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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 (Tw) on the mass flowrate and wall heat flux in a microchannel, the temperature of the incoming gas flow (Tin) 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 TwTin, the molecular number density of the flow drops rapidly near the inlet and the temperature of the gas flow increases along the channel. As Tw increases, the flow becomes more rarefied, the mass flowrate decreases, and the resistance at the entrance region increases. Furthermore, when Tw>Tin, a sudden jump of heat transfer flux and temperature are observed at the exit region of the channel.

Erosion-corrosion failures frequently found in piping systems can lead to the leakage of pipes, or even damage of the whole system. Erosion-corrosion is a form of material degradation that involves electrochemical corrosion and mechanical wear processes encountered on the surface of metal pipes. Fluid-structure interactions have a profound influence on such erosion-corrosion phenomena. This paper is focused on the multiphase flow-induced erosion-corrosion phenomena in pipes, with multiscale analysis, to study the interactions between the flow and the protective film inside the piping system. The shear stress and the pressure of the flow in a pipe with a step were first obtained using a multiphase flow dynamic analysis. The erosion-corrosion rules of the pipes under the multiphase flow were then summarized. Using the microscale flow simulation method, the fluid-structure interaction between the flow and the protective film at the critical position was modeled. The deformation of the protective films was shown to vary with the flow velocity and the corresponding flow regime. According to the simulation results of the fluid-structure interaction, the location, rate, and extent of the erosion-corrosion on pipe surfaces can be predicted. The prediction was also proven by actual instances. Moreover, the method can be used in optimizing the design of the inner sleeves of pipes.

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 today's 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.

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 (T w ) 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 T w ≪ 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 T w ≫ T ∞ , the number density of the flow drops rapidly near the inlet, and the temperature of the gas flow increases. As the T w increases, the flow becomes more rarefied, the mass flux decreases, and the resistance at the entrance region increases. Furthermore, when T w ≫ T ∞ , sudden jump in heat transfer flux and temperature are observed at the exit region of the channel.

Erosion results from interactions between the pipe surface and fluids traveling along the surface. Fluid-structure interactions have a profound influence on the erosion that takes place. The location, rate and extent of thinning or loss of a protective surface film depend strongly on the nature of the flow regime and interactions. Erosion-corrosion involves the modification, thinning and removal of protective films composed of corrosion product or scale deposits from a susceptible metal surface by fluid shear stress under high turbulence conditions. In the paper, multi-scale simulation of fluid-structure interactions between the flow and the protective films on the pipe surface is presented. The fluid shear stress and pressure of the flow in a pipe with a step is obtained by macro-fluid dynamic analysis. Viscous forces and the system’s pressure impose forces to the surface of the pipe. Using micro-simulation method, the fluid-structure interactions between the flow and the protective films is modeled. The deformation of the protective films is shown and changed with the different velocity and flow regime. Using the multi-scale simulation of fluid-structure interactions, the location, rate and extent of the erosion on the pipe surface can be predicted. The results are proved by the actual instances.Copyright © 2008 by ASME