Numerical Simulation of Thermal Transpiration in the Slip Flow Regime With Curved Walls

2012 ◽  
Author(s):  
Vlasios Leontidis ◽  
Lucien Baldas ◽  
Stéphane Colin
Keyword(s):  
Boundary Condition ◽  
Boundary Conditions ◽  
Flow Regime ◽  
Stokes Equations ◽  
Slip Flow ◽  
Velocity Slip ◽  
Operating Conditions ◽  
Thermal Creep ◽  
Geometrical Parameters ◽  
Slip Flow Regime

Nowadays, modeling gas flows in the slip flow regime through microchannels can be achieved using commercial Computational Fluid Dynamics codes. In this regime the Navier-Stokes equations with appropriate boundary conditions are still valid. A simulation procedure has been developed for the modeling of thermal creep flow using ANSYS Fluent®. The implementation of the boundary conditions is achieved by developing User Defined Functions (UDFs) by means of C++ routines. The complete first order velocity slip boundary condition, including the thermal creep effects due to an axial temperature gradient and the effect of the wall curvature, and the temperature jump boundary condition are applied. Motivation of the present work is the development of a simulation tool which will help in the pre-calculations and the preliminary design of a Knudsen micropump consisting of successively connected curved and straight channels and in a second step in the numerical optimization of the pump, in terms of geometrical parameters and operating conditions of the system.

scholarly journals Gas Microflows in the Slip Flow Regime: A Critical Review on Convective Heat Transfer

2011 ◽  
Vol 134 (2) ◽  
Author(s):  
Stéphane Colin
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Flow Regime ◽  
Convective Heat ◽  
Temperature Jump ◽  
Numerical Models ◽  
Slip Flow ◽  
Velocity Slip ◽  
Constant Wall Temperature ◽  
Slip Flow Regime

Accurate modeling of gas microvection is crucial for a lot of MEMS applications (microheat exchangers, pressure gauges, fluidic microactuators for active control of aerodynamic flows, mass flow and temperature microsensors, micropumps, and microsystems for mixing or separation for local gas analysis, mass spectrometers, vacuum, and dosing valves…). Gas flows in microsystems are often in the slip flow regime, characterized by a moderate rarefaction with a Knudsen number of the order of 10−2–10−1. In this regime, velocity slip and temperature jump at the walls play a major role in heat transfer. This paper presents a state of the art review on convective heat transfer in microchannels, focusing on rarefaction effects in the slip flow regime. Analytical and numerical models are compared for various microchannel geometries and heat transfer conditions (constant heat flux or constant wall temperature). The validity of simplifying assumptions is detailed and the role played by the kind of velocity slip and temperature jump boundary conditions is shown. The influence of specific effects, such as viscous dissipation, axial conduction and variable fluid properties is also discussed.

Rarefaction and Compressibility Effects on Steady or Transient Gas Flows in Microchannels

2004 ◽  
Author(s):  
Ste´phane Colin
Keyword(s):  
Boundary Conditions ◽  
Flow Regime ◽  
Pressure Fluctuations ◽  
Slip Flow ◽  
Thermally Driven ◽  
Gas Microflows ◽  
Significant Length ◽  
Slip Flow Regime ◽  
Vacuum Generation ◽  
Frequency Behavior

An analysis of the different significant length scales allows us to show the major part played by rarefaction in gas microflows, and the different flow regimes encountered in microchannels. The main theoretical and experimental results from the literature about steady pressure-driven gas microflows are summarized. Because it is very frequent in microchannels, the slip flow regime is more detailed and the question of appropriate choice of boundary conditions is discussed. It is shown that using second-order boundary conditions allows us to extend the applicability of the slip flow regime to higher Knudsen numbers that are usually relevant to transition regime. The case of pulsed flows is also presented, for this kind of flow is frequently encountered in micropumps. The influence of slip on the frequency behavior (pressure gain and phase) of microchannels is illustrated. When subjected to sinusoidal pressure fluctuations, microdiffusers reveal a diode effect which depends on the frequency. This diode effect may be reversed, when the depth is shrunk from a few hundred to a few micrometers. Thermally driven flows in microchannels are also described. They are particularly interesting for vacuum generation, using microsystems without moving parts.

