Investigation Of Coupled DSMC-SPH Multiscale Approach For The Micro Chemical Machinery

The micro chemical devices based on the MEMS/NMES technologies have attracted a great deal of attention in recent years. These highly integrated systems have been widely applied to various applications. For these exciting applications, a thorough understanding of the flow behaviors in microchannel is becoming increasingly important for accurate prediction of performance during the design process. However, simulation of gas flow through micro chemical devices is very difficult because the flow in the channel usually presents a multiscale effect, which means that the characteristic length of the micro devices varies from macroscale to macroscale. In the macro region of the micro devices, the flow is in the continuum regime and the flow behaviors can be predicted by the continuum theories. In the micro region, the rarefied gas flow is often encountered in the micro chemical devices, which necessitates the use of molecular approaches such as a direct simulation Monte Carlo method (DSMC). On one hand, the use of continuum theories for the entire devices can produce inaccurate results, because classical CFD based on the continuum theories could not accurately predict the flow and thermal behavior of the rarefied flow in the micro region; on the other hand, simulation of such multiscale problems by DSMC alone is difficult, since the flow in macro regions are not rarefied, the DSMC method has low computational efficiency. Therefore, single method based on the continuum theory or the rarefied theory is not suitable for the multiscale flow in the micro chemical devices. In this article, an efficient approach for such multiple length scale problems is proposed by combining DSMC with SPH. The DSMC method is used for simulating the rarefied flows in the micro region and the SPH is used for simulating the flows in the continuum region. The concern of this article is on the multiscale approach and the multiscale flow characteristics, and the main contents and research results are as follows:(1) The boundary treatment of DSMC and SPH are improved, respectively. According to the assumption of a certain pressure distribution in the cells, two new methods of the pressure boundary treatment for DSMC simulation are proposed. The methods are applied in the numerical simulations for the gas flows in micro-channel. Validity and accuracy of the new methods are verified by comparing to the analytical solutions of the micro-Poiseuille flow under slip condition. The new methods shows better convergence compared with previous boundary treatments. Based on setting buffer cells at inlet and outlet of the channel, a new treatment of flow boundary conditions is proposed for SPH method. The validity of the improved SPH method is also verified by simulating 2-D Couette flow and Poiseuille flow, respectively.(2) Using the direct simulation Monte Carlo (DSMC) with improved pressure boundary conditions, the heat and mass transfer characteristics of rarefied gas flows in micro straight channel and micro fluidic channel are investigated, respectively. For the micro straight channel, from the investigation of the pressure, velocity, temperature and the mass transfer characteristics, the effects of aspect ratio and wall temperature on flow behaviors in microchannels are studied parametrically; for the microfluidic flow, the effect of the wall temperature on the flow and heat behaviors in the microfluidic channel is studied.(3) A novel multiscale method that combines direct simulation Monte Carlo (DSMC) method with smoothed particle hydrodynamics (SPH) is proposed firstly. The rarefied region is treated by DSMC method and the continuum regions are treated by SPH method. The continuum and DSMC regions are combined by an overlapped Schwarz alternating method with Dirichlet-Dirichlet type boundary. To achieve this multiscale approach, the details such as calculation domain, the coupled iteration and the information exchange at the interface are studied. Also, the convergence criterion and the calculation flow of the multiscale are discussed. Verification of this new multiscale approach is implemented by compare the new method with the previous result. The effects of parameters on the convergence efficiency are investigated. The results show that the new method not only has the characteristic of high efficiency, good convergence and high accuracy, and also it can be used for other problems, such as complex boundary simulation, transient character analysis, heat and mass transfer, multicomponent flows, and so on.(4) Using the new multiscale approach, the multiscale flows in the micro filters and the membrane separation are studied. For the micro filters, the mainly concern is on the comtrollable structure, and the effects of the controlled motion on the flow behaviors are investigated. For the membrane separation, the different multiscale flow behaviors between H₂ and CO in the membrane are studied, which is used for obtaining ultra-high purity hydrogen from impure feed streams. The pressure, velocity, mass flowrate and rarefaction of H₂ and CO obtained by the method are compared in detail. The effect of the orifice size on the multiscale flow is discussed. Some unique phenomena are observed to be quite different from that of the macroscopic or microscopic flow and results can be used to understand the separation mechanism of the membrane for hydrogen production.