Jianjun Ye

Using Direct Simulation Monte Carlo With Improved Boundary Conditions for Heat and Mass Transfer in Microchannels

Micro-electromechanical systems and nano-electromechanical systems have attracted a great deal of attention in recent years. The flow and heat transfer behaviors of micromachines for separation applications are usually different from that of macro counterparts. In this paper, heat and mass transfer characteristics of rarefied nitrogen gas flows in microchannels are investigated using direct simulation Monte Carlo with improved pressure boundary conditions. The influence of aspect ratio and wall temperature on mass flowrate and wall heat flux in microchannels are studied parametrically. In order to examine the aspect ratio effect on heat and mass transfer behaviors, the wall temperature is set constant at 350 K and the aspect ratio of the microchannel varies from 5 to 20. The results show that as the aspect ratio increases, the velocity of the flow decreases, so does the mass flowrate. In a small aspect ratio channel, the heat transfer occurs throughout the microchannel; as the aspect ratio of the microchannel increases, the region of thermal equilibrium extends. To investigate the effects of wall temperature (T w ) on the mass flowrate and wall heat flux in a microchannel, the temperature of the incoming gas flow (T in ) is set constant at 300 K and the wall temperature varies from 200 K to 800 K while the aspect ratio is remained unchanged. Results show that majority of the wall heat flux stays within the channel entrance region and drops to nearly zero at the halfway in the channel. When T w T in , the molecular number density of the flow drops rapidly near the inlet and the temperature of the gas flow increases along the channel. As T w increases, the flow becomes more rarefied, the mass fiowrate decreases, and the resistance at the entrance region increases. Furthermore, when T w >T in , a sudden jump of heat transfer flux and temperature are observed at the I exit region of the channel.

New Treatment of Pressure Boundary Conditions for DSMC Method in Micro-Channel Flow Simulations

Using DSMC to simulate micro flows in micro-channels, the numerical treatment of boundary conditions is very important. In this paper, several previous numerical treatments of boundary conditions are discussed with their merits and demerits, and a new treatment method based on the assumption of certain pressure distribution in the cells for boundary conditions is proposed. As comparable validity tests, it is applied in the DSMC simulations for the Poiseuille micro flows in micro-channels with four types of classical pressure boundary conditions. The dimensionless velocity profiles are shown and compared with analytical solutions derived from the Navier-Stokes equations with slip boundary conditions. The pressure distributions along the centerline of the micro-channel with the different boundary conditions are presented, and the simulation solutions agree well with the slip analytical solutions. As the Knudsen number increased, a strong linearity of the pressure distribution can be evidently predicted by the new method. Compared with the inlet and outlet velocity distribution, it is shown that the new method has better efficiency than the previous methods in the convergence.© 2007 ASME

Effects of wall temperature on the heat and mass transfer in microchannels using the DSMC method

Micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS) have become the research focuses which attract a great deal of attention in recent years. The fluidic and thermal behaviors are usually different from those of the macro devices. In this paper, the heat and mass transfer characteristics of the rarefied nitrogen gas flows in microchannels are investigated using DSMC method. In order to study the effects of the wall temperature (Tw) on the mass flux and wall heat flux in the microchannels, the temperature of the incoming gas flow (T∞) is set constant at 300 K, and the wall temperature varies from 200 K to 800 K. For all of the simulated cases, majority of wall heat flux stays within the channel entrance region and drops to nearly zero when it reaches the middle region of the channel. When Tw ≪ T∞, with the restriction of the pressure driven condition and continuity of pressure, the number density of the flow has to decrease along the flow direction eventually after a short increase at the entrance region. When Tw ≫ T∞, the number density of the flow drops rapidly near the inlet, and the temperature of the gas flow increases. As the Tw increases, the flow becomes more rarefied, the mass flux decreases, and the resistance at the entrance region increases. Furthermore, when Tw ≫ T∞, sudden jump in heat transfer flux and temperature are observed at the exit region of the channel.

Improving DSMC with New Pressure Boundary Conditions for Heat and Mass Transfer of Microchannel Flows

A new treatment of pressure boundary conditions for the DSMC method is proposed for flow prediction in microchannels. Validity and accuracy of the new method are verified by comparing to the analytical solutions of the micro-Poiseuille flow under slip condition. The new method shows better convergence compared with previous boundary treatments. This advantage becomes more remarkable as the geometry of the microchannel becomes more complex. A study on a microchannel with sudden expansion is demonstrated using the new DSMC method. Wall temperature in the expanded region of the microchannel independently varies from 200 to 800 K to study the effects on the pressure distribution, velocity, mass flow rate, and heat flux of the microchannel flow. The results show that the wall temperature in the expanded region significantly affects the microchannel flow. Some unique phenomena are observed to be quite different from those of the macroscopic flow and the mechanism of these interesting phenomena is discussed.

Rarefaction and temperature gradient effect on the performance of the Knudsen pump

The prediction of the multiscale flow in the Knudsen pump is important for understanding its pumping mechanism. However, there is little research on such interesting multiscale phenomenon in the Knudsen pumps. In this paper, a novel numerical analysis method combining the direct simulation Monte Carlo (DSMC) method with the smoothed particle hydrodynamics (SPH) method is presented for simulating the multiscale flow, which is often encountered in the application of the Knudsen pumps. Validity and accuracy of the new method are given by comparing its results with that of the previous research. Using the coupled multiscale approach, the rarefaction and the temperature drive are studied, which are two main factors on the performance of the Knudsen pumps. To investigate the effect of rarefaction on the performance of the Knudsen pump, various pump operation pressures are compared. The flow characteristics and pumping ability at different rarefaction are analyzed, and the phenomenon of the multiscale flow is also discussed. Several cases with different linear or nonlinear temperature gradients are set to investigate the effect of temperature gradient on the performance of the Knudsen pump. The flow characteristics of the Knudsen pump such as the velocity, pressure increase, and the mass flowrate are presented. A unique phenomenon, the reverse transpiration effect caused by the nonlinear temperature gradient is studied, and the reason of the significant pressure increase in the pump channel is also analyzed. Since the multiscale gas flow is widely encountered in the microflow systems, the above method and its results can also be greatly beneficial and provide significant insights for the design of the MEMS devices.