Prediction of Leak Rates Through Porous Gaskets at High Temperature

2012 ◽  
Author(s):  
Lotfi Grine ◽  
Abdel-Hakim Bouzid
Keyword(s):  
Flow Regime ◽  
Room Temperature ◽  
Structural Characteristics ◽  
Slip Flow ◽  
Pressure Rise ◽  
Operating Conditions ◽  
Reasonable Accuracy ◽  
Fluid Properties ◽  
Slip Flow Regime ◽  
Different Gases

The ability of a gasket to maintain tightness under different operating conditions has been studied extensively in recent years. However most of the research studies conducted on leakage predictions was performed at room temperature. The aim of this work is to predict leakage through gaskets take into account the effect of the temperature on the fluid properties changes and gasket internal structural characteristics. The analytical model of slip flow regime to evaluate the mass leak rates through a porous gasket developed in [1] was used in this study. The results from the model were validated and compared with experimental data obtained from tests conducted on the Universal Gasket Rig with two different gases (Helium, Nitrogen). The leak rates measured are in the range of 1 to 10−4 mg/s, which is measurable using a pressure rise technique. As a second objective the influence of the gasket displacements caused by stress and temperature on the flow leakage was studied. A relationship between displacement or void thickness and leakage is clearly demonstrated. The slip flow regime model is capable of predicting leakage at temperature with reasonable accuracy.

Gas Microflows in the Slip Flow Regime: A Review on Heat Transfer

2010 ◽  
Author(s):  
Ste´phane Colin
Keyword(s):  
Heat Transfer ◽  
Flow Regime ◽  
Temperature Jump ◽  
Numerical Models ◽  
Slip Flow ◽  
Velocity Slip ◽  
Constant Heat Flux ◽  
Gas Analysis ◽  
Constant Wall Temperature ◽  
Slip Flow Regime

Accurate modeling of gas microvection is crucial for a lot of MEMS applications (micro-heat exchangers, pressure gauges, fluidic microactuators for active control of aerodynamic flows, mass flow and temperature micro-sensors, micropumps and microsystems for mixing or separation for local gas analysis, mass spectrometers, vacuum and dosing valves…). Gas flows in microsystems are often in the slip flow regime, characterized by a moderate rarefaction with a Knudsen number of the order of 10−2–10−1. In this regime, velocity slip and temperature jump at the walls play a major role in heat transfer. This paper presents a state of the art review on convective heat transfer in microchannels, focusing on rarefaction effects in the slip flow regime. Analytical and numerical models are compared for various microchannel geometries and heat transfer conditions (constant heat flux or constant wall temperature). The validity of simplifying assumptions is detailed and the role played by the kind of velocity slip and temperature jump boundary conditions is shown. The influence of specific effects, such as viscous dissipation, axial conduction and variable fluid properties is also discussed.

Prediction of Leak Rates Through Porous Gaskets at High Temperature

2013 ◽  
Vol 135 (2) ◽  
Author(s):  
Lotfi Grine ◽  
Abdel-Hakim Bouzid
Keyword(s):  
Flow Regime ◽  
Room Temperature ◽  
Structural Characteristics ◽  
Slip Flow ◽  
Pressure Rise ◽  
Operating Conditions ◽  
Reasonable Accuracy ◽  
Fluid Properties ◽  
Slip Flow Regime ◽  
Different Gases

The ability of a gasket to maintain tightness under different operating conditions has been studied extensively in recent years. However, most of the research studies conducted on leakage predictions was performed at room temperature. The aim of this work is to predict leakage through gaskets taking into account the effect of the temperature on the fluid properties and gasket internal structural characteristics. The analytical model of slip flow regime to evaluate the mass leak rates through a porous gasket developed by Grine and Bouzid (2011, “Correlation of Gaseous Mass Leak Rates Through Micro and Nano-Porous Gaskets,” ASME J. Pressure Vessel Technol.) was used in this study. The results from the model were validated and compared with the experimental data obtained from tests conducted on the Universal Gasket Rig with two different gases (helium and nitrogen). The leak rates measured are in the range of 1 to 0.0001 mg/s, which are measurable using the pressure rise technique. As a second objective, the influence of the gasket displacements caused by stress and temperature on the flow leakage was studied. A relationship between displacement or void thickness and leakage is clearly demonstrated. The slip flow regime model is capable of predicting leakage at temperature with reasonable accuracy.

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