ◽  
Navier Stokes ◽  
Jump Model ◽  
Micro Channels ◽  
Slip Flow Regime

A fully conjugate heat transfer analysis of gaseous flow, within slip flow regime, in short microchannel is presented. A Navier Stokes code, coupled with Maxwell and Smoluchowski slip and temperature jump model, is adopted. Due to the link between temperature and velocity field in highly compressible flows, results are presented for Nusselt number, heat sink thermal resistance and resulting wall temperature as well as Mach number profiles for different conditions, commenting on the relative importance of wall conduction, rarefaction and compressibility. Compressibility plays a major role, and the reduction in heat transfer rate due to axial conduction is quite remarkable.

Slip Flow Structures in Confined Geometries

2008 ◽  
Author(s):  
Steffen Jebauer ◽  
Justyna Czerwinska
Keyword(s):  
Gas Flow ◽  
Temperature Jump ◽  
Complex Structure ◽  
Slip Flow ◽  
Velocity Slip ◽  
Navier Stokes ◽  
Flow Structures ◽  
Complex Flow ◽  
Vortex Patterns ◽  
Slip Flows

This paper presents various flow structures related to velocity slip and temperature jump in very low Reynolds number gas flow. The structures differ significantly from the ones observed in continuum regime for laminar flow, especially if the geometry has complex structure, which is very often the case in microfluidic devices. We are modelling the flow as a continuum Navier-Stokes gas flow with additional velocity slip and temperature jump boundary conditions for curved surfaces for slip flows with Knudsen numbers Kn < 0.1. For complex channel geometries with obstacles and curved walls vortex patterns are observed that are related to the thermal stress slip flow. This type of flow is induced only when non-uniform temperature distributions inside flow domains are present. The investigated geometries consist of one or more cylinder walls with diameters of up to a few 100 μm placed inside of confined microchannels, with all setups being two-dimensional. In gaseous microdevices the resulting complex flow patterns can be utilised to enhance mixing or heat transfer.

scholarly journals Compressibility Effects in 2d Wall Heating Microchannel flow

2016 ◽  
Vol 35 ◽  
pp. 57-71
Author(s):  
Md Tajul Islam
Keyword(s):  
Boundary Conditions ◽  
Incompressible Flows ◽  
Stokes Equations ◽  
Control Volume ◽  
Pressure Ratio ◽  
Slip Flow ◽  
Navier Stokes ◽  
Slip Boundary ◽  
Wall Heating ◽  
Compressibility Effects

In this article we present a numerical solution of the Navier-Stokes equations and energy equation in parallel plate microchannels with the first order slip boundary conditions on the walls, adopting control volume scheme of CFD technique. Wall heating condition was considered on the walls. Noslip boundary conditions for compressible and incompressible flows were also solved to compare the effect of slip conditions. Compressibility effects were also investigated for compressible slip and compressible noslip flow conditions. A series of simulations were performed for different heights and lengths of channels and pressure ratios. Results are presented in graphs and tables and are compared with the available analytical and experimental results. It was found that the friction constants are the highest for noslip compressible flow and lowest for the slip flow against pressure ratio and mach numbers. Friction constant decreases continuously for compressible slip flow but it approaches to an asymptotic value of 96 for compressible noslip flow for the decrease of aspect ratio.GANIT J. Bangladesh Math. Soc.Vol. 35 (2015) 57-71

Models for Gaseous Slip Flow in Non-Circular Microchannels

2007 ◽  
Author(s):  
Zhipeng Duan ◽  
Y. S. Muzychka
Keyword(s):  
Friction Factor ◽  
Cross Sections ◽  
Slip Flow ◽  
Analytical Models ◽  
Roughness Effects ◽  
Practical Applications ◽  
Micro Channels ◽  
Mems Technology ◽  
Compressibility Effects ◽  
Circular Microchannels

Micro-scale fluid dynamics has received intensive interest due to the emergence of Micro-Electro-Mechanical Systems (MEMS) technology. Non-circular cross sections are common channel shapes that can be produced through a variety of micro-fabrication techniques. Non-circular microchannels have extensive practical applications in MEMS. Slip flow in noncircular microchannels has been examined by the authors and a review of several new models obtained by the authors is presented. These models are general and robust, and can be used by the research community for practical engineering design of microchannel flow systems. The reviewed models address: (i) fully developed slip flow in non-circular microchannels, (ii) hydrodynamically developing slip flow in non-circular microchannels, (iii) compressibility effects, and (iv) roughness effects. A model is proposed to predict the friction factor and Reynolds product fRe for fully developed and developing slip flow in most non-circular micro-channels. Compressibility effects on slip flow in non-circular microchannels have been examined and simple models are proposed to predict the pressure distribution and mass flow rate for slip flow in most non-circular microchannels. Finally, the effect of corrugated surface roughness on fully developed laminar flow in microtubes is examined. Simple analytical models are developed to predict friction factor and pressure drop in corrugated rough microtubes for continuum flow and slip flow.