Cold Stretching of Cryogenic Pressure Vessels From Austenitic Stainless Steels

Austenitic stainless steel (ASS) exhibits considerable work-hardening upon deformation while retaining the characteristics of the material. The high rate of austenite deformation hardening was utilized by cold stretching (CS) of cryogenic pressure vessels. A few percent deformation will give the vessel a considerable and homogeneous yield strength improvement, and the wall thickness may be greatly reduced. The authors have conducted extensive experimental and numerical studies on CS of cryogenic pressure vessels from ASS. A summary of our work as well as a brief introduction of the history, standards, safety, and advantages of CS are given in this paper. What should be further investigated, such as fatigue properties of cold stretched ASS especially under cryogenic temperature, design of cold stretched transportable cryogenic vessels based on life, are also presented.Copyright

Synthesis of scalable microdroplets with fluid instability of shearing flow in microfluidics

This paper reports a microfluidic reactor for synthesizing micro- and nano-droplets using fluid shearing instability of immiscible flow in microchannels. Three factors, flow rate between the two inputs, spout geometry and surface tension are numerically studied. Based on the simulation, a device with the optimal parameters was fabricated and tested. Both computational fluid dynamics (CFD) simulation results and experimental results suggested that this technique is capable of conveniently controlling the droplets sizes from hundreds of micrometers down to several micrometers or even nanometers. This pilot research offers a proof-of-concept demonstration using instabilities in the microchannels to synthesize large quantity of microdroplets. Although the microdroplets in this study were formed by DI water containing Tween20 and silicon oil, the design with modification could be applied for synthesis of other types of micro- and nano-droplets.

Using the improved DSMC method to predict the reliability of micro-flow channels

Reliability of microfluidic designs is an extremely important issue that defines the range of applicability of micro-devices. The reliability design of micro-channels presents new complications that require extensions of todays method to incorporate reliable prediction methodologies. In this paper, using the improved Direct Simulation Monte Carlo, a prediction method was proposed for the reliability design of micro-flow channels. The Direct Simulation Monte Carlo method appears to be useful in micro-flow simulations because the corresponding Knudsen numbers are often beyond those that can be simulated by continuum approaches. Based on the assumption of certain pressure distributions with the second-order on the boundaries in the micro flow cells, the improved DSMC method was used to obtain the corresponding pressure and viscous forces of the micro-flow. For all boundaries of the microstructure interacting with the fluid experienced loads from the flow, the load was expressed as the sum of the corresponding pressure and viscous forces obtained from the DSMC method. Using the Finite Element Method, the stress distribution and the deformation of the microstructure under the loads can then be given. Considering failure criteria of the material and structure, the reliability parameters were further estimated and analyzed. In order to demonstrate the effects of the method in optimizing the design of microfluidic devices, the performance and reliability predictions are made for a microchannel under backward-facing flows. The results show that the method is necessary and effective to predict weak spots of micro devices and prevent the extreme deformation in design of microchannels.

Using Improved DSMC Method in Simulating for the Heat and Mass Transfer in Microchannels

Micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS) have become the focus of a great deal of attention in recent years. The flow and heat behaviors of micro-machines are usually different from that of macro-machines. In this paper, the heat and mass transfer characteristics of rarefied nitrogen gas flows in microchannels have been investigated using improved DSMC method. The influence of aspect ratio and wall temperature on the mass flowrate and wall heat flux in microchannels are studied parametrically. In order to examine the aspect ratio effect on heat and mass transfer, the wall temperature is set constant at 350 K, and the aspect ratio of the microchannel varies from 5 to 20. The results show that as the aspect ratio increases, the velocity of the flow decreases, and the mass flowrate also decreases. In a small aspect ratio channel, the heat transfer occurs throughout the microchannel, and as the aspect ratio of the microchannel increases, the region of thermal equilibrium becomes larger. To investigate the effects of wall temperature on heat and mass transfer, the aspect ratio of the microchannel is held constant at 10 and the wall temperature is changed from 300 K to 800 K. The results show that, when the wall temperature increases, the pressure increases slightly and the number density drops rapidly near the inlet. In addition, the Knudsen number increases as the wall temperature increases. It indicates that the increasing wall temperature of microchannel enhances the rarefaction of the gas flow. Moreover, as the wall temperature increases, the mass flowrate decreases, and the temperature will increase more rapidly near the inlet, with more heat transfer between the gas flow and the wall.Copyright

Microscale Numerical Simulation on Dynamic Damage for the Deflagrated Pressure Vessels

The defects on the surface of the deflagrated pressure vessels leading to damage are usually in the form of pits, scratches or machine marks. A microscale numerical simulation method is presented for the dynamic damage process under the instantaneous impacted shock. The surface defects are described as the rectangle gap, the round gap and the groove. For fcc and other close-packed metals material, the embedded atom method (EAM), which has proven particularly good at modeling bulk and defect properties of metals and metal alloys, is used to describe the interactions among the metal atoms. The Molecular Dynamics (MD) code named LAMMPS is used to calculate the velocity of atoms and the pressure distribution in dynamic damage process. The results show the different damage effects of the metal material with different surface defects under the instantaneous impacted shock by the deflagration.Copyright