scholarly journals Obtaining Expressions for Time-Dependent Functions That Describe the Unsteady Properties of Swirling Jets of Viscous Fluid

Mathematics ◽  
2021 ◽  
Vol 9 (16) ◽  
pp. 1860
Author(s):  
Eugene Talygin ◽  
Alexander Gorodkov
Keyword(s):  
Blood Flow ◽  
Viscous Fluid ◽  
Swirling Flow ◽  
Fluid Flows ◽  
Theoretical Method ◽  
Flow Channel ◽  
Navier Stokes ◽  
Swirling Jets ◽  
Continuity Equations ◽  
Dynamic Velocity

Previously, it has been shown that the dynamic geometric configuration of the flow channel of the left heart and aorta corresponds to the direction of the streamlines of swirling flow, which can be described using the exact solution of the Navier–Stokes and continuity equations for the class of centripetal swirling viscous fluid flows. In this paper, analytical expressions were obtained. They describe the functions C0t and Г0t, included in the solutions, for the velocity components of such a flow. These expressions make it possible to relate the values of these functions to dynamic changes in the geometry of the flow channel in which the swirling flow evolves. The obtained expressions allow the reconstruction of the dynamic velocity field of an unsteady potential swirling flow in a flow channel of arbitrary geometry. The proposed approach can be used as a theoretical method for correct numerical modeling of the blood flow in the heart chambers and large arteries, as well as for developing a mathematical model of blood circulation, considering the swirling structure of the blood flow.

Lattice-Boltzmann Simulation of Two-Phase Fluid Flows

1998 ◽  
Vol 09 (08) ◽  
pp. 1383-1391 ◽  
Author(s):  
Yu Chen ◽  
Shulong Teng ◽  
Takauki Shukuwa ◽  
Hirotada Ohashi
Keyword(s):  
Lattice Boltzmann ◽  
Fluid Flows ◽  
Navier Stokes ◽  
Interface Model ◽  
Two Phase ◽  
Navier Stokes Equation ◽  
Lattice Boltzmann Simulation ◽  
Dam Breaking ◽  
Volumetric Stress ◽  
Phase Fluid

A model with a volumetric stress tensor added to the Navier–Stokes Equation is used to study two-phase fluid flows. The implementation of such an interface model into the lattice-Boltzmann equation is derived from the continuous Boltzmann BGK equation with an external force term, by using the discrete coordinate method. Numerical simulations are carried out for phase separation and "dam breaking" phenomena.

scholarly journals ESTIMATION OF THE SLIP LENGTH IN THE FLOW OF LIQUID IN MICRO-CHANNELS

2018 ◽  
Vol 40 (2) ◽  
pp. 12-19
Author(s):  
Y.Y. Kovetska
Keyword(s):  
Slip Length ◽  
Slip Flow ◽  
Gas Bubbles ◽  
Hydrophobic Surface ◽  
Dissipative Heating ◽  
Research Review ◽  
Surface Shear Stress ◽  
Surface Shear ◽  
Lower Viscosity ◽  
Micro Channels

Research review of phenomenon for slip flow in micro and nanocannels is presented in the paper. The analysis of theoretical and experimental data characterizing the slip length is carried out. In slip flow in microchannels the slip length is affected by the contact angle of the liquid with the surface, shear stress, pressure, dissipative heating, the amount and nature of the dissolved gas in the liquid, electrical characteristics, surface roughness. Studies of flow in microchannels with hydrophobic walls, which are based on molecular dynamics, showed that the slip length has order of 20 nm. This is much less than the values observed in the experiment. The introduction of an effective (apparent) slip length suggests the existence of a thin layer of gas bubbles near the hydrophobic surface or liquid layer with low value of viscosity and density. Since the idealized model for the total coverage of a hydrophobic surface by gas bubbles gives, as a rule, overestimated values of the slip length in comparison with experimental ones, some researchers consider the inhomogeneous coating of the wall by gas bubbles. In this case, the effect of a layer with a lower viscosity on the slip length turns out to be weaker.

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